Apr 2, 1987 - i n the e l p a s o l i t e l a t t i c e s C s ^ a l n C l ^ , ..... important class of ligands because they offer new possibilities in the field of ...... 772. 89. Ru[(dpp)Ru(phen) 2 ] 3 8 +. 760. 87. Ru[(dpp)Ru(tpy)Cl] 3 5 +. 758 ...... ^=fs. A~]. For the same donor (D = W (C 0) ^ ) the p o s i t i o n of e q u i l i b r i u m depends on.
Photochemistry and Photophysics of Coordination Compounds Edited by
H. Yersin and A.Vogler
W i t h 145 F i g u r e s a n d 3 8 T a b l e s
Springer-Verlag Berlin Heidelberg New York London Paris Tokyo
Proceedings of the Seventh International Symposium on the Photochemistry and Photophysics of Coordination Compounds E l m a u / F R G , March 29-April 2,1987
Priv.-Doz. Dr. Hartmut Yersin Institut fur Physikalische und Theoretische Chemie Universitat Regensburg Universitatsstr. 31, 8400 Regensburg, F R G Prof. Dr. A r n d Vogler Institut fur Anorganische Chemie, Universitat Regensburg 8400 Regensburg, F R G
Unhr.-Bibliofhek Regensburg
ISBN 3-540-17808-2 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-17808-2 Springer-Verlag New York Berlin Heidelberg This work is subject to copyright. A l l rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse o f illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions of the German Copyright Law of September 9,1965, in its version o f June 24,1985, and a copyright fee must always be paid. Violations fall under the prosecution act o f the G e r m a n Copyright Law. © Springer-Verlag Berlin Heidelberg 1987 Printed in Germany T h e use o f registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Printing and Binding: Druckhaus Beltz, Hemsbach/Bergstr. 2152/3140-543210
PREFACE
The
" S e v e n t h I n t e r n a t i o n a l Symposium on t h e P h o t o c h e m i s t r y
physics of Coordination Elmau
lying
Compounds"
i n a hidden
Partenkirchen,
valley
Federal Republic
was
and P h o t o -
h e l d i n t h e charming
of the Bavarian
Alps
o f Germany, from
above
Schloft
Garmisch-
March 29 t o
A p r i l 2,
1987. About thirty
ninety
p a r t i c i p a n t s from s e v e n t e e n c o u n t r i e s i n c l u d i n g
non-European s c i e n t i s t s
together
f o r this
symposium.
Forty-five
c o n t r i b u t i o n s were p r e s e n t e d . f o r many f o r m a l fic
oral
and t w e n t y - f i v e
These p r e s e n t a t i o n s
and t h e
came poster
opportunity
and i n f o r m a l d i s c u s s i o n s s t i m u l a t e d an i n t e n s e
scienti-
i n t e r a c t i o n between t h e p a r t i c i p a n t s .
This
meeting
(Koerner
followed
von G u s t o r f ) ,
(Wasgestian), 1984
Montreal
(Harriman).
[Ru(bpy)^]
s u c h as s o l a r
special
t h
Symposium
were
and p r o p e r t i e s o f complexes s u c h as
Cr(III)-complexes
of p o t e n t i a l (e.g.
were
applications
water s p l i t t i n g ) and
s u b j e c t s of t h i s meeting.
expanding f i e l d
of
Thus,
excited-state
chemistry. of
this
symposium was made
possible
the g e n e r o s i t y of Deutsche Forschungsgemeinschaft,
Chemischen CYANAMID
and s t o r a g e
were i m p o r t a n t
1974
K o l n 1978
and p h y s i c s o f c o o r d i n a t i o n compounds has become an i m p o r t a n t
organization
through
a series
that the r a p i d l y
7
photochemistry,
compounds as w e l l as Moreover,
i n Muhlheim
Scandola),
by t h i s
Furthermore,
energy c o n v e r s i o n technology
of inorganic
The
held
P a r i s 1982 ( G i a n o t t i ) and London
covered
organometallic
excited states.
was shown a g a i n
chemistry part
main f i e l d s
extensively.
photoresist
symposia
1980 ( S e r p o n e ) ,
and r e l a t e d
discussed
previous
F e r r a r a 1976 ( C a r a s s i t i ,
The
photo-redox processes, metal centered 2+
it
about
as f a r away as Japan and A u s t r a l i a
Industrie, GMBH ,
BASF
Degussa
AG,
AG ,
Bayer AG,
HOECHST
BMW
AG ,
AG,
financially Fonds
der
CIBA-GEIGY
AG,
DR. SEITNER-Me(Jtechnik,
Siemens AG, and Wacker-Chemie GmbH. Regensburg, J u l y 1987
Arnd V o g l e r
and Hartmut
Yersin
TABLE OF
Topic
CONTENTS
1: M e t a l - C e n t e r e d
Excited
States
P o l a r i z e d L u m i n e s c e n c e o f £pt(CN)2bipy] Single Crystals F i e l d and T e m p e r a t u r e E f f e c t s ( J . B i e d e r m a n n , M. W a l l f a h r e r , G. G l i e m a n n ) Light-Induced Excited Spin State Trapping i n Iron(II) ( S . D e c u r t i n s , P. G t i t l i c h , A. H a u s e r , H. S p i e r i n g ) I n f r a r e d Luminescence Spectroscopy ( C . R e b e r , H.U. G i i d e l )
of V
3 +
-
Magnetic 3
Complexes. 9
Doped C s N a Y X g
(X=Cl,Br)
2
17
Recent Progress i n Uranyl Photo-Physics (R. R e i s f e l d , C.K. J ^ r g e n s e n )
21
E f f e c t s of M a c r o c y c l i c and Cryptand Ligands Ions (N. S a b b a t i n i , S. P e r a t h o n e r , L . De C o l a ) Topic
2: P h o t o p h y s i c s
and P h o t o c h e m i s t r y
on P h o t o p h y s i c s
of Cr(III)
Complexes, Chromium(III) 31
Q u e n c h i n g o f T h r e e P h o t o a q u a t i o n Modes o f a C h r o m i u m ( I I I ) (A. D a m i a n i , P. R i c c i e r i , E . Z i n a t o ) S t e r e o c h e m i c a l C o n s t r a i n t s on t h e E x c i t e d S t a t e B e h a v i o r mium ( I I I ) ( J . F . E n d i c o t t , C.K. R y u , R.B. L e s s a r d , P . E , H o g g a r d ) E x c i t e d S t a t e Behavior as a Probe of Ground-State t i o n s i n C h r o m i u m ( I I I ) - P o l y p y r i d y l Complexes (M.Z. H o f f m a n , N. S e r p o n e )
Spectrum-Structure Complexes (T. S c h o n h e r r )
Splittings
Correlations
Multiphoton-Induced Picosecond p y r i d y l Complexes (N. S e r p o n e , M.Z. H o f f m a n )
+
25
Ligand F i e l d A n a l y s i s of the Doublet Excited States i n T r i s c h e l a t e d Complexes (A. C e u l e m a n s , N. B o n g a e r t s , L . G . V a n q u i c k e n b o r n e )
C o u n t e r i o n E f f e c t s on D o u b l e t ( P . E . H o g g a r d , K.-W. L e e )
of E u ^
Acidoammine^ 35 of
Ion-Pair
Chro39 Interac43
of Chromium(III)
Complexes 49
i n Hexacoordinated
Transition
Metal 55
Photophysics
of
Chromium(III)-Poly61
1
T o p i c 3: E x c i t e d S t a t e P r o p e r t i e s o f T r i s - 2 , 2 - B i p y r i d i n e and R e l a t e d C o m p l e x e s
Ruthenium(II)
On t h e O r b i t a l N a t u r e o f t h e L u m i n e s c e n t ated T r a n s i t i o n M e t a l Complexes
Orthometal-
Excited
State of
(V. B a l z a n i , M. M a e s t r i , A. M e i a n d r i , C. C o r n i o l e y - D e u s c h e l , P. J o l l i e t , U.
D. S a n d r i n i , L. Chassot, M a e d e r , A. v o n Z e l e w s k y )
71
C o r r e l a t i o n s B e t w e e n O p t i c a l and E l e c t r o c h e m i c a l P r o p e r t i e s o f R u ( I I ) P o l y p y r i d i n e Complexes: I n f l u e n c e of the Ligand S t r u c t u r e ( F . B a r i g e l l e t t i , A. J u r i s , V. B a l z a n i , P. B e l s e r , A. v o n Z e l e w s k y ) 79 Towards a Dynamic Model f o r t h e (M.A. C o l l i n s , E. K r a u s z )
Ru(bpy)3
2+
System 85
B r o a d - B a n d E m i s s i o n and Z e r o - P h o n o n L i n e s [ R u ( b p y ) 3 " ] (PFg) 2 A Comparison ( E . G a l l h u b e r , G. H e n s l e r , H. Y e r s i n )
of S i n g l e - C r y s t a l
_
Magnetic-Field [Ru ( b p y ) 3] 2 + H. Y e r s i n , E. (H. Highly Resolved pRu(bpy) ] X (G. H e n s l e r , E. 3
E f f e c t s and Gallhuber,
Highly G.
Resolved
93 Vibronic Structure
of
Hensler)
O p t i c a l Spectra
of
101
[0s(bpy) ] 3
2 +
Doped
into
2
Gallhuber,
H.
Yersin)
107
E x c i t e d S t a t e B e h a v i o r s o f R u t h e n i u m ( I I ) C o m p l e x e s as S t u d i e d by R e s o l v e d and T e m p e r a t u r e and S o l v e n t D e p e n d e n t E m i s s i o n Spectra ( S . T a z u k e , H.-B. K i m , N. K i t a m u r a )
Time 113
1
2 +
The L o w e s t E x c i t e d S t a t e s o f [ R u ( 2 , 2 ' - B i p y r a z i n e ) ( 2 , 2 - B i p y r i d i n e ) ] (H. K o b a y a s h i , Y. K a i z u , K. S h i n o z a k i , H. M a t s u z a w a ) 119 2
Quenching of E x c i t e d Ru(bpy)3 tures ( C z . S t r a d o w s k i , M. W o l s z c z a k )
with Methylviologen
a t Low
Tempera125
K i n e t i c s o f t h e C h e m i l u m i n e s c e n t O x i d a t i o n o f A q u e o u s B r " by Ru(bipyr) ( L . E l - S a y e d , D. S a l k i n S w a i m , W.K. W i l m a r t h , A.W. Adamson) 3 +
3
129
S y n t h e s i s and P h o t o p h y s i c a l S t u d i e s o f O r t h o - M e t a l a t e d p l e x e s I n c l u d i n g Two N o v e l P d ( I I ) / R h ( I I I ) D i m e r s (C.A. C r a i g , F . 0 . G a r c e s , R . J . W a t t s )
Pd(II)
Photoproperties of Ortho-Metalated I r ( I I I ) and (K.A. K i n g , F. G a r c e s , S. S p r o u s e , R . J . W a t t s )
Complexes
G r o u n d and E x c i t e d S t a t e (J.D. P e t e r s e n )
Rh(III)
Interactions i n Multimetal
Com135
141 Systems 147
P h o t o p h y s i c a l and P h o t o c h e m i c a l P r o p e r t i e s of Ruthenium(II) MixedL i g a n d C o m p l e x e s : P r e c u r s o r s t o H o m o n u c l e a r and H e t e r o n u c l e a r Multimetal Complexes C o n t a i n i n g Ruthenium(II), P l a t i n u m ( I I ) , Rhenium(I) and R h o d i u m ( I I I ) (D.P. R i l l e m a , H.B. Ross) 151 G r o u n d - and E x c i t e d - S t a t e A c i d - B a s e E q u i l i b r i a o f ( 2 , 2 ' - B i p y r i d i n e ) Tetracyanoruthenate(II) (M.T. I n d e l l i , C.A. B i g n o z z i , A. M a r c o n i , F. S c a n d o l a ) 159 Topic
4:
Photoredox
Processes
K i n e t i c s a n d M e c h a n i s m o f P h o t o c h e m i c a l F o r m a t i o n o f a. P y r a z i n e B r i d g e d F e ( I I ) P r o t o p o r p h y r i n IX P o l y m e r i c Compound ( C . B a r t o c c i , A. M a l d c t t i , R. A m a d e l l i , V. C a r a s s i t i )
167
C h a r g e - T r a n s f e r S t a t e s and Two-Photon P h o t o c h e m i s t r y Systems (R.M. B e r g e r , A.K. I c h i n a g a , D.R. McMillin)
of
Cu(NN)
+ 2
171
P h o t o i n d u c e d M u l t i e l e c t r o n R e d o x R e a c t i o n s i n t h e [PtC-dg] ^ " / A l c o h o l System: V i s i b l e L i g h t R e d u c t i o n of P l a t i n u m C e n t e r s ( A . B . B o c a r s l y , R.E. C a m e r o n , M. Z h o u ) Dynamic and S t a t i c O u t e r - S p h e r e ( L . C h e c c h i , C. C h i o r b o l i , M.A. Photochemistry (H. H e n n i g , D.
and I n n e r - S p h e r e Q u e n c h i n g P r o c e s s e s R a m p i S c a n d o l a , F. S c a n d o l a ) 181
and S p e c t r o s c o p y o f I o n P a i r R e h o r e k , R. B i l l i n g )
Backward E l e c t r o n T r a n s f e r w i t h i n E l e c t r o n T r a n s f e r Quenching ( T . O h n o , A. Y o s h i m u r a ) Photochemistry (J. Sykora)
C h a r g e T r a n s f e r Compounds 185
Geminate R a d i c a l P a i r
Formed
in
the 189
o f C o p p e r C o m p l e x e s and
i t s Catalytic
Aspects 193
I n t r a m o l e c u l a r E x c i t e d S t a t e E l e c t r o n T r a n s f e r from Cobalt(III)* (A.H. O s m a n , A. V o g l e r )
Naphthalene
to 197
P h o t o c h e m i s t r y o f C o o r d i n a t i o n Compounds o f M a i n G r o u p M e t a l s . R e d u c t i v e E l i m i n a t i o n of T h a l l i u m (III) Complexes (A. P a u k n e r , H. K u n k e l y , A. V o g l e r ) Topic
5:
177
Organometallic
205
Photochemistry
P h o t o p h y s i c s and P h o t o c h e m i s t r y o f T u n g s t e n C a r b y n e Complexes ( A . B . B o c a r s l y , R.E. C a m e r o n , A. M a y r , G.A. M c D e r m o t t ) The P h o t o i s o m e r i z a t i o n and P h o t o s u b s t i t u t i o n R e a c t i o n s of n i u m C l u s t e r H R u (CO) * ( / * - C 0 C H ) ( A . E . F r i e d m a n , P.C. F o r d ) 3
o
Ruthe-
3
M u l t i p l e E m i s s i o n f r o m (r\ " C H r ) Re (CO) L Complexes i n Room-Temperature S o l u t i o n (M.M. G l e z e n , A.J. Lees) 5
5
P h o t o c h e m i c a l l y Induced of T r a n s i t i o n Metals (C.G. K r e i t e r , K. L e h r )
the
213
2
217
(L = a S u b s t i t u t e d P y r i d i n e )
C-C-Bond F o r m a t i o n
221 i n the C o o r d i n a t i o n Sphere 225
T r i p l e t Q u e n c h i n g by M e t a l C a r b o n y l s (M. K u c h a r s k a - Z o n , A . J . P o e )
231
P h o t o e x c i t a t i o n o f W(C0)6 S o l u t i o n s C o n t a i n i n g cX-Diimine L i g a n d s . K i n e t i c s and M e c h a n i s m o f C h e l a t i o n f o r a S e r i e s o f Photoproduced W(CO)5(tf-Diimine) Intermediates ( A . J . L e e s , M.J. S c h a d t , L. Chan)
235
K i n e t i c s a n d M e c h a n i s m o f C-H A c t i v a t i o n F o l l o w i n g P h o t o e x c i t a t i o n o f (*r] -C H5) I r (CO) i n Hydrocarbon S o l u t i o n s * (D.E. M a r x , A . J . L e e s )
239
I d e n t i f i c a t i o n o f H -, D -, N2~Bonded I n t e r m e d i a t e s i n t h e Photocatalyzed Hydrogenation Reactions (A. O s k a m , R.R. A n d r e a , D . J . S t u f k e n s , M.A. Vuurman)
243
5
5
2
2
2
Cr(C0)
6
S p e c t r o s c o p y a n d P h o t o c h e m i s t r y o f N i (CO) 2 ( o t - D i i m i n e ) C o m p l e x e s ( P . C . S e r v a a s , D . J . S t u f k e n s , A. Oskam)
247
F e ( C O ) 3 ( R - D A B ) , a C o m p l e x w i t h Two C l o s e - L y i n g R e a c t i v e E x c i t e d States ( D . J . S t u f k e n s , H.K. v a n D i j k , A. Oskam)
253
The P h o t o c a t a l y t i c M e t a t h e s i s R e a c t i o n (T. S z y m a n s k a - B u z a r , J . J . Z i o ^ k o w s k i )
259
of
Olefins
Photochemical G e n e r a t i o n of N i n e t e e n - E l e c t r o n O r g a n o m e t a l l i c and T h e i r Use as R e d u c i n g A g e n t s i n M i c e l l a r S y s t e m s (D.R. T y l e r , V. M a c K e n z i e , A . S . G o l d m a n ) Probing Organometallic Photochemical P h o t o l y s i s of Mn (CO)« (A. V l t f e k , J r . ) I"
Mechanisms w i t h
Complexes 263
Quinones:
2
267
L a s e r F l a s h P h o t o l y s i s of P h o s p h i n e - S u b s t i t u t e d Dimanganese Compounds (K. Y a s u f u k u , N. H i r a g a , K. I c h i m u r a , T. K o b a y a s h i ) Topic
6:
Methods, A p p l i c a t i o n s ,
and
Other
Chelate
Photochemical Acid Glasses (R. R e i s f e l d ,
Behaviour M.
Eyal,
to
ns 277
Sulfide
in Liquid Solution 291 Metal
Complex 295
Semiconductors 301
f o r P h o t o l i t h o g r a p h i cA p p l i c a t i o n s 307
o f L u m i n e s c e n t Dyes i n S o l - G e l and R.
10
285
P r e s s u r e E f f e c t s on N o n r a d i a t i v e D e a c t i v a t i o n f r o m Excited States i n Solution (P.C. F o r d , J . D i B e n e d e t t o )
Inorganic Photoinitiators ( C . K u t a l , C.G. Willson)
ps
Polymers:
Temperature Dependent E m i s s i o n of Copper P o r p h y r i n s (M. A s a n o , 0. O h n o , Y. K a i z u , H. K o b a y a s h i )
H e t e r o g e n e o u s P h o t o c a t a l y s i s by M e t a l (H. K i s c h , W. H e t t e r i c h , G. T w a r d z i k )
271
Aspects
E l e c t r o n T r a p p i n g i n C o l l o i d a l T i 0 2 P h o t o c a t a l y s t s : 20 Kinetics ( C . A r b o u r , D.K. S h a r m a , C.H. L a n g f o r d ) The R a d i a t i o n S e n s i t i v i t y o f S e l e c t M e t a l M e c h a n i s t i c Changes a t H i g h e r E n e r g i e s (R.D. A r c h e r , C . J . H a r d i m a n , A.Y. L e e )
Carbonyl
Gvishi,
C.K.
Boric
J0rgensen)
I n d u s t r i a l A p p l i c a t i o n s of Organometallic (A. R o l o f f , K. M e i e r , M. R i e d i k e r )
313
Photochemistry
S y n t h e s i s and C h a r a c t e r i z a t i o n o f a / ^ - o x o - d i r u t h e n i u m C o m p l e x a s P r e c u r s o r t o an E f f i c i e n t W a t e r O x i d a t i o n C a t a l y s t ( F . P . R o t z i n g e r , S. M u n a v a l l i , P. C o m t e , J . K . H u r s t , M. G r a t z e l )
317 a ..
323
The A p p l i c a t i o n of D i f f u s e R e f l e c t a n c e L a s e r F l a s h P h o t o l y s i s t o M e t a l P h t h a l o c y a n i n e s i n an Opaque E n v i r o n m e n t (F. W i l k i n s o n , C.J. W i l l s h e r )
327
Quenching of (T. V i d o c z y ,
331
S i n g l e t Oxygen by S. N e m e t h )
Cobalt
Complexes
R e c e n t A d v a n c e s i n I n o r g a n i c and (R.E. W r i g h t )
Author
Organometallic
Photolithography*
Index
C o n t r i b u t i o n s m a r k e d w i t h a n a s t e r i x (#) a r e d e d i c a t e d t o P r o f . A.W. Adamson by h i s f o r m e r g r a d u a t e s t u d e n t s and p o s t d o c t o r a l associates.
335
339
TOPIC 1 Metal-Centered
Excited
States
POLARIZED LUMINESCENCE OF ( P t ( C N ) 2 b i p y ) TEMPERATURE EFFECTS
SINGLE CRYSTALS-MAGNETIC FIELD AND
J.Biedermann, M . W a l l f a h r e r , and G.Gliemann I n s t i t u t f U r P h y s i k a l i s c h e und T h e o r e t i s c h e Chemie, U n i v e r s i t a t Regensburg, 8400 Regensburg, FRG
ABSTRACT
The
polarized
ted.
On r a i s i n g
magnetic
luminescence
field
from
electric
field
vector,
.
temperature
crease
A
t o6 T reduces
crystal
[Pt(CN ^ b i p y ]
i s repor-
1.9 K t o 7 K o r i n c r e a s i n g t h e
from
0 t o 1 T the Ela-polarized
-175
cm
ofsingle
t h etemperature
a: crystallographic
emission
band ( E :
a a x i s ) i s blue
s h i f t e d by
i n c r e a s e t o 295 K o r a m a g n e t i c
field i n -
t h eemission
lifetime
of -10^
by a f a c t o r
and
2 -10
, respectively.
EXPERIMENTAL The
anhydrous
[ P t ( C N >2^ipy3 procedure
N N
:
z, b
Fig.
1.
Part o ft h e proposed
structure
of
the red modification of
[Pt(CN ) bipy] , 2
columnar
schematic.[1,2]
synthesized
described
[Pt(CN ) bipy] p
red modification was
belongs
by
of
by a
B i e l l i [31-
t o t h e group
of
planar
sional Fig.
d -transition
columns
1.
The
ments and
i n the
solid
apparatus
f o r the
metal
and
complexes which
state with
short
methods f o r the
magnetic
field
form quasi-one Pt-Pt
polarized
s t u d i e s are
dimen-
distances, emission
described
see
measure[4].
i n Ref.
RESULTS
Temperature Behavior 2
Figure
presents
[ P t (CN ) b i p y ] 2
Fig.
2.
E|[a
emission
the
and
Ela
e x
K.
Emission
emission
The
energies
s p e c t r a of of
the
dence
of crystal
V||
at
of
maxima
vs.
temperature
are
e m i s s i o n maximum d o e s n o t shifted
monotonically
temperature.
Within
the
and
plotted
depend the
on
in Fig.
temperature
I„
range
of a
=
^
e
x
z
K the
E||a
K i t is
blue
with
^ T ^ 295
36 f nm ) .
increasing K,
the
T it i s s h o r t e r t h a n 3 n s . B e t w e e n 1.9 K and 7 K t h e E l a II _1 maximum i s b l u e s h i f t e d by -175 cm and t h e c o r r e s p o n d i n g time 1^
i n c r e a s e s by
emission with It up
a factor
of - 2 0 .
i n c r e a s i n g temperature. becomes s h o r t e r by
t o 295
Above T > 7
increases monotonically
K.
The
a factor
and
the
lifetime of
•10^
K the
intensity at
when t h e
T =
energy 1^ 1.9
numbers emission
[ P t (CN ^ b i p y ]
A b o v e 20
K
depen-
wave
B e l o w 20
decreases
1.9
emission
the
crystal
$.
temperature.
intensity
E||a
the
of
single
crystal
Temperature
and
maxima
nm).
single
E l a and
3.
Fig.
polarized
single
(* =364
K
Polarized
polarized
1.9
at T =
spectra
[Pt(CN^bipy] T=1.9
of the
intensity of the
becomes K is
temperature
life-
emission
Ela
weaker 2 ms ,
increases
Polarized
The
Emission
15100
of Magnetic
Fields
e n e r g y a n d t h e i n t e n s i t y o f t h e E l a e m i s s i o n maximum a t 1 . 9 K
functions Fig.
i n the Presence
4
of
the strength 5,
and
of a magnetic
respectively.
field 0
Between
Hla
i s
and 1 T a
presented
blue
of
4. M a g n e t i c
(bipy)]
maximum
^ IT] 5
Fig.
of the E l a
emission
of a
[PtCCN)^
single
c r y s t a l a t T=1 . 9 K
single
cm
T=1.9
i s observed.
emission The
remains
Above
constant,
of
indicating a saturation
o f -28 r e a c h i n g
a constant of strength 2
influenced
by magnetic
6 Z +
strength n
position
magnetic
of -10
by a f a c t o r
dependence of
field
K (* =3
increases
value
.
H l a
at
-
of the E l a
effect
between
(Fig.
4).
0 and 4 T by a
at 4 T ( F i g . 5).
H = 6 T yields
6
[ P t ( C N ) ^ ( b i p y )]
crystal i n
H = 1 T the spectral
of
lifetime
a
H
of the E l a
™ )•
factor
field
of
I
e x
r e l a t i v e i n t e n s i t y I (H)/I^(H=0)
a
Intensity
magnetic
=364 nm ) . H l a . ex ^
-175
5.
dependence
field
w a v e n u m b e r v^
the
emission
U
in
shift
-
L [T] 5 H 6
Fig.
as
Application
a reduction
of
the
i,
The E||a e m i s s i o n
i s found
t o be n o t
fields.
DISCUSSION
The
observed
properties
complexes
o f C^
described
by
interchain
lowest
the
excited
c a n be d e s c r i b e d
symmetry.
a valence
coupling
respectively.[5] account,
v
Taking
by a system
electron ground
states
coupling state
c a n be c l a s s i f i e d
of
interacting
of the e l e c t r o n i c
band and a c o n d u c t i o n
of the molecular
resulting states
The e n e r g i e s
states are
band
generated
Pt5d
and
and s p i n - o r b i t
has symmetry according
to
by
an
P t 6 p ^ / C N tc* , coupling
A ( A
)
.A^ ,B^(
and 5
into the ) and
The
E l a emission
parentage, band.
which
These
Fig.
6.
Fig
6.
2 v
),
1.9
At
is
(self-trapped ) states A^.A^B^
shown a t t h e
emission
belongs
to
spin-orbit
components
of
IV: m a g n e t i c
a l l o w e d . At
A^
and
the
side
(I
transition
A^
H||y
temperatures
>
become t h e r m a l l y
^B^
II)
-> A'^
of
which
radiative (symmetry
) , I I I : magnetic
(symmetry
field
of
conduction
and
coupling neglected
coupling included ),
edge o f t h e
hand
[ P t ( C N ) ^ ( b i p y ) ] and
spin-orbit
(schematic).
lower
left
e n e r g y - l e v e l diagram
only v i b r o n i c a l l y
orbit
are
(symmetry C
H||z
from
p o s i t i o n e d below the
K the
Proposed
II:
field
are
states
transitons C
originates
(symmetry C 1.9
K
the
).
higher
repopulated
spin-
and
emit
directly
(non-vibronically i n t o the ground s t a t e , r e s u l t i n g i n a blue -1 shift o f -175 cm , an i n c r e a s e o f t h e e m i s s i o n i n t e n s i t y and a r e d u c t i o n of the l i f e t i m e . The b l u e s h i f t i s on t h e o r d e r o f one v i b r a tional
quantum.
transition
The
right
hand
[ P t (CN ^ b i p y ] magnetic
(case
C
is
a magnetic
Ajj/B^-ES] new
emission
corresponds
lifetime
T .,
side
lowers
of
Fig. 6
the
As
field
into
induced
a result
presents
the
opened.
This
lifetime
decrease
of the
the
field
v
from
E l a emission
field at
of the
1.9
levels
H||z
and
ground
induced K.
lowering and
states,
excited
state blue
the III)
s t a t e s A^
of these
lowest
of
H||y
(case
symmetry
excited
mixing the
non-excited
energy
s u b g r o u p C^
of the
corresponding
magnetic
singlet-singlet
Hla. For
A consequence
channel
vibrationally
explains
the
to the
interaction
of the
radiative
to the
< 3 ns ) .
s y m m e t r y C^
IV), respectively.
additional
A/A"(A^)
E||a
(short
i n a homogeneous m a g n e t i c
field
and
g
The
B'
A/A'(Ajj)
shift
and
a
state is the
ACKNOWLEDGEMENT
We
would
Deutsche
like
t o thank
t h e Fonds
Forschungsgemeinschaft
d e r Chemischen I n d u s t r i e
f o r support
and t h e
o f o u r work.
References: [1]
T e x t o r , M., O s w a l d , Untersuchungen
[2]
H. R., R o n t g e n o g r a p h i s c h e
an z w e i M o d i f i k a t i o n e n
Chem. 4 0 7 , 2 4 4 ( 1 9 7 4 ) .
platin(II).
Z. a n o r g .
Biedermann,
J . , Wallfahrer,
temperature
effects
allg.
[3]
M. , G l i e m a n n , G., M a g n e t i c - f i e l d a n d
on t h e o p t i c a l
bipyridyl-platinum(II ) single in
und s p e k t r o s k o p i s c h e
von D i c h l o r o - 2 , 2 ' - d i p y r i d y l -
properties
crystals.
of
dicyano-2,2'-
J . of Luminescence
(1987)
press.
Bielli, copic
E.,
Gidney,
studies
platinum(II)
P.M., G i l l a r d , R.D., H e a t o n ,
on some c o m p o u n d s with
2,2'-bipyridyl
(including and i t s
B.T., S p e c t r o s -
dimorphic s o l i d s ) of
analogeous.
J.
C.
S.
D a l t o n 4, 2 1 3 3 ( 1 9 7 ^ ) • [4]
Hidvegi, the
I . , Ammon, W. v . , G l i e m a n n ,
luminescence
yH 0. 2
J.
Chem. P h y s .
Gliemann,
G.,
2
4
5
2
field
e f f e c t s on
M [Pt(CN) ]•
7 6 , 4 3 6 1 ( 1 9 8 2 ) ; Ammon, W. v . , H i d v e g i , I . ,
Magnetic
Ln LPt(CN) 3 -yH 0
G., M a g n e t i c
of quasi-one-dimensional c r y s t a l s
field
single
effects
crystals.
J.
on t h e Chem.
luminescence Phys.
80,
of 2837
(1984) . [5]
Gliemann,
G.,
Yersin,
H
., S p e c t r o s c o p i c p r o p e r t i e s
o n e - d i m e n s i o n a l t e t r a c y a n o p l a t i n a t e ( I I ) compounds. Bonding [6]
and
6 2 , 87 (1985 ) .
Gliemann, sition
of the quasi
Structure
G., M a g n e t i c
metal
complexes.
field
effects
on t h e l u m i n e s c e n c e
of tran-
Comments I n o r g . Chem. 5., 2 6 3 ( 1 9 8 6 ) .
LIGHT-INDUCED EXCITED SPIN STATE TRAPPING IN I RON(II) COMPLEXES
S . D e c u r t i n s , P . G u t l i c h , A.Hauser, and H . S p i e r i n g I n s t i t u t f u r Anorganische Chemie und A n a l y t i s c h e Chemie, Johannes G u t e n b e r g - U n i v e r s i t a t , 6500 M a i n z , FRG
In the course of our s t u d i e s on the t h e r m a l l y induced high s p i n (HS) t r a n s i t i o n i n i r o n ( I I ) complexes / l / , metry, we have observed low temperature, i n t o the
^ l g
i n
t
^
G a
PP
r o x
i
m a t
-i
0 1 1
low s p i n (LS) °f ^
i n 1984 a new p h o t o p h y s i c a l e f f e c t /2/: I f , a t s u f f i c i e n t l y
t h e s o l i d s p i n c r o s s o v e r complex i s i r r a d i a t e d with green
+ ^
sym-
l i g a n d f i e l d a b s o r p t i o n band, the thermodynamica.ily
light
s t a b l e LS
s t a t e can be converted t o the metastable HS s t a t e and trapped w i t h p r a c t i c a l l y i n f i n i t e l i f e t i m e . We have c a l l e d t h i s unusual phenomenon "Light-Induced E x c i t e d Spin S t a t e Trapping (LIESST)". The f i r s t example, where we have seen the LIESST e f f e c t , i s [ F e ( p t z ) ^ ] ( B F ^ ^ ( p t z = 1 - p r o p y l t e t r a z o l e ) /2/. T h i s c o o r d i n a t i o n compound i s known t o e x h i b i t t h e r m a l l y - i n d u c e d s p i n t r a n s i t i o n w i t h h y s t e r e s i s of ca. 7 K width around 130 K /3,4/. The t r a n s i t i o n from the HSC^T^) s t a t e t o t h e LS(^A.) s t a t e i s accompanied by a d r a matic c o l o r change from white t o p u r p l e . A c c o r d i n g l y , the s i n g l e - c r y s t a l a b s o r p t i o n s p e c t r a of t h i s complex are q u i t e d i f f e r e n t i n the two s p i n s t a t e s as can be seen from F i g . 1 /5/. At 273 K, there i s only one a b s o r p t i o n band around 12250 cm ^ a r i s i n g from t h e q u i n t e t - q u i n t e t t r a n s i t i o n
~* E i n the HS molecules. At 8 K
t h i s a b s o r p t i o n band has completely disappeared
i n favour of two s i n g l e t - s i n g l e t
t r a n s i t i o n s , centered a t 18400 and 26650 cm \ corresponding t o ^A, + *T. and 1 1 i i T^ i n t h e LS molecules. A f t e r b l e a c h i n g the c r y s t a l with white l i g h t f o r c a . 2 min one sees a g a i n the t y p i c a l HS a b s o r p t i o n spectrum. Around 10 K, the trapped
2 7 3 K b. b. 8 K b. b.
8 K o.b.
F i g . 1: S i n g l e - c r y s t a l absorpt i o n s p e c t r a ( u n p o l a r i z e d ) of [ F e ( p t z ) ] ( B F ) before b l e a c h i n g (b.b.) a t 273 K and 8 K,and a f t e r b l e a c h i n g (a.b.) f o r 2min w i t h white l i g h t (tungsten lamp) a t 8 K (from / 5 / ) . 6
10
15
20
25
3
1
30 x 10 cm"
4
2
metastable HS s t a t e does not decay to any n o t i c e a b l e e x t e n t w i t h i n s e v e r a l days, e x c l u d i n g any t u n n e l l i n g p r o c e s s e s . Back r e l a x a t i o n to the thermodynamically s t a b l e LS(^A-^) s t a t e occurs only i f the temperature i s r a i s e d to 50-55 K.
I f the sample i s
heated f u r t h e r , one f i n a l l y observes the w e l l e s t a b l i s h e d t h e r m a l l y - i n d u c e d t r a n s i t i o n around 130 K.
LS
HS
We have f o l l o w e d the LIESST e f f e c t i n [ F e ( p t z ) ^ ] ( B F ^ ) a l s o by magnetic s u s c e p t i b i l i t y measurements /5/ and ~^Fe Mossbauer s p e c t r o s c o p y /2/. F i g u r e 2 shows a sequence of Mossbauer s p e c t r a of [ F e ( p t z ) ^ ] ( B F ^ ) f o r the l i g h t - i n d u c e d LS HS c o n v e r s i o n 2
2
at 15 K (a •> b ) , the t h e r m a l l y induced back r e l a x a t i o n HS -> LS around 50-55 K ( c , d ) , and the t h e r m a l l y induced LS •> HS c o n v e r s i o n around 130 K (e f ) . The HS s t a t e p r o duced by LIESST has the same Mossbauer parameters as the thermodynamically s t a b l e HS s t a t e above 130 K (compare s p e c t r a b and f i n F i g . 2 ) . 100
100
90
1'
80 100
T1
f
95 90 85 0> OS
F i g . 2: Mossbauer s p e c t r a of [ F e ( p t z ) ] ( B F ) . (a) Before
90
6
80
100
80
90 85 -2
0
a f t e r second h e a t i n g to 50-55 K and c o o l i n g to = 15 K; (e)
100
I(J
2
( c ) a f t e r b l e a c h i n g to 50-55 K and c o o l i n g t o = 15 K; (d)
100 95
A
b l e a c h i n g (measuring temperature 15 K ) ; (b) a f t e r b l e a c h i n g M f o r 1 h at 15 K (T = 15 K ) ;
a f t e r subsequent h e a t i n g to 97 K ( T = 97 K ) ; ( f ) a f t ee rr M
2
4
v/mms
0
2
The mechanism of LIESST can be e x p l a i n e d
t o 148 K
(from
111).
(T^
148
K)
4
v/mms"
- 1
heating 1
on the b a s i s of F i g . 3: I r r a d i a t i n g the c o l d
and \ - \ ( s . F i g . 1). sample induces s p i n - a l l o w e d t r a n s i t i o n s " A, - \ lg ^8 ^ ^-5 i The e x c i t e d s p i n - s i n g l e t s t a t e s are s h o r t - l i v e d and can decay back to the ground s t a t e w i t h i n nanoseconds. There i s , however, an a l t e r n a t i v e decay path, favoured by 3 s p i n - o r b i t c o u p l i n g , which l e a d s to a p o p u l a t i o n of the s p i n t r i p l e t s t a t e s T.. 3 and T ( i n t e r s y s t e m c r o s s i n g ) . These a g a i n decay v i a i n t e r s y s t e m c r o s s i n g , e i t h e r 1 5 to the A t ground s t a t e or to the m e t a s t a b l e T s t a t e . There i s no r a d i a t i v e de5 1 5 cay path from the T to the A. s t a t e , and the T (HS) s t a t e remains trapped lg lg 2g X
g
s
?
0 2 g
l g
n
0
w i t h p r a c t i c a l l y i n f i n i t e l i f e t i m e as long as the temperature i s s u f f i c i e n t l y so t h a t the energy b a r r i e r between the ^T^g
a n d
t h e
A
i
g
low
p o t e n t i a l s u r f a c e s , which
are w e l l separated by the l a r g e d i f f e r e n c e i n the m e t a l - l i g a n d
bond l e n g t h of ca.
0.2 8 between the two s p i n s t a t e s /6/, i s not t h e r m a l l y overcome. But the trapped HS s t a t e can be pumped back t o the LS s t a t e by i r r a d i a t i n g w i t h red l i g h t (of c a . 5 5 850 nm) i n t o the T -> E a b s o r p t i o n band /7/. 0
The
\
1 2
A-^
r e l a x a t i o n k i n e t i c s was
examined t o pure [ F e ( p t z ) ^ ] ( B F ^ ) ^ c r y s t a l s as
w e l l as f o r mixed c r y s t a l s [ F e ^ n ^ ^ p t z ^ ] ( B F ^ ) I t was
shown t h a t (a) f o r x & 0.1
2
u s i n g o p t i c a l spectroscopy
a s i n g l e - i o n treatment of both the s p i n
/8/.
equili-
F i g . 3: P o t e n t i a l surface diagram according to e x p e r i m e n t a l and c a l c u l a t e d energies of the l i g a n d f i e l d s t a t e s of [Fe(ptz) ] ( B F , ) (from r e f . / 5 / ) .
?
NUCLEAR
COORDINATE
brium ( w i t h A H ^ = H^g-H^g = 510(12) cm and
the T •> n
AS. = S HL
H g
-S
= 5.1(2) cm
l s
/K at T = 100K
r e l a x a t i o n of the l i g h t - i n d u c e d trapped HS s t a t e (with Arrhenius
a c t i v a t i o n energy E° = 810(30) cm ^ and frequency f a c t o r A £z 10"Vs) i s a p p r o p r i a t e a and (b) w i t h i n c r e a s i n g x c o o p e r a t i v e e f f e c t s become more and more important f o r both the HS ^ LS e q u i l i b r i u m and the r e l a x a t i o n of the trapped HS s t a t e . These statements become apparent from F i g s . 4-7.
F i g . 4: R e l a t i v e i n t e n s i t y of the ^A^
*Tj t r a n s i t i o n v s . temperature
f o r (*) [ F e ( p t z ) ] ( B F ) 6
4
[Fe Z _ (p ) ](BF ) x
n i
x
t Z
6
A
2
2
and (o) (x~0.1)
(from / 8 / ) . The abrupt t r a n s i t i o n f o r the pure i r o n complex i s t y p i c a l f o r a f i r s t - o r d e r phase t r a n s i t i o n due t o c o o p e r a t i v e e f f e c t s and agrees w i t h the r e s u l t s from Mossbauer and magnet i c s u s c e p t i b i l i t y measurements / 4 / . For the d i l u t e m a t e r i a l c o o p e r a t i v e e f f e c t s a r e l e s s important, and the s p i n conversion curve can be i n t e r preted as a continuous LS +-+ HS e q u i -
l i b r i u m w i t h AG = G^g-G^g = ^HS~^LS~~ -k T l n K = - ( HS- LS> = H L - H L - - k T In [ d - Y ) / Y ] ( Y ^ - mole T
S
S
A H
B
B
T A S
L S
H L
L S
f r a c t i o n of LS m o l e c u l e s ) . —
experimentol exponentiol
11 fJ / // till/ '/ 'X mi/ y strongly suggests that the photochemical and nonradiative decay modes of ^Ti/ E are coupled and involve the same intermediates. We propose here that the "seven-coordinate" aqua intermediate (Jamieson 1981; Serpone 1983b) e x i s t s i n the ground-state (reaction 3) and undergoes acid-base equilibrium ( r e a c t i o n 4) i n the neutral pH range. E x c i t a t i o n of these species r e s u l t s i n ^T^/^E with the same configuration. The nonradiat i v e behavior of ^T^/^E as a function of pH r e f l e c t s the subtle change of the v i b r a t i o n a l modes of the complex as H2O i s deprotonated; the photochemical pathway can be viewed as a r e l a t i v e l y minor branch o f f the same p o t e n t i a l energy surface. 3 +
3
0
s
2
3
Cr(NN) +
+
3
H0 2
3
H 0•••Cr(NN) + 2
3
< 4
» f
H 0'•'Cr(NN) 2
3+
[3]
3
3
"HO•••Cr(NN) + 3
+
H+
[4]
At high [ C l ~ ] , r e a c t i o n 5 could compete with reaction 3 i n a c i d i c s o l u t i o n , b r i n g i n g C I into the inner-sphere and r e s u l t i n g i n a diminution i n T b . In a l k a l i n e s o l u t i o n , the d r i v i n g force of reaction 4, and the i n t e r a c t i o n of 0H~ with the metal center and, perhaps, the n i t r o g e n - h e t e r o c y c l i c ligands, would minimize the presence of C I i n the inner-sphere. -
0
s
-
Cr(NN)
3+ 3
,CI"
«
"
"CI•••Cr(NN)
3+ 3
[5]
ACKNOWLEDGEMENTS This research was supported i n part by the O f f i c e of Basic Energy S c i ences, D i v i s i o n of Chemical Sciences, U.S. Department of Energy, and m part by the Natural Sciences and Engineering Research Council of Canada. The authors acknowledge the work of t h e i r associates, Drs. G. Neshvad, M. Bolte, and R. Sriram
REFERENCES B a l z a n i V, C a r a s s i t i V (1970) Photochemistry of c o o r d i n a t i o n compounds. Academic Press, New York B o l l e t t a F, Maestri M, Moggi L, Jamieson MA, Serpone N, Henry MS, Hoffman MZ (1983) Photochemical, photophysical, and thermal behavior of tris(1,10-phenanthroline)chromium(III) i o n i n aqueous s o l u t i o n . Inorg Chem 22: 2502-2509 Brunschwig B, S u t i n N (1978) Reactions of the e x c i t e d s t a t e s of subs t i t u t e d polypyridinechromium(III) complexes with oxygen, i r o n ( I I ) ions, ruthenium(II) and - ( I I I ) , and osmium(II) and - ( I I I ) complexes. J Am Chem Soc 100: 7568-7577 Ghosh, PK, Brunschwig BS, Chou M, Creutz C, S u t i n N (1984) Thermal and l i g h t - i n d u c e d reduction of R u ( b p y ) 3 i n aqueous s o l u t i o n . J Am Chem Soc 106: 4772-4783 Henry MS (1977) Prolongation of the l i f e t i m e of the E s t a t e of t r i s (2,2'-bipyridine)chromium(III) i o n by anions i n aqueous s o l u t i o n . J Am Chem Soc 99: 6138-6139 Henry MS, Hoffman MZ (1978) S o l u t i o n medium e f f e c t s on the photophysics and photochemistry of p o l y p y r i d y l complexes of chromium( I I I ) . Adv Chem Ser 168: 91-114 Jamieson MA, Serpone N, Maestri M (1978) Hydroxide i o n a s s i s t e d aquat i o n of t r i s ( 2 , 2 ' - b i p y r i d i n e ) c h r o m i u m ( I I I ) i o n . Inorg Chem 17: 24322436 Jamieson MA, Serpone N, Hoffman MZ (1981) Recent advances i n the photochemistry and photophysics of chromium(III) p o l y p y r i d y l complexes i n f l u i d media. Coord Chem Rev 39: 121-179 Jamieson MA, Langford CH, Serpone N, Hersey MW (1983a) Medium e f f e c t s i n chromium(III) photochemistry. Dynamic vs. s t a t i c processes i n t r i s ( b i p y r i d i n e ) c h r o m i u m ( I I I ) and trans-diammine(tetrathiocyanato)chromium(III) ions i n a c e t o n i t r i l e - w a t e r mixtures. J Phys Chem 87: 1004-1008 Jamieson MA, Serpone N, Hoffman MZ, B o l l e t t a F (1983b) Ground-state quenching of (^T^/^E)Cr(NN)3 . Anion and temperature dependence. Inorg Chim A c t a 72: 247-252 Kane-Maguire NAP, Langford CH (1976) E f f e c t s of quenchers on the emission and photoracemization of t r i s ( 1 , 1 0 - p h e n a n t h r o l i n e ) chromium(III). Inorg Chem 15: 464-466 L i l i e J , Waltz WL (1983) P u l s e d - l a s e r photochemical study of t r i s (2,2'-bipyridine)chromium(III) i o n i n a c i d i c and a l k a l i n e aqueous media. Inorg Chem 22: 1473-1478 L i l i e J , Waltz WL, Lee SH, Gregor LL (1986) P u l s e d - l a s e r photochemi c a l study of tris(1,10-phenanthroline)chromium(III) i o n i n a c i d i c and a l k a l i n e aqueous media. Inorg Chem 25: 4487-4492 Maestri M, B o l l e t t a F, Serpone N, Moggi L, B a l z a n i V (1976) K i n e t i c s of l i g a n d s u b s t i t u t i o n of t r i s ( 2 , 2 ' - b i p y r i d i n e ) c h r o m i u m ( I I I ) i n aqueous s o l u t i o n . Inorg Chem 15: 2048-2051 Maestri M, B o l l e t t a F, Moggi L, B a l z a n i V, Henry MS, Hoffman MZ (1978) Mechanism of the photochemistry and photophysics of the t r i s ( 2 , 2 ' bipyridine)chromium(III) i o n i n aqueous s o l u t i o n . J Am Chem Soc 100: 2694-2701 Neshvad G, Hoffman MZ, Bolte M, Sriram R, Serpone N (to be published) Photophysics of chromium(III)-polypyridyl complexes. Behavior of the ^ T i / ^ E e x c i t e d s t a t e s a a probe of ground-state i o n - p a i r i n t e r a c t i o n s . Inorg Chem Serpone N, Jamieson MA, Henry MS, Hoffman MZ, B o l l e t t a F, Maestri M (1979) E x c i t e d s t a t e behavior of p o l y p y r i d y l complexes of chromium( I I I ) . J Am Chem Soc 101: 2907-2916 Serpone N, Jamieson MA, Sriram R, Hoffman MZ (1981) Photophysics and photochemistry of p o l y p y r i d y l complexes of chromium(III). Inorg Chem 20: 3983-3988 3+
2
3+
Serpone N, P o n t e r i n i G, Jamieson MA, B o l l e t t a F, Maestri M (1983a) Covalent hydration and pseudobase formation i n t r a n s i t i o n metal p o l y p y r i d y l complexes: R e a l i t y or myth? Coord Chem Rev 50: 209-302 Serpone N, Hoffman MZ (1983b) Photochemistry and photophysics of chromium(III)-polypyridyls. A case study. J Chem Educ 60: 853-860 Sriram R, Hoffman MZ, Jamieson MA, Serpone N (1980) Ground state quenching of E excited s t a t e s of Cr(bpy)3 + C r ( p h e n ) 3 . - J A™ Chem Soc 102: 1754-1756 Van Houten J , Porter GB (1978) A u t o c a t a l y t i c chain reaction i n the photochemical decomposition of tris(2,2'-bipyridyl)chromium(III) i n dimethylformamide. Inorg Chem 18: 2053-2054 Wrona PK (1984) E q u i l i b r i u m constants of chromium(III) and chromium(II) inner- and outer-sphere complexes with c h l o r i d e , bromide, and iodide ions. Inorg Chem 23: 1558-1562 2
3
a
n
d
3+
T
a
s
a
F i g . 1. o b s function of the t o t a l concentration of CI" (addition of NaCl) f o r Ar-purged s o l u t i o n s a t 5° and pH 3.3 ( c o n t r o l l e d with HC1) . #, 15 fiU C r (phen) - ; A, 10 /*M C r ( 5 Mephen) +; O, 10 /xM C r ( 5 P h p h e n ) ; V , 10 aM Cr(4,7-Me phen) +; • 10 fiU Cr(3,4,7,8-Me phen) . 3
f
3
3
3
34
3
3
2
>.4
0.0
0.4
0.6
1.2
U
2.0
-log [ c r ]
2.4
2.8
3.2
3.6
3
3+
4
3
COUNTERION EFFECTS ON DOUBLET SPLITTINGS OF CHROMIUM(111) COMPLEXES
P.E.Hoggard
and Kyu-Wang Lee
Department o f C h e m i s t r y , North Dakota S t a t e U n i v e r s i t y , F a r g o , ND 58102, USA
Splittings
within
ions a r e s e n s i t i v e not
very
splitting
1981,
1986).
in
t ot h eexact
strongly affected
field
within
the
splittings,
It
appears,
field
theory
into
a
data.
i nTable
E state
e
splitting
that matter
7r v a l u e s
i s
field
model
p ianisotropyi s
compounds,
beyond
with
d e v i a t i o n from the a b i l i t y of
bounds on t h e
conclusion that ligand predicting (Flint
should
field
splittings 1977).
with
Table 1
n o t , however,
be t h r o w n
t h a t c a n r e a s o n a b l y be
1 are problematic. by s p i n - o r b i t
50 cm ^ w o u l d
ligand
I n an octahedral c o u p l i n g , so t h a t
be e x p e c t e d
I n t h e pentaammine s e r i e s ,
b e t w e e n NH^ a n d t h e o t h e r
+
effects
i sunsplit
complex.
o
simple
centimeters
Thet h e o r y
are s t i l l
much s m a l l e r t h a n
differences
+
a
f o r i f the exact
a r e seen,
of e f f e c t i v e l y
octahedral
e
a ligand
Ligand
t o dominate
that e
calculations.
o f t h edata 2
visible
1981).
the widespread
splittings
easily
from
be a c c o u n t e d
splittings
of reciprocal
as t h e r e
environment,
found
no p i a n i s o t r o p y , a n d l i t t l e
incapable
some o f t h e s e
as long
calculated
(Hoggard
approximation,
c a n be e x p e c t e d
we h a v e
t oe x p l a i n (given reasonable
andcausing
incorporated
All
e^ e f f e c t s although
t h a t f o r some v e r y
symmetry,
magnitudes o f tens
out
t othef i r s t
but
ofthe ligand
(Hoggard 1986)
substantial
i s simply
presents
are,
c o u p l i n g (Hoggard
however,
high
parameters), theory
ligand
butcan i np r i n c i p l e
orthoaxiality, ligand
geometry,
t h a t c a n make l a r g e c o n t r i b u t i o n s t o t h e d o u b l e t
i s known
relatively
bands i n d
o f t h e c o o r d i n a t i n g groups
of s p l i t t i n g s
spin-orbit
factor
geometry
lODq,
t h es p l i t t i n g s ,
a better predictor including
metal-ligand angular
thet r a n s i t i o n s
subshell,
doublet
by d i f f e r e n c e s i n t h e v a l u e s
parameter,
Since
determining
another
the intraconfigurational
f o r any
t h e d i f f e r e n c e s i n lODq
a r e o f t e n l a r g e enough
of thef i r s t
t o produce
q u a r t e t b a n d . B u t i t i s n o t lODq
f o r thedoublet
splittings,
and i n f a c t t h e
o f NH«, C l ~ , B r " , I " , and H 0 a r e n o t m a r k e d l y ?
Unk-Bibliofhek
Regensburg
Table 1 2 Eg S p l i t t i n g s
Nearly Orthoaxial
in
Splitting,
Complex
K [Cr(CN) ] 3
3
5
[Cr(NH ) Br]Br 5
[Cr(NH ) I]I 3
5
2
[Cr(NH ) H 0](C10 ) 3
5
different.
2
4
very
close
luminescence
complexes with
1974 1973
225
Flint n
305
tt
205
ti
to
spectra
different
of
at
most
50
cm
,
and
zero.
of
a number o f
cations
reveal
hexacyanochromium(III)
a considerable
variation
s p l i t t i n g s ( S c h l a f e r 1 9 7 1 ) , f r o m 78 cm ^ f o r t h e L i _1 f o r the Ph,As s a l t . S i m i l a r v a r i a t i o n s are seen 3-
§ 41
Flint
T h u s c a l c u l a t i o n s show s p l i t t i n g s
more t y p i c a l l y
The
3
Ref e r e n c e
^
49 2
2
cm
Complexes
175
6
[Cr(NH ) Cl]Cl 3
Chromium(III)
+
salt
in
to
+
cm
for
4
[Cr(CN)^]
substituted into
1977). T h i s thus
used
Figgis
suggests
the
a strong
known c r y s t a l
1981),
and
have
i n order
in
5
2
Table
1 might
be
orbitals
derived
from
The
Angular
contributions assume t h a t the
represent
sigma
counterions ion,
represented appropriate. electron at
at
any
need
as
only
point
(e
crystal
hypothesis
energetic
of
a
have 1974;
of
that
the
e f f e c t s on
the
counterions.
i n the
for e
of
a
( r , 6,
0
4,-3 E transition 2g g 1
absorption low
temperature
uring
an
spectra of
nents
the
rich
1
of
E
culated
K
1
due
Low
salts
3 a l l five
and
were
v i b r o n i c sidebands
angle
of
relation 10//2 • between the
energy p a r t of
[Cr(NK-j)^] (CdCl ) 3 6 5 r
the at
the
(9
c
u
b
of
f o r meas-
the
hexaammine
s p i n - o r b i t compoincluding
i n both cases
parts
by
f o r the of
(cf. F i g .
complete
• TT/1 80°
spectrum of
LF
d-electron
- 6)
trigonal
ligand
trigonal
the
chromophore can
octahedral
for
absorption
CrN^
absorption 4.2K.
4.2K
identified
certain limitations radial
applied
i n the
-copper
to reasonable
from the parameter (K+K') / Dq -
1
g
which provides
m a t e r i a l s , was
i n Ref. 2
Trigonal distortions
where 6 denotes the
device,
schemes have been r a t i o n a l i z e d
calculations considering
wavefunctions.
solid
in detail 2
f o r about s i x t y
p a r a m e t e r s K and
Figure
of
optical
v i b r o n i c band p a t t e r n
doublets)
electronic level
field
a new
v i b r o n i c s t r u c t u r e of the 2 less intense T. lines in
the
p e n t a c h l o r o c a d m i u m and
elaborated
(Kramers
assingments
Recently,
spectroscopy
extremely
c o m p l e x . As
The
spectra.
rare because the u s u a l l y covers
be
,
cal(1)
a x i s and
one
crystalline
1).
of the m e t a l - l i g a n d v e c t o r s well
as
ground
experimental and
state splittings
findings,
0.23° i n t h e
along
the
( 4 ) . The
and
6
trigonal
axis.
This two
reflects
lattices,
coincides w i t h X-ray r e s u l t s c o n v e n t i o n a l LFT
can
Otherwise
(usually
the the
theory
are
the
compression
copper
of
up
also
that
to high
ir-bonding can of
the
salt
i t i s concluded
accu-
be
neg-
electron repulsion
used) o r e x t e n s i o n s (vide
as
Cd-salt
distortions
f o r the
energies
symmetry and
required
i n the
a slight
expected value
experimental
a high
energies
0.19°
indicating
more s o p h i s t i c a t e d t r e a t m e n t s
spheric parameters are
cular o r b i t a l
t o be
(0.22°). T h e r e f o r e ,
rationalize
r a c y when c o m p l e x e s p o s s e s s
doublet
i n e x c e l l e n t agreement w i t h
i s determined
Cu-salt, respectively,
chromium c o m p l e x i n t h e
lected.
are
calculated
referring
to
mole-
infra).
4 + Angular
Overlap
C a l c u l a t i o n s f o r Os w i t h the
ions
Complexes
In
comparision
is
known a b o u t d - e l e c t r o n i c s t a t e s o f h e a v i e r m e t a l
pounds. Though i n r e c e n t w i t h a b s o r p t i o n and compounds
(e.g.
calculation
Refs.
of
5-8),
first
transition
spectroscopy
less
ions i n complex
com-
of hexacoordinated
assingments even of
levels
p e r i o d , much
s e v e r a l p a p e r s have been p u b l i s h e d d e a l i n g 4 +
transitions
energy
spin-orbit
years
luminescence
intraconfigurational
cluding
from the
are
still
open
the
best i n v e s t i g a t e d
f o r d i s c u s s i o n . The
f o r o c t a h e d r a l l y surrounded
c o u p l i n g , was
performed
Os
i n 1968
first
osmium(IV), i n -
(Dorain et a l . )
using
4 c o n v e n t i o n a l LFT. treated
up
Low
t o now.
To
compounds as w e l l sitions
and
group t h e o r y tures
and
s p e c t r a , band
even
the
of
the
given
levels
low
symmetry
vibronic
electronic
experimental
the molecules.
into
low
transtill
p r e d i c t i o n s of
p a r a m e t e r s o f osmium to the
properties of
splitting
theoretically
assignments are
to e l u c i d a t e the
, when d o p e d
experimental
complexes,
ones By
struc-
by
this
pro-
symmetry K S n C l ^
host
2
pattern of
the
spin-orbit
sta-
T (6,7) i s r e p r o d u c e d by u s e o f a n t i b o n d i n g p a r a -1 9" _i meters e = 9 5 0 0 cm and e = 1 8 0 0 cm . I n v e s t i g a t i o n s on t h e e n e r g y l e v e l schemes o f m i x e d h e x a h a l i d e compounds s u p p o r t t h i s r e s u l t ( 9 ) . In t h e f o l l o w i n g , t h e o b t a i n e d f i n d i n g s a r e a p p l i e d t o t h e w e l l r e s o l v e d 3 2— r
(8).
from
splittings
f o r z e r o - p h o n o n and
u n k n o w n AOM
geometrical
been
i n c o n t r a d i c t i o n to the
energy
f o r OsCl,-
that the
resulting
rules
i n order
hitherto
calculated
found
complexes have not
(6). Nevertheless,
partially
determine
adaptation of
lattice,
assign
selection
(8). Therefore,
have f i t t e d
c e d u r e was
tes
as
have been used
speculative
we
symmetry d
1 1
T
( a l l from i n t h i s molecule
a
-|g)
transitions
in
[OsCl^ox]
, ox
strong d e v i a t i o n from o c t a h e d r a l
a p p e a r s due
to the
rigidity
of
siderations
a bite
angle
about
of
the
= oxalate,
surrounding
o x a l a t e i o n . From g e o m e t r i c a l 74°
i s expected.
In our
AOM
con-
calcula-
tions for
t h ec h l o r i d e antibonding
should
h a s been a c h i e v e d
cant
o f t h echelate ligand
though easy t o handle
a good
field
while series
perfect f i t of the experimental angle
a. The o b t a i n e d
t h einfluence o f covalency
demonstrates that spectrum-structure
above
o f thespectrochemical
by v a r y i n g t h e b i t e
( a = 78 ± 1°) r e f l e c t s
as t h er i g i d i t y
a r eused a s given
lines
h o l d . F i g . 2 shows t h a t an a l m o s t
findings result well
parameters
t h eo x a l a t e parameters t h eguide
( a > 74°) a s
(a o--o/ T,-0.65ns \ • h-S
02 .f />
"
V
1
T~50ps
\ -v ••.
\ x
11t2ns
-J 400
*\ _
L.
440
480
520
560
600
640
680
WAVELENGTH, nm
F i g . 4: W a v e l e n g t h - r e s o l v e d t r a n s i e n t e m i s s i o n s p e c t r a t a k e n a t 0 p s f r o m t h e 266-nm a n d 355-nm l a s e r e x c i t a t i o n o f C r ( 5 , 6 - M e p h e n ) 3 . B o t h s p e c t r a were t a k e n under i d e n t i c a l c o n d i t i o n s except f o r t h e e x c i t a t i o n wavelength. 3 +
2
( d e c a y ) o f t h e 460-nm b a n d ; T ^ 50 p s . F i g u r e 6 s u m m a r i z e s t h e t r a n s i e n t a b s o r p t i o n r e s u l t s a s AA v s . t i m e . C l e a r l y , t h e r e a r e t w o d e c a y i n g t r a n s i e n t s a t ^ 4 6 0 nm a n d t h e r e i s o n e a t 5 4 1 nm ( s e e i n s e r t i n F i g . 6 ) . M o r e i m p o r t a n t , t h e r e i s a n a b s o r p t i o n r i s e f r o m ^ 1 n s t o 10 ns, which i s o u t s i d e o f + 2 c a t both wavelengths. E x c i t a t i o n o f C r ( 5 , 6 - M e p h e n ) 3 " " a t 266 nm s h o w s a s p e c t r u m ( F i g . 7 a ) r e m i n i s c e n t o f s o l v a t e d e l e c t r o n s ( t h u s a CTTS s t a t e i s p o p u l a t e d ; e q n 2 ) ; t h e t r a n s i e n t s p e c t r u m o f t h e f r e e l i g a n d i n 1M HC1 ( F i g . 7b) a l s o s h o w s f e a t u r e s 3
2
1
CK5,6-Me phen)3 in H 0 —i i i \ t i i 2
475
625
2
675
WRVELENGTH
(NM)
(b)
(a)
F i g . 5: ( a ) T r a n s i e n t a b s o r p t i o n s p e c t r a a t -50 p s ( A ) , -20 p s ( B ) , 0 ps (C) , a n d +50 p s (D) d e l a y t i m e s f r o m t h e 355-nm e x c i t a t i o n o f C r ( 5 , 6 - M e p h e n ) 3 • F r o m ( S e r p o n e 1 9 8 7 ) . (b) D i f f e r e n c e s p e c t r a a t v a r i o u s t i m e s a g a i n s t t h a t a t 0 p s ; i n s e r t shows t h e i n v e r s e d e c a y o f t h e 4 6 0 nm b a n d . 3 +
2
3
Cr(5 6-Me phen) * in H 0
24
k
•i 20
-
^
2
3
2
error .
'
541
1
6
transition are also
t h e above e x p e r i m e n t a l
c a n be f o c u s s e d
Ru
from a m e t a l
the lowest
complexes a r e o f the type R u ( L L ' )
where L L ' i s a l i g a n d
The b a s i s f o r
"spatially
and l i g a n d
processes,
o f t h e LUMO o r b i t a l .
energy and symmetry p r o p e r t i e s
The
resembles
centered
the ligand
properties
to correlate
and e m i s s i o n
t h e lowest energy
i n themetal-to-1igand and r e d u c t i o n
electrochemical
absorption
t h e p r o m o t i o n o f an e l e c t r o n
l i g a n d . T h e same m e t a l
the oxidation
studied
p o t e n t i a l s and v i c e v e r s a .
on t h e f a c t t h a t
TT o r b i t a l
centered
to obtain
t o the lowest antibonding,
m
free
that
t h e c o r r e l a t i o n s between o p t i c a l and
from e l e c t r o c h e m i c a l
TT o r b i t a l
t o be o f i n t e r e s t
( O h s a w a , D o d s w o r t h ) a r e much
t r a n s i t i o n involves
ligand the
processes continues
o f f e r the opportunity
such c o r r e l a t i o n s l i e s
centered
c o m p l e x e s a s s e n s i t i z e r s i n a number o f
generating
the reduction
and t h e r e d u c t i o n
potential f o r the free potential for
ligands,
the complexes,
E / (red), 1
2
TI*(LL):
S K
n*(LL')
III Z IU
< E
z u
1 / 2
(red)
: (ox) i/2
m o
i
2+
LL'
Ru(LLULL')
F i g . 1. O r b i t a l d i a g r a m f o r t h e e n e r g y c o r r e l a t i o n s b e t w e e n t h e c a l c u l a t e d e n e r g y o f t h e LUMO o f L L ' , t h e e l e c t r o c h e m i c a l p r o p e r t i e s of t h e f r e e l i g a n d L L ' , and t h e s p e c t r o s c o p i c and e l e c t r o c h e m i c a l p r o p e r t i e s o f t h e complexes.
T a b l e 1. R e d u c t i o n P o t e n t i a l Unoccupied Molecular O r b i t a l
Ligand
E
1
/
2
(LL')
and C a l c u l a t e d P r o p e r t i e s o f t h e Lowest (LUMO) o f t h e F r e e L i g a n d s .
a
LUMO
energy/eV
a) b) c) d) e) f) g) h) i) 1)
i-b iq bpy phen 4 ,4 '-dpb bpym pq biq bpz DP taphen
-2.20 -2.22 -2.04 -2.06 -1.80 -1.94 -1.74 -1.70 -1.18° -1 .26
-9.56 -9.71 -9.77 -9.70 -9.94 -10.01 -10.19 -10.47 -10.66 -10.78
(pN,pN)
(0.35, (0.44, (0.44, (0.41, (0.35, (0.44, (0.39, (0.45, (0.10, (0.12,
b
0.35) 0.44) 0.44) 0.41) 0.35) 0.34) 0.39) 0.45) 0.10) -0.12)
symmetry
*
X
a ) Room t e m p e r a t u r e d a t a i n a c e t o n i t r i l e , u n l e s s o t h e r w i s e n o t e d ; b ) MO a t o m i c c o e f f i c i e n t s o n c h e l a t i n g N p o s i t i o n s ; c ) i n d imethyIformamide.
Fig. bpy
2. L i g a n d s
(LL') and complexes:
and R u ( i - b i q ) ( b p y ) 2
Ru(bpy)2(4f4'-dpb) R
u(bpy) (
R
u(bpy) (bpz)
2
p
q
2
)
2
+
2
against
2 +
r
2 +
2 +
2
2 +
,
> (b)
2 +
,
( d ) 4,4'-dpb and
2 +
,
( f ) pq and
3
2
( i ) DP a n d R u ( b p y ) ( D P )
, (h) bpz and and ( j ) taphen and
.
1
*TT ( L L ) LUMO o f t h e f r e e
correlations
LUMO o f t h e f r e e
complex
( c ) phen and R u ( p h e n )
2
2 +
and R u ( i - b i q ) ^
( e ) bpym a n d R u ( b p y ) ( b p y m )
f
the calculated
causes good
,
(a) i - b i q
(g) b i q and R u ( b p y ) ( b i q )
r
Ru(bpy) (taphen)
the
2 +
a r e found
ligand
indicating that
i n the former
( c l o s e l y resembling that
ligands.
case
of the free
In both
reduction
occurs to
a n d t o t h e LUMO o f t h e
ligand)
i n the latter
AE,/
Fig.
c a s e . As one c a n s e e t h e two c o r r e l a t i o n s
effect lying
unoccupied
affected
Under C
the
ligand
due t o t h e
separation
approximates
^abs
hv
e m
leading to
arguments,
On t h e
t h e HOMO ( m e t a l c e n t e r e d ) and
interaction.
symmetry
transition
symmetry
c e n t e r e d ) o r b i t a l s , F i g . 1, a r e e x p e c t e d t o be most
by t h i s
2 v
o r b i t a l s o f matching
and i f a s i n g l e the excited
singlet)
h
this i s likely
o f t h e f o r m e r and d e s t a b i l i z a t i o n o f t h e l a t t e r .
o f energy
LUMO ( l i g a n d
well
F o r t h e complexes
i n F i g . 3 are characterized
o f i n t e r a c t i o n between o c c u p i e d m e t a l d o r b i t a l s and l o w e s t
stabilization basis
, tV
F i g . 4 . See t e x t .
3. See t e x t .
by d i f f e r e n t s l o p e s .
2
configuration
state,
Ru — >
LL' t r a n s i t i o n
t h e energy o f the a b s o r p t i o n ( t o
and e m i s s i o n ( f r o m t h e t r i p l e t ) maxima o f t h e MLCT * i n v o l v i n g t h e T T ( L L ' ) LUMO i s g i v e n by
( S )
(T)
=
AE =
A
E
1 / 2
1
/
2
+ A + B
w h e r e ^\/2 potential account
~
e
E
t 1 /2 ^
o x
^ "
E
i / 2 ^
r
e
d
^ '
E
l/2^
o
x
^
o f t h e c o m p l e x , and A and B i n c l u d e
solvation energies,
coulombic
energies.
Figure
inner
and o u t e r
i
s
t
h
e
terms
oxidation that
take
4 shows t h e r e l a t i o n
between t h e
spectroscopic
and e l e c t r o c h e m i c a l
observed w i t h
a few e x c e p t i o n s
( f u l l points
i n the figures).
discussion concerning
the reported
deviations w i l l
detailed elsewhere
As
quantities. Linear
relations are A more be
given
(Barigelletti).
one c a n s e e t h e l i g a n d s t r u c t u r e a p p a r e n t l y
properties of Ru-polypyridine * symmetry o f t h e of F i g u r e s ligands
into
sphere b a r r i e r s , and
complexes through
family with
i n designing
predicted
Acknowledgments. This Research C o u n c i l Swiss N a t i o n a l
electrochemical
linear relations
EHMO c a l c u l a t i o n s o n
new c o m p l e x e s o f t h e
by t h e I t a l i a n
Pubblica
free
Ru-polypyridine
and s p e c t r o s c o p i c
work was s u p p o r t e d
and M i n i s t e r o d e l l a
Science
that
a number o f
t h e e n e r g y and
TT ( L L ' ) LUMO. B a s e d o n t h e r e p o r t e d
3 a n d 4, i t i s s u g g e s t e d
can help
determines
properties. National
I s t r u z i o n e a n d by t h e
Foundation.
References
Barigelletti
F, J u r i s
A, B a l z a n i V, B e l s e r
P, v o n Z e l e w s k y A ( t o b e
published) D o d s w o r t h E S , L e v e r A B P ( 1 9 8 6 ) Chem P h y s L e t t
1 2 4 : 152 a n d
references
therein J u r i s A, B a r i g e l l e t t i A ( t o be p u b l i s h e d ) Ohsawa Y, H a n c k KW,
F, C a m p a g n a S, B a l z a n i V , B e l s e r Coord
P, v o n Z e l e w s k y
Chem R e v
D e A r m o n d MK
( 1 9 8 4 ) J E l e c t r o a n a l Chem 1 7 5 : 229
TOWARDS A DYNAMIC MODEL FOR THE R U ( B P Y )
M.A.Collins
2 + 3
SYSTEM
and E.Krausz*
Research School o f C h e m i s t r y , A u s t r a l i a n N a t i o n a l C a n b e r r a 2601, AUSTRALIA
U n i v e r s i t y , G.P.O.Box 4 ,
INTRODUCTION
The R u ( b p y )
2 +
(RBY) chromophore/luminophore
has been the s u b j e c t of a great d e a l o f
study by a range o f s p e c t r o s c o p i c techniques. Some recent evidence,- such as the time resolved luminescence o f Ferguson and Krausz (FK) (1982, 1986a) magnetic
circular
p o l a r i z a t i o n o f luminescence (FK 1982, 1986a,b,c) anomalous Zeeman e f f e c t s (FK 1987),(Krausz 1987), magnetic c i r c u l a r d i c h r o i s m r e s u l t s o f Ferguson, Krausz and Vrbancich (1986) and a l s o s o l i d phase e x c i t e d s t a t e Raman by Krausz (1984), have pointed t o the p o s s i b i l i t y of v i b r o n i c c o u p l i n g i n t h i s system. Summarizing our c u r r e n t i d e a s , much o f the spectroscopy o f RBY r e q u i r e s a d e l o c a l i z e d d e s c r i p t i o n o f the metal to l i g a n d charge t r a n s f e r e x c i t e d s t a t e , yet other e v i d e n c e , p a r t i c u l a r l y e x c i t e d s t a t e Raman measured i n s o l u t i o n p o i n t s to e x c i t a t i o n o f a s i n g l e b i p y r i d i n e l i g a n d . C r i t i c a l o b s e r v a t i o n s o f luminescence changes i n p a s s i n g from r i g i d to l i q u i d phases (FK 1986a,b,1987b) i n d i c a t e s an e n v i r o n m e n t a l l y induced l o c a l i z a t i o n process. Some aspects o f the luminescence below 10 K bear a remarkable resemblance t o the changes i n p a s s i n g from r i g i d to f l u i d environments and i n g e n e r a l p r o v i d e evidence f o r s t r o n g v i b r o n i c c o u p l i n g . Our c o n t e n t i o n i s then, that although d e l o c a l i z e d , the e x c i t a t i o n has a tendency to l o c a l i z e , which becomes more pronounced
i n long l i v e d e x c i t e d s t a t e s below 10 K and
furthermore l e a d s t o the luminophore " d i g g i n g i t s e l f a h o l e " i n f l u i d environments. Thus some i n h e r e n t tendency to l o c a l i z e i s compensated
by a r e l a x a t i o n o f the
environment, which i t s e l f causes a g r e a t e r degree o f l o c a l i z a t i o n e t c . . In order to access the parent ( v i b r o n i c ) problem t h e o r e t i c a l l y , a g r e a t d e a l of s i m p l i f i c a t i o n of our d e s c r i p t i o n s o f the system must be made. We are d e v e l o p i n g a model t h a t c o n t a i n s the b a r e s t e s s e n t i a l s of the known p r o p e r t i e s o f RBY and yet can s t i l l be lead to a u s e f u l v i b r o n i c a n a l y s i s . We c o n s i d e r a s i n g l e ,
non-degenerate
deformation mode o f (each) bpy l i g a n d and couple i t to a s i m p l i f i e d m e t a l - l i g a n d charge t r a n s f e r e x c i t a t i o n process. P o t e n t i a l s u r f a c e s of the r e s u l t i n g
(total-
fnolecule) modes a r e then d e r i v e d a l o n g w i t h v i b r o n i c energy l e v e l s . Some attempts a t c a l c u l a t i n g i n t e n s i t y (Rranck-Condon)
f a c t o r s between v i b r o n i c l e v e l s
Figure 1.
Model for vibronic analysis.
(at 0 K) are absorption
then made, i n order to access the observed Stokes s h i f t between
and
discussed
e m i s s i o n . The relevance to e x i s t i n g e x p e r i m e n t a l i n f o r m a t i o n
is
and o u t l i n e s o f f u t u r e developments g i v e n .
THEORY
C o n s i d e r a t r i g o n a l arrangement of the metal M and In the ground e l e c t r o n i c s t a t e , we c o n s i d e r associated
H
w i t h each l i g a n d (or m e t a l - l i g a n d 3 2 E p /2m - *n n=l
=
g
where p
+
2
+ kq /2 n M
three i d e n t i c a l l i g a n d s
bond). We w r i t e
k'q q n n-l M
M
x
i s the momentum o p e r a t o r of the v i b r a t i o n on the nth l i g a n d , q
n
v i b r a t i o n a l coordinate,
n
m
n
coupling
m
an e l e c t r o n i c e x c i t e d s t a t e o f the ML complex, i n an 3
e x c i t o n model, as a l i n e a r combination o f e x c i t e d s t a t e s a s s o c i a t e d
w i t h each
l i g a n d . The e x c i t e d s t a t e i n our case r e f e r s to the MLCT s t a t e w i t h the
Let
force
to q q - I f a l l the
normal modes o f v i b r a t i o n i n the ground s t a t e are degenerate, then no q q terms w i l l o c c u r . We d e s c r i b e
(1)
7
the
n
m the e f f e c t i v e mass o f the l i g a n d v i b r a t i o n , k the
c o n s t a n t and k' i s a f o r c e constant f o r terms p r o p o r t i o n a l
e l e c t r o n occupying a n
2 3*
j u s t one v i b r a t i o n a l degree o f freedom
transferred
orbital.
|g > be the ground e l e c t r o n i c s t a t e o f l i g a n d n and n
|e > the e x c i t e d s t a t e o f n
l i g a n d n. Thus the e l e c t r o n i c e x c i t e d s t a t e i s w r i t t e n as \Y> = c |e >|g >|g > + c | >|e >|g > + C3| >|g |e > 1
1
2
3
a useful notation
2
gl
2
3
gl
2
(2)
3
i s |g>=|g^>|g >|g > and a* the r a i s i n g o p e r a t o r which c r e a t e s an 2
3
e x c i t a t i o n on the nth l i g a n d (going
from |g > to |e >« Thus equation (2) becomes n
n
3
|T>
= I ca
+
n
n
|g>
(3)
n=l a
i s the r e v e r s e ( d e s t r u c t i o n o p e r a t o r ) and
n
we assume that d i f f e r e n t s t a t e s
are
o r t h o n o r m a l . Thus
e
a
a
e
e
< nl n J n> = < nKlSn> = and
< e
e
n
l n> =
1
4
as no s t a t e e x i s t s below the ground s t a t e a | g > = 0 so that n
= a
n
a
< e
„ K n l 8 n > ' = a
a
n
0
5
When the nth l i g a n d i s e x c i t e d , the v i b r a t i o n a l energy a s s o c i a t e d changed: the Born-Oppenheimer (BO)
p o t e n t i a l energy s u r f a c e
with q
f o r (say)
R
ring
is
s t r e t c h i n g i s d i f f e r e n t f o r e x c i t e d s t a t e bpy" from ground s t a t e bpy. We denote the change i n p o t e n t i a l energy f o r the n'th l i g a n d v i b r a t i o n as A V ( q ) . n
w r i t e the e l e c t r o n i c H a m i l t o n i a n of the e x c i t e d ML^
Thus we
can
complex as
3 H = H + E a a AV(q ) g - n n n' n=l +
V M
v
(6) '
The number o p e r a t o r ( a * a ) w i l l g i v e the eigenvalue 1 i f the nth l i g a n d i s e x c i t e d n
so that the t o t a l energy w i l l be changed by A V ( q ) . We now
c o n s i d e r the e f f e c t of
n
resonant energy t r a n s f e r . Since a l l three l i g a n d s are s i m i l a r , e l e c t r o n i c e x c i t a t i o n can be t r a n s f e r r e d from one l i g a n d to another. This s h a r i n g changes the energy. The o p e r a t o r a _ ^ a n
n
n - l t h l i g a n d . Thus we a r r i v e at our 3 H = Z{p /2m in n=l 2
Hamiltonian
2
+
+ k q / 2 + k'q q ^n n n-l n
(3) and
+
+
+ a a AV(q ) + M[a ,a + a a J) n n n' n-1 n n n-l
M
M
where M i s the energy g a i n (or l o s s ) due equations
total
a c t i n g on |Y> moves the e x c i t a t i o n from the nth to the
1
(7)
J
to resonant
energy t r a n s f e r . Combining
(7) we can w r i t e the BO energy s u r f a c e as the e x p e c t a t i o n v a l u e
of the H a m i l t o n i a n f o r the e l e c t r o n i c s t a t e , n e g l e c t i n g the n u c l e a r k i n e t i c energy ( p ) i n ( 7 ) ; t r e a t i n g the n u c l e a r c o o r d i n a t e s q n
3 = I k q / 2 + k ' q q . - n n n-l n=l 2
1
M
H
M
as parameters we d e r i v e
n
2
+ |c | A V ( q ) + M[c*c n n n n-1 1
1
V M
7
S o l v i n g the s e c u l a r equation H| MO^e)Y> g i v e s
1
- + c* - c } n-1 n J
2
E = z + I kq /2 +
= E
v
'
(8)
k^n^n-l
where e s a t i s f i e s
M M
1
AV(q )-e M 2
M M AV(q )-
(9)
3
The c u b i c e q u a t i o n i n e i s e a s i l y s o l u b l e i f AV^ energies s
1
2
3 = 2M,
-M,
-M
=
2
3 0> g i v i n g the e l e c t r o n i c
(the -M l e v e l i s doubly degenerate).
I f AV i s non
v a n i s h i n g , we assume i t can be expanded i n a power s e r i e s AV(q)
=
VQ
+ v q + v q 1
for s i m p l i c i t y we
2
+
(10)
t r u n c a t e the s e r i e s a f t e r the q u a d r a t i c term; s i g n i f i c a n t v i b r o n i c
e f f e c t s can be d e s c r i b e d at t h i s l e v e l of approximation. equation a r i s i n g from the s e c u l a r determinant truncation,
The s o l u t i o n s to the c u b i c
can be e v a l u a t e d , u s i n g t h i s
by c o n s i d e r a b l e tedious but computer a s s i s t e d a l g e b r a i c m a n i p u l a t i o n ,
to o b t a i n the e n e r g i e s
2 f
3
z
1
e
2
= v
Q
+ 2M + v
= v
Q
- M + v
2
lPl
s
3
= v
2
2
2
lPl
2
/ > l 3 + v ( p ^ + r ) / 3 + | v | r/J6 - r v|/18M 2
+ [v |v |/3v 2
2
/ > l 3 + v ( p ^ + p + p ) / 3 + v^(p|+p )/9M
1
(11)
x
2
2
- v /18M]r cos39
1
2
2
2
- M + v /l6 - r v^/18M
Q
lPl
2
+ [-v |v |/3v 2
1
a r e
where f>i 2 3
t
*
ie
2
+ v^/18M]r cos39
1
n o r m a
Pi = (q +q +q )/>l3, 1
1
2
l
coordinates with frequencies ^
3 defined
2
a s
p =(q -q /2-q /2)^l2/>l3 and p =(q -q )/>l2
3
2
1
2
3
3
and the p o l a r c o o r d i n a t e s have the u s u a l The energy s u r f a c e f o r e
2
(12)
3
definition,
p =rcos9 and 2
p =rsin6 3
has minima at r^O. The s u r f a c e has three l o c a l minima as
3
a f u n c t i o n of 9, the w e l l known Mexican hat s u r f a c e , F i s h e r
Introducing dimensionless
(1984).
v a r i a b l e s , and T being the n u c l e a r k i n e t i c energy
o p e r a t o r , we w r i t e
h
1
= (T+8 -v -2M)/"hw
x
n
= >l(mw / h)p
u
1
= v />l(3m(A) h) ; u
1
0
J
2
1
;
n
;
2
= 2v /(3mco ) u
2
2
h
2
h
n
=
3
(T+s -v +M)/tiw 3
0
2
(13)
2
2
= v /(9Mm« )
3
2
R
the n u c l e a r dynamics a r e d e s c r i b e d
Q
Hamiltonians
2
2
2
2
[P +(w x /(A) ) ]/24-y x +y x^/2+[P +p|/R ]/2+R [l/2+y /2+y ] 1
1
2
1
1
2
2
3
= [P^(w x /w ) ]/2+u x1+u x^/2+[P +P^/R ]/2+R [Up -p ]/2 + 2
2
;
2
= p /^l(mw n)
n
p o l a r c o o r d i n a t e s P , R, 9, P
2
=
x
2
2
by the three n u c l e a r BO
h
= (T+e -VQ+M)/"hw
2
R - ^l(ma> /h)r ; P
2
In dimensionless
h
1
1
2
2
1
2
2
2
2
3
(14)
|u |R/>l2 [p |M |/y -y ]R cos(39)/2 2
1
h
+
2
1
1
3
2
3
2
2
2
2
= [P^ ( w x / w ) ] / 2 u x 4 - u x ? / 2 [ P P / R ] / 2 + R [ l y - y ] / 2 +
1
1
2
+
1
1
2
+
+
+
2
3
-
2
|M |R/>l2 [-y |u |/y M ]R cos(39)/2 1
+
2
1
1+
3
DISCUSSION
The three e l e c t r o n i c e i g e n v a l u e s
z^
2
3
determine three e l e c t r o n i c s t a t e s at each
n u c l e a r c o n f i g u r a t i o n v i a the s e c u l a r equations
( 9 ) . These s t a t e s form a complete
b a s i s f o r the exact v i b r o n i c v a v e f u n c t i o n s , F i s h e r (1984), which have y e t t o be evaluated. Here we d i s c u s s some f e a t u r e s o f the a d i a b a t i c n u c l e a r dynamics and e i g e n s t a t e s determined by the d e r i v e d BO Hamiltonians
h^ 3 2
The nondegenerate v i b r a t i o n , w i t h c o o r d i n a t e X p shows the same o r i g i n and frequency s h i f t s i n a l l three e x c i t e d e l e c t r o n i c s t a t e s : The ground and e x c i t e d s t a t e s u r f a c e s are as shown i n F i g u r e 2 u n l e s s ^ ^ " ( ^ i /
w 2
)
v
»
n
e
n
anharmonic terms a r e s i g n i f i c a n t
and the e x c i t e d s t a t e s u r f a c e may d i s p l a y a double w e l l form. The displacement o f the minimum, p r o p o r t i o n a l to U p w i l l c o n t r i b u t e to a p r o g r e s s i o n of t h i s mode i n the a b s o r p t i o n spectrum as w e l l as a Stokes s h i f t of the order o f 2
((^Uj/o^) - ( c o / ) . 2
The v i b r a t i o n s w i t h c o o r d i n a t e s x and X 3 a r e degenerate i n the ground s t a t e and 2
remain so i n the s t a t e d e s c r i b e d by h p where o n l y a frequency these v i b r a t i o n s e f f e c t a J a h n - T e l l e r d i s t o r t i o n
s h i f t occurs. However
which s p l i t s the degeneracy of the
e l e c t r o n i c s t a t e s o f energy -M, to produce d i s t i n c t BO Hamiltonians
h and h^. I f 2
only l i n e a r e l e c t r o n - n u c l e a r c o u p l i n g i s i n c l u d e d , u =P2=0> the degeneracy of the 2
v i b r a t i o n a l c o o r d i n a t e i s s t i l l r e t a i n e d i n both h^ and h - The r a d i a l dependence o f 2
these p o t e n t i a l energy s u r f a c e s i s shown i n F i g u r e 3, c l e a r l y i l l u s t r a t i n g the displacement
of the minimum i n h^. While v i b r o n i c c o u p l i n g between s t a t e s on these
two s u r f a c e s w i l l be s i g n i f i c a n t f o r a l l v i b r a t i o n a l l e v e l s , the lowest l e v e l s should be approximately
d e s c r i b e d w i t h i n the BO approximation
energy
i f the energy
s p l i t t i n g , p r o p o r t i o n a l to U p i s s u f f i c i e n t l y l a r g e . Assuming that \x^>2 and
Figure 2
Potential energy as a function of the non-degenerate coordinate.
Figure 3
Potential energy as a function of the "Mexican Hat" radius.
1^,1^-0, the J a h n - T e l l e r d i s t o r t i o n i s expected to produce a Stokes s h i f t o f the 2
order of 3y /4-2. I n t e r e s t i n g l y , t h i s c o n t r i b u t i o n to the t o t a l Stokes s h i f t may smaller
vibration
x^.
When U 2 3 = 0 » =u
tne
motion of the angular c o o r d i n a t e
However, the second order c o u p l i n g constants ^ p r o p o r t i o n a l to cos(39) which hinders
y >0, so that the two
9 i s that of a f r e e r o t o r .
and u^ i n t r o d u c e
states
l i g a n d , we expect that UpU2>0 and
terms i n h^ p r o p o r t i o n a l to cos(39) may
3
a potential
the r o t a t i o n a l motion. For e x c i t e d
which i n v o l v e a e l e c t r o n t r a n s f e r to the bpy
The
be
that the c o n t r i b u t i o n caused by the o r i g i n s h i f t of the nondegenerate
be of opposing s i g n s .
d i r e c t i o n of the d i s t o r t i o n a s s o c i a t e d w i t h t h i s angle 9 would depend on
a c t u a l magnitude of u 2
9
and y
v
the
We note that the angular p o t e n t i a l energy i s
p r o p o r t i o n a l to R , so that the cos(39) term i s much more s i g n i f i c a n t i n the e x c i t e d s t a t e where the o r i g i n s h i f t i n c r e a s e s
the average value of
third
R.
While the a n g u l a r momentum i s not constant when U2>3*0, i t f l u c t u a t e s about a u
2
quantized
value,
(1 , 1=1,2,3...) when y
r a d i a l motion i n 113
2
and y^ are s m a l l . In t h i s l i m i t ,
can be approximately d e s c r i b e d
the
i n terms of the r a d i a l p o t e n t i a l
i n F i g u r e 3,
w h i l e the angular motion i s that of a hindered r o t o r at the average
r a d i u s R. The
wavefunctions of t h i s hindered r o t o r may
Mathieu f u n c t i o n s , to d e s c r i b e maxima i n c o s ( 3 9 ) .
the t u n n e l l i n g and
be obtained
i n terms of
" f r e e " s t a t e s below and
Without going i n t o d e t a i l s the Franck Condon (FC)
t r a n s i t i o n s from the ground e l e c t r o n i c / v i b r a t i o n a l s t a t e , where 1=0, s t a t e s of h3
can be evaluated.
of the order of 1 i f 1=0 so on. While one
in
€3.
It i s straightforward The
above the
factors for to the
rotor
to show that the FC f a c t o r i s
f a c t o r i s 0 i f 1-1,
(U3-M21 y-^ | / y
1
)
2
i f 1=2
can i n f e r from the form of the r a d i a l p o t e n t i a l energy t h a t
r a d i a l w a v e f u n c t i o n FC f a c t o r s f o r t r a n s i t i o n s to upper v i b r a t i o n a l l e v e l s o f £3 f a v o r a b l e , o n l y the component of these l e v e l s w i t h 1=0
and
the are
are r e a d i l y e x c i t e d .
L o c a l i z a t i o n corresponds to t r a p p i n g w i t h i n a s i n g l e minimum of the cos(39) p o t e n t i a l w e l l w i t h i n the s t a t e d e s c r i b e d
by h^. Of course e i g e n s t a t e s
t r i g o n a l symmetry of the H a m i l t o n i a n so that no e i g e n s t a t e one w e l l . Any considers
r e t a i n the
can be l o c a l i z e d w i t h i n
l o c a l i z e d wavefunction i s t h e r e f o r e a time dependent s t a t e . I f
one
the e x c i t a t i o n to be i n i t i a l l y l o c a l i z e d , the c o r r e s p o n d i n g wave packet
w i l l spread on a t i m e s c a l e determined by t u n n e l l i n g between the e q u i v a l e n t
minima i n
the cos(39) p o t e n t i a l . I t i s c l e a r that t h i s t u n n e l l i n g time i s long when y^ i s l a r g e and
1Vi^I
i s
l a r g e . The
dominant i n f l u e n c e i s of the l i n e a r c o e f f i c i e n t
FUTURE DIRECTIONS
A n a l y t i c s o l u t i o n s to the e i g e n s t a t e s difficult
to e v a l u a t e .
and
FC f a c t o r s f o r the r a d i a l f u n c t i o n s
Numerical e v a l u a t i o n of the f u l l v i b r o n i c problem i s
are now
f e a s i b l e as we have a n a l y t i c forms f o r the e l e c t r o n i c m a t r i x elements and p o t e n t i a l energy s u r f a c e s ,
together w i t h a BO b a s i s s e t . Numerical c a l c u l a t i o n w i l l a l s o
e v a l u a t i o n o f magnetic moments (and p o l a r i z a t i o n s ) of e i g e n s t a t e s . i n p a r t i c u l a r w i l l be s t r o n g l y i n f l u e n c e d by v i b r o n i c c o u p l i n g
These
allow
properties
parameters and should
allow a u s e f u l d e s c r i p t i o n o f recent experiments to be developed.
REFERENCES
Ferguson J , Krausz E, (1982) The assignment of the Luminescent States of +
R u ( b p y ) . MCPL and Time-Resolved Luminescence 3
at 2K, Chem Phys L e t t
Ferguson J , Krausz E, (1986a) Time Resolved Luminescence
93: 21-25.
and MCPL of R u ( b p y )
2 +
in
Glassy S o l v e n t s a t the F l u i d - G l a s s T r a n s i t i o n , Chem Phys L e t t 127: 551-556.
Ferguson J , Krausz E, (1986b) MCPL Evidence f o r the T r a n s i t i o n from D e l o c a l i z e d to L o c a l i z e d Luminescent States o f R u ( b p y )
2 +
i n Glass-Fluid
Media,
Inorg Chem 25: 3333-3335.
Ferguson J , Krausz E, (1986c) The E x c i t a t i o n Dependence of the Luminescence of R u ( b p y )
2 +
and MCPL
i n R i g i d S o l u t i o n s . J Lumin 36: 129-141.
Ferguson J , Krausz E, (1987a)
Magnetically
Perturbed, Temperature-Dependent and
Time Resolved Aspects o f the P o l a r i z e d Luminescence
of R u ( b p y )
2 +
Below 10 K.
Chem Phys 112: 271-283.
Ferguson J , Krausz E, (1987b) P o l a r i z e d Luminescence
A b s o r p t i o n , Luminescence
and Magnetic C i r c u l a r
o f Dicarboethoxy D e r i v a t i v e s of R u ( b p y )
2 +
i n R i g i d and
F l u i d S o l u t i o n s : Evidence f o r E n v i r o n m e n t a l l y Induced Charge L o c a l i z a t i o n . J Phys Chem 91: ( i n p r e s s ) .
Ferguson J , Krausz E, V r b a n c i c h J , (1986) Magnetic C i r c u l a r Dichroism and the Assignments o f M e t a l - L i g a n d Charge T r a n s f e r Systems, Chem Phys L e t t
Fisher G,(1984)
States
i n Ru(bpy)
2 +
and R e l a t e d
131: 463-467.
V i b r o n i c C o u p l i n g , Academic Press,
London.
Krausz E, (1984) E x c i t e d S t a t e Raman Spectra i n the Luminescent States o f R u ( b p y ) ^ . Chem Phys L e t t
116: 501-504.
Krausz E, (1987) Zeeman E f f e c t s i n A b s o r p t i o n and Luminescence l i n e s i n R u ( b p y ) o ( P F ) . Chem Phys L e t t ( i n p r e s s ) . 6
?
o f the Zero Phonon
BROAD-BAND EMISSION AND ZERO-PHONON L I N E S OF [Ru(bpy) ](PF ) - A COMPARISON 3
6
SINGLE-CRYSTAL
2
E.Gallhuber, G.Hensler,
and
H.Yersin
I n s t i t u t f l i r P h y s i k a l i s c h e und T h e o r e t i s c h e C h e m i e , 8400 R e g e n s b u r g ,
FRG
INTRODUCTION In
this
paper
single-crystal ture
r e p o r t on r e s u l t s o f
[Ru(bpy) ](PF ) . 3
6
1
has
been d e t e r m i n e d :
molecular Thus,
we
For t h i s
2
The
complex
t h r e e f o l d axes a r e p a r a l l e l
[Ru(bpy) ](PF ) 3
spectroscopy,
6
investigations
on
compound t h e c r y s t a l ions
l i e on D
3
sites
strucand
t o the c r y s t a l l o g r a p h i c c
s i n g l e c r y s t a l s are w e l l - s u i t e d f o r
2
which a l l o w s
grouptheoretical
neat
all axis.
polarized
a s s i g n m e n t s o f the e x c i t e d
2
s t a t e s by use of t h e s e l e c t i o n r u l e s may
be much b e t t e r r e s o l v e d due
ening.
Indeed,
a t low
phonon l i n e s ' ^ ' ^ ,
. Moreover,
to a r e d u c t i o n
single-crystal
temperatures i t i s p o s s i b l e to r e s o l v e
which d e l i v e r s c o n s i d e r a b l y
spectra
of inhomogeneous b r o a d the
more i n f o r m a t i o n
lowest e x i t e d s t a t e s t h a n had been deduced from the broad-band 5 6 of t h e complex d i l u t e d i n o r g a n i c g l a s s e s or host m a t r i c e s . f o l l o w i n g we
want t o d e m o n s t r a t e
spectra
t h e zero-phonon
their
and
the p a r a l l e l i s m between the
lines
of
[Ru(bpy) ](PF^) 3
t e m p e r a t u r e dependence and m a g n e t i c - f i e l d
2
with
zero-
about
the
spectra In the
broad-band respect
to
behavior.
TEMPERATURE DEPENDENCE
Broad-band Figure emission
Spectra 1 shows the t e m p e r a t u r e - d e p e n d e n t
spectra
resolution.
(Eic,
E = electric
The main band
On t e m p e r a t u r e i n c r e a s e becomes dominant
a t 1.4
band
K
field
vector)
(band I) has
i n o f band
by a f a c t o r of about
of t h e
r u n a t low
' ' ,
i n which
a b l y more a l l o w e d the
different
4 t o 5 up
s i d e a t 570
t o 10
from t h e l o w e r one,
i n t h e i n s e t of F i g .
increase
nm
nm. and by
K. two-state
from t h e s e c o n d s t a t e i s c o n s i d e r -
transitions involve different
shown s c h e m a t i c a l l y temperature
the t r a n s i t i o n
than the t r a n s i t i o n
spectral
I I i s accompanied
T h i s t e m p e r a t u r e dependence i s r e a d i l y i n t e r p r e t e d by a 9 R f\ model
total
i t s maximum a t 585
I I grows i n on the b l u e
above 2 K. The g r o w i n g
an i n t e n s i t y i n c r e a s e
development
1.
and
i n which
vibronic patterns Thus,
the b l u e
as
is
shift
on
r e s u l t s from a d i s p l a c e m e n t o f the c e n t r e
of the
18000 J
1
cm" i
v L
15000 l2£ . ^ ! ! . e x p ( - A E / k T ) !«• 1r
from
k
I2E'
follows
*2L -150
+ 80
slope: (7.5±1) cm-
1
= AE
-2 0.4
0.2
Fig.
2. A r r h e n i u s p l o t
two l o w e s t
s t a t e s I E ' and
2E*.
The s p e c t r a
and
h e i g h t s . The i n t e n s i t i e s
the
emission
vibronic spectra
corresponding least the
spectra
of
from
cm of
intercept line
of the corresponding
distribution.
spectra
ratio
through
o f these
from
f o r the r a t i o
transitions
crys-
maximum
s p e c t r a are
(the
(T
and
0-
found 5 '
and III>
equilibrium.
1
cm"
1
1
1
\emission
and H
1
The
(abs.)
1
left
733 767
16800
scales
16600
16200 i
(T =
2 K)
16000
i n -
The (see the
1
cm"
are strongly enlarged.
2
II') =0.8 II) • 0.6 IH>
of the wavefunctions
of wavelengths mixing
3
v
IE') =0.6 II) -0.8 in>
16400 i
[Ru(bpy) ](C10^)
H =0T
of single-crystal hand
spectra
II) -*I0> • 481 cm'
A
659 667
17000 i
the m a g n e t i c - f i e l d induced 4 c o n d i t i o n s a r e summarized i n r e f . .
from
17200 i
331 352 372 440 481
respectively.
result
= 6 T,
I11 >
1
| Av = 161 cm'
17400
and a b s o r p t i o n
ni
17600
experimental
The
T
sets).
= 0
I I ' > and
H
states
at
568.5
(emission
x
1. E m i s s i o n
U
Figure
567.5
!
1
111 cm' 1 i /1 \ 1 H = 6T abs,' I \ T\ i», ! line I r \ A
/
\ /1 ]l> 11 III
v
H = 0T
17600
!• A
1
8.2 I cm" I /?*
1 1 line n i i
abs./ / / /
17620
Therefore,
The to
) a r e summarized 6
e n e r g i e s o f Raman m o d e s .
gies
safely
coupled
allows to assign
t o t h e zero-phonon
supported
by t h e f a c t
that
The good
and t h e i r
energy
i n T a b l e I and agreement
separations compared
t h e o b s e r v e d modes t o v i b r a t i o n a l
transition.
Further,
t h e Raman i n t e n s i t i e s
this
modes
assignment
of just
the
c l o s e c o r r e s p o n d e n c e between t h e e n e r g i e s o f t h e
vibronic
t h e Raman modes a l s o
o f l i n e I as
peaks
modes o f [Ru(bpy)-] (CIO.)
Energy separations
[cm
]
[cm
1
]
Raman 6
data [cm
1
]
at T = 2 K
Resonance Raman enhancement factors and
remarks
0
-
17444
161
165
17402
203
198
4
17274
331
337
8
17605
Moreover,
transition.
Table I . V i b r a t i o n a l
1
t h e assignment
.
( l i n e I)
17253
352
-
17233
372
372
17165
440
(464)
17124
481
478
zero-phonon
16946
659
659
16938
667
668
line
4 (20) II>->I0>+481
cm
4 150 II' >-»IO>+667
16872
733
730
>5
16838
767
765
2
16591
1014
1012
4
16579
1026
1025
8
16432
1173
1173
20
16333
1272
1278
6
16282
1323
1317
12
16110
1495
1490
25
16048
1557
1558
58
is
modes
and
substantiates
experiment
these
strongly
Emission
i n t h e r e s o n a n c e Raman
to
between t h e s e e n e r -
are
zero-phonon
enhanced
f r o z e n out
m a i n l y from I I > .
e n e r g i e s o f t h e d o m i n a t i n g peaks
l i n e I ( a t 17605 cm
the
III> i s e s s e n t i a l l y
a t T = 2 K t h e e m i s s i o n from
and t h e s p e c t r u m r e s u l t s
cm
peaks a
With mixing II'>.
application
of
III>
of high
and II>,
mainly
the
spectroscopic
Further,
properties
i0>
* II>
at
reproduced sorption
H = 0 T.
of
I0>
-» I I I ' >
to
8.2 cm"
1
magnetic
higher
fields,
From
the
spectrum
state
at
structure
the
state.
1
lower
AE 5
(at H = 6 T ) . '
these t r a n s i t i o n s occur
ab-
Due t o t h e Zeeman e f f e c t ,
s h i f t e d to
respectively,
t o 11 cm"
spectrum,
induced
energy
and
increasing
8
Also
at exactly
from
under
high
t h e same e n e r g i e s
presented
discussion
i t follows
that
the
vibronic
III>.
Thus,
from
a comparison o f the
the
vibronic
a t H = 6 T one f i n d s
o f t h e o b s e r v e d p r o m o t i n g modes a r e t h e same f o r t h e
example,
states.
distinctly
However,
t h e 481 cm
I I > -» I0>
the
of
for
T = 2 K and H = 6 T ( F i g . l b ) i s m a i n l y g o v e r n e d by
lowest e x c i t e d
tion
part
a t H = 0 T ( F i g . l a ) with the s t r u c t u r e
most
III>-
absorption.
wavefunction of state
while
one a l s o
finds distinct
mode i s more s t r o n g l y t h e 667 cm
mode
promote t h e t r a n s i t i o n III>
coupled
to the t r a n s i -
(and some low
-» 10>.
two
differences.
energy
ones)
Since the d i s t r i b u t i o n
p r o m o t i n g modes i s d i f f e r e n t f o r t h e two e m i t t i n g
states
one
e a s i l y understand the blue s h i f t
s p e c t r a with a p p l i c a t i o n of high The discussed properties 2+ [Ru(bpy)^]
to
spectra
a t H = 6 T (T = 2 K) and a t
the thermal r e p o p u l a t i o n 5
c o n s t a n t o f III > It
of the non-resolved broad band 10 magnetic f i e l d s . o f t h e two l o w e s t e x c i t e d states of
suggest the occurrence of s i m i l a r v i b r o n i c
emission
(due
two
strong
one e x p e c t s a g r o w i n g i n
the enlarged
energy,
( a t H = 0 T)
state
shows t h e m a g n e t i c - f i e l d
I0> -» I I ' > i s s l i g h t l y
i n e m i s s i o n and
that
Indeed,
t o the very lowest e x c i t e d
energy
rate
a
excited
I0> -» I I ' > which i s n o t o b s e r v a b l e
i n Fig. l b , clearly
the
the
lowest
of the o r i g i n a l
with i n c r e a s i n g magnetic f i e l d s
the zero-phonon a b s o r p t i o n
can
one o b t a i n s
t h e r a d i a t i v e d e c a y r a t e from s t a t e III> i s 4 5 t h a n from s t a t e I I > ' , t h e t r a n s i t i o n I I ' > -» I0> r e f l e c t s 7-9
of
of
fields
the perturbed
Due t o t h e f a c t t h a t
much l a r g e r
For
magnetic
giving
is
states
and t h e r e l a t i v e l y
worthwhile t o mention that I I > and III>
t o t h e v i b r a t i o n a l modes,
This
i n d i c a t e s very s i m i l a r geometrical and
III>
with respect
any
pronounced
of
(H = 0 T) radiative
by e x p e r i m e n t .
the emission spectra
do n o t e x h i b i t
regard II>,
high
I0> ) . I n f a c t , t h i s i s s t a t e d
with
!0>,
structures
T « 5 K
from
the
progressions
which a r e l i s t e d
in
Table I.
configurations
of the
t o t h e c o r r e s p o n d i n g normal
states coordi-
nates . Transitions states
could
between
the ground s t a t e
exhibit different properties
and h i g h e r
lying
of the v i b r a t i o n a l
excited coupling. 3
This
has r e c e n t l y
been shown f o r [ R u ( b p y ) ^ ] ( P F ^ ) ^ s i n g l e c r y s t a l s .
It
is
found
that
the a b s o r p t i o n -1
l y i n g about 800 cm 1
1600
cm
can
conclude
above
(bpy-ring
compared
coordinate.
to change
the lowest e x c i t e d
stretching)
on
to that
spectrum t o a s t a t e
a shift
progression.
one,
of the e q u i l i b r i u m
by about 0.02
the i n t e r - r i n g 12
A
ref.
p o s i t i o n of
C-C
11)
i s d o m i n a t e d by
From t h i s p r o g r e s s i o n
o f t h e ground s t a t e a l o n g the
( F o r example,
(2A' see
this
corresponding
separation
is
a
one state
normal
estimated
)
CONCLUSION This detailed
paper p r e s e n t s h i g h l y insight
chromophore.
into
the
very lowest e x c i t e d
distinct
modes promote
to the t r a n s i t i o n from configurations lying
dutrie" cial
111>.
cm
1
for
more s t r o n g l y
Both s t a t e s to that
state
like
allow 2+
[Ru(bpy)^]
magnetic
fields
the t r a n s i t i o n
from
seem t o have s i m i l a r geomet-
o f the ground s t a t e w h i l e
the e q u i l i b r i u m
bpy-ring
a u t h o r s would
Gliemann
the
which
I I > w h i l e o t h e r s c o u p l e more p r o n o u n c e d
state
compared
excited
r e s p e c t t o a 1600 ACKNOWLEDGMENTS
The
of
I t i s shown by i n v e s t i g a t i o n s under h i g h
several
higher
emission spectra
the v i b r o n i c s t r u c t u r e
that
rical
resolved
stretching
position i s shifted
s u p p o r t o f t h i s work.
The
a
with
coordinate.
to express t h e i r thanks t o
and t h e " S t i f t u n g Volkswagenwerk"
for
Professor
"Verband d e r Chemischen a r e acknowledged
for
G. In-
finan-
support.
REFERENCES
[1]
Kober,
E.M.;
[2]
Kober, E.M.;
[3]
Yersin,
Chem. 1986,
134, [4]
[5]
90,
R.S.;
Meyer,
T.J.
J . Phys.
3722
Meyer,
H.;
Lumpkin,
T.J.
Gallhuber,
I n o r g . Chem. 1985, E.; H e n s l e r , G.
24,
106
Chem. Phys. L e t t .
1987,
Chem. Phys. L e t t .
1985,
497
Gallhuber, 120,
C a s p a r , J.V.;
E.;
Hensler,
G.;
E.;
Hensler,
G.;
Yersin,
H.
445
Gallhuber,
Yersin,
H.
Contribution
Proceedings [6]
Poizat,
[7]
G l i e m a n n , G.
O.;
Sourisseau,
C.
J . Phys. Chem. 1984,
Comments I n o r g . Chem. 1986,
5,
263
88,
3007
i n these
[8]
Hensler,
G.; G a l l h u b e r ,
E.; Y e r s i n , H.
Inorg.
Chem. 1987, 26, i n
press [9]
Hensler,
G.;
Gallhuber,
E.;
Y e r s i n , H.
C o n t r i b u t i o n i n these
Proceedings [10]
Elfring,
W.H.;
C r o s b y , G.A.
B a k e r , D.C.; C r o s b y , G.A. [11] Y e r s i n , H.; G a l l h u b e r , [12]
Schonherr, submitted
E.
J . Am. Chem. Soc. 1981, lOJL
2683.
Chem. Phys. 1974, 4, 428 J . Am. Chem. S o c , 1984, 106, 6582
T.; Degen, J . ; G a l l h u b e r ,
E.; H e n s l e r ,
G.; Y e r s i n , H.
HIGHLY RESOLVED OPTICAL SPECTRA OF [ O s ( b p y ) ]
DOPED INTO [ R u ( b p y ) ] X
2 +
3
3
2
G.Hensler, E . G a l l h u b e r , and H . Y e r s i n Institut
f u r P h y s i k a l i s c h e u n d T h e o r e t i s c h e C h e m i e , 8400 R e g e n s b u r g , FRG
INTRODUCTION
Many
i n v e s t i g a t i o n s were c a r r i e d o u t t o d e s c r i b e t h e e l e c t r o n i c 2+ 2+ of [Rufbpy)^] and [ O s t b p y ) ^ ] complexes due t o t h e i r
properties
i n t e r e s t i n g photochemical 1-4 covery
of
lowest
the
MLCT
and p h o t o p h y s i c a l b e h a v i o r .
electronic
states
0-O-transitions
and t h e ground
delivered
the energy
resolved
low-temperature
state
in
The r e c e n t d i s -
occuring neat
between
single
the
crystals,
p o s i t i o n s and s e p a r a t i o n s o f t h e l o w e s t excited 2+ s t a t e s o f [Ru(bpy)~] . I n t h i s c o n t r i b u t i o n we want t o p r e s e n t h i g h l y 2+
[Ru(bpy)^]X extremely
(X = P F ,
2
c
6
structured
e m i s s i o n s p e c t r a of [Os(bpy)^] ^
0
^ single
crystals.
doped
into
To o u r knowledge
s p e c t r a a r e d i s c u s s e d here
f o r the f i r s t
these
time.
RESULTS and DISCUSSION 2+ F i g u r e l a shows t h e E i c - p o l a r i z e d into
[Ru (bpy) ~] (PF.-) ~ (E = e l e c t r i c
the c r y s t a l ) . is
not
e m i s s i o n o f [Os{bpy)^]
field
vector,
The e m i s s i o n o f [ R u ( b p y ) ]
lies
3
reproduced
here
(EIIH)
leads to a d r a s t i c 2
reproduces
high energy intensity of about The
most
conspicuous
(The
i s (37 ± 2) c m .
with
crystal
lines
With
spectrum
of
the
f i e l d s H±c ( F i g . lb) .
dependence i n t h e r e g i o n o f t h e i n c r e a s i n g magnetic
increases d r a s t i c a l l y
field
the
up t o a f a c t o r
f e a t u r e of these emission spectra
structure.
- 1
from t h r e e d i f f e r e n t
not
axis of
(at H = 6 T ) .
occurrence of a t r i p l e lines
field
part of the emission.
3
of h i g h magnetic
change o f t h e o v e r a l l
the magnetic
of the dominating 10
c = needle
a t h i g h e r e n e r g i e s and
( b u t s e e t h e two o t h e r c o n t r i b u t i o n s
authors i n these p r o c e e d i n g s ) . A p p l i c a t i o n
Fig.
doped
sites
The energy
These t r a n s i t i o n s
structures of [ R u ( b p y ) ] ( P F )
application
3
of
are interpreted
of the guest molecules
isomorphous.^) T h i s i n t e r p r e t a t i o n magnetic
fields
6
2
i s the
s e p a r a t i o n between
i n the
to
these result
host matrix.
and [ O s ( b p y ) ^ ] ( P F ) 6
i s c o n f i r m e d by t h e f a c t the t r i p l e
structure
is
2
are that also
14 500 —1 i
14000 Li
13500 1i
cm"
13000 1I
1
H =
12500 i
v
OT
a
Ail *15
H = 6T
b
700
725
750
nm
775
X
800
2+ F i g . 1. E i c - p o l a r i z e d emission s p e c t r a of [Ru(bpy) ](PF ) at H = 0 T and H = 6 T 3
6
2
[Os(bpy) ] (T = 2 K, 3
doped into Ru:Os » 100;
363.8 nm. For a = 632.8 nm the r e s u l t s are i d e n t i c a l ) . The "jt ex i n t e n s i t i e s are not comparable. The Ellc p o l a r i z e d e m i s s i o n intensity =
v
i s weaker by a f a c t o r o f a b o u t 14500
cm
14450
-1
80.
14 400
H = 0T
T
•*"
I
I
1
"
1
—
" • " i
i
I
'
r
i
i
r
2T
j
\
Fig.
* 1
T
1
r
6T
{
£
, 1
690
,
•
, "
"
"
T
,
T
692
"
J,
sion
spectra into
—
r
nm
(Eic, of 1
,
694
"-"
X
f l a n k of the
doped various
,J 688
2. B l u e
2
t h e narrow field 1
cm" .
[Os(bpy) ] 3
[Ru(bpy)^] ( P F g )
magnetic f i e l d Hie,
higher
of
EIIH). The lines
emis2+
2
strengths half-width
appearing
strengths
at
is
at
about
observed
in
Further,
this
the
absorption
a s s i g n m e n t was
spectra
measurements, l e a d i n g t o o n e - s i t e Consequently, [Ru(bpy) 3(PF ) 3
6
different coupled
result
2
sites
The
with
the
transition
(for
magnetic-field into
considerable Figure
the
for
This
2
[Ru(bpy) ](C10 ) 3
is
4
electronic
' .
2
The
lowest
(see
[Os(bpy) ] 3
and
have isomorphous s t r u c t u r e s . ) about 330
i n a n a l o g y t o the
cm
1
,
m a g n e t i c f i e l d e f f e c t (see F i g . 3b) 13500 uooo cm ed i n F i g . 1, b u t l e s s d r a s t i c .
can
get
doped
into
—T
d
4
Furthermore, to
the
[Ru(bpy) ]X 3
the fi
salts
2
2
(PF )2 3 ' .
demonstrat12 500
H = 0T
r— -
result 3
i s s i m i l a r to t h a t 13000
lying
([Os(bpy) ](C10 )
compared
corresponding
The
through
seems t o
sites. 5
2
but higher
transition o a l s o r e f . ). 2+
s p e c t r u m i s much s i m p l e r similar
of
to
zero-phonon
this
s p e c t r u m of
of r e l a t i v e l y
transitions vibrations,
i s strongly forbidden
emission
three
are i n t e r p r e t e d i n analogy 6 7
l o w e s t e x c i t e d one
r e d - s h i f t e d by
compound, b e i n g
site)
into
structure.
strength mixed-in
shows t h e
a distribution
emission
3
doped
s p e c t r a of
intramolecular
a d m i x t u r e of wave f u n c t i o n s
the
oscillator 3a
[Ru(bpy) ]X
2 +
3
the
energies.
site selection
[Os(bpy) ]
respective purely
a specific
t h a t of
3
and
of
from a s u p e r p o s i t i o n of
induced
[Ru(bpy) ](ClO^) . from
spectra.
features
p a t t e r n of a t r i p l e
found
corresponding r e c e n t l y by
emission
changes under m a g n e t i c f i e l d s
behavior
states
emission
very
t o a l a r g e number of phonons and
thus p r o d u c i n g
the
the
a t the
verified
a
1
\
H = 6T b
x6
800
775
750
725
700
2+ Fig. at
3.
Emission
H = 0 T and
comparable.
spectra
H = 6 T
of
[Os(bpy) ]
(T = 2 K;
3
doped i n t o
Ru:Os « 5 0 ) .
[Ru(bpy) ](C10 )
Intensities
3
are
4
2
not
Figure 4 delivers
a synopsis of the temperature 2+
emission of [Osfbpy)^] 1
I
( a t 14169 cm >
increasing 211
cm
1
[Ru(bpy) ](C10 ) . 3
represents the t r a n s i t i o n
temperature (line
doped i n t o
dependence o f t h e
4
At T = 2 K l i n e
2
of h i g h e s t
two f u r t h e r peaks appear, 61 cm
III) at higher energies,
energy. 1
(line
respectively.
With
I I ) and
These
three
transitions
a r e d e t e c t e d i n a b s o r p t i o n a t t h e same e n e r g i e s and t h e r e Q f o r e r e p r e s e n t d i f f e r e n t zero-phonon l i n e s .
Finally,
we want t o f o c u s on t h e v i b r a t i o n a l
satellites
observed
2+
m
the emission
In Table
spectra of [Os(bpy) ] 3
doped i n t o
I we compare t h e e n e r g i e s o f v i b r a t i o n s
[Ru(bpy) ](C10 ) . 3
coupled
4
2
to the lowest 1 0
electronic t r a n s i t i o n f o r t h e c a s e o f H = 0 T t o IR and Raman data 2+ 11 2+ of [Ru(bpy)-] and Raman d a t a o f [Os(bpy)~] . At zero magnetic -1 -1 field, 1445
several
IR
modes a r e
cm ^, 1564 cm
, while
present
( e . g . 1126 cm
f o r H = 6 T those v i b r a t i o n s 1 0
w h i c h show a s t r o n g Resonance Raman e n h a n c e m e n t ' 1325
1
cm ,
1553 cm
1
) .
This
,
result
indicates
1 1
a r e dominant
(e.g.
that
1244 cm
the
1
1172 cm" , vibronic
coupling
of
the t r a n s i t i o n s
from d i f f e r e n t
e x c i t e d s t a t e s t o IR
and
Raman modes, r e s p e c t i v e l y , i s d i s t i n c t .
Emission of 2 +
[Os(bpy).] [cm
line
satellites
]
[cm
I 14169 14009 13959 13924 13891 13850 13796 13751 13727 13687 13526 13496 13440 13402 13143 13100 13044 13011 12997 12925 12844 12724 12678 12616 12605 12561
Table
Vibrational
a
a
I . Comparison
[Os(bpy) ] Raman d a t a
2
+
]
1
[cm
2 +
of 3
[cm
1
2 +
Raman
]
[cm
(464) 643 658 730 765 1025 1069
1243
-
1029
-
1160 1173
-
1175
-
1317
1322
-
1441 1485
-
1490 1558
-
1491 1558
-
1565 1600 satellites
)
--
-
(468) 642 658 732 774 1024 1066 1121 1159
1
-
372
-
2 +
-
-
371 421
of 3
(198) 252 281
-
11
[Os(bpy) ]
-
(195)
into
10
[Ru(bpy) ]
]
-
1607
1610
i n the emission
spectra of
[Ru(bpy).](CIO.) ( a t T = 2 K) w i t h IR and 2 + 2 + and [ O s ( b p y ) 3 complexes. The marked 0
of the [Ru(bpy) ] 3
v i b r o n i c peaks
Raman
3
of observed
doped
of
[Ru(bpy) ]
0 160 210 245 278 319 373 418 442 482 643 673 739 767 1026 1069 1125 1158 1172 1244 1325 1445 1491 1553 1564 1608
a
1 0
IR
3
(a) a r e weak a t H = 0 T and s t r o n g a t H = 6 T.
CONCLUSION
[Ru (bpy) -] X~
CIO.) r e p r e s e n t s an appropriate 4 f o r s p e c t r o s c o p i c studies of [Os(bpy) ] . Due t o weak elec3
matrix
(with
2
X = PF^, 6
3
tron-phonon c o u p l i n g i n t h i s d i l u t e d to
obtain
guest
highly
complex.
different
crystalline
resolved emission
and a b s o r p t i o n
Three zero-phonon t r a n s i t i o n s
electronic
states
and
system i t i s p o s s i b l e 9
the
ground
spectra
of
o c c u r i n g between state
are
the three
detected.
Further,
many
spectra,
with
vibronic t h e energy
satellites
are resolved
s e p a r a t i o n s from
i n the
emission
the O - 0 - t r a n s i t i o n i n agree-
ment w i t h t h e e n e r g y v a l u e s p u b l i s h e d f o r IR a n d Raman modes.
ACKNOWLEDGEMENT
The
authors
would l i k e
Gliemann
f o r support
dustrie"
and
financial
to express
of t h i s
the " S t i f t u n g
their
thanks
work. T h e " V e r b a n d Volkswagenwerk"
t o P r o f e s s o r G.
der Chemischen I n -
are acknowledged f o r
support.
REFERENCES
(1)
H. Y e r s i n , E. G a l l h u b e r ,
(2)
E. G a l l h u b e r , G. H e n s l e r , H. Y e r s i n , Chem. P h y s . L e t t . ,
(3)
G. H e n s l e r ,
46,
G. H e n s l e r ,
J. Physique
( P a r i s ) , 1985,
p.C7-453. 1985, 120,
445. E. G a l l h u b e r , H. Y e r s i n , I n o r g . Chim. A c t a ,
1986, 113,
91. (4)
H. Y e r s i n , E. G a l l h u b e r ,
(5)
M. Z a b e l , U n i v e r s i t y
(6)
G. H e n s l e r ,
134,
in
G. H e n s l e r ,
Chem. P h y s .
Lett.,
497. Regensburg,
E. G a l l h u b e r ,
private
communication.
H. Y e r s i n , I n o r g . Chem.,
1987, 26,
press.
(7)
E. G a l l h u b e r ,
(8)
G. G l i e m a n n ,
(9)
H. Y e r s i n , E . G a l l h u b e r , G. H e n s l e r ,
(10)
O. P o i z a t , C. S o u r i s s e a u , J . Phys. Chem., 1984, 88., 3007.
109,
(11)
G. H e n s l e r ,
H. Y e r s i n , J . Am.
Chem.
S o c , 1987,
i n press.
J . V. C a s p a r , T.
1987,
"Comments I n o r g . Chem." 1986, 5, 263.
T. D. W e s t m o r e l a n d ,
submitted
for publication.
G. H. A l l e n ,
P. G.
Bradley,
J . Meyer, W. H. Woodruff, J . Am. Chem. S o c , 1982, 106, 3492.
EXCITED STATE BEHAVIORS OF RUTHENIUM(11) COMPLEXES AS STUDIED BY RESOLVED AND TEMPERATURE AND SOLVENT DEPENDENT EMISSION SPECTRA
S . T a z u k e , H . - B . K i m , and
TIME
N.Kitamura
Research L a b o r a t o r y o f R e s o u r c e s U t i l i z a t i o n , Tokyo 4259 N a g a t s u t a , M i d o r i - k u , Yokohama 227, JAPAN
Institute
of
Technology,
INTRODUCTION Ruthenium(II) complexes are widely i n v e s t i g a t e d e l e c t r o n t r a n s f e r s e n s i t i z e r s (Kalyanasundaram 1982). The m a i n r e a s o n s a r e ; i ) t h e app r o p r i a t e o x i d a t i o n and r e d u c t i o n p o t e n t i a l s , i i ) the l o n g e x c i t e d s t a t e l i f e t i m e , i i i ) t h e n e g l i g i b l e p h o t o a n a t i o n , and i v ) t h e handy wavelength of photoabsorption. While the f a c t o r s d e t e r m i n i n g the properties m e n t i o n e d a b o v e a r e i n p r i n c i p l e d e t e r m i n e d by e n e r g y l e v e l s o f various e l e c t r o n i c s t a t e s a n d i n t e r a c t i o n s b e t w e e n a n y two e n e r g y l e v e l s ( i . e . , t r a n s i t i o n moment, o v e r l a p , and c o u p l i n g w h i c h d e c i d e a b s o r p t i o n and r a d i a t i v e / n o n r a d i a t i v e p r o c e s s e s ) , the p h o t o p h y s i c s of R u ( I I ) complexes i s not yet f u l l y u n d e r s t o o d . The m a j o r p a r a m e t e r s t o d e t e r m i n e t h e e l e c t r o n i c s t a t e i s c e r t a i n l y the n a t u r e of l i g a n d s . In the p a s t , we synthesized a s e r i e s of R u ( I I ) complexes w i t h v a r i o u s diazadiimine l i g a n d s and showed t h a t t h e r e d o x p o t e n t i a l s i n t h e g r o u n d and excited s t a t e s , t h e w a v e l e n g t h o f a b s o r p t i o n and e m i s s i o n , and t h e e x c i t e d s t a t e l i f e t i m e c o u l d be a l t e r e d ( K i t a m u r a 1 9 8 3 a , 1 9 8 3 b ; K a w a n i s h i 1984 ; Tazuke 1985). W h i l e t h e r e d o x p o t e n t i a l s and t h e a b s o r p t i o n and e m i s s i o n e n e r g i e s were c o r r e l a t e d w i t h l i g a n d p r o p e r t i e s , the dynamics of t h e r e l a x a t i o n f r o m t h e e x c i t e d s t a t e was puzzling. I n t h i s r e p o r t , we a r e g o i n g t o d i s c u s s t h e r e l a x a t i o n p r o c e s s e s o f excited RufbpyK and R u L ? ( C N ) ( L i s 2,2 ' - b i p y r i d i n e ( b p y ) o r 1,10phenanthroline (phen)). T h e s e a r e , f i r s t l y , s o l v e n t e f f e c t s on r e l a x a t i o n p r o c e s s e s , s e c o n d l y , the r e l a x a t i o n p r o c e s s e s from the initially populated excited MLCT s t a t e by means o f t i m e - r e s o l v e d emission spectroscopy. T h r o u g h t h e s e i n v e s t i g a t i o n s , we a r e t r y i n g t o p i n p o i n t the dynamics of the r e l a x a t i o n p r o c e s s e s of the e x c i t e d R u ( I I ) complexes. 2
I.
SOLVENT E F F E C T S ON
EXCITED STATE PROPERTIES.
One way t o m o d u l a t e t h e MLCT e x c i t e d s t a t e p r o p e r t i e s o f m e t a l complexes i s the v a r i a t i o n of s o l v e n t s . As r e p o r t e d p r e v i o u s l y , b o t h a b s o r p t i o n and e m i s s i o n e n e r g i e s o f R u L ( C N ) ( L = d i i m i n e l i g a n d s ) show higher energy s h i f t w i t h i n c r e a s i n g the Gutmann s s o l v e n t a c c e p t o r n u m b e r , AN ( B e l s e r 1 9 8 5 ) . W i t h i n c r e a s i n g AN o f s o l v e n t s , o-donor s t r e n g t h o f t h e c y a n i d e l i g a n d s d e c r e a s e s and c o n s e q u e n t l y , t h e m e t a l t p g o r b i t a l s move t o l o w e r e n e r g y , b r i n g i n g a b o u t t h e h i g h e r e n e r g y s h i f t o f b o t h a b s o r p t i o n and e m i s s i o n . S o l v e n t e f f e c t s on t h e r e d o x p o t e n t i a l s of R u ( p h e n ) ( C N ) are c o n s i s t e n t w i t h the i n t e r p r e t a t i o n (Kitamura 1 987a). The r e d u c t i o n p o t e n t i a l ( i . e . , t h e TT energy of the p h e n l i g a n d ) i s a l m o s t u n a f f e c t e d by s o l v e n t s w h e r e a s t h e o x i d a t i o n p o t e n t i a l ( i . e . , the t o r b i t a l on t h e m e t a l ) d e p e n d s o n s o l v e n t AN. The s p e c t r a l d a t a a n d t h e o x i d a t i o n p o t e n t i a l s a r e c o r r e l a t e d a s s h o w n i n F i g . 1. Changes i n energy l e v e l s s h o u l d a l s o i n f l u e n c e the e x c i t e d s t a t e lifetime. F r o m t h e l i f e t i m e a n a l y s i s by e q . ( 1 ) , t h e t e m p e r a t u r e 2
2
1
2
2
2
a
13.5-
+
+
E 1 / 9 ( R u / R u ° ) vs. F c / F c °
Fig.1 R e l a t i o n s h i p between E or E (•) a n d t h e o x i d a t i o n potential of Ru(phen) (CN) .
(O)
F
i
9 -
2
S
o
l
v
e
n
t
e f f e c t s on I n k
n r
.
a t > s
9
T"
1
= k
+ k
r
n
9
+ k ' e x p ( - A E /RT)
r
(1 )
independent n o n r a d i a t i v e r a t e constant, k^ , and t h e a c t i v a t i o n energy of t h e e m i s s i o n l i f e t i m e , A E , were o b t a i n e d . F i g u r e 2 shows a I n k vs emission energy ( E )plot. The e n e r g y gap l a w (Meyer 1986) i s a p p l i c a b l e f o ra s e r i e s o f s o l v e n t s w h i l e t h e d a t a i n hydrogen bonding s o l v e n t s d e v i a t e from t h e l i n e a r r e l a t i o n . The hydrogen b o n d i n g i n t e r a c t i o n between t h e c y a n i d e l i g a n d s and s o l v e n t s i s c e r t a i n l y i m p o r t a n t for the r e l a x a t i o n of excited Ru(phen) (CN) • On t h e o t h e r h a n d , t h e r e i s a r e a s o n a b l e c o r r e l a t i o n b e t w e e n AE^ a a nndd AN ((FFiigq.. 3 ) . T h e o b s e r v e d I the estin a ' ~ — ^ The e n e r g y d i f f e r e n c e was f e r e n c e b e t w e e n t h e d - d a n d MLCT s t a t e s r
a
e
n
r
m
2
2
r e p o r t e d t o be 5 0 0 0 cm i n an N,N-dimethylformamide/dichloromethane mixture ( B e l s e r 1985). R e c e n t l y , t h e p r e s e n c e o f t h e f o u r t h MLCT e x c i t e d s t a t e ( M L C T ' ) w h i c h l i e s 6 0 0 - 1 0 0 0 cm"' a b o v e M L C T s t a t e w a s s u g g e s t e d ( K o b e r 1984 ; Meyer 1 9 8 6 ) . I n l o w AN s o l v e n t s , t h e MLCT s t a t e l o c a t e s a t l o w e r e n e r g y a s g o m p a r e d w i t h t h a t i n h i g h AN s o l v e n t s s o t h a t t h e r e l a x a t i o n v i a d-d i s u n l i k e l y b u t p r o c e e d s t h r o u g h t h e MLCT' giving a small 4 a* W i t h i n c r e a s i n g AN o f s o l v e n t s , t h e r e l a x a t i o n proceeds v i a d-d and t h e o v e r a l l a c t i v a t i o n energy i n c r e a s e s . Howevej, i t i s experimentally n o t p o s s i b l e t o d i v i d e o v e r a l l AE into AE ( d - d ) and AE ( M L C T ' ) . I f b o t h t h e d - d a n d MLCT' s t a t e s p a r t i c i p a t e s i n t h e r e l a x a t i o n , p h o t o a n a t i o n i n a ^ s e r i e s o f s o l v e n t s w i l l be r e l a t e d t o t h e f r a c t i o n o f d e c a y v i a t h e d-d state. The r e s u l t s i n T a b l e 1 a g r e e w i t h t h i s view i n d i c a t i n g that photoinduced ligand s u b s t i t u t i o n i s f a c i l i t a t ed w i t h i n c r e a s i n g AN. 3
E
Table. 1 Photoreaction with KSCN(0.1M).
"1
m
i o X
•
J
1
•
2
• •
a
LU
•
••
•
1
< —L_
10
20
1
,
30
,
1
i
40
50
AN
Fig.
3 Solvent
e f f e c t s o n AE„
N,N-dimethylacetamide dimethylsulfoxide N-methylformamide methanol Relative water.
9
13.6 19.3 32.1 41 . 3
t o t h e y i e l d of
9
x
AN
Solvent
a) in
o f Ru(phen) ( C N ) z ^
0.0 13.7 14.5 27.4
Ru(bpy)?*
„a)
II.
T I M E DEPENDENT S H I F T OF
EMISSION. 3
S o l v e n t d i p o l e r e l a x a t i o n i n t h e M L C T e x c i t e d s t a t e c a n be f o l lowed by t i m e - r e s o l v e d e m i s s i o n s p e c t r o s c o p y . I n t e r r e l a t i o n between the r o l e o f s o l v e n t d i p o l e r e l a x a t i o n and l o c a l i z a t i o n / d e l o c a l i z a t i o n o f an e x c i t e d e l e c t r o n i s a p o i n t o f c o n t r o v e r s y . I n t h e ns t i m e r e g i m e , t h e emission spectrum of R u ( b p y ) shows t i m e - d e p e n d e n t (TD) s h i f t a t l o w t e m p e r a t u r e ( F e r g u s o n 1985 ; K i t a m u r a 1 9 8 6 ) . T h i s p h e n o m e n o n was a t t r i b u t e d t o s o l v e n t d i p o l e r e l a x a t i o n and p r o b a b l y , i n p a r t , t o t h e t r a n s i t i o n from the charge d e l o c a l i z e d e x c i t e d s t a t e t o the charge l o calized state. A l t h o u g h t h e TD s h i f t s l i g h t l y d e p e n d s o n t h e n a t u r e o f a c o u n t e r a n i o n ( X = C 1 ~ , C 1 0 " , o r P F ^ " ) o f R ^ b p y ^ X n ( K i t a m u r a 1 986 ) , t h i s d i s c u s s i o n c o u l d n o t be a p p l i e d t o R u L ( C N ) , w h i c h s h o w s e v e n a b i g g e r TD s h i f t t h a n R u ( b p y ) a s s h o w n i n F i g . 4. The TD s h i f t i s s p e c i f i c a l l y o b s e r v e d i n a n a r r o w t e m p e r a t u r e r a n g e j u s t below and above t h e g l a s s t r a n s i t i o n t e m p e r a t u r e o f t h e m a t r i x (T ). For q u a n t i t a t i v e d i s c u s s i o n , we d e f i n e d t h e r a t e o f r e l a x a t i o n a s t h e r e c i p r o c a l o f t i m e r e q u i r e d f o r h a l f r e l a x a t i o n ( k ^ ) and t h e A r r h e n i u s p o l t s a r e shown i n F i g . 5. The p l o t s a r e d e v i d e d i n t w o p a r t s a t 130 K. B e l o w 130 K, t h e a p p a r e n t a c t i v a t i o n e n e r g i e s f o r TD s h i f t , AE , 570 a n d 1360 cm~^ f o r R u ( b p y ) C l and R u ( b p y ) ( C N ) , r e s p e c t i v e l y . The a c t i v a t i o n e n e r g y o f v i s c o u s f l o w f o r a n e t h a n o l / m e t h a n o l m i x t u r e b e l o w 120 K i s 1900 cm ( C a r l i n 1985) so t h a t t h e p r e s e n t AE v a l u e s a r e t o o s m a l l t o be a s c r i b e d t o t h e a c t i v a t i o n e n e r g y f o r t h e phenomena r e l a t e d t o t h e v i s c o s i t y o f t h e medium. S i n c e t h e r a t e o f r e l a x a t i o n depends on t h e nature of s o l v e n t s , solvent d i p o l e r e l a x a t i o n i n the MLCT e x c i t e d s t a t e i s p r i m a r i l y r e s p o n s i b l e f o r t h e TD s h i f t . 2 +
3
4
2
2
2 +
3
a
r
e
a
3
2
2
2
As i u d g e d b y t h e a c c u m u l a t e d e v i d e n c e t h a t a n e x c i t e d e l e c t r o n i n Ru(bpy) or analogous complexes i s d e l o c a l i z e d i n r i g i d m a t r i c e s w h i l e t h a t i s l o c a l i z e d i n f l u i d m e d i a , t h e TD s h i f t i n n e a r T of the s o l v e n t s h o u l d be c l o s e l y r e l a t e d t o s o l v e n t d i p o l e r e l a x a t i o n o r more p r e c i s e ly, solvent assisted electron localization. I n e t h a n o l / m e t h a n o l a t 150 K, t h e e m i s s i o n f r o m R u ( b p y ) o b e y s a s i n g l e e x p o n e n t i a l f u n c t i o n and the decay t i m e i s i n d e p e n d e n t of the m o n i t o r i n g w a v e l e n g t h . However, f o r Ru(bpy) a t 125 K, t h e d e c a y p r o f i l e o b s e r v e d a t 580 nm d o e s n o t a g r e e w i t h t h a t m o n i t o r e d a t 752 nm a s s h o w n i n F i g . 6. S i m i l a r r e s u l t s have been r e p o r t e d by F e r g u s o n e t a l . ( 1 9 8 6 a , 1 9 8 6 b ) . The e f f e c t s a r e m o r e pronounced and c o m p l i c a t e d i n t h e c a s e o f R u ( p h e n ) ( C N ) ^ (Kitamura 1987b). The e m i s s i o n d e c a y m o n i t o r e d a t 580 nm i s m u l t i - o r e v e n n o n e x p o n e n t i a l and t h e decay p r o f i l e v a r i e s w i t h t h e m o n i t o r i n g wavelengths ( F i g . 7). The l o n g e r t h e m o n i t o r i n g w a v e l e n g t h , t h e l o n g e r t h e apparent e m i s s i o n l i f e t i m e . Furthermore, the wavelength dependent emiss i o n d e c a y i s o b s e r v e d a t 150 K. The f o r m a t i o n a n d d e s t r u c t i o n o f t h e hydrogen b o n d i n g i n t e r a c t i o n between t h e c y a n i d e l i g a n d s and a l c o h o l i c solvents around T ( K i t a m u r a 1 9 8 7 a ) c o u l d be r e f l e c t e d o n t h e e m i s s i o n decay p r o f i l e . A l t h o u g h i t i s n o t c l e a r l y s e e n i n F i g . 6 a n d 7, t h e e m i s s i o n f r o m b o t h c o m p l e x e s e x h i b i t s r i s e and d e c a y when m o n i t o r e d a b o v e 700 nm. S i n c e t h e e m i s s i o n s p e c t r u m s h o w s TD s h i f t a t t h i s temperature, the change i n the e m i s s i o n decay p r o f i l e w i t h the m o n i t o r ing wavelength i s not s u r p r i s i n g . H o w e v e r , t a k i n g t h e r e p o r t o f MCPL e x p e r i m e n t s ( F e r g u s o n 1 9 8 6 ) i n t o a c c o u n t , we c o n c l u d e t h a t t h e p h e n o m e n a are c l o s e l y r e l a t e d w i t h the t r a n s i t i o n from the d e l o c a l i z e d e x c i t e d s t a t e t o t h e l o c a l i z e d one. +
3
3
2 +
3
2
A s we h a v e a l r e a d y s h o w n , t h e d e c a y p r o f i l e o f t h e e m i s s i o n b e t w e e n 100 a n d 140 K i s c o m p l i c a t e d a n d c o u l d n o t be f i t i n a s i n g l e o r m u l t i e x p o n e n t i a l mode. I t i s i n d e e d n o n - e x p o n e n t i a l . The m e a n i n g o f n o n e x p o n e n t i a l d e c a y s h o u l d be u n d e r s t o o d i n t w o w a y s . F i r s t l y , because of t h e TD s h i f t o f e m i s s i o n , t h e d e c a y p r o f i l e m o n i t o r e d a t a f i x e d w a v e length provides a f a l s e feature. The t o t a l e m i s s i o n i n t e n s i t y i n t e g -
«
;
' fln K XX) K
o
590
a 110 K
a
-
\ :600
b) -
. \
2 • 610 •
-
•
•
E
• • 120K
LU
620 -» m
-
500 1000 1500 Delay Time (ns) •
640
•
125K 130 K
00
° HOK
1500
1000
500
0
Delay Time (ns) F i g . 4 Time and t e m p e r a t u r e dependence o f t h e e m i s s i o n maximum o f R u ( b p y ) C l (a) a n d R u ( b p y ) ( C N ) (b) i n an ethanol-methanol mixture. 3
150
2
2
110
130 a)
Ru(bpy) X 3
2
2
110 K
150
130
IA , ^
b)RuL (CN) 2
2
^5
cn o
X 8
9 1000/T
7
8
9
1/K
F i g . 5 Temperature dependence o f k ; (a) X - C I ( • ) , C l o : < A ) , and P F , ( o ) , (b) L = bpy(o) and p h e n ( A ) . r
0
2
4 6 8 time l i s
F i g . 6 Emission decay of R u ( b p y ) .
0
profile
3
2 4 6 time |is
F i g . 7 E m i s s i o n decay of Ru(phen) (CN) . 2
8 profile
2
r a t e d o v e r t h e w h o l e w a v e l e n g t h r e g i o n must be p l o t t e d a g a i n s t t i m e . S e c o n d l y , p r o v i d e d t h a t t h e p r o c e d u r e m e n t i o n e d above c o u l d be a d o p t e d , the d e c a y p r o f i l e m i g h t be m u l t i - e x p o n e n t i a l . This i s the true case of time-dependent r a t e . Such n o n - e x p o n e n t i a l decay has been d i s c u s s e d i n a number o f u n i m o l e c u l a r f l u o r e s c e n c e d e c a y o f o r g a n i c c o m p o u n d s s u c h a s t w i s t e d i n t r a m o l e c u l a r CT c o m p o u n d s ( H e i s e l 1 9 8 3 , 1 9 8 5 a , 1 9 8 5 b ) a n d i n t r a m o l e c u l a r e x c i m e r / e x c i p l e x (Tazuke 1986). The work a l o n g t h i s l i n e i s now i n p r o g r e s s .
References B e l s e r P, Z e l e w s k y AV, J u r i s A, B a r g e l l e t t i F, B a l z a n i V ( 1 9 8 5 ) E x c i t e d s t a t e p r o p e r t i e s o f some new r u t h e n i u m ( I I ) c y a n o p o l y p y r i d i n e c o m p l e x e s i n v a r i o u s s o l v e n t s . Gazz Chim I t a l 115: 723-729 C a r l i n CM, D e A r m o n d MK ( 1 9 8 5 ) T e m p e r a t u r e - d e p e n d e n t p h o t o s e l e c t i o n - I n t r a m o l e c u l a r e x c i t o n motion i n [ R u ( b p y ) 3 ] . J Am Chem Soc 1 0 7 : 53-57 F e r g u s o n J , K r a u s z ER, M a e d e r M ( 1 9 8 5 ) C h a r g e l o c a l i z a t i o n i n t h e luminescent state of R u ( b p y ) i n fluid solutions. J Phys Chem 8 9 : 1 8 5 2 - 1 8 5 4 F e r g u s o n J , K r a u s z E ( 1 9 8 6 a ) T i m e - r e s o l v e d l u m i n e s c e n c e a n d MCPL of R u ( b p y ) i n glassy solvents at the fluid-glass transition. Chem P h y s L e t t 1 2 7 : 5 5 1 - 5 5 6 F e r g u s o n J , K r a u s z E ( 1 9 8 6 b ) MCPL e v i d e n c e f o r t h e t r a n s i t i o n f r o m delocalized t o l o c a l i z e d luminescent state of R u ( b p y ) in g l a s s - f l u i d m e d i a . I n o r g Chem 2 5 : 3 3 3 3 - 3 3 3 5 H e i s e l F, M i e h e J A ( 1 9 8 3 ) D y n a m i c a l s t u d y o f t w i s t e d i n t r a m o l e c u l a r charge t r a n s f e r i n p - d i m e t h y l a m i n o b e n z o n i t r i l e s o l u t i o n s . Chem P h y s L e t t 100 : 183-188 2 +
2 +
3
+
3
+
3
H e i s e l F, M i e h e J A ( 1 9 8 5 a ) p - D i m e t h y l a m i n o b e n z o n i t r i l e s o l u t i o n . 1 Time-dependent r a t e i n i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r r e a c t i o n . Chem P h y s 9 8 : 2 3 3 - 2 4 1 . H e i s e l F, M i e h e J A , M a r t i n h o JMG ( 1 9 8 5 b ) p - D i m e t h y l a m i n o b e n z o n i t r i l e i n p o l a r s o l u t i o n . 2 Quantum y i e l d m e a s u r e m e n t s a n d q u a s i - s t a t i o n a r y k i n e t i c study of the e l e c t r o n t r a n s f e r r e a c t i o n . Chem P h y s 9 8 : 243-249 Kalyanasundaram K (1982) P h o t o p h y s i c s , p h o t o c h e m i s t r y and s o l a r energy c o n v e r s i o n w i t h t r i s ( b i p y r i d y l ) r u t h e n i u m ( I I ) and i t s a n a l o g u e s . C o o r d Chem R e v 4 6 : 1 5 9 - 2 4 4 K a w a n i s h i Y, K i t a m u r a N, K i m Y, T a z u k e S ( 1 9 8 4 ) L i g a m d d e s i g n o f ruthenium(II) complexes aiming a t e f f i c i e n t e l e c t r o n t r a n s p o r t s e n s i t i z a t i o n . R i k e n S c i P a p e r s 78: 212-219 K i t a m u r a N, K a w a n i s h i Y, T a z u k e S ( 1 9 8 3 a ) S p e c t r o s c o p i c a n d e l e c t r o c h e m i c a l s t u d i e s on r u t h e n i u m ( I I ) c o m p l e x e s c o n t a i n i n g d i a z a d i i m i n e l i g a n d s . Chem P h y s L e t t 9 7 : 1 0 3 - 1 0 6 K i t a m u r a N, K a w a n i s h i Y, T a z u k e S ( 1 9 8 3 b ) H i g h l y e f f i c i e n t p h o t o r e d u c t i o n o f m e t h y l v i o l o g e n by t r i s ( b i s - d i a z a d i i m i n e ) ruthenium(II) complexes. Chem L e t t : 1 1 8 5 - 1 1 8 8 K i t a m u r a N, K i m HB, K a w a n i s h i Y, O b a t a R, T a z u k e S ( 1 9 8 6 ) T i m e r e s o l v e d e m i s s i o n s p e c t r a o f R u f b p y J ^ C l ? and c i s - R u ( b p y ) ( C N ) a t l o w t e m p e r a t u r e . J P h y s Chem 9 0 : 1 4 8 8 - 1 4 9 1 K i t a m u r a N, S a t o M, K i m HB, O b a t a R, T a z u k e S ( 1 9 8 7 a ) Solvatochromism i n the e x c i t e d s t a t e of c i s - d i c y a n o b i s ( 1 , 1 0 p h e n a n t h r o l i n e ) r u t h e n i u m ( I I ) c o m p l e x . I n o r g Chem : s u b m i t t e d K i t a m u r a N, K i m HB, S a t o M, T a z u k e S. ( 1 9 8 7 b ) I n p r e p a r a t i o n K o b e r EM, M e y e r T J ( 1 9 8 4 ) A n e l e c t r o n i c s t r u c t u r a l m o d e l f o r t h e e m i t t i n g MLCT e x c i t e d s t a t e s o f R u ( b p y ) and O s ( b p y ) . I n o r g Chem 2 3 : 3 8 7 7 - 3 8 8 6 Meyer T J (1986) P h o t o c h e m i s t r y o f m e t a l c o o r d i n a t i o n complexes ; m e t a l t o l i g a n d c h a r g e t r a n s f e r e x c i t e d s t a t e s . P u r e A p p l Chem 58: 1193-1206 T a z u k e S, K i t a m u r a N, K a w a n i s h i Y ( 1 9 8 5 ) P r o b l e m s o f b a c k e l e c t r o n t r a n s f e r i n e l e c t r o n t r a n s f e r s e n s i t i z a t i o n . J Photochem 29: 123-138 T a z u k e S, H i g u c h i Y, T a m a i N o , K i t a m u r a N, T a m a i N a , Y a m a z a k i I ( 1 9 8 6 ) F o r m a t i o n and r e l a x a t i o n o f e x c i t e d complex i n p o l y m e r s . Macrom o l e c u l e s 19: 603-606 2
+
3
+
3
2
THE
LOWEST EXCITED STATES OF (Ru(2 , 2 ' - b i p y r a z i n e )
Hiroshi
K o b a y a s h i , Youkoh K a i z u ,
Department o f C h e m i s t r y , Tokyo Tokyo 1 5 2 , JAPAN
(2,2'-bi p y r i d i n e )
Kazuteru Shinozaki,
)*
2
+
and H i d e y o M a t s u z a w a
I n s t i t u t e o f T e c h n o l o g y , 0-okayama, M e q u r o - k u , y
PROTONATION AND DEPROTONATION OF E X C I T E D
COMPLEXES
e x c i t e d s t a t e o f [ R u ( b p y ) (CN) ]
1
The
lowest
the
"metal-to-bipyridine" charge-transfer excited state.
2
2
(bpy,
2 2 -bipyridine) i s r
Absorption
spectrum o f t h e complex v a r i e s w i t h p r o t o n a t i o n on t h e c o o r d i n a t e d cyano g r o u p s
i nacidified
of deprotonated hydrogen-ion
media, w h i l e
species with a constant
1976).
In highly acidic
as [Ru(phen) (CN) ]
emission
chelate
4
band w i t h v i b r a t i o n a l
complexes.
results
band
with
I t was p r o p o s e d
ligand-localized
spectrum
i svery
(Peterson
[Ru(bpy) (CN) ] 2
2
e x h i b i t s an similar
thecorresponding
to the
Rh(IH)
tris-
that theprotonation i nthe r i g i d
i n an i n v e r s i o n o f t h e lowest 3
species
g l a s s a t 77 K,
s t r u c t u r e which
found
a
complete
(phen, 1 , 1 0 - p h e n a n t h r o l i n e )
2
(IT , TT ) e m i s s i o n
ligand
glass
2
A rapid,
o f theprotonated
H S0 -methanol
2
exhibits
regardless of the
[H ] i n s o l u t i o n .
follows excitation
as w e l l
yield
+
concentrations
deprotonation
emission
(TT,TT*) e x c i t e d s t a t e s
charge-transfer and t h e
(Peterson
1978). 22R e c e n t l y we p r e p a r e d a n i o n c o m p l e x e s [ R u b p y t C N ) ^ ] and [Ruphen(CN) ] . The c o m p l e x e s e x h i b i t m e t a l - t o - b i p y r i d i n e o r - p h e n a n t h r o l i n e charge2+ 4
transfer
bands v e r y
[Ru(bpy) (CN) ] 2
solutions protonated
with
and a f a s t ,
results
+
than
?
I n t h e media
the protonated
i n an one-electron
observed
intense
solution,
theoriginal
short-lived
spin-allowed
emission
transition
+
A further
media
complex.
Ru(II)
blue
a r e observed
e x c i t e d complexes a r e r a t h e r
species.
increase o f [H ]
+
( [H ]
A paramagnetic > 8 . 3 M) e m i t s a t which
has t h e 3+
o f [Rh(bpy) l 3
NMR s p e c t r u m c o n f i r m e d
from
cyano groups i s
lifetime
an intense s h o r t - l i v e d emission 3 theligand (TT,TT*) e m i s s i o n
Ru(nr)
acidic
> 2 M a continuous
+
with
state o f the paramagnetic
,
In dilute
o x i d a t i o n o f t h e complex.
K (Matsuzawa u n p u b l i s h e d ) .
acidic
[Ru(bpy)^]
follows excitation of
6 M > [H ]
complex y i e l d e d i n h i g h l y a c i d i c
structure
with
analogues.
deprotonation
the deprotonated
ambient temperature 7
observed
band and an i n c r e a s e o f decay
increasing [H ]:
Ru(n[)
complete
complex.
o f emission
long-lived
t o those
phenanthroline
s u c h a s [H ] < 0 . 5 M, o n e o f t h e c o o r d i n a t e d
the p r o t o n a t e d shift
similar
and t h e i r
2
at
the spin-doublet
B y a d d i t i o n o f OH~" t o t h e
complex i s recovered.
[Ru(HI)bpy(CN) ]~ 4
The
i sassigned
from t h e t r i p - d o u b l e t s t a t e o f l i g a n d
3
as a
(TT,TT*)
origin.
The i n t e r a c t i o n
with
makes a l l l i g a n d s i n g l e t s (trip-doublets) rise
spin doublet
become d o u b l e t s
and quartets
into
doublets
The p r o t o n a t i o n
gives
t o an e l e c t r o n detachment b u t n o t an i n v e r s i o n o f t h e lowest and t h e l i g a n d - l o c a l i z e d
Both protonated
and deprotonated 2+
1,10-phenanthroline)] proton-dissociation
are emissive constants
media
state.
(Giordano
1978).
On t h e o t h e r
1
4,4 -dicarboxylic o f coordinated
obtained
by a b s o r p t i o n
displacement
ligand
and e x c i t e d s t a t e s a r e
measurements w i t h
decreases
of
E X C I T E D S T A T E S OF
band
9
intensity
o f c a r b o x y l i c a c i d on
titration
curves
were
measurements and a
was a t t r i b u t e d t o a s h i f t
(Giordano
[Rubpz(bpy) ]
i n the excited
[Ru(bpy) (2,2 -bipyridine-
e q u i l i b r i u m upon e l e c t r o n i c e x c i t a t i o n
charge-transfer
[H ] i n
1
the acidity
curves
+
varied
i s more a c i d i c
Two d i f f e r e n t
and emission
of the t i t r a t i o n
protonation
9
i n aqueous s o l u t i o n s and t h e
hand, e x c i t a t i o n
bipyridines.
excited states.
[Ru(bpy) (4,7-dihydroxy-
i n t h e ground
The c o m p l e x
acid)]
^(Tr,Tr*)
forms o f
determined by s p e c t r a l and l i f e t i m e
one
and t r i p l e t s
(trip-quartets).
charge-transfer
the
on t h e c e n t r a l metal i o n
of the
i n the metal-to-
1977).
2 +
2
The
lowest
excited state of [Rubpz(bpy) ]
the
"metal-to-bipyrazine"
2 +
charge-transfer
1
(bpz,
2
2,2 -bipyrazine)
i s
(MLCT) e x c i t e d s t a t e .
Irradiation
o f 4 5 7 . 9 nm l i n e i n t h e r e d c o m p o n e n t o f t h e s p l i t v i s i b l e 3 -1 MLCT b a n d ( b a n d I , 2 0 . 5 x 1 0 cm ) s h o w s a r e s o n a n c e Raman m o d e s o f b i p y r a z i n e , w h i l e i r r a d i a t i o n o f 4 0 6 . 7 nm l i n e i n t h e b l u e c o m p o n e n t 3 -1 band (band I I , 24.8x10 cm ) e x h i b i t s those o f b i p y r i d i n e . [Rubpz2+ (bpy) ] e x h i b i t s emission from t h e lowest e x c i t e d t r i p l e t s t a t e . 2+ 9
Phosphorescence e x c i t a t i o n films
show a h i g h
temperature
spectra
o f [Rubpz(bpy) ] 2
positive polarization
i n t h e band
I , which
(P^2/5)
localized
is
i n a m i r r o r - i m a g e o f t h e weak a b s o r p t i o n
the
S-T
within the coordinated
second-derivative (band I ) .
(singlet-triplet)
absorption
i n PVA
indicates the electronic excitation
is
MLCT b a n d
doped
even a t room
bipyrazine.
Phosphorescence 3 -1
band
(16.2x10
cm
band
) by
spectrum t o the r e d o f the intense
The weak b a n d
i s assigned
as t h e s p i n - f o r b i d d e n
component band o f " m e t a l - t o - b i p y r a z i n e "
MLCT
transition. 2+ [Rubpz(bpy) ] 2
ture: D 0, 2
460
emits
the lifetime
phosphorescence
and y i e l d
i n s o l u t i o n even a t room
i n H 0 a t 2 5 °C a r e 2
T
tempera-
= 8 8 n s , = 0 . 0 0 6 ;
T = 1 9 0 n s , = 0 . 0 1 2 ; C H O H , T = 2 0 0 n s , = 0 . 0 2 6 ; C H C N , T = 3
n s , = 0. 0 2 7 ;
respectively yields,
propylenecarbonate,
(Shinozaki unpublished).
3
T = 4 0 0 n s , =
0.026,
From t h e measured
t h e r a d i a t i v e (k^) and n o n r a d i a t i v e
l i f e t i m e s and
decay r a t e constants ( k
n
r
)
1
are
evaluated according to k = ((JT" a n d knr 4 - 1 6 -1 A 6.8x10* s , k =*11.3xl0 s ; D 0 , k =6.3xl0 b
( l - c j O x " : H~0, k = - i £ i s , k =5.2xl0 s " ; 2
4
9
X
1
=
r
X
r
1
6
X
4
1
1
CH-OH, k =6.7x10* s , k =4.9xlO s ; CH CN, k =5.9xl0 s " , k = 6 -1 i - i £ - i 2.1x10 s ; propylenecarbonate k =6.5xl0 s , k =2.4x10° s . Q
n
r
r
n
r
4
f
The
radiative rate
i s about
7x10
s
independent o f t h e media.
However, t h e l i f e t i m e i s c o n t r o l l e d by t h e r a t e relaxation The
which
increases
i n protic
l i f e t i m e s were measured
ture
(Shinozaki
i n H^O
unpublished).
of
nonradiative
solvents.
and D 0
as a f u n c t i o n
2
A plot of In K
( = 1/T)
o f tempera-
against
1A
T
B
w e l l f i t s t h e e q u a t i o n k = k ° e x p ( - A E / k T ) : k ° = 3 . 8 8 x l 0 s " , AE =270 - 1 0 7-1 -1 cm ; k =2.70x10 s , AE =34 0 cm . P r o t i c s o l v e n t molecules form 7
1
D
D
D
hydrogen bonds w i t h Even
the peripheral
nitrogens
i fthe b a s i c i t y of bipyrazine
from r u t h e n i u m
to bipyrazine
water protons oxygen. -i
(m /m ) D
/
o r deuterons
i s increased
i n the excited
are oscillating
From t h e p l o t o f I n k^/k^ v e r s u s n JJ
9
/
f+/f =1.4 and t h u s
H
f#/f =l.
Q
protonated
species,
minimum b e y o n d protonated
should
state,
transfer
the hydrogen-bonded
i n a minimum n e a r b y 1/k^T, we 13
be t r a n s f e r r e d
to another
1
H
3 +
[RubpzH(bpy) ]
1
D
e m i t s no
2
water
obtained
(AE =270 cm"" ; AE =340 cm
an energy b a r r i e r
species
by charge
bipyrazine.
To f o r m t h e n o n e m i s s i v e
Q
the proton
of coordinated
) .
The
phosphorescence.
A semiempirical
SCKO c a l c u l a t i o n w a s c a r r i e d o u t o n t h e (TT,TT*) e x c i t e d 13 * s i n g l e t and t r i p l e t s t a t e s o f coordinated b i p y r a z i n e . The ' (TT ,TT ) e x c i t e d s t a t e s w e r e a l s o c a l c u l a t e d o n t h e two s t a g e s o f p r o t o n a t i o n 3 * of t h e p e r i p h e r a l n i t r o g e n s . The l o w e s t (TT,TT ) s t a t e o f coordinated 3+ 3 —1 bipyrazine
i n [RubpzH(bpy) ]
(kpy)
triplet
(14xl0
nonemissive stabilized the
3
3
state 1
cm" )
state
cm
) i s lower
1
cm" ), while
low-lying In this
that
t h e MLCT e x c i t e d
o f coordinated
The q u e n c h i n g
) excited
(TT,TT
p a r t i c u l a r case, 1 , 3
activation to the low-lying the l i f e t i m e of the excited
A l l s o p p 1978; D u r h a m 1982; C a s p e r PROTONATION OF [Rubpz(bpy) ] 2+
state
than of
2
state.
The
much l o w e r
than
i n the protonated
relaxation i nthe o f monoprotonated
i ti s not necessary (d,d*)
complexes 1983;
the lowest
[RubpzH ~
bipyrazine i s
t o one o f t h e two n i t r o g e n s
triplet.
i s attributable to fast nonradiative 3 *
nonemissive
thermal
3
i s a t around
by p r o t o n a t i o n
bipyrazine.
limits
(12xl0
(TT,TT*) e x c i t e d
l o w e s t MLCT e x c i t e d
complex
(11x10
2
MLCT e x c i t e d
Allen
excited
t o assume a
states
which
( V a n H o u t e n 1975,
1976;
1984).
2 +
2
In a c i d i c media, nitrogens
[Rubpz(bpy) 1' 2
of coordinated
Predominant
i n 6.0
M>
i s p r o t o n a t e d on t h e p e r i p h e r a l
bipyrazine. +
[ H ] >3.5
The m o n o p r o t o n a t e d
M, w h i l e
the diprotonated
species species
i s i n
+
[H ]
M.
>8.0
dilute
The
protonated
a c i d s o l u t i o n s u c h t h a t no
absorption
measurements,
1
H^o" " i n s o l u t i o n . function
of
pH
However the the of
Two
by
different
of
the
time-resolved excitation, 3 +
which ns)
i s rather
tion
i n the
in acidic
short
curves
the
protonated
i s not
of
the
Since
species
H^0
+
obtained
of
i s so
the
as
controlled protonation dissociation
indication By
of
measurements
species
after
nonemissive
determined *
as
1.1 ns, 2+
[Rubpz(bpy) ]
(88
2
accelerated
that
the
protona-
excited-state
the
excited
2 +
recovery of [Rubpz(bpy) ] *" Measurements determined the 3+ 9
[RubpzH(bpy)2]
COMPLEX FORMATION I N
and
THE
thus
f o l l o w i n g the
rate of
lifetime,
diffusion
complex.
[Rubpz(bpy) ]
is
2
deactivation.
In
i s slower than the emission rate of proton dissociation
y i e l d e d by
EXCITED
a
nonradiative relaxation
i s g o v e r n e d by and
by
measurements.
the
that of
the
a
by
quenched
ground-state
a c r u e o u s m e d i a was
media
are
direct
accomplished w i t h i n the
in acidic
i s detected
the
9
proton
the
curves
intensity
In
a c i d s o l u t i o n where the g r o u n d - s t a t e species i s 2+ [Rubpz(bpy) ] , t h e e x c i t e d c o m p l e x i s q u e n c h e d by 2 +
exclusively diffusion
species
bipyrazine basicity.
in contrast with
encounter of
In a d i l u t e
emission
phosphorescence.
i s partly
excited-state lifetime
e q u i l i b r i u m i s not quenching
controlled
of
the
(Shinozaki unpublished).
process
titration
and
repopulation
the
9
protonated
phosphorescence
absorption
displacement
*[RubpzH(bpy) ]
by
the
MLCT-induced enhancement o f
pulse
the
c o m p l e x e s e m i t no
the
d e a c t i v a t i o n of
recovered
fact,
the
decay. 8 -1 (k__ =2xio s ) 9
excited
species.
STATE
A b s o r p t i o n s p e c t r a v a r y w i t h i-ncreasincr c o n c e n t r a t i o n o f h y d r a t e d Ag ion [Ag ] i n a s o l u t i o n o f [ R u b p z ( b p y ) ] . The f i r s t s t a g e o f t h e 3+ +
+
2 +
9
spectral in
variation
which a
peripheral emission + [Ag of
i s ascribed
hydrated
Ag
the
formation
of
t o one
[RubpzAg(bpy) ] 9
of
the
[RubpzAg(bpy) ] emits to the red of the 2+ [Rubpz(bpy) ] . E m i s s i o n l i f e t i m e s were measured
from
9
9
lifetimes
i n the
,
bipyrazine
+
nitrogens.
] v a r i e d w i t h i n the the
to
ion i s coordinated
first
stage
a b s e n c e and
of
complex formation.
presence of
Ag
+
The
with
ratio
ion, T Q / T ,
i n c r e a s e s w i t h [Ag ] b u t t h e p l o t o f T / x a g a i n s t l o g [ A g ] shows a + -1 p l a t e a u w h e n [Ag ] i s i n c r e a s e d up t o 2 ^ 4 x 1 0 M. The l i f e t i m e o f [RubpzAg(bpy) ] a t 25 °C was d e t e r m i n e d a s 66 n s f r o m t h e p l a t e a u +
+
n
3 +
9
+ v a l u e x^. E m i s s i o n s p e c t r u m v a r i e s w i t h [Ag ] , w h i l e i t i s i n v a r i a n t r e g a r d l e s s o f whether i t i s e x c i t e d i n the a b s o r p t i o n band o f 2+ 3+ [Rubpz(bpy) ] or that of [RubpzAg(bpy) ] - This i n d i c a t e s the e q u i l i b r i u m of complex formation i s accomplished i n a s h o r t e r p e r i o d than the time constant of the e x c i t e d - s t a t e l i f e t i m e s . 2
2
From t h e d e c a y l i f e t i m e s formation 25
ground
of [Ag ],
t h e complex-
i n t h e e x c i t e d s t a t e w a s d e t e r m i n e d K*=130 M*"
On t h e o t h e r
s t a t e was o b t a i n e d
°C, u=3.0 M .
25
+
f o ra variety
1
constant
°C, u=3.0 M.
measured
hand, t h e f o r m a t i o n
1
appreciably
promoted
coordinated
bipyrazine increases
at
i nthe
b y a b s o r p t i o n m e a s u r e m e n t s a s K=2 0 M*"
The complex f o r m a t i o n than
constant
at
i n the excited state i s
t h a t o f t h e ground
state,
the basicity
since the
u p o n t h e MLCT
electronic
excitation.
References Allen
GH, W h i t e R P , R i l l e m a DP, M e y e r T J ( 1 9 8 4 ) S y n t h e t i c c o n t r o l o f
excited-state ligand J
properties.
1
2,2 -bipyrazine,
T r i s - c h e l a t e complexes c o n t a i n i n g t h e 1
2 , 2 - b i p y r i d i n e , and
1
2,2 -bipyrimidine.
Am Chem S o c 1 0 6 : 2 6 1 3 - 2 6 2 0 .
Allsopp
SR, C o x A , Kemp T J , R e e d WJ
solution.
Part-1.
(1978)
Inorganic
Temperature a c t i v a t i o n
photophysics
o f decay processes
i n
i n the
1
luminescence o f t r i s ( 2 , 2 - b i p y r i d i n e ) r u t h e n i u m ( I I ) and t r i s ( 1 , 1 0 phenanthroline) ruthenium (II)
ion.
J Chem S o c F a r a d a y T r a n s I 7 4 :
1275-1289. 2+ C a s p e r J V , M e y e r T J (1983) P h o t o c h e m i s t r y effects.
of Ru(bpy)
.
3
Solvent
J Am Chem S o c 1 0 5 : 5 5 8 3 - 5 5 9 0 .
Durham B, C a s p e r J V , N a g l e J K , M e y e r T J ( 1 9 8 2 ) P h o t o c h e m i s t r y Ru(bpy)
2 + 3
.
G i o r d a n o P J , B o c k CR, W r i g h t o n MS of
ruthenium(II)
Increased
of
J Am Chem S o c 1 0 4 : 4803--4810.
complexes o f
acidity
(1978) E x c i t e d s t a t e p r o t o n
transfer
4,7-dihydroxy-l,10-phenanthroline.
i n the excited state.
J Am Chem S o c 1 0 0 : 6 9 6 0 -
6965. Giordano P J , Bock Excited the
CR, W r i g h t o n MS, I n t e r r a n t e L V , W i l l i a m s
state proton
acid
transfer
t r a n s f e r o f a metal
dissociation
constant
f o ra m e t a l - t o - l i g a n d
state of a ruthenium(II)
RFX
(1977)
complex: d e t e r m i n a t i o n
complex.
of
charge
J Am Chem S o c 9 9 :
3187-3189. M a t s u z a w a H, K a i z u Y, K o b a y a s h i emission Peterson
in
2
S H , Demas J N
transition
metal
aqueous a c i d .
Peterson
H
( t o be p u b l i s h e d )
P r e p a r a t i o n and
spectra o f K [Rubpy(CN) ] and K [Ruphen(CN) ]. 4
2
4
(1976) E x c i t e d s t a t e a c i d - b a s e
complexes:
reactions of
1
Dicyanobis(2,2 -bipyridine)ruthenium(II)
J Am Chem S o c 9 8 : 7 8 8 0 - 7 8 8 1 .
S H , Demas J N
(1978) E x c i t e d - s t a t e a c i d - b a s e
1
dicyanobis(2,2 -bipyridine)ruthenium(II) phenanthroline) ruthenium (II) .
reactions of
and d i c y a n o b i s (1,10-
J Am Chem S o c 1 0 1 : 6 5 7 1 - 6 5 7 7 .
Shinozaki
K, K a i z u
published) the
Y, H i r a i
Protonation
lowest excited
Shinozaki
[RubpzH(bpy) ] 2
Van
Houten
3 +
(19 76) chemical in
lifetime
of a nonemissive
R J (1975) The e f f e c t o f l i g a n d
on t h e e x c i t e d
transfer
state
to solvent.
properties solution.
properties
i o n i n aqueous
Temperature dependence
aqueous
2
Y, K o b a y a s h i H, S u m i t a n i M, Y o s h i h a r a
Excited-state
bipyridyl)ruthenium(II) electron
( t o be i n
K
complex
.
J , Watts
deuteration
H, K o b a y a s h i H
formation of [Rubpz(bpy) ]
state.
K, O h n o 0, K a i z u
(to be p u b l i s h e d )
H, M a t s u z a w a
and complex
and
solvent
of the tris(2,2'-
solution.
J Am Chem S o c 9 7 :
Evidence f o r 3843-3844;
of the photophysical
and photo-
of the tris(2,2'-bipyridyl)ruthenium(II) i o n J Am Chem S o c 9 8 :
4853-4858.
QUENCHING OF E X C I T E D R u ( b p y ) 2
Cz.Stradowski
WITH METHYLVIOLOGEN AT LOW
+
TEMPERATURES
and M . W o l s z c z a k
I n s t i t u t e of Applied Radiation Chemistry, Technical 93-590 L o d z , W r o b l e w s k i e g o 1 5 , POLAND
University
(Politechnika),
There i s a growing i n t e r e s t i n t h e s t u d i e s o f e l e c t r o n t r a n s f e r r e a c t i o n s , i n w h i c h t h e r e a c t a n t s a r e s e p a r a t e d by s e v e r a l m o l e c u l a r r a d i i . A m o n g t h e m p h o t o i n d u c e d e l e c t r o n t r a n s f e r was i n t e n s e l y s t u d i e d ( M i l l e r e t a l . 1982).Well-know 'react i o n t r a n s f e r between e x c i t e d Ru(bpy) ( t r i s ( 2 , 2 ' - b i p y r i d i n e ) r u t h e n i u m ( I I ) d i c a t i o n ) a n d MV"** (methylviologen , 1,1'-dimethy1-4,4'-bipyridinium d i c h l o r i d e ) served as a m o d e l s y s t e m ( M i l o s a v l j e v i c a n d Thomas 1 9 8 5 ) . I t h a s b e e n d e m o n s t r a t e d that m e t h y l v i o l o g e n quenches the luminescence o f Ru(bpy)| i n polymers a t a m b i e n t ( M i l o s a v l j e v i c a n d Thomas 1 9 8 5 ) a n d l o w ( G u a r r e t a l . 1 9 8 5 ) t e m p e r a t u r e s . T h e q u e n c h i n g was a s c r i b e d t o e l e c t r o n t u n n e l l i n g o v e r l a r g e d i s t a n c e s (>10 A ) f r o m e x c i t e d R u ( b p y ) | t o m e t h y l v i o l g e n . T h e s i m i l a r i n t e r p r e t a t i o n was u s e d t o e x p l a i n t h e q u e n c h i n g o f R u ( b p y ) | i n v i s c o u s l i q u i d (Guarr e t a l . 1 9 8 3 ) . However,the d i r e c t p r o o f o f electron t r a n s e f e r v i a long-range tunnelling,showing the a c c e l l e r a t ion o f l u m i n e s c e n c e d e c a y i n t h e p r e s e n c e q u e n c h e r a t 77 K i s s t i l l l a c k i n g . T h e a i m o f t h e p r e s e n t w o r k was t o s t u d y t h e q u e n c h i n g o f l u m i n e s c e n c e i n s o l i d p h a s e down t o 77 K. 2
3
f
+
+
2
The l u m i n e s c e n e o f R u f b p y ) ^ " was s t u d i e d i n e t h y l e n e g l y c o l - w a t e r m i x t u r e , 2 : 1 b y v o l u m e (EG/H^O) a n d i n s e v e r a l p o l y m e r foils.The p r e p a r a t i o n o f t h e samples and l u m i n e s c e n c e measurements a r e d e s c r i b e d e l s e w h e r e ( S t r a d o w s k i a n d W o l s z c z a k ^1987 ).The F i g . l summarizes t h e d a t a o f luminescence o f R u ( b p y ) i n EG/H^O a s m e a s u r e d by t i m e - r e s o l v e d l a s e r p h o t o l y s i s . T h e k i n e t i c s o f t h e l u m i n e s c e n c e , o b s e r v e d a f t e r 10 n s l a s e r p u l s e ( A e x c = 530 nm) was m o n i t o r e d a t 610 nm.The d e c a y o f t h e l u m i n e s c e n c e was a l w a y s monoexponential.The l i f e t i m e s o f l u m i n e s c e n c e a t t e m p e r a t u r e s 290-200 K a r e p r e s e n t e d i n F i g . l . i t i s e v i d e n t t h a t a t t e m p e r a t u r e s 290-200 K t h e p r e s e n c e o f t h e quencher a c c e l l e r a t e d markedly t h e decay o f t h e luminescence.The s y s t e m EG/H2O i s h i g l y v i s c o u s a n d p o s s i b i l i t y o f t h e d i f f u s s i o n o f Ru(bpy)|"** a n d M V * d u r i n g t h e l i f e t i m e o f l u m i n e s c e n c e i s v e r y l i m i t e d . N e v e r t h e l e s s , o n e o b s e r v s e f f i c i e n t quenching o f l u m i n e s c e n c e under t h i s c o n d i t i o n s , w h i c h i s favour o f long-range t u n n e l l i n g as a mechanism o f quenching.On t h e o t h e r h a n d , b e l o w 200 K t h e p r e s e n c e o f the q u e n c h e r h a s v e r y l i t t l e o r no e f f e c t o n t h e r a t e o f l u m i n e s c e n c e d e c a y , T h e d e c a y a t 77 K was s t i l l m o n o e x p o n e n t i a l . T h e lifetime of the l u m i n e s c e n c e w a s 3.1 u s , i n d e p e n d e n t o f t h e q u e n c h e r a d d i t i o n u p t o 0.15 M M V . T h e r e f o r e , w e c o n c l u d e t h a t a t t e m p e r a t u r e r a n g e 7 7 - 2 0 0 K the d y n a m i c q u e n c h i n g o f l u m i n e s c e n c e o f R u ( b p y ) ^ b y m e t h y l v i o l o g e n does n o t o c c u r . I n o r d e r t o f i n d o u t t h e q u e n c h i n g m e c h a n i s m , t h e measurements o f t h e dependence o f luminescence i n t e n s i t y as a f u n c t i o n of M V c o n c e n t r a t i o n w e r e c a r r i e d o u t . T h e i n t e n s i t y o f l u m i n e s c e n c e was m e a s u r e d w i t h a c o n v e n t i o n a l f l u o r i m e t e r . T h e r e s u l t s a r e s u m a r i z e d i n F i g . 2 . C u r v e 1 i n F i g . 2 s h o w s t h a t a t 77 K t h e q u e n c h i n g i s v e r y i n e f f i c i e n t , a n d independent o f t h e n a t u r e o f t h e medium.In ^ e a r l i e r s t u d i e s t h e q u e n c h i n g o f t h e l u m i n e s c e n c e o f R u ( b p y ) | b y MV i n s o l i d s was a s c r i b e d t o l o n g - r a n g e t u n n e l l i n g o f e l e c t r o n f r o m e x c i ted R u ( b p y ) | * t o M V ( M i l o s a v l j e v i c a n d Thomas 1 9 8 5 ; G u a r r e t a l . 1 9 8 5 ) . +
5
2
2 +
2 +
+
Z +
P r e s e n t d a t a s h o w t h a t t h i s i n t e r p r e t a t i o n c a n n o t be u s e d f o r d a t a a t 77 K . T h r e e f o l l o w i n g o b s e r v a t i o n a r e i n d i s a g r e e m e n t w i t h s u c h i n t e r p r e t a t i o n : ( 1 ) T h e d e c a y o f l u m i n e s c e n c e o f R u ( b p y ) | a t 77 K was not a c c e l l e r a t e d i n the presence o f ( 2 ) The q u e n c h i n g c u r v e s h o w n i n F i g . 2 , i s w e l l d e s c r i b e d by l u m i n e s c e n c e q u e n c h i n g i n c o n t a c t p a i r s . The r a d i u s o f s u c h c o n t a c t p a i r , c a l c u l a t e d f r o m c u r v e 1 i n F i q . 2 i s 7.1 A , i . e l e s s t h e n sum o f g e o m e t r i c r a d i i o f R u t b p y ) ^ a n d MV (10 A ) . Such r e s u l t i s m e a n i n g l e s s from t h e p o i n t o f view of l o n g - r a n g e t u n n e l l i n g . ( 3 ) L a c k o f a n y t r a c e s o f MV " r a d i c a l c a t i o n , t h a t i s t h e p r o d u c t o f q u e n c h i n g a t 77 K . T h e r e f o r e , w e p o s t u l a t e t h a t t h e q u e n c h i n g o f l u m i n e s c e n c e o f R u ( b p y ) | b y M V a t 77 K v i a l o n g - r a n g e t u n n e l l i n g i s n e g l i b i l e . I n e f f i c i e n c y o f e l e c t r o n t u n n e l l i n g between e x c i t e d Ru(bpy)| and MV was a s s i g n e d b y S e e f e l d ( S e e f e l d e t a l . 1 9 7 7 ) t o h i g h e n e r g y b a r r i e r b e t w e e n r e a c t a n t s ( c a . 2.5 e v ) . T h u s , t h e q u e n c h i n g a t 77 K i n o u r s y s t e m s h o u l d b e a s c r i b e d t o " s t a t i c " t y p e o f q u e n c h i n g , o c c u r i n g w i t h i n c o n t a c t p a i r s formed i n the ground state.The P e r r i n o r s t a t i c t y p e o f q u e n c h i n g o c c u r i n g i n f i n i t e l y f a s t w i t h i n an a c t i v e s p h e r e , r e s u l t s i n a m o n o e x p o n e n t i a l l u m i n e s c e n c e decay and l e a d s t o a decrase of the i n i t i a l i n t e n s i t y of luminescence.The l i f e t i m e of luminescence remains u n a f f e c t e d by t h i s type of quenching. f
f
4
+
+
+
2 +
2 +
On t h e o t h e r h a n d , t h e c u r v e s 2,3 a n d 4 i n F i g . 2 i n d i c a t e t h a t a t r o o m t e m p e r a t u r e t h e q u e n c h i n g i n s o l i d s t a t e i s much m o r e e f f i c i e n t t h e n a t 77 K . T h e same o b s e r v a t i o n c o n c e r n s v i s c o u s l i q u i d a t l o w t e m p e r a tures (see F i g . 1).Obviously,the p o s s i b i l i t y of the d i f f u s i o n of r e a c t a n t s d u r i n g t h e l i f e t i m e o f e x c i t e d R u ( b p y ) | i s v e r y l i m i t e d under c o n d i t i o n s a p p l i e d i n the p r e s e n t work.Thus,we c o n c l u d e t h a t t h e r m a l l y a c t i v a t e d e l e c t r o n t u n n e l l i n g t a k e s p l a c e . W e h a v e r e c o r d e d some t r a c e s o f M V * c a t i o n r a d i c a l i n p o l y m e r f o i l s a t room t e m p e r a t u r e , w h i c h a d d i t i o n a l l y s u p p o r t s t h i s s u g g e s t i o n . L e t u s d i s c u s s b r i e f l y some p o s s i b l e mechanisms of the t h e r m a l a c t i v a t i o n . +
c,M +
F i g . 2. N o r m a l i z e d i n t e n s i t y o f l u m i n e s c e n c e o f R u ( b p y ) | as a f u n c t i o n o f m e t h y l v i o l o g e n c o n c e n t r a t i o n . C u r v e (1) a t 77 K ( (o) EG/ H 0,(t) c e l l o p h a n e , ( x ) p o l y ( v i n y l a l c o h o l ) , c u r v e s 2,3 and 4 a t room t e m p e r a t u r e ( ( A ) p o l y ( v i n y l a l c o h o l ) i n vacuum, (•) p o l y ( v i n y l a l c o h o l ) in a i r , ( A ) dry c e l l o p h a n e ) .
The f i r s t mechanism i n v o l v e s the i n f l u e n c e o f t e m p e r a t u r e on e l e c t r o n energy l e v e l s o f e l e c t r o n d o n o r , e x c i t e d R u l b p y ) ^ .Thanks t o the f i n e work o f Y e r s i n and G a l l h u b e r ( Y e r s i n and G a l l h u b e r 1984) one knows a g r e a t d e a l about e n e r g y l e v e l s of R u ( b p y ) 3 a t v a r i o u s t e m p e r a t u r e s . However,the q u e s t i o n o f dynamics o f e l e c t r o n l o c a l i z a t i o n o n t o one o f bpy l i g a n d s a t low t e m p e r a t u r e s i n p o l a r s o l v e n t i s s t i l l u n r e s o l v e d ( F e r g u s o n e t a l . 1 9 8 5 ) . The r e o r g a n i s a t i o n o f t h e s o l v a t i o n s h e l l arround e x c i t e d R u ( b p y ) | i n a g l a s s , p o l y m e r i c f i l m , o r the s o l i d s t a t e at low t e m p e r a t u r e s i s l o n g on t h e time s c a l e f o r e x c i t e d s t a t e decay (Meyer 1986).At h i g h e r t e m p e r a t u r e s r e o r g a n i z a t i o n o f t h e medium i s completed a t t i m e s much s h o r t e r t h a n the l i f e t i m e o f l u m i n e s c e n c e . R e l a x a t i o n t i m e o f t h e medium i s s e v e r a l o r d e r s o f magnitude s m a l l e r at room t e m p e r r a t u r e t h e n a t 77 K and e n e r g y l e v e l m a t c h i n g (making e l e c t r o n t r a n s f e r e f f i c i e n t ) c o u l d be much f a s t e r . +
+
The s e c o n d a c t i v a t i o n mechanism i n v o l v e s i n f l u e n c e o f t e m p e r a t u r e on energy l e v e l s i n e l e c t r o n a c c e p t o r , i . e M V . T h e e n e r g y gap between e x c i t e d Ru (bpy )| and M V ' c a t i o n r a d i c a l i s 0.4 eV ( Mi losa,vl j e v i c and Thomas 1 9 8 5 ) . T h i s amount o f e n e r g y s h o u l d be d i s s i p a t e d i n t o t h e medium d u r i n g e l e c t r o n t r a n s f e r , m o s t l y v i a phonon e m i s s i o n . T h i s P r o c e s s i s a l s o b e l i e v e d t o be t h e r m a l l y a c t i v a t e d . z+
+
+
Both f a c t o r s d i s c u s s e d above a r e dependent on the medium n a t u r e . T h i s agrees w e l l w i t h c u r v e s i n F i g . 2 . I t i s d e m o n s t r a t e d t h a t a t room t e m p e r a t u r e t h e q u e n c h i n g i s d i f f e r e n t i n v a r i o u s media w h i l e a t 77 K t h i s i s not c a s e .
A d d i t i o n a l proof f o r the c r u c i a l r o l e of thermal a c t i v a t i o n i nthe q u e n c h i n g o f e x c i t e d Ru c o m p l e x e s c a n b e i n f e r r e d f r o m t h e s t u d i e s o f R u { b p y k (N-N-Me)2 (Sullivan e t a l .1978).In t h i s s t r u c t u r e e l e c t r o n a c c e p t o r o c c u p i e s a p a r t o f t h e l i g a n d sphere o f t h e complex. A t 77 K t h e l u m i n e s c e n c e c h a r a c t e r i s t i c s r e s e m b l e d t h o s e o f R u ( b p y ) | * . A t room t e m p e r a t u r e , h o w e v e r , m u c h w e a k e r , s h o r t - 1 i v e d and r e d - s h i f t e d l u m i n e s c e n c e w a s o b s e r v e d . T h i s e f f e c t was e x p l a i n e d i n terms o f t h e r m a l l y a c t i v a t e d e l e c t r o n t r a n s f e r from bpy l i g a n d t o remote pyridinium s i t e ( S u l l i v a n e t al.1978). +
Generally,the present observation suggest that the quenching of l u m i n e s c e n c e o f Ru(bpy)| b y m e t h y l v i o l o g e n a t 77 K h a s m a i n l y s t a t i c character.At higher temperatures the quenching v i a thermally activated long-range t u n n e l l i n g occurs i n s o l i d phase.The e f f i c i e n c y o f t h i s p r o c e s s d e p e n d s s i g n i f i c a n t l y upon t h e n a t u r e o f s u r r o u n d i n g medium. +
ACKNOWLEDGMENT One o f t h e a u t h o r s ( C z . S ) g r a t e f u l l y a c k n o w l e d g e s measure t i m e - r e s o l v e d luminescence a t Max-PlanckStrahlenchemie,Mulheim,W.Germany.
the p o s s i b i l i t y Institutfur
to
REFERENCES. F e r g u s o n J , K r a u s z E R , M a e d e r M (1985) C h a r g e l o c a l i z a t i o n i n t h e l u m i n e s c e n t s t a t e s o f R u ( b p y ) | * i n f l u i d s o l u t i o n s . J P h y s Chem 89: 18521854. G u a r r T , M c G u i r e M , S t r a u c h S , M c L e n d o n G (1983) C o l l i s i o n l e s s p h o t o i n d u ced e l e c t r o n t r a n s f e r from ruthenium t r i s ( b i p y r i d i n e ) * homologues to methyl viologen (MV ) i n r i g i d g l y c e r o l s o l u t i o n . J Am Chem S o c 105:616-618. G u a r r T , M c G u i r e M E , M c L e n d o n G ( 1985) L o n g r a n g e p h o t o i n d u c e d e l e c t r o n t r a n s f e r i n a r i g i d p o l y m e r . J Am Chem S o c 107:5104-5111. M i l l e r J R , P e e p l e s J A , S c h m i t t M J , C l o s s GL (1982) L o n g - d i s t a n c e f l u o r e s c e n c e q u e n c h i n g b y e l e c t r o n t r a n s f e r i n r i g i d s o l u t i o n s . J Am Chem Soc 104:6488-6493. M i l o s a v l j e v i c BH,Thomas J K (1985) P h o t o c h e m i s t r y o f c o m p o u n d s a d s o r b e d into cellulose.5.Solid-state reduction of methylviologen photosensit i z e d by t r i s ( 2 , 2 ' - b i p y r i d i n e ) r u t h e n i u m ( I I ) . J P h y s Chem 89: 18301835. M e y e r T J (1986) P h o t o c h e m i s t r y o f m e t a l c o o r d i n a t i o n complexes:metal to l i g a n d charge t r a n s f e r e x c i t e d states.Pure A p p l Chem 58:11931206. S e e f e l d K - P , M o b i u s D,Kuhn H (1977) E l e c t r o n t r a n s f e r i n m o n o l a y e r with incorporated r u t h e n i u m ( I I ) c o m p l e x e s . H e l v C h i m A c t a 60:2608-2632. S t r a d o w s k i C z , W o l s z c z a k M ( 1987 ) Q u e n c h i n g o f e x c i t e d R u ( b p y ) f with m e t h y l v i o l o g e n i n s o l i d p h a s e . t o be p u b l i s h e d . S u l l i v a n B P , A b r u n a H D , F i n k l e a H O , S a l m o n D J , N a g l e J K , M e y e r T J (1978) M u l t i p l e e m i s s i o n from charge t r a n s f e r e x c i t e d s t a t e s o f ruthenium ( I I ) - P o l y p y r i d i n e complexes.Chem Phys L e t t 58:389-393. Y e r s i n H , G a l l h u b e r E (1984) On t h e l o w e s t e x c i t e d s t a t e s o f [ R u ( b p y ) ] ( P F ) s i n g l e " c r y s t a l s . J Am Chem S o c 106: 6582-6586. 2 +
+
3
6
2
KINETICS OF
THE
CHEMILUMINESCENT OXIDATION OF
L.El-Sayed, D.Salkin
AQUEOUS BR"
S w a i m , W . K . W i l m a r t h , and +
BY
U n i v e r s i t y o f Southern C a l i f o r n i a ,
A
of
number
of
examples
for
which
the
chemi1 urninescent
emission
comes
3
A.W.Adamson*
Department o f C h e m i s t r y , CA 9 0 0 8 9 - 1 0 6 2 , USA
known,
Ru(bipyr)
, CL,
from
an
Los
redox
Angeles,
reactions
excited
are
now
state
coordination 3+ compound. The b e s t known c a s e i s t h a t o f t h e r e d u c t i o n o f RuL^ , L = 2 , 2 ' - b i p y r i d i n e , by v a r i o u s reductants, w i t h e m i s s i o n from the lowest
2+ * charge
transfer
Gafney
1975;
excited
Martin
state
1972;
of
the
Vogler
product,
1981;
[ RuL^
Bolletta
1981,
]
(Lytle,
1982;
1971;
El-Sayed
1987). As o t h e r e x a m p l e s , e m i s s i o n f r o m t h e d o u b l e t t h e x i s t a t e o f 3 ~l" 2 ~f" 3+ CrL^ h a s b e e n o b s e r v e d on o x i d a t i o n o f C r L « by RuL~ (Vogler 1981) as w e l l a s t h e CL o x i d a t i o n o f M o , C I , , or r e d u c t i o n of 6 14 Mo^Cl^
(El-Sayed
generated on
back
1987).
reaction The
typical
studies
are
rare.
and
and
a
two
diates which
redox
electron
and can
are
An
a
slow
useful
example
such
enough
the
1984;
as
were
between
a
about
the
of
1978,
in
by
kinetic
non-
radical
kinetic
Br
be product
Rubinstein
detailed
properties
aqueous
may
state
one-electron
involve
conventional
oxidation
and
interested
reactions
for
species
excited
Luong
however,
1980)
such
reactant
emitting
fast
information is
of
an
Nocera is
(Stanbury
reactions,
pair
give
reaction
reductant;
often
yield
mediates.
CL
co-workers
complementary
to
( B o l l e t t a 1982;
1981).
Wilmarth
Also,
e1ectrochemica11y,
oxidant interme-
studies,
of
such
inter-
Fe(5-Br-
3 + Phen)^ joint with
RuL 2
has
to
give
study 3 + 3
RuL
been
the
Fe(II)
(El-Sayed
1983),
+
=
3
+
2
Br"
investigated,
2
and
Br
kinetics
(Salkin
2
of
along
+
2
the
1983)
and
analogous
in
a
reaction
Br ~)
emphasis
on
the
Posthumus contribution. To whom c o r r e s p o n d e n c e s h o u l d
latter
be
(1)
3
We
study,
that
(or
the
this
with
Br
of
the
with
RuL^*
analytical reaction.
accompanies
*
the
, 3
that
+
complex
chemiluminescence, report
aspect.
addressed.
briefly
here
CL, on
EXPERIMENTAL
All
chemicals
(as
in
the
tilled
used
case
over
were
of
of
reagent
grade
[RuL^] ( C 1 0 ^ ) ^ ;
sodium
persulfate
the
and
the
or
were
solvent
carefully
water
used
purified was
dis-
vapor
passed through a tube at 3+ 800°C w i t h oxygen c a r r i e r gas. S o l u t i o n s o f RuL were p r e p a r e d 2+ i m m e d i a t e l y b e f o r e u s e by o x i d i z i n g a n a c i d i f i e d Ruh^ solution with PbO ( t h e e x c e s s t h e n f i l t e r e d o f f ) . T h e o x i d i z e d c o m p l e x was also 2+ ?
prepared No
pho t o c h e m i c a l l y
difference
by
in kinetic
photolysis
behavior
Chemiluminescent
intensities,
were
means
measured
which
a
second could
by
solution reactant
be
analytical of
mixtures
then
the
is a
Slawson
minor
correction
allow
of
the
It
was
same the
for
AND
as
was
was
1,
given CL
1°
and
made
established of
of
the
the
some
in Table reaction
(initial
Separate studies analytical
to
with
which
R
the
to
be
dark
RuL^
as
a
having
u
placed
the
determine
and
a
CL
L
product
at
Various
the
and
the
The
from
nm.
of
filters
chamber
spectrum.
453
time,
into
solution
reaction the
of
chamber
conditions.
of
3
±
solution.
function a
s p e c t r o p h o t o m e t r i c a 1ly, 2+
the
Reacting
0.1°C.
water
is
in
the
slightly both
the
reaction.
r e d u c t i o n but
(to within
plots
of
solvent
has
that
absence
Br
(Sutin
chemiluminescent. analytical
and
effect
on
A
the
D i s s o l v e d oxygen
little
the
1.
the
CL
10%
CL
1976,
relatively rate
data
i n c r e a s e s the
the
Br
CL
reduction.
in
2
was
from
reaction
essentially 2-f
RuL^
(1)
and
was
the
also
indeed
that
being
precision). CL
apparent
Figures
spectrum
emission
analytical
about
decrease of
the
photoexcited
s t o i c h i o m e t r y of
Some
to
and
counter
window
as
aerated
DISCUSSION
that
Fig.
initial
under
the
followed
reaction
1976),
this
first
observed
the
slow
solvent
RESULTS
injected
thermostatted
1983;
both
an
observed.
r e a c t a n t would
absorbance
were
was
quantum
d e t e c t o r , so
reaction
There
to
one
a
i n t e r p o s e d between
photomultip1ier
growth
of
of
of
and
intensity, first 3
I,
order
illustrate
with
rate
time
are
shown
constants,
that
the
rate
in are
law
for
is
CL
intensity)
(to
reaction
be (1)
=
reported to
be
- 3 (constant)(Br ) (RuL elsewhere)
showed
3
the
3+2 ) rate-law
(2) for
the
Fig. of vs. of
1.
Semi-logarithmic
chemiluminescence time
f o r t h e Br 3+ Ru(L) i n 0.053
plot
Fig.
intensity
CL
reduction
due
M
HC10
Fig.
3.
Dependence
intensity
(Br")
4 "
1 2 3 4 K>*[Ru(bipy)J*] . M
2.
(corrected
to s o l v e n t
Table
initial f o r that
r e d u c t i o n ) on
.
Dependence
of
intensity
on
i n i t i a l CL 3+ (RuL ' ) .
8
corrected
of
1
Rate
Constants f o r the Analyt 3+ Ru ( b i pyr) by B r .
ical
and
Chemiluminescent
Reduction of
3
t , °c
2
10 [Br"]
1 0 2
k
CL'
S
-1
10
2
k . .. f s AN
-1
17.5
1 . 30
1.13
± 0. 1
0.49
±
0.09
27.5
1. 82
2.53
± 0. 15
1.69
±
0.15
1. 30
1.53
± 0. 09
0.99
±
0.03
0. 781
0.91
± 0. 07
0.578
1.30
2.60
± 0. 10
1.13
37.4
Not
parallel
runs
± ±
k
CL
/ k
AN
2.25
*
0.00 0.03
2 . 30
-
d
It
ln(RuL
follows
3 +
from
expectation k
give
C
using
L
)/dt
3
A
k
Eqs.
is =
/ ^
=
=
A N
(2)
k (Br
and
,
3
±
luminol —5 be 4 . 2 x 1 0 moles 3 + RuL reduced.
system of
as
)
2
confirmed
The
N
the
k (Br
2
+
( 3 ) t ha t - d ( I / I ° ) / d t
qualitatively 2
) +
1
by
the
chemiexcitation
3
3 +
k (Br") (RuL 3
-
data yield
k
=
C L
of
2
(3)
k^.
Table
was
)
This
1,
which
determined,
reference
and
found
state,
per
mole
one
of
excited
( B r u n d r e t t 1974), 2 -f" ^ [RuL, ] , produced
to of
3
Our
results
pathways a
minor
indicate
for
that
[RuL
about
]
3
lies CL
path
law,
but
make
excited
that
Br
CL
pathway
is
the
same
as
a n a l y t i c a l r e a c t i o n , and that v i a this -4 o n l y a b o u t 10 of r e a c t i o n e v e n t s p r o d u c e
one,
the
the
the
product. Note 2+ *
for
that
must
the 48
must
also state
AG°
for
kcal
reaction
above
therefore
the
not
contain
a
formation
step
is
ground
.only
product
(1)
is
-
state.
conform
which
only
to
path,
One
kcal,
Any
is
state while
mechanism
observed
sufficiently
possible.
which
excited 8
the
the
rate
energetic
possibility
to
is
• r a d i c a l s p r o d u c e d as i n t e r m e d i a t e s i n the a n a l y t i c a l reac3+ 2-f can r e a c t w i t h R u L to form [ R u L ( L B r ) J , w h e r e L B r i s some of b r o m i n a t e d b i p y r i d i n e . R e a c t i o n of t h i s l a s t complex with 2+ * another B r ^ ' r a d i c a l c o u l d now h a v e s u f f i c i e n t e n e r g y t o g i v e [ML3 ] tion form
+
Br
3
as
3
kinetics
The
would
present
direct the
products.
A
detailed
conform
system
connection
analytical
2
to
Eqs.
appears can
and
be
mechanism
to
(2)
be
made
and
the
can
written
such
that
the
(3).
only
one
app r o x i m a t e l y
corresponding
be
reported
between
CL
reaction.
in
part
for which
the
a
mechanisms
of
ACKNOWLEDGEMENT
This
investigation
National
Science
was
supported
by
a
grant
from
the
U.S.
Foundation.
REFERENCES
Bolletta of
F,
Rossi
Tris(2,2
f
Metal-Centered Bolletta
F,
Reduction
A,
Excited
Balzani of
Balzani
V
(1981)
Chemi1uminescence
-bipyridine)chromium(II):
V
Bromate
dine)ruthenium(II).
State.
(1982) by J
Am
Inorg
Chemical Chim
Oscillating
Malonic Chem
Acid
Soc
104:
Acta
on
53:
by
4250-4251.
of
a
L23-24.
Chemiluminescence
Catalyzed
Oxidation
Generation
from
the
Tris ( 2,2'-bipyri-
Bolletta tion
F, C i a n o
Metal
chemical
M,
B a l z a n i V,
Complexes
Reduction
Serpone
as L i g h t
N
Emission
(1982)
Polypyridine
Sensitizers
of the P e r s u l f a t e I o n .
Inorg
in
Chim
Transi-
the Acta
Electro6 2 : 2 0 7-
213 . Brundrett
RB , W h i t e
Derivatives El-Sayed
L
Dissertation,
L, Adamson +1+3 bipyridine)^ * HD,
Adamson
Experiment. Luong
Transfer
J Am
Reactions
University
AW
(1975) Educ
of Southern
Processes
Chemiluminescence,
52:
L, W r i g h t o n
(1978)
Ground
Electrogenerated
Soc
5790-5795.
JE
nescence
Quenching
DM
(1971)
Hart
E J , Adamson
from
the Reaction
pyridyl)ruthenium(III). Nocera and
DG,
Gray
HB
(1984)
An
Illuminating
State
Elec-
Involving fac-Tri-carbonylchloro(1,10-
of the Lowest
Martin
California.
and E x c i t e d
Transfer
100:
7497-7502.
Coordination
480-481.
MS
Electron
FE, Hercules
Soc 96,
(1987)
phenanthro 1ine)rhenium(I) .
Lytle
Chem
Involving
Chemiluminescent R e a c t i o n s of Ru(2,2'-1-3 and Mo^Cl^^ ' . I n o r g Chim A c t a accepted.
J Chem
J L , Nadjo
tron
AW
and Chemiluminescen.ee o f
and I s o l u m i n o l .
Chemiluminescent
El-Sayed
Gafney
(1974 ) S y n t h e s i s
of Luminol
(1983)
Compounds.
EH
Photobiol AW,
Gafney
Chem
State.
and
J Am
Chem
13: 123. H, H a l p e r n
of the Hydrated
J Am
Chemi1urninescence
Excited
S o c 94:
J
(1972)
Electron
Chemilumi-
with
Tris(bi-
9238-9240.
Electrochemical Reduction
of Molybdenum(II)
Tungsten(II)
H a l i d e C l u s t e r Ions. Electrogenerated Chemilumi2of M o ^ C l , . J Am Chem S o c 1 0 6 : 824,-825. o 14 I, Bard AJ (1981) E l e c t r o g e n e r a t e d Chemi1urninescence . 37, 2+
nescence Rubinstein Aqueous
E e l Systems
Organic
Acids.
J Am
(1983)
Outer
Salkin Main
DS
Group
Based Chem
T
on R u ( 2 , 2 -
Soc 103: 512-516
Sphere
Substrates.
bipyridine)^
Electron
and c i t a t i o n s
Transfer
Dissertation,
and O x a l a t e
Reactions
University
or
therein, of
Some
of Southern
Calif-
ornia. Slawson
V, A d a m s o n
Stanbury of
DM,
Wilmarth
Thiocyanate
Creutz with
C,
AW
Sutin
(1976) WK,
Khalaf
and I o d i d e N,
Hydroxide
(1976)
and i t s
unpublished
work.
S, Po HN,
by I r i d i u m ( I V ) .
Reaction
of
Application
Byrd
J E (1980)
Inorg
Chim
Acta
53;
Reactions
L35-L37.
Oxidation
19:
2715-2722.
tris(bipyridine)ruthenium(III) i n a Solar
Energy
P r o c N a t A c a d S c i USA 7 2 : 2 8 5 8 - 2 8 6 2 . S u t i n N, C r e u t z C ( 1 9 8 3 ) p r i v a t e communication. V o g l e r A, E l - S a y e d L , J o n e s RG, N a m n a t h J , A d a m s o n Chemiluminescent
Chem
Involving Coordination
AW
Storage
(1981)
System.
New
Compounds.
Inorg
SYNTHESIS AND PHOTOPHYSICAL STUDIES OF ORTHO-METALATED P d ( I I ) TWO NOVEL P d ( I I ) / R h ( I I I ) DIMERS
COMPLEXES
INCLUDING
C . A . C r a i g , F . O . G a r c e s , and R.J.Watts Department o f C h e m i s t r y , U n i v e r s i t y o f C a l i f o r n i a , Santa
B a r b a r a , CA 9 3 1 0 6 , USA
INTRODUCTION The p h o t o p h y s i c s o f c y c l o p a l l a d a t e d compounds h a s r e c e i v e d a t t e n t i o n in the literature recently (Wakatsuki 1985). We were initially interested i n e x p l o r i n g t h e photophysics o f a complex prepared by Kasahara (1968); [Pd(ppy)CI] . The e l e c t r o n i c t r a n s i t i o n s o f this d i m e r i c complex a s w e l l a s f o r s e v e r a l monomeric d e r i v a t i v e s have been characterized as i n t r a ligand transitions of the ortho-metalated 2-phenylpyridine (ppy) l i g a n d ( C r a i g 1987). Our a t t e n t i o n was then focused on combining two d i f f e r e n t metals o f n o n - e q u i v a l e n t valence i n t o a s i n g l e m o l e c u l e . We r e p o r t h e r e t h e s y n t h e s i s , c h a r a c t e r i z a t i o n and photophysics o f two n o v e l mixed-metal o r g a n o m e t a l l i c complexes where p a l l a d i u m ( 1 1 ) and r h o d i u m ( I I I ) have been c o u p l e d v i a a halide bridge. 2
Figure 1. Structures t(ppy)Pd(Cl) Rh(bzq) ] (B). 2
of
[(ppy)Pd(CI)~Rh(ppy)
]
( A ) , and
2
Ortho-metalated transition metal c e n t e r s l i n k e d by a u-dichloro bridge, where t h e metal i s R h ( I I I ) o r i r ( I I I ) , have been r e p o r t e d t o cleave q u i t e r e a d i l y a t t h e b r i d g e when b i d e n t a t e c h e l a t i n g ligands are brought i n c o n t a c t w i t h t h e parent dimer i n s o l u t i o n (Nonoyama 1974). Similar cleavage occurs i n t h e presence o f monodentate coordinating l i g a n d s o r i n s o l v e n t s s u c h a s DMF, DMSO o r acetonitrile r e s u l t i n g i n monomeric d e r i v a t i v e s o f t h e p a r e n t complexes. Not surprisingly, t h i s i s t r u e f o r P d ( I I ) a s w e l l (Cope 1 9 6 5 ; P a r s h a l l 1 9 6 9 ; Trofimenko 1 9 7 2 ; G u t i e r r e z 1 9 8 0 ; Nonoyama 1 9 8 2 ; S p r o u s e 1 9 8 4 ) . We u t i l i z e d t h i s t e c h n i q u e t o p r o d u c e monomeric d e r i v a t i v e s o f t h e P d ( I I ) and R h ( I I I ) ortho-metalated dimers [Pd ( p p y ) C I ] , [Rh(ppy) C1] and [Rh(bzq) C 1 K where: (ppy)=2-phenylpyridine ana (bzq)=benzo[h]quinol i n e . WhSn i r r a d i a t e d i n e q u i m o l a r p r o p o r t i o n c e r t a i n m o n o m e r i c d e r i v atives o f t h e s e compounds combine t o form heterometallie dichlorobridged dimers. The n e w l y p r e p a r e d compounds have the following 2
2
2
formulas: [ ( p p y ) P d ( C I ) Rh(ppy) ] ( A ) , and [ ( p p y ) P d ( C I ) R h ( b z q ) ] ( B ) , w i t h p r o p o s e d s t r u c t u r e s i l l u s t r a t e d i n F i g . 1. H NMR f o r t h e s e n o v e l complexes have been o b t a i n e d and r e v e a l complex splitting patterns. Photophysical data s t r o n g l y suggests that the e m i t t i n g s t a t e s of (A) a n d (B) a r e p r i n c i p a l l y l i g a n d l o c a l i z e d a n d e a c h m e t a l c e n t e r r e t a i n s i t s unique e m i t t i n g manifold. 2
2
EXPERIMENTAL Synthesis
of
[(ppy)Pd(CI) Rh(ppy) ) 2
2
[Pd(ppy)Cl] (0.1441 g) was c o m b i n e d w i t h ( 0 . 1 5 6 7 g) [Rh(ppy}^Cl] ( N o n o y a m a 1 9 7 1 ) i n 100 m l d i c h l o r o m e t h a n e . C a r b o n m o n o x i d e w a s buDblea through t h i s solution f o r three hours. The s o l u t i o n was then broad band irradiated w i t h a 150 W Hg s o u r c e f o r 1 h o u r a n d t h e dichloromethane e v a p o r a t e d u s i n g a f l o w of n i t r o g e n gas. The r e s i d u a l y e l l o w powder was dissolved i n 125 m l o f chloroform, filtered, and the chloroform evaporated. The s o l i d was t h e n r e c r y s t a l l i z e d from d i chloromethane and hexanes t o g i v e 0 . 2 3 2 9 g f i n e n e e d l e c r y s t a l s ( 90 % y i e l d ) . A n a l . C a l c u l a t e d f o r [(ppy)Pd(C1)-Rh(ppy) ]•1/2CH C1 : C, 5 1 . 2 8 ; H, 3 . 1 8 ; N, 5 . 3 5 . F o u n d : C, 5 1 . 4 3 ; H, 3 . 1 2 ; N, 4.92. 2
2
2
Synthesis
of
2
[(ppy)Pd(CI) Rh(bzq) 3 2
2
The procedure was similar t o t h a t used for (A). [ P d ( p p y ) C l J and [Rh(bzq) C1] were combined i n equimolar q u a n t i t i e s , stirred in the presence o f CO f o r 3 h o u r s a n d t h e n i r r a d i a t e d f o r 45 minutes. Upon obtaining t h e s o l i d i t was d i s s o l v e d i n d i c h l o r o m e t h a n e and hexanes and p a s s e d t h r o u g h a c o l u m n c o n t a i n i n g S e p h a d e x LH-20 r e s i n i n order to separate the mixed metal dimer from unreacted s t a r t i n g materials. Mass s p e c t r a l a n a l y s i s gave t h e f o l l o w i n g p e a k s : 791,49 4,459,179,155. Anal. Calculated f o r [(ppy)Pd(CI)-Rh(bzq)A•1/4CH-C1 : C, 5 5 . 1 0 ; H, 3 . 0 1 ; N, 5 . 1 7 . F o u n d : C, 5 5 . 2 5 ; H, 3 . 0 1 ; N, 4.76. 2
2
2
Absorption d a t a w e r e o b t a i n e d w i t h a C a r y 15 s p e c t r o p h o t o m e t e r o r HP 8452A Diode Array Spectrophotometer. Emission lifetime d a t a ^ were o b t a i n e d w i t h apparatus d e s c r i b e d elsewhere (Sprouse 1984). A l l H NMR s p e c t r a w e r e m e a s u r e d w i t h a N i c o l e t N T - 3 0 0 FT NMR spectrometer.
R E S U L T S AND
DISCUSSION
Compounds (A) and (B) a r e f o r m e d f o l l o w i n g initial generation of [Pd(ppy)COC1] and [ R h ( L ) C O C l ] monomeric a d d u c t s of t h e p a r e n t d i m e r s (L=(ppy) or (bzq)) ( C r a i g 1987). We k n o w f r o m infrared data that irradiation of the s o l u t i o n c o n t a i n i n g the carbon monoxide monomers r e s u l t s i n p h o t o l y s i s o f the t e r m i n a l l y bound carbon monoxide ligands on both p a l l a d i u m and rhodium. This suggests formation of two coordinatively unsaturated fragments which then recombine to form compounds (A) and ( B ) . P r e f e r e n t i a l f o r m a t i o n o f the mixed metal species may be e x p l a i n e d u s i n g s i m p l e s t e r i c a r g u m e n t s . We propose that the sterically hindered coordinatively unsaturated rhodium fragment combines readily with the less bulky palladium fragment. Attempts to repeat these reactions using [Pd(ppy)Cl] and flr(ppy) ci] under i d e n t i c a l c o n d i t i o n s have been u n s u c c e s s f u l i n forming the mixed metal species because of the difficulty in photolyzing CO o f f of the i r i d i u m . We f e e l t h a t s u c c e s s i n forming Pd(II)/Rh(III) d i m e r s i s d u e t o f a c i l e CO l o s s u n d e r p h o t o l y s i s foll o w e d by d i m e r i z a t i o n . 2
2
2
B o t h o f t h e mixed m e t a l d i m e r s e x h i b i t u n i q u e H NMR s p e c t r a w h i c h a r e not superposed s p e c t r a of the parent [Pd(ppy)Cl] and [Rh(L) Cl] dimers. H NMg f o r b o t h mixed metal complexes i n t e g r a t e f o r 24 p r o ^ tons. 2D COSY H NMR have been o b t a i n e d f o r (A) and (B) and from t h i s data we a r e able t o a s s i g n a l l o f the chemical s h i f t s i n the proton NMR ( C r a i g 1 9 8 7 ) . 9
Room t e m p e r a t u r e a b s o r p t i o n and 77 K e m i s s i o n d a t a f o r (A) and (B) a r e shown i n F i g . 3. S u p e r p o s i t i o n o f t h e s t a r t i n g d i m e r s p e c t r a with those o f the P d ( I I ) / R h ( I I I ) dimers i l l u s t r a t e obvious similarities between t h e mixed m e t a l systems and t h e i r p a r e n t dimer c o m p l e x e s . The low t e m p e r a t u r e e m i s s i o n s p e c t r a o f [ P d ( p p y ) C l ] and [ R h ( L ) C l ] have been a s s i g n e d a s i n t r a l i g a n d t r a n s i t i o n s (Sprouse 1984; C r a i g 1987). 2
2
2
The emission s p e c t r u m f o r complex (B) e x h i b i t s two distinguishable emission p r o f i l e s . L i f e t i m e s were o b t a i n e d a t t h e maximum peak i n t e n sity positions o f 460 nm and 484 nm (see T a b l e 1 ) . The lifetime obtained a t 484 nm i s slightly less than that measured for [Rh(bzq) C l ] p whereas t h e l i f e t i m e a t 460 nm i s a p p r o x i m a t e l y d o u b l e t h a t o f f p d ( p p y ) C l ] D u e t o t h e s i m i l a r i t i e s o f t h e two components o f t h i s e m i s s i o n s p e c t r u m t o t h e p a r e n t d i m e r m a t e r i a l s i t a p p e a r s as i f each metal c e n t e r e d f r a g m e n t o f t h e h e t e r o - d i m e t a l l i e dimer retains the emission c h a r a c t e r i s t i c s of i t s r e s p e c t i v e parent complex. The lifetime i n c r e a s e a t 460 nm f o r complex (B) r e l a t i v e t o [ P d ( p p y ) C l ] c o u l d be e x p l a i n e d by a d e c r e a s e i n t h e n o n r a d i a t i v e r a t e o f d e a c t i v a tion v i a t h e d i c h l o r o - b r i d g e f o r Pd(ppy) w h i c h i s now bound t o a much h e a v i e r R h ( b z q ) ~ s p e c i e s . The o p p o s i t e e f f e c t i s a l s o o b s e r v e d f o r t h e rhodium portion o f t h e dimer w h i c h i s now c o u p l e d to a lighter palladium moiety. A second source o f t h e l i f e t i m e d i f f e r e n c e s might r e s u l t f r o m d e a c t i v a t i o n pathways v i a s o l v e n t - c h l o r i d e b r i d g e i n t e r a c f
2
tion. The s m a l l e r p a l l a d i u m fragment coupled t o the b u l k i e r rhodium fragment would exclude solvent-chloride i n t e r a c t i o n r e l a t i v e to the [Pd(ppy)Cl]dimer resulting in less non-radiative deactivation through t h i s pathway. The R h ( b z q ) C l p o r t i o n o f c o m p l e x (B) w o u l d b e solvated t o a g r e a t e r d e g r e e a b o u t the c h l o r i d e b r i d g e as a r e s u l t o f having the l e s s s t e r i c a l l y h i n d e r e d p a l l a d i u m fragment bound to i t . This would have the e f f e c t of i n c r e a s i n g the s o l v e n t - c h l o r i d e i n t e r a c tion relative to [Rh(bzq)-CI] a l l o w i n g f o r a g r e a t e r degree of nonr a d i a t i v e d e a c t i v a t i o n r e s u l t i n g i n a decrease i n the l i f e t i m e a t 484 nm. 2
2
300
400
500
300 (nm)
WAVELENGTH Figure 3. Absorption (a) [ ( p p y ) P d ( C D - R h ( p p y ) ] ; (d) [ ( p p y ) P d ( C l ) 2 R h ( b z q ) ] ; 2
400
and 77 K (b) [ P d ( p p y ) C l ] ; (e) [ P d ( p p y ) C 1 ] ;
500
emission spectra (c) [ R h ( p p y ) C I ] (f) [ R h ( b z q ) ^ C l ]
2
of,
2
The e m i s s i o n s p e c t r a f o r d i m e r ( A ) , [ P d ( p p y ) C l ] and [ R h ( p p y ) - C I ] are nearly identical. The lifetimes of t h e s e compounds a r e also very similar. T h e s e r e s u l t s make i t i m p o s s i b l e f o r u s t o d e t e c t a double exponential decay f o r c o m p l e x (A) a n d we t h e r e f o r e r e p o r t a single lifetime. If this complex has n o n - e q u i l i b r a t e d electronic excited states analogous to dimer (B), a l i f e t i m e which i s near that of [ R h ( p p y ) - C l l - and [ P d ( p p y ) C 1 3 - w o u l d be e x p e c t e d . This l i f e t i m e would be a w e i g h t e d a v e r a g e d e p e n d e n t on t h e quantum y i e l d o f e m i s s i o n for each h a l f of the dimer. E x a c t quantum y i e l d s of the p a r e n t d i m e r s are 2
2
n o t known a t 77 K, however t h e 484-460 nm peak i n t e n s i t y r a t i o s o f t h e e m i s s i o n s p e c t r a f o r complex (B) and t h e a b s o r p t i v i t y o f [ R h ( p p y ) - C I ] versus [Pd(ppy)Cl] s u g g e s t s t h a t t h e l i f e t i m e s h o u l d be nearef to that of the [Rh(ppy) C I ] - dimer t h a n the corresponding palladium dimer. Additionally the lifetime i s e x p e c t e d t o be l e s s than the [Rh(ppy)-CI]- dimer (vida s u p r a ) . 2
T a b l e 1. Photophysical data f o r parent dimers, and f o r t h e f r e e l i g a n d s (ppy) and (bzq)
compounds
(B)
ABSORPTION FEATURES (nm)
E
2-pheny1pyridine
240,
295
440
a
1.9(s)
benzo[h]quinoline
278, 352,
303 364
480
a
2.2 ( s )
[Pd(ppy)Cl]
254, 265(sh) 305 ( s h ) , 315 365
2
[Rh(ppy) Cl]
2
[Rh(bzq) Cl]
2
2
2
77 K EMISSION (nm)° 0-0
(A) and
460
77 K . LIFETIMES
a
a
116(us)
240, 310(sh) 461 333 ( s h ) , 380(sh) 393, 450
93(us)
484 255, 277, 330 ( s h ) , 390(sh) 410, 440 (sh)
2.6(ms)
[(ppy)Pd(Cl) Rh(ppy) ]
245, 265(sh) 305, 315 (sh) 365, 393 455(sh)
460
85 (us)
[(ppy)Pd(Cl) Rh(bzq) l
247, 257 3 0 5 ( s h ) , 315 408
460 484
237(us) 2.5(ms)
2
2
2
2
a. P h o s p h o r e s c e n c e b. 4/1/1 V/V/V Ethanol/Methanol/Dichloromethane glass
CONCLUSION Carbon monoxide i s u s e f u l i n c l e a v i n g u - d i c h l o r o - b r i d g e d systems of the t y p e s mentioned. T h i s c a n be used as a method t o d e r i v a t i z e t h e s e d i m e r s (Ryabov 1985; P f e f f e r 1981; C o n s t a b l e 1980). More i m p o r t a n t l y however, f a c i l e CO b r i d g e c l e a v a g e f o l l o w e d by p h o t o l y s i s i s a s i m p l e and efficient means w i t h w h i c h t o p r e p a r e mixed m e t a l systems. The restriction at t h i s p o i n t a p p e a r s t o be t h a t t h e CO l i g a n d must be somewhat p h o t o l a b i l e . I t may be p o s s i b l e however t o overcome t h i s by a l t e r i n g the l i g a n d used i n the cleavage r e a c t i o n . T h i s may actually become an advantage as one m i g h t i m a g i n e t a i l o r i n g reaction and photolysis c o n d i t i o n s a s a method t o s e l e c t i v e l y p r e p a r e mixed metal dimers.
The s i m i l a r i t y o f t h e a b s o r p t i o n and e m i s s i o n s p e c t r a f o r (A) and (B) to those of [ P d ( p p y ) C l ] , [Rh(ppy) C1] and [Rh(bzq) C1] , together with lifetime data i s c o n c l u s i v e evidence that the lowest energy excited s t a t e s of these novel dimers are i n t r a ligand t r a n s i t i o n s of the o r t h o - m e t a l a t e d 2 - p h e n y l p y r i d i n e and b e n z o [ h ] q u i n o l i n e ligands. It might have been e x p e c t e d t h a t t h e s e new d i m e r s would have one equilibrated e x c i t e d s t a t e a l t h o u g h e v i d e n c e has been p r e s e n t e d here a r g u i n g f o r n o n - e q u i l i b r a t e d e x c i t e d s t a t e s i n t h e s e c o m p l e x e s . We a r e currently a t t e m p t i n g t o p r e p a r e new d i m e r s o f t h i s t y p e u s i n g s i m i l a r ortho-metalated dimer p r e c u r s o r s . 2
2
2
2
2
ACKNOWLEDGMENT T h i s work was s u p p o r t e d by t h e O f f i c e o f B a s i c E n e r g y S c i e n c e s , S t a t e s Department o f E n e r g y , P r o j e c t DE-AT03-78ER70277
United
REFERENCES Constable AG, McDonald WS, S a w ^ i n s LC, Shaw BL (1980) Transitionmetal-carbon bonds. Part 45. Attempts to c y c l o p a l l a d a t e some a l i p h a t i c o x i m e s , N N - d i m e t h y l h y d r a z o n e s , k e t a z i n e s and oxime O - a l l y l ethers. Crystal s t r u c t u r e s o f [Pd-(CH-C (CHO-C (=NOH)CHO-C1-] and [Pd(CH C(=NNMe )C(CH ) ) ( a c a c ) ] . J C S D a l t o n 1992-2000 Cope AC, Siekman RW T1965) F o r m a t i o n o f c o v a l e n t bonds f r o m p l a t i n u m or palladium t o c a r b o n by d i r e c t s u b s t i t u t i o n . J Am Chem Soc 87: 3272-3273 C r a i g CA, G a r c e s FO, W a t t s RJ (1987) u n p u b l i s h e d r e s u l t s . G u t i e r r e z MA, Newkome GR, S e l b i n J (1980) C y c l o m e t a l l a t i o n . P a l l a d i u m 2 - a r y l p y r i d i n e c o m p l e x e s . J O r g a n o m e t a l Chem 202: 341-350 Kasahara A (1968) Sigma-bonded 2 - p h e n y l p y r i d i n e p a l l a d i u m complex. B u l l Chem Soc J p n 41: 1272 Nonoyama M, Yamasaki K (1971) Rhodium(III) complexes of b e n z o [ h ] q u i n o l i n e and 2 - p h e n y l p y r i d i n e . N u c l Chem L e t t e r s 7: 943-946 Nonoyama M (1974) S y n t h e s i s o f s e v e r a l bis(benzo[h]quinoline-10-YL-N) r h o d i u m ( I I I ) c o m p l e x e s . J O r g a n o m e t a l Chem 82: 271-276 Nonoyama M (1982) C y c l o p a l l a d a t i o n and cyclorhodation of N-(3t h i e n y l ) p y r a z o l e . J O r g a n o m e t a l Chem 229: 287-292 P a r s h a l l GW (1970) I n t r a m o l e c u l a r a r o m a t i c s u b s t i t u t i o n i n t r a n s i t i o n m e t a l c o m p l e x e s . A c c Chem Res 3: 139-144 Pfeffer M, Grandjean D, L e -Borgne G (1981) Reactivity of cyclopalladated compounds. 6. Synthesis of heterodimetallic s p e c i e s w i t h Pd-Co, Pd-Mo, o r Pd-Fe b o n d s . X - r a y c r y s t a l s t r u c t u r e of (Dimethylphenylphosphine)tricarbonyl(n-cyclopentadienyl) molybdenum ( 8 - m e t h y l q u i n o l i n e - C , N ) p a l l a d i u m ( I I ) ( P d - M o ) . Inorg Chem 20:4426-4429 Ryabov AD (19 85) The a p p l i c a t i o n o f c y c l o p a l l a d a t e d compounds in s y n t h e s i s . 54: 153-170 S p r o u s e S, K i n g KA, S p e l l a n e P J , W a t t s R J (1984) P h o t o p h y s i c a l e f f e c t s o f m e t a l - c a r b o n sigma bonds i n o r t h o - m e t a l a t e d c o m p l e x e s o f Ir(III) and R h ( I I I ) . J Am Chem Soc 106: 6647-6653 Trofimenko S (1973) Some s t u d i e s o f t h e c y c l o p a l l a d a t i o n r e a c t i o n . I n o r g Chem 12: 1215-1221. W a k a t s u k i Y, Y a m a z a k i H, G r u t s c h PA, Santhanam M, Kutal C (1985) Study of intramolecular sensitization and other excited-state pathways i n o r t h o m e t a l l a t e d a z o b e n z e n e c o m p l e x e s o f p a l l a d i u m ( I I ) . J Am Chem Soc 107: 8153-8159 2
PHOTOPROPERTIES OF ORTHO-METALATED
I r ( I I I ) AND R h ( I I I ) COMPLEXES
K.A.King, F.O.Garces, S.Sprouse, and R.J.Watts Department o f C h e m i s t r y , U n i v e r s i t y o f C a l i f o r n i a , Santa B a r b a r a , CA 93106, USA
INTRODUCTION Recent s t u d i e s have served t o provide i n i t i a l c h a r a c t e r i z a t i o n s o f t h e photoproperties o fa v a r i e t y o f ortho-metalated complexes o f I r ( I I I ) ( S p r o u s e 1 9 8 4 ; K i n q 1984, 1 9 8 5 ) , R h ( I I I ) ( B a l z a n i 1986; S p r o u s e 1 9 8 4 ) , R u ( I I ) (Reveco 1985; C o n s t a b l e 1986), P t ( I I ) (Maestri 1985; Chassot 1986), and Pd(II) (Wakatsuki 1985) w i t h 2-phenylpyridine (ppy), benzo[h]quinoline (bzq) a n d r e l a t e d l i g a n d s . While a p o r t i o n o f t h e i n i t i a l impetus f o r these s t u d i e s f o l l o w e d from i n t e r e s t i n t h e characterization o fortho-metalated complexes o ft h ew i d e l y used ligand, 2 , 2 ' - b i p y r i d i n e (bpy) (Watts 1 9 7 7 ; S p e l l a n e 1983; W i c k r a m a s i n g h e 1 9 8 1 ; Nord 1 9 8 3 ; B r a t e r m a n 1 9 8 4 ; S k a p s k i 1985; Cohen 1985; Slama-Schwok 1985; Grutsch 1986; F i n l a y s o n 1986), t h e more traditional orthometalatinq ligands such a s ppy and b z q i m p a r t upon t h e i r m e t a l complexes photoproperties quite d i s t i n c t from those of either N,N' c h e l a t e d o rN,C-ortho-metalated bpy complexes. Of p a r t i c u l a r s i g n i f i cance i st h e a b i l i t y o fthese t r a d i t i o n a l o r t h o - m e t a l a t i n g l i g a n d s t o impart upon their metal complexes t h e thermodynamic potential t o f u n c t i o n a s s t r o n g r e d u c i n q agents i nt h e i r low-enerqy e x c i t e d s t a t e s . Analoqous complexes o f N , N - c h e l a t i n g bpy tend t o be f a r s u p e r i o r oxidizing a q e n t s b u t much p o o r e r r e d u c i n q a q e n t s i n t h e i r low-enerqy excited states. Recent s t u d i e s i n o u r l a b o r a t o r y have focused upon ortho-metalated complexes o f I r ( I I I ) and Rh(III) w i t h ppy asw e l l a s the methyl-substituted ligands 2 - ( p - t o l y l ) p y r i d i n e (ptpy) a n d 3methy1-2-phenylpyridine (mppy). The s t r u c t u r e s o f t h e s e l i g a n d s a r e i l l u s t r a t e d i nF i q . 1 below. 1
2-phenylpyridine PPY
Figure
1.
3-methy1-2-phenylpyridine MPPY
S t r u c t u r e s o fOrtho-metalating
2- (p-tolyl) -pyridine PTPY
Ligands
These s t u d i e s p r o v i d e some i n i t i a l i n s i g h t s i n t o t h e r e l a t i v e e f f e c t s of s u b s t i t u t i o n o f t h e phenyl and p y r i d y l r i n g s on t h e p h o t o p r o p e r t i e s of ortho-metalated complexes o fthese l i g a n d s .
RESULTS
AND
DISCUSSION
A v a r i e t y o f m o n o m e t a l l i c complexes c a n be prepared (Nonoyama 1 9 7 1 , 1974) by f a c i l e r e a c t i o n s o f c h e l a t i n g l i g a n d s w i t h d i m e r i c species o f the type [M(X-ppy) C1] , w h e r e X - p p y i s p p y , p t p y o r mppy. For these studies we c h o s e t o u s e b p y a s t h e c h e l a t i n g l i g a n d b e c a u s e i t s presence i n t h e c o o r d i n a t i o n sphere o f t h e metal i o nassures that both good siqma-donor (X-ppy) and p i - a c c e p t o r (bpy) l i q a n d s a r e s i m u l t a neously coordinated. S e v e r a l i n t e r e s t i n g phenomena a r i s e i n c o m p l e x e s containinq t h i s combination, i n c l u d i n q : a) l o w - e n e r q y m e t a l - t o - l i g a n d charqe-transfer (MLCT) t r a n s i t i o n s ; b ) MLCT t r a n s i t i o n s i n w h i c h a metal e l e c t r o n may b.e p r o m o t e d t o a p i * o r b i t a l o f e i t h e r b p y o r t h e p y r i d i n e r i n g o f ppy; and c) e x c i t e d s t a t e s capable o f f u n c t i o n i n q as e i t h e r good e l e c t r o n donors o r e l e c t r o n a c c e p t o r s . ?
?
Evidence f o r d u a l e m i s s i o n s a r i s i n q f r o m MLCT e x c i t e d states associated with bpy and w i t h p p y i s found i n t h e t i m e - r e s o l v e d emission spectrum of Ir(ppy) (bpy) i n low temperature qlasses as illustrated i n F i q . 2 below. +
2
F i q u r e 2. T i m e - r e s o l v e d E m i s s i o n S p e c t r a o f I r ( p p y ) (bpy) i n E t h a n o l / M e t h a n o l a t 77 K E x c i t e d a t 3 7 7 nm w i t h a P u l s e d Nitroqen Laser. a ) 1 0 0 n s a f t e r e x c i t a t i o n ; b ) 15 / i s a f t e r e x c i t a t i o n .
Similar results have been found f o r Ir(mppy) (bpy) and for Ir(ptpy)-(bpy) . I n each case t h e r e l a t i v e l y unstructured emission a t short d e l a y t i m e s (100 n s ) i s t h e r e s u l t o f o v e r l a p p i n q e m i s s i o n s of states r e s u l t i n q from c h a r q e - t r a n s f e r o f a metal e l e c t r o n t o a c h e l a ting (MLCT/bpy) o r o r t h o - m e t a l a t i n q (MLCT/X-ppy) l i q a n d . At lonqer delay t i m e s (10-15 p s ) a more s t r u c t u r e d e m i s s i o n characteristic of the lower-enerqy MLCT/bpy s t a t e i s e v i d e n t . Further evidence f o r these assiqnments has been found by m o n i t o r i n q t h e emission s p e c t r a o f the i r ( I I I ) complexes as a f u n c t i o n o f e x c i t a t i o n wavelenqth. I n these measurements, excitation a t w a v e l e n q t h s l o n g e r t h a n 4 7 0 nm l e a d s to the same s t r u c t u r e d MLCT/bpy e m i s s i o n a s i s s e e n i n F i g . 2b above, while shorter wavelenqth e x c i t a t i o n gives unstructured luminescence 2
characteristic of o v e r l a p o f t h e h i q h e j e n e r q y MLCT/ppy and lowere n e r q y MLCT/bpy e m i s s i o n s . R h ( p p y ) ( b p y ) , on t h e o t h e r h a n d , d i s p l a y s a sinqle emission c h a r a c t e r i s t i c of a l i q a n d - l o c a l i z e d s t a t e associated w i t h ppy (LL/ppy) i n b o t h t i m e - r e s o l v e d e m i s s i o n spectroscopy and i n e x c i t a t i o n - e m i s s i o n s p e c t r o s c o p y . These r e s u l t s i n d i c a t e t h a t the i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r (Meyer, 1978) f r o m MLCT/ppy to MLCT/bpy i s slow i n I r ( l l l ) complexes i n r i q i d g l a s s e s whereas r e s o n ance enerqy t r a n s f e r ( B a l z a n i , 1980) from (LL/bpy) t o (LL/ppy) i n the Rh(III) complex i s r a p i d . The r e s u l t i s a t t r i b u t e d t o a viscosity dependent Franck-Condon b a r r i e r ( D e l l i n q e r , 1975, 1976; W a t t s , 1978) to i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r which a r i s e s from a large d i s t o r tion of the MLCT(bpy) s t a t e r e l a t i v e t o the qround state. Evidence f o r t h i s d i s t o r t i o n i s f o u n d i n the room t e m p e r a t u r e e m i s s i o n s o f the ir(III) complexes in fluid solutions, which d i s p l a y larqe Stoke's shifts and a s i n q l e e m i s s i o n c h a r a c t e r i s t i c o f t h e MLCT/bpy excited state under these c o n d i t i o n s . In c o n t r a s t , the LL/ppy and LL/bpy states in Rh(ppy)-(bpy) are r e l a t i v e l y u n d i s t o r t e d compared t o the qround s t a t e , and r e s o n a n c e e n e r q y t r a n s f e r p r o c e e d s r a p i d l y without s i g n i f i c a n t Franck-Condon b a r r i e r s . 2
Half-wave p o t e n t i a l s f o r the several Ir(III) and R h ( I I I ) compiled i n Table 1 below.
Table
1.
f i r s t o x i d a t i o n and f i r s t reduction complexes from c y c l i c voltammoqrams
H a l f - w a v e P o t e n t i a l s f o r I r ( I I I ) and R h ( I I I ) C o m p l e x e s T E A H - s a t u r a t e d A c e t o n i t r i l e a t Room T e m p e r a t u r e E
Complex +
2
(bpy)
Ir(ptpy) (bpy) + Rh(ppy) (bpy) 9 z
2
a. b.
'
V vs
+
+
1.28
-1.38
1.21
-1.41
SCE
-1.42
1.18 1.60
in
1 + /0
2 + /1 +
Ir(ppv) (bpy) Ir(mppy)
a
i / r
of are
-1.38
b
A s c a n r a t e o f 1.0 V / s was u s e d i n e a c h m e a s u r e m e n t A n o d i c peak p o t e n t i a l o f i r r e v e r s i b l e wave
The reductive p r o c e s s (+1/0) i s r e v e r s i b l e i n a l l o f the complexes which were s t u d i e d , whereas the o x i d a t i v e process i s r e v e r s i b l e i n the I r ( I I I ) complexes but i r r e v e r s i b l e i n the Rh(III) complex. The reduct i v e p r o c e s s i s b e l i e v e d t o o c c u r a t t h e bpy l i q a n d w h e r e a s t h e oxidative process (+1/+2) i s t h o u a h t t o b e m e t a l - c e n t e r e d i n t h e Ir(III) complexes. The h i g h p o t e n t i a l o x i d a t i v e p r o c e s s i n t h e R h ( I I I ) comp l e x i s i r r e v e r s i b l e a n d o x i d a t i o n may o c c u r a t t h e p p y l i q a n d i n t h i s instance. Variations of 100 mv i n t h e E values for the 1 + /2 + o x i d a t i v e p r o c e s s amonq t h e t h r e e I r ( I I I ) c o m p l e x e s r e f l e c t t h e d e g r e e to which the methyl s u b s t i t u e n t s e n r i c h the e l e c t r o n d e n s i t y at the metal center. / 2
By c o m b i n i n q v a l u e s o f t h e p o t e n t i a l s f o r t h e o x i d a t i v e a n d reductive processes i n Table 1 w i t h estimates of the e x c i t e d s t a t e e n e r q i e s of these c o m p l e x e s t a k e n f r o m low t e m p e r a t u r e e m i s s i o n maxima (2.34 V) , standard e l e c t r o c h e m i c a l p o t e n t i a l s f o r the luminescent e x c i t e d state of e a c h s p e c i e s c a n be e s t i m a t e d . This procedure leads to estimated v a l u e s o f E° (2 + / * l + ) i n t h e r a n g e -1.06 V v s SCE [Ir(ppy) (bpy) ] t o
-1.16 V v s SCE [ I r ( p t p y ) ( b p y ) ] a n d t o v a l u e s f o r E°(*l+/0) o f a b o u t 0.95 V v s SCE f o r a l l f o u r c o m p l e x e s . These e s t i m a t e s assume that e n t r o p y chanqes upon e x c i t a t i o n a r e n e g l i g i b l e , and althouqh they a r e expected t o p r o v i d e adequate estimates o f t h e thermodynamic driving force f o r e x c i t e d s t a t e redox processes, t h e y do n o t r e f l e c t kinetic limitations t o e l e c t r o n t r a n s f e r which miqht a r i s e from Franck-Condon barriers or non-adiabatic behavior (Sutin 1983). In order t o more fully c h a r a c t e r i z e the k i n e t i c s o f e x c i t e d s t a t e o x i d a t i o n and reduct i o n r e a c t i o n s , ^tern-Volmer s t u d i e s o f the quenchinq o f the emission of I r ( p p y ) (bpy) b y a s e r i e s o f o x i d a t i e q u e n c h e r s (nitrobenzenes) (Bock 1975T and a s e r i e s o f r e d u c t i v e quenchers (phenylamines and methoxybenzenes) ( M a r s h a l l 1984) were p e r f o r m e d . The r e s u l t s o f t h e s e s t u d i e s a r e summarized i n F i q . 3 below. 2
a
°DMA
10
0
°PT
b
-0
pDNB
0
mDNB
DPA 0
AN
-
o NBA o CNB
0 0
TMB
NB -
0
MNB
-
DMB° i
i
0.6
0,8
1.0 1.2 QUENCHER
i
i
'
-1.2 0.8 -1.0 REDUCTION P O T E N T I A L (V vsSCE)
F i q u r e 3. Stern-Volmer Quenchinq o f I r ( p p y ) ( b p y ) i n A c e t o n i t r i l e by R e d u c t i v e (a) a n d O x i d a t i v e (b) Q u e n c h e r s . (a) P T , P h e n o t h i a z i n e ; DMA, N , N - d i m e t h y l a n i l i n e ; DPA, diphenylamine; A N , a n i l i n e ; TMB, 1 , 2 , 4 - t r i m e t h o x y b e n z e n e ; DMB, 1 , 4 - d i m e t h o x y b e n z e n e (b) pDNB, p-Dinitrobenzene; mDNB, m - D i n i t r o b e n z e n e ; NBA, m-nitrobenzaldehyde; CNB, p-chloronitrobenzene; NB, nitrobenzene; MNB, p-methylnitrobenzene 2
K i n e t i c estimates of the e x c i t e d s t a t e redox p o t e n t i a l s taken from the breakinq r e q i o n between t h e d i f f u s i o n c o n t r o l l e d l i m i t and t h e l i n e a r r e q i o n i n d i c a t e a v a l u e o f E°(2+/*l+) o f - 1 . 1 V v s SCE a n d a v a l u e of E ° ( * l + / 0 ) o f +0.98 V v s S C E . These v a l u e s a r e i n qood aqreement w i t h the thermodynamic estimates above, and t h e r e l a t i v e l y narrow b r e a k i n q reqion i n the plots i n Fiq. 3 suggest t h a t r e o r q a n i z a t i o n a l b a r r i e r s toward e i t h e r o x i d a t i v e o r r e d u c t i v e e l e c t r o n t r a n s f e r a r e s m a l l .
SUMMARY Combined p h o t o p h y s i c a l , photochemical, and e l e c t r o c h e m i c a l s t u d i e s o f these and r e l a t e d o r t h o - m e t a l a t e d complexes o f a v a r i e t y o f t r a n s i t i o n metals i n d i c a t e t h a t t h i s c l a s s of molecular species provides a u s e f u l basis f o r the study of e f f e c t s of metal-carbon siqma-bondinq on electron-transfer processes. While the covalent nature of these metal-carbon bonds may well assure stronq electronic couplinq of
e x c i t e d s t a t e s i n t h e s e c o m p l e x e s , t h e s e b o n d s may a l s o i m p o s e larqe qeometric d i s t o r t i o n s on e x c i t e d s t a t e s a s s o c i a t e d w i t h l i q a n d s t r a n s to them. T h e a s s o c i a t e d F r a n k - C o n d o n b a r r i e r s may, i n viscous solvents, lead t o dual emissions such as those observed i n t h e Ir(III) complexes. The s t r o n q siqma-donor a b i l i t i e s o f o r t h o - m e t a l a t i n q liqands qreatly enriches the e l e c t r o n denisty a t the metal center i n t h e i r complexes, and leads t o species which are often strong reducinq aqents i n t h e i r excited states. When t h e s e l i g a n d s a r e c o m b i n e d w i t h q o o d electron accepting l i q a n d s , such as bpy, i n the c o o r d i n a t i o n sphere o f a s i n q l e metal, complexes w h i c h c a n be used as e i t h e r e x c i t e d s t a t e o x i d i z i n q or reducinq aqents r e s u l t . Kinetic Stern-Volmer quenching studies indicate that t h e s e s p e c i e s do i n d e e d p a r t i c i p a t e i n outer sphere electron transfer reactions with drivinq forces comparable t o the values e s t i m a t e d from combined s p e c t r o s c o p i c - e l e c t r o c h e m i c a l r e s u l t s . Ortho-metalated c o m p l e x e s show p r o m i s e f o r a p p l i c a t i o n as photocatalysts where t h e r e i s a need f o r s u b s t a n c e s w h i c h c a n a b s o r b visible liqht and convert a larqe f r a c t i o n of the absorbed enerqy i n t o reduci n g power i n r e l a t i v e l y l o n q - l i v e d e x c i t e d s t a t e s .
ACKNOWLEDGMENT T h i s work was s u p p o r t e d b y t h e O f f i c e o f B a s i c E n e r q y S c i e n c e s , S t a t e s D e p a r t m e n t o f E n e r q y , P r o j e c t DE-AT03-78ER70277
United
REFERENCES B a l z a n i V, ( p r i v a t e c o m m u n i c a t i o n 1986) B a l z a n i V , B o l l e t a F , S c a n d o l a F ( 1 9 8 0 ) J Am Chem S o c 1 0 2 : 2 1 5 2 - 2 1 6 3 B o c k CR, M e y e r T J , W h i t t e n DG ( 1 9 7 5 ) J Am Chem S o c 9 7 : 2 9 0 9 - 2 9 1 1 B r a t e r m a n P S , H e a t h GA, M a c K e n z i e A J , N o b l e B C , P e a c o c k RD, Y e l l o w l e s s L J ( 1 9 8 4 ) I n o r q Chem 2 3 : 3 4 2 5 - 3 4 2 6 C h a s s o t L , v o n Z e l e w s k y A, S a n d r i n i D, M a e s t r i M, B a l z a n i V ( 1 9 8 6 ) J Am Chem S o c 1 0 8 : 6 0 8 4 - 6 0 8 5 C o h e n H, S l a m a - S c h w o k A, R a b a n i J , W a t t s R J , M e y e r s t e i n D (1985) J P h y s Chem 8 9 : 2 4 6 5 - 2 4 6 7 C o n s t a b l e E C , H o l m e s J M ( 1 9 8 6 ) J O r q a n o m e t Chem 3 0 1 : 2 0 3 - 2 0 8 D e l l i n q e r B , K a s h a M ( 1 9 7 5 ) Chem P h y s L e t t 3 6 : 4 1 0 - 4 1 4 D e l l i n q e r B , K a s h a M ( 1 9 7 6 ) Chem P h y s L e t t 3 8 : 9-14 F i n l a y s o n MF, F o r d P C , W a t t s R J ( 1 9 8 6 ) J P h y s Chem 9 0 : 3 9 1 6 - 3 9 2 2 G r u t s c h P A , K u t a l C ( 1 9 8 6 ) J Am Chem S o c 1 0 8 : 3 1 0 8 - 3 1 1 0 K i n q KA, F i n l a y s o n MF, S p e l l a n e P J , W a t t s R J ( 1 9 8 4 ) S c i P a p I n s t P h y s Chem R e s 7 8 : 9 7 - 1 0 6 K i n q K A , S p e l l a n e P J , W a t t s R J ( 1 9 8 5 ) J Am Chem S o c 1 0 7 : 1 4 3 1 - 1 4 3 2 M a e s t r i M, S a n d r i n i D, B a l z a n i V , C h a s s o t L , J o l l i e t P, v o n Z e l e w s k y A ( 1 9 8 5 ) Chem P h y s L e t t 1 2 2 : 3 7 5 - 3 7 9 M a r s h a l l J L , S t o b a r t S R , G r a y HB ( 1 9 8 4 ) J Am Chem S o c 1 0 6 : 3 0 2 7 - 3 0 2 8 M e y e r T J ( 1 9 7 8 ) A c c Chem R e s 1 1 : 9 4 - 1 0 0 N o n o y a m a M ( 1 9 7 4 ) B u l l Chem S o c J p n 4 7 : 7 6 7 - 7 6 8 N o n o y a m a M, Y a m a s a k i K ( 1 9 7 1 ) I n o r q N u c l Chem L e t t 7: 9 4 3 - 9 4 6 N o r d G, H a z e l l A , H a z e l l RG, F a r v e r O ( 1 9 8 3 ) I n o r q Chem 2 2 : 3 4 2 9 - 3 4 3 4 R e v e c o P, S c h m e h l R H , C h e r r y WR, F r o n c z e k FR, S e l b i n J (1985) I n o r q Chem 2 4 : 4 0 7 8 - 4 0 8 2 S k a p s k i AC, S u t c l i f f e V F , Y o u n q GB ( 1 9 8 5 ) J Chem S o c Chem Commun 6 0 9 611 S l a m a - S c h w o k A, G e r s h u n i S, R a b a n i J , C o h e n H, M e y e r s t e i n D ( 1 9 8 5 ) J P h y s Chem 8 9 : 2 4 6 0 - 2 4 6 4
S p e l l a n e P J , W a t t s R J , C u r t i s C J (1983) I n o r q Chem 22: 4060-4062 S p r o u s e S, K i n q KA, S p e l l a n e P J , W a t t s R J (1984) J Am Chem Soc 106: 6647-6653 S u t i n N, C r e u t z C (1983) J Chem E d 60: 809-814 W a k a t s u k i Y, Yamazaki H, G r u t s c h PA, Santhanam M, K u t a l C (1985) J Am Chem Soc 107: 8153-8159 Watts R J , H a r r i n q t o n J S , v a n H o u t e n J (1977) J Am Chem Soc 99: 21792187 Watts R J , M i s s i m e r D (1978) J Am Chem Soc 100: 5350-5357 W i c k r a m a s i n q h e WA, B i r d PH, S e r p o n e N (1981) J Chem Soc Chem Commun: 1284-1286
GROUND AND
EXCITED STATE
INTERACTIONS IN MULTIMETAL SYSTEMS
J.D.Petersen Department o f C h e m i s t r y , Clemson U n i v e r s i t y , Clemson, SC 29634-1905, USA
T h i s a r t i c l e d i s c u s s e s d a t a from t h i s and o t h e r l a b o r a t o r i e s w i t h r e s p e c t t o t h e e l e c t r o n i c communication a c r o s s v a r i o u s a r o m a t i c , n i t r o g e n h e t e r o c y c l i c l i g a n d s i n p o l y m e t a l l i c complexes. As t h e n a t u r e and c o m p l e x i t y o f t h e b r i d g i n g l i g a n d i n t r a n s i t i o n m e t a l comlexes i n c r e a s e s o u r u n d e r s t a n d i n g o f the f a c t o r s t h a t a f f e c t g r o u n d - a n d e x c i t e d - s t a t e e l e c t r o n i c c o u p l i n g must be more f u l l y understood. The m a j o r i t y o f t h e work s i t e d h e r e i s f r o m o u r l a b o r a t o r i e s and h a s i n v o l v e d numerous g r a d u a t e s t u d e n t s and p o s t d o c t o r a l f e l l o w s , most n o t a b l y A n d r e a W a l l a c e , Karen Brewer, and Rory Murphy.
BACKGROUND 2+ The e x c i t e d - s t a t e r e a c t i v i t y o f complexes r e l a t e d t o R u ( b p y ) ^ (where bpy = 2 , 2 - b i p y r i d i n e ) has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n b e c a u s e o f t h e l o n g - l i v e d , e x c i t e d - s t a t e o f t h i s s p e c i e s a t room t e m p e r a t u r e i n f l u i d s o l u t i o n , and t h e a b i l i t y o f t h i s complex and i t s d e r i v a t i v e s t o undergo f a s c i l e e x c i t e d - s t ^ t e e l e c t r o n - o r energy -transfer reactions. The f a c t t h a t R u ( b p y ) absorbs v i s i b l e l i g h t ( i n t e n s e m e t a l - t o - l i g a n d c h a r g e - t r a n s f e r t r a n s i t i o n ) h a s made i t a prime c a n d i d a t e f o r a v a r i e t y o f s o l a r - e n e r g y - d r i v e n , f u e l p r o d u c t i o n schemes. 1
3
R e c e n t l y , t h e c a p a b i l i t i e s o f t h e s e complexes have been expandecj by t h e p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f complexes l i k e R u ( d p p ) (where dpp = 2 , 3 - b i s ( 2 - p y r i d y l ) p y r a ^ i n e ) . Like the intensely s t u d i e d Ru(bpy) c e n t e r , Ru(dpp)~ absorbs v i s i b l e l i g h t , has a l o n g - l i v e d , e m i s s i v e , e x c i t e d s t a t e , and i s s t a b l e t h e r m a l l y jiji b o t h o x i d i z e d and r e d u c e d f o r m s . One a d v a n t a g e t h a t R u ( d p p ) has over Ru(bpy)~ i s t h a t t h e f o r m e r i s c a p a b l e o f b i n d i n g an a d d i t i o n a l m e t a l c e n t e r t o e a c h o f t h e dpp l i g a n d s t o f o r m t h e r m a l l y s t a b l e , p o l y m e t a l l i c complexes. In a d d i t i o n , mixedl i g a n d , p o l y m e t a l l i c complexes have been p r e p a r e d u s i n g dpp and o t h e r r e l a t e d l i g a n d s s u c h as 2 , 2 - b i p y r i m i d i n e (bpm), and 2,3-bis(2*-pyridyl)quinoxaline(dpq). W h i l e p o l y m e t a l l i c complexes a f f o r d t h e advantage t h a t s y n t h e t i c a l l y you can b r i n g a l l r e a c t i o n p a r t n e r s t o g e t h e r i n a s i n g l e m o l e c u l a r u n i t (thus a v o i d i n g t h e i n h e r e n t i n e f f i c i e n c y o f b i m o l e c u l a r p r o c e s s e s ) , these systems a r e complex m o l e c u l a r u n i t s w h i c h a r e n o t w e l l u n d e r s t o o d . The f a c t t h a t some o f t h e s e p o l y m e t a l l i c systems e m i t a t room t e m p e r a t u r e i n f l u i d s o l u t i o n w h i l e o t h e r d o n ' t , and t h e i n c o n s i s t e n t t r e n d s i n e l e c t r o c h e m i s t r y i n g o i n g from m o n o m e t a l l i c t o b i m e t a l l i c t o p o l y m e t a l l i c s p e c i e s have g e n e r a t e d numerous and v a r i e d h y p o t h e s e s on t h e e x i s t e n c e and t h e n a t u r e o f m e t a l - m e t a l c o m m u n i c a t i o n i n t h e s e l i g a n d - b r i d g e d systems. +
3
1
3
1
GROUND STATE COMMUNICATION The e l e c t r o c h e m i s t r y o f mono- d i - and p o l y m e t a l l i c complexes o f t h r e e b r i d g i n g l i g a n d s , bpm ( 2 , 2 - b i p y r i m i d i n e ) , dpp and dpq a r e summmarized i n T a b l e I . I n t h e f i r s t two d a t a columns o f T a b l e I , some i n t e r e s t i n g t r e n d s / a r e o b s e r v e d f o r t h e m e t a l o x i d a t i o n s . The comparison ° f T { ^ f i c o u p l e i n a m o n o m e t a l l i c complex w i t h t h e f i r s t Ru couple i n the corresponding b i m e t a l l i c analog shows s l i g h t l y d i f f e r e n t - b e h a v i o r d e p e n d e n t on BL. In t h e c a s e o f BL = bpm, t h e f i r s t Ru c o u p l e i s 0.13 V more p o s i t i v e f o r t h e b i m e t a l l i c [Ru ( b p y ) ] b p m than the mono-metallic Ru(bpy) bpm complex. F o r t h e o t h e r BL e n t r i e s i n T a b l e I t h e d i f f e r e n c e s a r e much s m a l l e r (0.00-0.05 V ) . G a f f n e y and c o w o r k e r s have a t t r i b u t e d t h i s t o g r o u n d - s t a t e m e t a l - m e t a l communication i n the b i m e t a l l i c systems. When BL = bpm, good e l e c t r o n i c c o m m u n i c a t i o n i s e x h i b i t i e d between t h e two r u t h e n i u m c e n t e r s c a u s i n g t h e 0.13 V s h i f t i n p o t e n t i a l and l e a d i n g t o l o s s o f e m i s s i o n (room temperature, f l u i d s o l u t i o n ) f o r the bpm-bridged bimetallic complex. On t h e o t h e r h a n d , dpp and dpq b r i d g e d s p e c i e s show o n l y s m a l l p o t e n t i a l s h i f t s f r o m m o n o m e t a l l i c a n a l o g s (0.00-0.05 V) due t o a l a c k o f g r o u n d - s t a t e c o m m u n i c a t i o n , and t h u s t h e room t e m p e r a t u r e e m i s s i o n i s o b s e r v e d i n t h e p o l y m e t a l l i c complexes and n o t quenched by t h e p r e s e n c e o f t h e s e c o n d m e t a l c e n t e r . 1
R u
4
2
+
2
2
TABLE I . E l e c t r o c h e m i s t r y o f Mono-, D i - , T r i - and Complexes o f Ruthenium P o l y a z i n e Complexes Ei (1) ° b)
Complex
X
H
2 +
Ru(bpy) b p m . [Ru(bpyf ] bpm Ru[(bpm)6utbpy) ] 9
9
2
Ru(bpy) dpp
b + 3
2 +
2
4
+
[Ru(bpyf ] dpp Ru[ ( d p p ) M b p y ) ] 3 9
9
R
2+ Ru(phen) dpp [Ru(phenf ] dpp Ru[(dpp)R6(phen) 2
9
4
9
2
2 +
Ru(bpy) dpq [Ru(bpyf l dpq 2
2
2
Ru(phen) dpq [Ru(phenf dpq 2
+
2+ .
4
+
]3
y
+
c)
X
V D
r
e
-1.02 -0.41
1.33 1. 38 1.50
1.5$ 1.8
-1.06 -0.66 -0.56
1.39 1-44 1.43
1.6| 1.8
-1.07 -0.64 -0.5
1.42 1.47
1.62
-0.77 -0.37
1.42 1.48
1.64
-0.79 -0.40
e
2
H
1.69
1.40 1.53 irreversible
p
V2)°
Polymetallic
e
r
r
d
2
a) b) c) d) e) f)
In a c e t o n i t r i l e w i t h 0.1 M s u p p o r t i n g e l e c t r o l y t e u n l e s s noted otherwise. P o t e n t i a l f o r o x i d a t i o n of f i r s t Ru(II) c e n t e r ( i n V v s . P o t e n t i a l f o r o x i d a t i o n o f s e c o n d R u ( I I ) c e n t e r where a p p r o p r i a t e ( i n V v s . SCE). P o t e n t i a l f o r f i r s t r e d u c t i o n o f complex ( i n V v s . S C E ) . Corresponds c o u l o m e t r i c a l l y t o a t h r e e - e l e c t r o n p r o c e s s . O u t s i d e s o l v e n t window.
SCE)
F o r t h e t e t r a m e t a l l i c c o m p l e x e s , t h e same t r e n d s a r e o b s e r v e d . W i t h bpm as t h e b r i d g i n g l i g a n d , i . e . , Ru [ (bpm) Ru ( b p y ) , no room t e m p e r a t u r e e m i s s i o n i s o b s e r v e d and t h e e l e c t r o c h e m i s t r y i s irreversible. When dpp i s the b r i d g i n g l i g a n d , t h e t e t r a m e t a l l i c complexes e m i t a t room t e m p e r a t u r e and have e l e c t r o c h e m i s t r y c o n s i s t e n t w i t h m i n i m a l communication between t h e p e r i p h e r a l m e t a l c e n t e r s . W h i l e r e s o n a n c e Raman s t u d i e s may i n d i c a t e t h a t t h e h i g h d e g r e e o f e l e c t r o n i c communication i n bpm and l a c k o f e l e c t r o n i c c o m m u n i c a t i o n i n dpp and dpq b r i d g e d systems i s due t o l i g a n d c o n f o r m a t i o n , t h a t may n o t be t h e t o t a l p i c t u r e . E l e c t r o c h e m i c a l o x i d a t i o n o f t h e b i m e t a l l i c complexes o f F e ( I I ) and R u ( I I ) show t h a t t h e e l e c t r o n i c communication as measured by c o m p r o p o r t i o n a t i o n c o n s t a n t i s r o u g h l y t h e same f o r bpm, dpp, and dpq s y s t e m s ( T a b l e I I ) .
Table I I . Comproportionation Complexes.
Constants
Complex
E^mV
[Ru(bpy) ] bpm^/5+/6+ [Ru(bpy) ] d p p ' ' [Ru(bpy)2]2 4+/5+/6+ 2
2
2
dpq
[Rujphe,
2
2
dp j: P
:
[Ru(phen) ] dpq 2
2
[Fe(CN) ] b p m j : ^ : ^ [Fe(CN)*]^dpp 2
4
/ J
1 1
a)
In a c e t o n i t r i l e
b)
E^(2)
u
-E^U)^
w i t h 0.1 from
for Various
13
K
160 180 150 210 160
com 5.1xl0 l.lxiof 3.4x10^ 3.5x10^ 5.1x10
140 150
2.3xl0 3.4xlO
M supporting
Table
Bimetallic
2
2 z
e l e c t r o l y t e unless
I.
Thus, w h i l e e m i s s i o n s t u d i e s i n d i c a t e t h a t e x t e n s i v e g r o u n d - s t a t e c o m m u n i c a t i o n o c c u r s o n l y f o r bpm, t h e e l e c t r o c h e m i s t r y g i v e s ambiguous r e s u l t s .
EXCITED STATE PROPERTIES The bpm, dpp, and dpq m o n o m e t a l l i c complexes and t h e dpp and dpq b i - and t e t r a m e t a l l i c complexes e m i t a t room t e m p e r a t u r e i n f l u i d s o l u t i o n w i t h l i f e t i m e s i n t h e 20 - 300 ns r e g i o n . The e m i s s i o n i s c h a r g e t r a n s f e r i n n a t u r e and i n v o l v e s t h e l o w e s t e n e r g y ir* o r b i t a l (bpm, dpp o r dpq) and t h e Ru d core. R e g a r d l e s s of the c o m p l e x i t y o f t h e s y s t e m , t h e e m i s s i o n ^ e n e r g y depends on whether t h e bpm, dpp, o r dpq l i g a n d i s t e r m i n a l o r b r i d g e d and i s n o t s e n s i t i v e t o t h e number o f m e t a l c e n t e r s p r e s e n t ( T a b l e I I I ) . As an example o f t h i s phenomenon, a l l o f t h e p o l y m e t a l l i c complexes c o n t a i n i n g b r i d g i n g dpp ( T a b l e I I I ) have e m i s s i o n maxima between 746 and 772 nm.
TABLE I I I . E m i s s i o n Maxima and L i f e t i m e s o f Mono-, B i - a n d T e t r a m e t a l l i c Complexes i n A c e t o n i t r i l e a t Room T e m p e r a t u r e
Complex
* max
em
, nm
T,
Monometallics 2 +
Ru(bpm)
639
3
2 +
Ru(bpy) bpm
710
2
2
Ru(dpp)
+
623
131 76 183
3
R u ( b p y ) d p p.2+ 2
Ru(phen) d p p Ru(dpq)
2 +
2 + 3
Ru(bpy) dpq
2 +
2
Ru(phen) dpq
2 +
2
660
226
652
252
716
82
766
71
756
83
756
134
dpp P o l y m e t a l l i c s [Ru(bpy) ] dpp 2
4 +
2
[Ru(phen) ] dpp 2
4 +
746
2
4
Ru(phen) dppRu(bpy)
+
2
Ru[(dpp)Ru(bpy) ]3 2
Ru[(dpp)Ru(phen) ] 2
8 + 3
Ru[(dpp)Ru(tpy)Cl]
5 + 3
752
153 113
772
89
760
87
758
50-200
dpq P o l y m e t a l i l c s [Ru(bpy) ] dpq 2
4 +
2
[Ru(phen) l dpq 2
4 +
2
Ru(phen) dpqRu(bpy) 2
4 + 2
822
}
s e -
1, 2 a n d 3 a n d B L i s a
l i g a n d .
involved
of
bpm
2 , 3 - b i s < 2 - p y r i d y 1 ) q u i n o x a l i n e =
BiBz ImH
weakly
gap c o r r e l a t i o n s .
2 , 2 ' - b i p y r a z i n e
BilmH^
The
a d -
f o l l o w s :
bpy
=
a r e
quantum
Ligands
bpm
=
issues
COMPLEXES
A b b r e v i a t i o n s
BL y
p r o p e r t i e s o f
and s u b s t i t u t i o n
bpy
bpz
report
t o c o o r d i n a t e
o f t h e mononuclear
i n d i c a t e d
i nt h i s
c o o r d i n a t i o n
ions
Ligands
F i g u r e
(Sahai
have
remote
i no u r l a b o r a t o r i e s
p r o p e r t i e s
complexes
complexes
with
(Rillema
One thermal
acetone
b i d e n t a t e
mixed-ligand
1982, 1983; Haga method
t o give
h a s
1983)and photo-
h a s involved
< e q . 1) f o l l o w e d
ligand
complexes
formation o f
b y r e a c t i o n
C R u ( b p y ) -,-BL 1
with
.
a n excess
A more
0
S=CHJCCH R u ( b p y ),,,C1^
*Person
+ 2AgPF,.
t o c o n t a c t
f o r r e p r i n t s
3
> CRu (b p y )
-
+ 2Ag.Cl
+ 2PF, , :
[13
general
route
r e a c t i o n excess of
of
of
to
the
BL
a
v a r i e t y
ligand
ruthenium(I
of
appropriate
I>
in
ruthenium(I
ruthenium
ethylene
complexes
I)
complexes
precursor
g l y c o l
c o n t a i n i n g
according the
ethylene
+
g l y c o l
x's R u C l
EG
the
procedures
Photochemical C l ~
and
was
hV, C Ru < bp z )
were
y i e l d
with
an
ligand
BL3
C !
C23
+
f
who
g l y c e r o l
as
the
g l y c o l .
to
synthesize
loss
an
used
+
was
complexes
effected
acetochloro
excess
of
the
in
that
the
intermediate
BL
ligand
via
could
presence (eq.
eq.
3),
2
CI "
3'~ *
3
used Ligand
to
reacted
(1983),
ethylene
t h e r m a l l y .
a c e t o n i t r i l e
then
Haga
than
r e a c t i o n s
obtained
which
of
rather
Formation
[ Ru < b p y ) ( BL ) ^3
3
medium
be
B 5
CRu(BL> 3 -
followed
of
=
)
an
2.
A
a
r e a c t i o n
not
BL,
3H O
3
eq.
CRu(bpy
7S
Ru(bpy)Cl,
to
involved
with
bibenzimidazo1e
R u ( b p y ) Cl vst
has
complex
>
CRu ( bp z )
R u ( b p m )
5.3
and
e.g.
of
2.
the
F i g .
ligands, the
p r o p e r t i e s
3 +
negative
ligand
stronger
Ru
F i g .
following
"
more
stronger
R u
3
" p o t e n t i a l s
of
(the
the
the
level
ground
£ 3 - . +
and
of
pK^,
of of
bpz
B L l i g a n d s
a l l
a d d i t i o n ,
the
other the
E
t r i s C Ru ( BL
The
x
)
3
3
1
p o s i t i v e
e.g. shows
are
ruthenium are
Ru'-*"""
that
more
complexes.
' &••*•*• p o t e n t i a l
p o t e n t i a l s
+
u
and
(Allen
f i g u r e
p o t e n t i a l s 3
the
the
manifold
state)
p o t e n t i a l s , -
0
chelate
R 3*-"3 *
most
„
0
by
between
e m i t t i n g
redox
+
determined
ground
redox
^ > •* '
s i m i l a r
that
the
state
3
were
d i f f e r e n c e
state
of
£ 3 /
=
CRu ( b p y ) 3
shows
whereas
energy
excited
v i b r a t i o n a l
+
in
other
p y r i d i n e
- v a l u e s
p o t e n t i a l s
value
the
in
involves
the
*
3
the
p r o p e r t i e s
extremes.
£ «a •+•-'-»•••
F i g .
q u a n t i t i e s . CRu ( b p z
/
E
1987)
.c:. -
CRu(bpy )
Energy
,
u
to
1968))
these
zeroth
-
(pK^.
The
between
R i l l e m a
£&>.•+•«•/•+• the
bpz (Ford
v i b r a t i o n a l
of
3
3
containing
2
Ru(bpy)(CN)(CNH)
+
H
+
«
2
+
+ H
3
320 nm
Fe(III)PP 100
fold
The
photoreduction
ranging
excess of
o f F e ( I I I ) P P was The
c a r r i e d out w i t h pyz/Fe c o n c e n t r a t i o n r a t i o e s
results unequivocally
F e ( I I ) P P ( p y z ) ^ s p e c i e s i s formed i n any
case,
i n d i c a t e t h a t the monomeric
i r r e s p e c t i v e o f the p y r a z i n e
t h a t the p o l y n u c l e a r s p e c i e s i s formed v i a a secondary thermal
the r a t e o f which i n c r e a s e s w i t h d e c r e a s i n g o f the s e c o n d a r y r e a c t i o n was 300/1
t o 1,500/1. In these
much s l o w e r
than
the p y r a z i n e
c o n c e n t r a t i o n . The
c o n d i t i o n s the p o l y m e r i z a t i o n p r o c e s s
the r e d u c t i o n o f F e ( I I I ) P P . One
been reduced when p o l y m e r i z a t i o n b e g i n s .
performed w i t h an excess o f Sodium d i t h i o n i t e and was
f o l l o w e d by m o n i t o r i n g
r e s u l t s show t h a t the p o l y m e r i z a t i o n
b a s i s , we
The
complete r e d u c t i o n the
to
from be
that v i r t u a l l y a l l of
disappearance
the absorbance decrease
f o l l o w s a second o r d e r r a t e law.
propose the f o l l o w i n g mechanism:
K
kinetics
i s observed
can thus envisage
F e ( I I I ) P P was
rate of Fe(II)PP(pyz)
concentra-
reaction,
s t u d i e d w i t h pyz/Fe c o n c e n t r a t i o n r a t i o e s r a n g i n g
F e ( I I I ) P P has
The
a
pyrazine.
from 5000/1 to 1/1.
t i o n and
of
s o l u t i o n s a t p H ~ 1 0 : a) w i t h a 5000 f o l d excess o f p y r a z i n e ; b) w i t h
a t 550 On
this
nm.
C
+
B
(6)
C
+
A
(7)
where, A = F e ( I I ) P P ( p y z ) ; B = Fe(II)PPpyz; C = F e ( I I ) P P p y z F e ( I I ) P P p y z ; 2
C
= pyzFe(II)PPpyzFe(II)PPpyz;
D =
D' =
pyzFe(II)PPpyzFe(II)PPpyzFe(II)PPpyz;
Fe(II)PPpyzFe(II)PPpyzFe(II)PPpyz.
Supposing t h a t E q u i l i b r i u m 1 i s very r a p i d l y e s t a b l i s h e d , t h a t , due p y r a z i n e , r e a c t i o n s 6 and 7
to the excess
of
are n e g l i g i b l e compared with r e a c t i o n 5, and i n t r o d u c i n g
the steady s t a t e a p p r o x i m a t i o n
f o r C, C ,
D
Eq. 8 g i v e s the r a t e o f d i s a p p e a r -
ance o f A ( F e ( I I ) P P ( p y z ) ). -d ( j Q
/dt = (n-l)k K
2
2
Q T j / [pyz]
2
(n-l)k K [ A ]
/
(8)
[£yl]
where n i s the number o f the i r o n c e n t e r s i n the polymeric compound. C o n s i d e r i n g t h a t the steady s t a t e c o n c e n t r a t i o n o f i n t e r m e d i a t e s C, C , very low,
D,
... are l i k e l y to be
n can be taken c o n s t a n t and r e p r e s e n t i n g the number o f the monomeric u n i t s
i n the f i n a l
p r o d u c t . Then, s i n c e the v a r i a t i o n o f the p y r a z i n e c o n c e n t r a t i o n i s
n e g l i g i b l e due
to the excess o f the b a s i s with r e s p e c t to F e ( I I ) P P ( p y z ) ,
-drA~|dt —'
= k
obs
2 r"A""| w i t h k
obs
2 — — 2 - ( n - l ) k K /I pyzj + (n-l)k K / T p y z l 2 — — 3
(9)
L
The v a l i d i t y o f k i n e t i c e x p r e s s i o n 9 has been t e s t e d by p l o t t i n g k then making the b e s t f i t o f the experimental data to the form
obs
vs.
pyz
and
2 y = a/ £pyzj
+
b/J^pyz_J
In F i g . 2 the good f i t o f the experimental p o i n t s with the t h e o r e t i c a l curve i s evident.
k , obs
x 10
0.01
0.03
0,05
[pyz]
_ F i g . 2 - Best f i t
p l o t of experimental k ^ v a l u e s
2
to the form y=a/[pyz] +
b/Qpyz)
Another p e c u l i a r i t y o f the F e ( I I ) P P p o l y n u c l e a r compound i s i t s r e a c t i v i t y r e s p e c t t o oxygen: i n a l k a l i n e s o l u t i o n
with
(pH^vlO) the polymer i s r a p i d l y o x i d i z e d and
d i s g r e g a t e d to g i v e the r e a c t a n t F e ( I I I ) P P . At pHs
c l o s e to n e u t r a l i t y
(^8)
the
s p e c t r a l behaviour and
^
a f t e r oxygenation
bands decreased
shifted;
i s as f o l l o w s : i ) the spectrum
i i ) the a d d i t i o n o f an excess
o f the reduced
e x h i b i t s the OC
i n i n t e n s i t y and the 800-nm a b s o r p t i o n l e s s i n t e n s e and b l u e
polymer; i i i )
o f d i t h i o n i t e r e v e r t s t h e spectrum
the spectrum
to that
o f F e ( I I I ) P P i s o b t a i n e d back by a d d i n g
f e r r i c y a n i d e . These r e s u l t s can be taken as an evidence
t h a t oxygen o x i d i z e s
[Fe(II)PPpyz"] t o a F e ( I I ) - F e ( I I I ) mixed v a l e n c e p o r p h y r i n p o l y n u c l e a r compound. — —' n T h i s compound appears t o be s t a b l e f o r s e v e r a l days. I t forms w i t h p o l y - L - l y s i n e a 1
s o l i d adduct
t h a t p r e s e n t s a spectrum
i d e n t i c a l with t h a t observed
g i v i n g evidence o f i t s p o l y m e r i c s t r u c t u r e . The p a r t i a l
i n solution,
oxidation of —
Vfe(II)PPpyzl — n
by oxygen may be i n t e r p r e t e d as a r i s i n g from a d i f f e r e n t r e a c t i v i t y o f the t e r m i n a l F e ( I I ) atomswith r e s p e c t t o the o x i d a t i o n , compared w i t h t h e o t h e r ones. Thus, the mixed v a l e n c e polymer s h o u l d have the s t r u c t u r e F e ( I I I ) P P p y z F e ( I I ) P P p y z . . . . F e ( I I ) P P p y z F e ( I I I ) P P . In t h i s s t r u c t u r e t h e p y r a z i n e I T system c a l i z e d compared w i t h t h a t o f the reduced
i s expected
t o be l e s s
delo-
polymer, so g i v i n g the r e a s o n o f t h e ob-
s e r v e d d i m i n u t i o n i n i n t e n s i t y and the b l u e s h i f t o f the 800-nm band. The r a p i d and complete o x i d a t i o n observed b r i d g e b r e a k i n g caused rin
by OH
i n alkaline solution i s likely
s p e c i e s . U n f o r t u n a t e l y , any attempt
reduced
t o be due t o the p y r a z i n e
which i s known t o be a good l i g a n d
f o r F e ( I I I ) porphy-
t o i s o l a t e the mixed v a l e n c e as w e l l as the
p o l y n u c l e a r compounds f a i l e d because o f the r a p i d o x i d a t i o n and d i s g r e g a t i o n
o c c u r r i n g as soon as the s o l i c b were s e p a r a t e d from
the excess
o f pyrazine.
References B a r t o c c i C, Scandola F, F e r r i A, C a r a s s i t i V (1980) P h o t o r e d u c t i o n o f Hemin i n A l c o h o l - C o n t a i n i n g Mixed S o l v e n t s . J Am Chem Soc 102: 7067-7072 B a r t o c c i C, M a l d o t t i A, T r a v e r s o 0, B i g n o z z i CA, C a r a s s i t i V (1983) P h o t o r e d u c t i o n o f Chlorohemin
i n Pure P y r i d i n e . Polyhedron
2: 97-102
B a r t o c c i C, A m a d e l l i R, M a l d o t t i A, C a r a s s i t i V (1986) P h o t o r e d u c t i o n o f F e ( I I I ) P r o t o p o r p h y r i n IX i n Ethanol-Water Polyhedron
solutions containing Bifunctional
Ligands.
5: 1297-1301
Fuhrhop JH, Baccouche M, Bunzel M (1980) A P y r a z i n e - B r i d g e d Heme Dimer A b s o r b i n g a t 800
nm. Angew Chem I n t Ed E n g l 19: 322-323
M a l d o t t i A, B a r t o c c i C, A m a d e l l i A, C a r a s s i t i V (1983) An ESR S p i n T r a p p i n g g a t i o n on the P h o t o r e d u c t i o n o f Chlorohemin
Investi-
i n Mixed S o l v e n t s . I n o r g Chim A c t a 74:
275-278 M a l d o t t i A, B a r t o c c i C, C h i o r b o l i C, F e r r i A, C a r a s s i t i V (1985) The Role o f Oxygen i n the Mechanism o f t h e I n t r a m o l e c u l a r Photoredox R e a c t i o n o f F e ( I I I ) P r o t o p o r p h y r i n IX i n A l k a l i n e Aqueous E t h a n o l . J Chem Soc Chem Comm: 881-882 S c h n e i d e r 0, Hanack M (1983) A x i a l p o l y m e r i s i e r t e s ( P h t a l o c y a n i n a t o ) e i s e n ( I I ) m i t Pyrazin, 4 , 4 • - B i p y r i d i n , 1,4-Diisocyanobenzol
oder
1,4-Diazabicyclo(2.2.2)octan
a l s B r i i c k e n l i g a n d e n ; D a r s t e l l u n g , C h a r a k t e r i s i e r u n g und e l e k t r i s c h e
Leitfahigkeiten
Chem Ber 116: 2088-2108 Wang JH, B r i n i g a r WS (1960) D e s i g n
and S y n t h e s i s o f a C a t a l y s t
d a t i o n o f Cytochrome c. Proc N a t l Acad S c i USA 46: 958-963
f o r the A e r o b i c O x i -
CHARGE-TRANSFER STATES AND
TWO-PHOTON PHOTOCHEMISTRY OF
R . M . B e r g e r , A . K . I c h i n a g a , and Department o f
Cu(NN)
+ 2
SYSTEMS
D.R.McMillin
C h e m i s t r y , P u r d u e U n i v e r s i t y , West L a f a y e t t e ,
IN 4 7 9 0 7 ,
USA
1 0
Because Cu(I) has a d electronic configuration, the metalc e n t e r e d a b s o r p t i o n b a n d s t e n d t o o c c u r i n t h e UV s p e c t r a l r e g i o n . But b e c a u s e t h e C u ( I I ) o x i d a t i o n s t a t e i s q u i t e a c c e s s i b l e , m e t a l to-ligand charge-transfer (CT) a b s o r p t i o n bands appear in the visible spectrum when the ligands present low-lying acceptor orbitals. For example, Cu(I) complexes involving chelating heteroaromatic l i g a n d s (NN l i g a n d s ) such as b i p y r i d i n e (bpy) or 1,10-phenanthroline (phen) o r one o f t h e i r d e r i v a t i v e s a r e b r i g h t l y colored. In fact, several different CT excited states are a c c e s s i b l e i n these systems. One r e a s o n i s t h a t t h e l i g a n d s p r e s e n t 2 l o w - l y i n g a c c e p t o r l e v e l s w h i c h O r g e l ( 1 9 6 1 ) d e s i g n a t e d a s x a n d V> orbitals. (They t r a n s f o r m as s y m m e t r i c and a n t i s y m m e t r i c o r b i t a l s , respectively, under r o t a t i o n about the t w o - f o l d a x i s ( F i g . 1).) Further complexity arises i n Cu(NN) systems because the d s h e l l of the metal i s s p l i t into at least four sublevels. F i n a l l y , there i s some evidence that a charge-transfer-to-solvent state can be p o p u l a t e d when a h i g h i n t e n s i t y p u l s e d l a s e r s o u r c e i s u s e d . +
2
I n t h i s r e p o r t we w i l l a t t e m p t t o a s s i g n t h e v i s i b l e a b s o r p t i o n bands of a s e r i e s of copper p h e n a n t h r o l i n e s . A s t h e CT s p e c t r a a r e p o o r l y r e s o l v e d , e v e n a t low t e m p e r a t u r e s , i n most c a s e s t h e s p e c t r a will be a s s i g n e d under the assumption of the highest available s y m m e t r y w h i c h i s D }. However, when p h e n y l g r o u p s a r e p r e s e n t i n t h e 2,9 p o s i t i o n s o f t h e p h e n a n t h r o l i n e c o r e , t h e c o m p l e x e s e x h i b i t a s i g n i f i c a n t d i s t o r t i o n a t low t e m p e r a t u r e s and i n t h e s o l i d s t a t e . T h i s d i s t o r t i o n i s a p p a r e n t l y f a v o r e d by i n t r a m o l e c u l a r i n t e r l i g a n d stacking interactions. 2c
EXPERIMENTAL The l i g a n d a b b r e v i a t i o n s a r e as f o l l o w s : 1,10-phenanthroline, phen; 2,9-dimethyl-l,10-phenanthroline, dmp; 2,9-dimethyl-4,7d i p h e n y l - 1 , 1 0 - p h e n a n t h r o l i n e , bcp; 2 , 9 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n e , d p p ; 2, 4, 7 , 9 - t e t r a p h e n y l - l , 1 0 - p h e n a n t h r o l i n e , t p p . The l i g a n d s w e r e obtained from commercial s o u r c e s o r p r e p a r e d by a l i t e r a t u r e m e t h o d ( D i e t r i c h - B u c h e c k e r e t a l . 1981), and t h e c o m p l e x e s • w e r e p r e p a r e d as before (McMillin etal. 1977). Absorption measurements were carried out with a Cary 17D spectrophotometer, and t h e l o w - t e m p e r a t u r e data were o b t a i n e d w i t h a n O x f o r d I n s t r u m e n t s DN-704 c r y o s t a t . A NdrYAG l a s e r ( Q u a n t a Ray DCR-1) served as a high intensity pulsed light source. For p h o t o l y s i s s t u d i e s t h e s a m p l e was d e o x y g e n a t e d i n a q u a r t z c u v e t t e and s t i r r e d d u r i n g i r r a d i a t i o n . T h e r e p r a t e was 10 H z , a n d the w i d t h a t h a l f h e i g h t o f e a c h p u l s e was a b o u t 7 n s .
BAND A S S I G N M E N T S I N D
2 d
SYMMETRY
I n t h e e n e r g y l e v e l scheme d e p i c t e d i n F i g . 1 t h e r e a r e s e v e n symmetry-allowed CT t r a n s i t i o n s . However, two t r a n s i t i o n s a r e e x p e c t e d t o c a r r y most o f t h e i n t e n s i t y . T h i s f o l l o w s f r o m t h e CT theory o f Mulliken which p r e d i c t s that t h e o s c i l l a t o r strength i s m a i n l y due t o t h e c h a r g e - t r a n s f e r term and i s c o n c e n t r a t e d i n
F i g u r e 1. Upper l e f t : The r e l a t i v e w e i g h t s and a t o m i c o r b i t a l s i n t h e x* d V>* o r b i t a l s o f p h e n . Cu(phen) i n D symmetry. Right: one e l e c t r o n Cu(phen) i n D j symmetry. T h e z - p o l a r i z e d CT < I ) i n d i c a t e d by arrows. a
n
+
2
2 d
+
2
1
A
a
2 c
r
p h a s e s o f t h e p?r Lower l e f t : energy l e v e l s f o r transitions ( B 1
2
e
those t r a n s i t i o n s which a r e p o l a r i z e d metal and t h e l i g a n d centers (Mulliken P h i f e r and M c M i l l i n 1986). When t h i s labels are e(xz,yz) > e(V>*) a n d b ( x a s t e r i s k i n d i c a t e s t h a t 7r-antibonding involved. 1
along the axis j o i n i n g the 1 9 5 2 ; Day a n d S a n d e r s 1 9 6 7 ; i s t h e z - a x i s , t h e symmetry -y ) > a ( x * ) where t h e orbitals of the ligand are
2
2
2
+
The absorption spectrum of Cu(dmp) a t 90 K i n a 4:1 ethanol/methanol glass i s presented i n Fig.2. The s p e c t r u m r e v e a l s a s t r o n g b a n d a t 4 6 0 nm w i t h a v i b r o n i c s a t e l l i t e a t 4 4 0 nm. In a d d i t i o n t h e r e i s a w e a k e r maximum a t 3 9 0 nm a n d a s h o u l d e r a t 5 1 0 nm. Similar spectra are obtained from C u ( p h e n ) and C u ( b c p ) ( T a b l e 1 ) . I f t h e bands a r e l a b e l e d as Bands I , I I and I I I i n o r d e r o f i n c r e a s i n g energy, t h e o n l y unambiguous assignment i s t h a t t h e t r a n s i t i o n t o e(V>*) i s c o n t a i n e d i n B a n d I I . This f o l l o w s because the transition t o ij>* i s e x p e c t e d t o be quite intense. In p a r t i c u l a r , i t s h o u l d b e 2-3 t i m e s a s i n t e n s e a s t h e t r a n s i t i o n t o x b e c a u s e t h e 2p7r o r b i t a l o f n i t r o g e n p a r t i c i p a t e s m o r e s t r o n g l y i n the V>* o r b i t a l (Phifer a n d M c M i l l i n 1986) . Band I I I p r o b a b l y represents t h e p a r t i a l l y resolved t r a n s i t i o n t o x*• This assignment i s i n a c c o r d w i t h t h e r e l a t i v e t r a n s i t i o n e n e r g i e s e x p e c t e d on t h e b a s i s o f F i g . l and w i t h t h e f a c t t h a t an a n a l o g o u s t r a n s i t i o n i s n o t r e s o l v e d i n t h e s p e c t r u m o f b p y a n a l o g u e s w h e r e t h e x* orbital i s known t o o c c u r a t h i g h e r e n e r g i e s . I f t h e symmetry i s D , Band I must be a s s i g n e d as x,y-polarized. The r e l a t i v e intensity i s c o n s i s t e n t w i t h t h i s assignment, and t h e r e i s a p r e c e d e n t f o r x , y p o l a r i z e d t r a n s i t i o n s i n t h e s p e c t r a o f W(C0) (NN) complexes ( S t a a l e t a l . 1978). On t h e o t h e r h a n d , B a n d I c o u l d a l s o a c q u i r e i n t e n s i t y from t h e charge t r a n s f e r term i f t h e complexes a r e s u b j e c t t o lowsymmetry d i s t o r t i o n s i n solution. In the solid state Cu(phen) (Healy etal. 1985) and C u ( d m p ) (Dobson e t a l . 1984) have been found t o adopt f l a t t e n e d s t r u c t u r e s with lattice-dependent angles b e t w e e n mean l i g a n d p l a n e s w h i c h a r e s i g n i f i c a n t l y s m a l l e r t h a n 90°. However, the flattening distortions have been connected with intermolecular stacking i n t e r a c t i o n s which do not survive i n solution (Goodwin e t a l . 1986). More e x p e r i m e n t a l work w i l l be needed t o a s c e r t a i n whether any l o w symmetry d i s t o r t i o n s o c c u r i n solution. 2
+
+
2
2
2 d
4
+
2
+
2
700
Wavelength (NM)
+
F i g u r e 2. Absorption spectra of Cu(dmp) (-—) ( ) a t 9 0 K i n a 4:1 e t h a n o l / m e t h a n o l g l a s s . 2
and C u ( d p p )
+ 2
Table
NN
bcp
a
The
LOW
Electronic Absorption
band
phen
dmp
1.
A,
nm
6
I Ila lib III
500 463 435 380
(sh)
I Ila lib III
510 460 440 390
(sh)
I Ila lib III
540 495 460 425
(sh)
€ values
are
SYMMETRY
Data at
90
NN
a
K
band
dpp
II' Ilia' Hlb'
tpp
II III
17,200 12,500 8, 100
A,
1 1
nm
550 436 413
3 , 000 4,400 3,900
590 460
9, 800 10,300
16,500 10,800 4,800
31,600 20,200 10,700
not
corrected
for solvent
contraction
SPECTRA
As can be seen in Fig. 2 the low-temperature absorption spectrum of Cu(dpp) has a very d i f f e r e n t shape i n t h a t two i n t e n s e CT a b s o r p t i o n b a n d s a r e r e s o l v e d i n t h e v i s i b l e r e g i o n . If t h e s e b a n d s c a n b e a s s i g n e d a s t r a n s i t i o n s t o t h e V* a n d x* orbitals of the l i g a n d s , i n view of the r e l a t i v e i n t e n s i t i e s the lower energy band would be assigned to the V* transition. Extended Huckel calculations reveal that the presence of the phenyl groups has l i t t l e e f f e c t on t h e r e l a t i v e e n e r g i e s o f t h e V* a n d x* orbitals, hence the i n c r e a s e d s p e c t r a l r e s o l u t i o n p r o b a b l y r e f l e c t s a change in the separation among t h e d 2 _ y 2 and t h e d and d y orbitals. The s p l i t t i n g i n c r e a s e s u g g e s t s a symmetry change, and i n l i n e w i t h t h i s a r g u m e n t a r e l a t e d c o m p l e x o f a c a t e n a n d l i g a n d has been shown to have a d i s t o r t e d , almost t r i g o n a l pyramidal c o o r d i n a t i o n geometry in the s o l i d state. In t h i s case the r e l a t i v e p o s i t i o n i n g of the ligands appears to be determined by interligand stacking i n t e r a c t i o n s (Cesario e t a l . 1985). Because i n t r a m o l e c u l a r stacking i s i n v o l v e d , t h e l o w s y m m e t r y s t r u c t u r e m i g h t be e x p e c t e d t o o c c u r in solution as well. The Cu(dpp) system also has phenyl s u b s t i t u e n t s i n t h e 2 and 9 p o s i t i o n s , h e n c e i t m i g h t be e x p e c t e d t o have a s i m i l a r s t r u c t u r e . Indeed, the copper catenate, Cu(dpp) and Cu(tpp) a l l exhibit similar low temperature absorption spectra. The indicated conclusion i s that Cu(I) complexes of phenanthroline l i g a n d s bearing 2,9-phenyl s u b s t i t u e n t s are prone to l o w - s y m m e t r y d i s t o r t i o n s due t o i n t r a m o l e c u l a r , i n t e r l i g a n d s t a c k i n g interactions. Because of the a n t i c i p a t e d symmetry differences, p r i m e d a n d u n p r i m e d n u m e r a l s a r e u s e d t o l a b e l t h e b a n d s i n T a b l e 1. Parenthetically, we should note that i n the limit of a large d i s t o r t i o n i t i s not p o s s i b l e to a t t r i b u t e the s e p a r a t e l y resolved t r a n s i t i o n s t o x* a n d V* orbitals. 2 +
2
x
x z
Z
+
2
+
2
+
2
TWO
PHOTON A B S O R P T I O N
Although deaerated s o l u t i o n s of Cu(dmp) + i n C H C 1 are q u i t e p h o t o s t a b l e u n d e r r o o m l i g h t a n d u n d e r e x p o s u r e t o 355 nm r a d i a t i o n f r o m a 1000 W Xe a r c lamp, F i g . 3 shows t h a t t h e s o l u t i o n s a r e r a p i d l y b l e a c h e d when i r r a d i a t e d w i t h a h i g h i n t e n s i t y p u l s e d l a s e r source. The b l e a c h i n g i s e v e n m o r e r a p i d w h e n 3 5 4 . 7 nm light is used. Product a n a l y s i s shows t h a t t h e n e t r e a c t i o n e n t a i l s the oxidation of Cu(dmp) t o t h e C u ( I I ) s t a t e and t h e r e d u c t i o n o f t h e s o l v e n t w i t h concomitant formation of c h l o r i d e . At r e l a t i v e l y low powers (< 3 mJ per pulse a t 3 5 4 . 7 nm) the r a t e of bleaching approximately d e p e n d s on the square of the l a s e r power. This establishes that the bleaching requires the absorption of two photons: 2
2
2
+
2
Cu(dmp)
+ 2
+
2 hv
+ CH C1 2
2
> Cu(dmp)
2 + 2
+ CI" + Prod.
(1)
A t h i g h e r i n t e n s i t i e s t h e b l e a c h i n g r a t e becomes a l i n e a r f u n c t i o n of l a s e r power because a l l of the Cu(dmp) molecules in the i r r a d i a t e d v o l u m e h a v e b e e n d r i v e n i n t o t h e r e l a t i v e l y l o n g - l i v e d CT excited state. Consistent with this interpretation, saturation i s much h a r d e r t o a c h i e v e f o r C u ( p h e n ) w h i c h has a r e l a t i v e l y s h o r t l i v e d CT excited state. The m e c h a n i s t i c d e t a i l s have y e t t o be e s t a b l i s h e d , b u t i t i s p o s s i b l e t h a t two p h o t o n a b s o r p t i o n populates a c h a r g e - t r a n s f e r - t o - s o l v e n t (CTTS) e x c i t e d s t a t e . I n any c a s e t h e s o l v e n t dependence of the bleaching r a t e (CH C1 > C H C N >> C H 0 H ) p a r a l l e l s t h e r e a c t i o n r a t e s o f t h e s o l v a t e d e l e c t r o n w i t h t h e same s e r i e s of solvents. M o r e o v e r , CTTS t r a n s i t i o n s h a v e b e e n o b s e r v e d i n t h e UV s p e c t r a of the r a d i c a l anion forms of v a r i o u s aromatic s y s t e m s ( J o s c h e k and G r o s s w e i n e r 1966). +
2
+
2
2
0.9
2
3
3
-
CD U
c CD
ID
C_
o in -Q
F i g u r e 3. P h o t o l y s i s o f C u ( d m p ) + a t 20° in CH C1 . The s a m p l e was irradiated with 3.2 mJ p u l s e s o f 5 0 2 . 9 nm r a d i a t i o n f o r 1 min i n t e r v a l s where the r e p e t i t i o n r a t e i s 10 Hz. 2
2
0.0 350
550
450
Wavelength
(nm)
2
ACKNOWLEDGMENT T h i s r e s e a r c h was s u p p o r t e d t h r o u g h G r a n t No. CHE-8414267.
by t h e N a t i o n a l Science
Foundation
REFERENCES C e s a r i o M, D i e t r i c h - B u c h e c k e r CO, G u i l h e m J , P a s c a r d C, S a u v a g e , J P (1985) M o l e c u l a r s t r u c t u r e o f a c a t e n a n d a n d i t s c o p p e r ( I ) catenate: Complete overturn o f t h e i n t e r l o c k e d m a c r o c y c l i c l i g a n d s b y c o m p l e x a t i o n . J Chem S o c Chem Commun 244-247. Day R, S a n d e r s N ( 1 9 6 7 ) T h e s p e c t r a o f c o m p l e x e s o f c o n j u g a t e d ligands. Part I I . Charge-transfer i n substituted phenanthroline c o m p l e x e s : I n t e n s i t i e s . J Chem S o c (A) 1 5 3 6 - 1 5 4 1 D i e t r i c h - B u c h e c k e r CO, M a r n o t P A , S a u v a g e J P ( 1 9 8 2 ) D i r e c t s y n t h e s i s of d i s u b s t i t u t e d aromatic polyimine chelates. T e t L e t t 23: 5291-5294 D o b s o n J F , G r e e n BE, H e a l y PC, K e n n a r d CHL, P a s k a w a t c h a i C, W h i t e AH (1984). The s t e r e o c h e m i s t r y o f b i s ( a , a ' - d i i m i n e ) - c o p p e r ( I ) c o m p l e x e s : T h e c r y s t a l amd m o l e c u l a r s t r u c t u r e s o f b i s ( 2 , 9 dimethyl-1,10-phenanthroline)copper(I) bromide hydrate, bis(4,4 6,6 -tetramethyl-2,2 -bipyridine)copper(I) chloride dihydrate, and bis(2,9-dimethyl-l,10-phenanthroline)copper(I) n i t r a t e d i h y d r a t e ( a r e d e t e r m i n a t i o n ) . A u s t J Chem 3 7 : 6 4 9 - 6 5 9 G o o d w i n K V , M c M i l l i n DR, R o b i n s o n WR ( 1 9 8 6 ) C r y s t a l a n d m o l e c u l a r structure of [Ag(tmbp) ]BF . Origin of flattening distortions i n d c o m p l e x e s o f t h e t y p e M ( N N ) . I n o r g Chem 2 5 : 2 0 3 3 - 2 0 3 6 H e a l y P C , E n g e l h a r d t LM, P a t r i c k V A , W h i t e AH ( 1 9 8 5 ) L e w i s - b a s e a d d u c t s o f g r o u p I B m e t a l ( I ) compounds. P a r t 19. C r y s t a l structures o f bis(1,10-phenanthroline)copper(I) p e r c h l o r a t e and d i b r o m o c u p r a t e ( I ) . J Chem S o c D a l t o n T r a n s 2 5 4 1 - 2 5 4 5 Joschek H-I, Grossweiner L I (1966) O p t i c a l g e n e r a t i o n o f h y d r a t e d e l e c t r o n s f r o m a r o m a t i c c o m p o u n d s . I I . J Am Chem S o c 8 8 : 3 2 6 1 - 3 2 6 8 M c M i l l i n DR, B u c k n e r MT, A h n BT ( 1 9 7 7 ) A l i g h t - i n d u c e d r e d o x reaction of bis(2,9-dimethyl-l,10-phenanthroline)copper(I). Inorg Chem 1 6 : 9 4 3 - 9 4 5 M u l l i k e n , RS ( 1 9 5 2 ) M o l e c u l a r c o m p o u n d s a n d t h e i r s p e c t r a . I I . J Am Chem S o c 7 4 : 8 1 1 - 8 2 4 O r g e l LE (1961) D o u b l e b o n d i n g i n c h e l a t e d m e t a l c o m p l e x e s . J Chem S o c 3 6 8 3 - 3 6 8 6 P h i f e r C C , M c M i l l i n DR ( 1 9 8 6 ) T h e b a s i s o f a r y l s u b s t i t u e n t e f f e c t s o n c h a r g e - t r a n s f e r a b s o r p t i o n i n t e n s i t i e s . I n o r g Chem 2 5 : 1 3 2 9 1333 S t a a l L H , S t u f k e n s D J , Oskam A (1978) A s t u d y o f t h e e l e c t r o n i c p r o p e r t i e s o f M ( C O ) D A B (M = C r , Mo, W; DAB = d i a z a b u t a d i e n e ) . I . E l e c t r o n i c a b s o r p t i o n , r e s o n a n c e Raman, i n f r a r e d , c - and N NMR s p e c t r a . I n o r g C h i m A c t a 2 6 : 2 5 5 - 2 6 2 1
1
1
2
4
1 0
+
2
4
1
3
1 5
PHOTOINDUCED MULTIELECTRON REDOX REACTIONS IN THE V I S I B L E LIGHT REDUCTION OF PLATINUM CENTERS
( P t C l ) " / A L C O H O L SYSTEM2
£
6
A . B . B o c a r s l y , R . E . C a m e r o n , and M e i s h e n g Zhou Department o f C h e m i s t r y , P r i n c e t o n
University,
Princeton,
N . J . 0 8 5 4 4 , USA
A l t h o u g h the p h o t o c h e m i s t r y o f o c t a h e d r a l h a l o p l a t i n u m (IV) complexes has been e x t e n s i v e l y i n v e s t i g a t e d , i t has always been viewed i n terms o f l i g a n d s u b s t i t u t i o n processes. However, the preference of platinum f o r the ( I I ) and (IV) o x i d a t i o n s t a t e s , a n d t h e p r e v i o u s o b s e r v a t i o n (Cox e t a l . , 1972) t h a t photoa q u a t i o n o f [ P t C i ] " c a n i n v o l v e a P t ( I I I ) i n t e r m e d i a t e suggests t h a t such complexes may be capable o f photoinduced m u l t i e l e c t r o n charge t r a n s f e r . Such a capability has now been demonstrated i n the p h o t o i n i t i a t e d r e a c t i o n of [ P t C i ] ~ w i t h a l c o h o l s . Up t o f o u r e l e c t r o n s can be t r a n s f e r r e d i n t h i s process l e a d i n g to the f o r m a t i o n of platinum metal (Cameron and B o c a r s l y , 1986) and aldehydes o r ketones depending on whether a primary or secondary a l c o h o l i s i n i t i a l l y employed. Of s p e c i a l i n t e r e s t i s the f i n d i n g t h a t primary a l c o h o l s are n o t o v e r o x i d i z e d to the a c i d . F u r t h e r aldehyde formation can be c a r r i e d out c a t a l y t i c a l l y by a d d i t i o n o f 0 / C u C i t o the r e a c t i o n m i x t u r e as a r e o x i dant o f lower p l a t i n u m o x i d a t i o n s t a t e s back t o P t ( I V ) (Cameron and B o c a r s l y , 1985) . 2
6
2
6
2
2
The p h o t o c h e m i c a l p r o d u c t i o n of p l a t i n u m metal i s an unusual f i n d i n g . The photochemistry o f P t ( I V ) i s t y p i f i e d by an i n a b i l i t y t o generate p l a t i n u m metal. Only r e c e n t l y has p r o d u c t i o n o f metal been noted. T h i s was o b t a i n e d by V o g l e r and H l a v a t c h (1983) who employed the r e l a t i v e l y n o v e l [ P t ( N ) ] ' comp l e x which generates N and metal upon i l l u m i n a t i o n . Since the a b i l i t y t o gen e r a t e a w e l l d e f i n e d p l a t i n u m s u r f a c e i s both s c i e n t i f i c a l l y n o v e l and o f p r a c t i c a l i n t e r e s t w i t h r e s p e c t to heterogeneous c a t a l y s i s and e l e c t r o n i c l i t h o g r a p h y , p h o t o c h e m i s t r y l e a d i n g t o t h i s g o a l i s o f fundamental i n t e r e s t . 2
3
6
2
MECHANISM OF PLATINUM PHOTOREDUCTION 2
The f o u r e l e c t r o n photoinduced r e d u c t i o n o f [ P t C i ] ~ i n aqueous a l c o h o l does not r e q u i r e h e a t i n g and occurs a t a l l wavelengths a t which the complex absorbs l i g h t . Thus, i r r a d i a t i o n a t e n e r g i e s as low as 514 nm, r e p o r t e d t o e x c i t e a [PtCi ] : iAig • T ] p l i g a n d f i e l d t r a n s i t i o n (Swihart and Mason, 1970) y i e l d s p l a t i n u m m e t a l . A l a r g e v a r i e t y o f a l c o h o l r e a c t a n t s undergo two e l e c t r o n o x i d a t i o n i n the presence o f [ P t C i ] ~ . The r e a c t i o n o f [ P t C i j ~ w i t h e t h a n o l and 2-propanol a r e p r o t o t y p i c a l . 6
2 _
3
6
2
2
6
6
The
f a t e o f t h e p l a t i n u m s p e c i e s i n these r e a c t i o n s has been f o l l o w e d u s i n g Pt-FTNMR and U V - v i s i b l e s p e c t r o s c o p y . F i g u r e 1 d e t a i l s the time dependence of t h e NMR s e n s i t i v e p l a t i n u m s p e c i e s d u r i n g the p h o t o r e d u c t i o n o f P t C i " by 2-propanol, a l o n g w i t h o r g a n i c product f o r m a t i o n as determined by gas chromotography. As c a n be seen from these data the i n i t i a l l y observed p l a t i n u m phot o p r o d u c t i s a P t ( I I ) s p e c i e s i d e n t i f i e d as P t C i " u s i n g U V - v i s i b l e s p e c t r o s copy. There e x i s t s a f a i r l y l o n g i n d u c t i o n p e r i o d p r i o r t o metal f o r m a t i o n . The e x a c t l e n g t h o f t h i s p e r i o d depends on the l i g h t i n t e n s i t y employed, and the reagent c o n c e n t r a t i o n s . Once an i n i t i a l amount o f metal i s produced the r e a c t i o n r a p i d l y goes t o c o m p l e t i o n . 1 9 5
2
6
2
4
In g e n e r a l , p l a t i n u m f o r m a t i o n i s not found t o occur u n t i l a ~ 90% y i e l d o f [ P t C i ] ~ has accumulated. T h i s suggests t h a t [ P t C i ] " a c t s as an i n h i b i t o r toward p l a t i n u m m e t a l f o r m a t i o n . I n order t o g a i n i n s i g h t i n t o t h i s process the thermal r e d u c t i o n c h e m i s t r y o f [ P t C i ] ' was examined. [ P t C i ] was found t o r e a c t w i t h 2-propanol i n a redox r e a c t i o n t o produce p l a t i n u m metal 2
2
4
6
2
4
2 _
4
2
and acetone i n a 1:1 s t o i c h i o m e t r y . A d d i t i o n o f [ P t C i ] ' ( o r K C i ) had a s t r o n g i n h i b i t i n g e f f e c t on m e t a l f o r m a t i o n d e m o n s t r a t i n g the source o f the [ptCi ] " inhibition. On the o t h e r hand, a d d i t i o n o f h i g h s u r f a c e a r e a p l a t i num m e t a l enhanced the r e a c t i o n r a t e ; t h u s , t h i s r e a c t i o n i s a u t o c a t a l y t i c i n p l a t i n u m m e t a l . These two f i n d i n g s t a k e n t o g e t h e r e x p l a i n the asymmetry o f the P t ( I I ) v e r s u s time p r o f i l e shown i n F i g . 1. 6
2
6
w i t h 2-propanol as a f u n c t i o n o f time. A l l c o n c e n t r a t i o n s are i n m i l l i m o l a r . The "+" r e p r e s e n t s the [ P t C i ] ~ c o n c e n t r a t i o n , represents [ P t C i ^ ] " , and r e p r e s e n t s one t h i r d o f the t o t a l o r g a n i c concen tration. The l a t t e r i s a sum o f acetone and a c e t a l d e h y d e c o n c e n t r a t i o n s . M e t a l i s o b s e r v e d t o form a t ~ 150 minutes. 2
6
2
2
U n l i k e the r e d u c t i o n o f [ P t C i ^ ] " , which o n l y y i e l d s acetone as the o r g a n i c p r o d u c t , a c e t a l d e h y d e i s i n i t i a l l y the major p r o d u c t d u r i n g the p h o t o r e d u c t i o n o f P t C i ^ " by 2-propanol. F o r m a t i o n o f t h i s s p e c i e s i s i n d i c a t i v e o f methyl f r e e r a d i c a l l o s s from 2-propanol. The e x i s t e n c e o f such a p r o c e s s i s c o n f i r m e d by r e a c t i o n o f t - b u t a n o l w i t h [ P t C J ? ] " . T h i s r e a c t i o n y i e l d s acetone ( a g a i n s i g n i f y i n g methyl l o s s ) and [ P t C i ] " , d i r e c t l y i n d i c a t i n g t h a t [ P t C i ] " a s s o c i a t e d p h o t o c h e m i s t r y i s r e s p o n s i b l e f o r the f o r m a t i o n o f methyl free r a d i c a l s . The e x i s t e n c e o f m e t h y l f r e e r a d i c a l s i n t r o d u c e s two major mechanistic implications. F i r s t , the oxygen based 2-propanol f r e e r a d i c a l , •0-CH(Me) , must be g e n e r a t e d . T h i s i s u n u s u a l s i n c e i t i s the carbon based r a d i c a l w h i c h i s more s t a b l e . Second, the charge t r a n s f e r c h e m i s t r y must i n v o l v e one e l e c t r o n p r o c e s s e s . T h i s l a t t e r c o n c l u s i o n i s c o n f i r m e d by the photochemical r e a c t i o n of c y c l o b u t a n o l with [ P t C i ] ~ . This alcohol i s a known f r e e r a d i c a l c l o c k (Meyer and Rocek, 1972) which produces r i n g opened p r o d u c t s upon one e l e c t r o n o x i d a t i o n . Our o b s e r v a t i o n i n t h i s r e a c t i o n o f one e l e c t r o n o x i d i z e d o r g a n i c p r o d u c t s i m p l i c a t e s a P t ( I I I ) s p e c i e s as a p r i m a r y photoproduct. 2
2
6
2
4
2
6
2
2
6
S i n c e o x i d a t i o n o f an a l c o h o l v i a an u n s t r u c t u r e d o x i d a n t such as Ci« (a p o s s i b l e r e a c t i v e s p e c i e s i n the p r e s e n t system) would l e a d t o the t h e r m o d y n a m i c a l l y f a v o r e d carbon based f r e e r a d i c a l , the a v a i l a b l e d a t a suggest t h a t r e a c t i o n proceeds v i a p h o t o i n d u c e d f o r m a t i o n o f a p l a t i n u m a l k o x y bond f o l l o w e d by o p t i c a l h o m o l y s i s t o g e n e r a t e the i n d i c a t e d p r o d u c t s as o u t l i n e d below: hi/ 2
[PtCi ] "* 6
• R'RCH-OH
2
[ P t C i ( O C H R R ' ) ] ~ + HCi 5
(1)
hi/ [PtCi (OCHRR') 5
2
#
• [ P t C i ] " + 0CHRR' 5
(2)
[PtCi 3 " 2
2
[ P t C i ] " + O-CRR'
+ *OCHRR'
5
2
2[PtCi ] "
(3a)
4
2
2
[PtCi ] - + [PtCi ] "
5
4
•OCHRR'
(3b)
6
0=CR + R' ( f o r R'=Me)
(4)
The e x i s t e n c e o f r e a c t i o n s 3a,b i s s p e c u l a t i v e , however, c o n s i s t e n t w i t h the expected b e h a v i o r o f a P t ( I I I ) s p e c i e s . Support f o r t h i s mechanism comes from the o b s e r v a t i o n t h a t i r r a d i a t i o n o f i s o l a t e d [ P t C i ( O R ) _ ] " complexes (where R = 2 - p r o p y l ) l e a d s t o p l a t i n u m metal p r o d u c t i o n . Thus, r e a c t i o n (2) i s d i r e c t l y v e r i f i e d . M e t a l f o r m a t i o n i s expected t o proceed from the thermal r e a c t i o n o f t h e p r o d u c t s o f r e a c t i o n (3) w i t h t h e a l c o h o l as d i s c u s s e d p r e v i o u s l y . A l t h o u g h t h i s l a t t e r r e a c t i o n proceeds v i a a thermal pathway, i t i s p h o t o c h e m i c a l l y a c c e l e r a t e d by v i s i b l e l i g h t . 2
x
6
x
CATALYTIC PRODUCTION OF ALDEHYDES AND KETONES A l t h o u g h t h e o b s e r v a t i o n t h a t aldehyde can be produced w i t h o u t o v e r o x i d a t i o n to t h e a c i d i s s y n t h e t i c a l l y i n t e r e s t i n g , the u t i l i z a t i o n o f an expensive r e agent ( K [ P t C i ] ) removes any s y n t h e t i c appeal. I n o r d e r t o generate a synt h e t i c a l l y u s e f u l p r o c e s s t h e r e a c t i o n must be c a t a l y t i c i n p l a t i n u m . S i n c e r e o x i d a t i o n o f p l a t i n u m m e t a l i s d i f f i c u l t i t i s n e c e s s a r y t o i n t e r c e p t an i n t e r m e d i a t e o x i d a t i o n s t a t e o f p l a t i n u m . F u r t h e r , i t i s d e s i r a b l e t o t r a p the •OCHRR' p r o d u c t p r i o r t o methyl l o s s i n cases where t h i s i s a p o s s i b l e react i o n pathway. T h i s i s accomplished by adding a C u C i / 0 redox c a t a l y s t t o the system. As shown i n Table 1 under these c o n d i t i o n s aldehyde and ketone products a r e generated i n h i g h y i e l d w i t h o u t methyl l o s s o r o t h e r f r e e r a d i c a l s i d e p r o d u c t s . C o n t r o l experiments i n d i c a t e t h a t n e i t h e r C u C i / 0 alone n o r [ P t C i ] * / 0 alone produces a c a t a l y t i c c y c l e . 2
6
2
2
2
2
2
6
2
Table I : T y p i c a l Product Y i e l d s Alcohol
a
Product
Ethanol 2-Propanol Cyclopentanol Cyclohexanol Cyclobutanol Benzyl Alcohol 2-Hexanol Cinnamyl A l c o h o l
Acetyldehyde 2-Propanone Cyclopentanone Cyclohexanone Cyclobutanone Benzaldehyde 2-Hexanone Cinnamaldehyde
% Yield 94 98 98 92 94 93 84 88
b
0.05 0.06 0.04 0.02 -
0.03 0.02 0.02
1:2 K [ P t C i ] / C u C l c a t a l y s t . R e a c t i o n s were c a r r i e d out i n a c e t o n i t r i l e , acetone, o r water s o l v e n t s , k R e a c t i o n s were c a r r i e d out i n an acetone s o l v e n t u s i n g a t u n g s t e n h a l o g e n source s u p p l i e d w i t h UV and IR c u t - o f f f i l t e r s . Quantum y i e l d f o r p r o d u c t f o r m a t i o n a t 488 nm. a
2
6
2
c
As c a n be seen from d a t a i n Table I t h i s system i s capable o f c a r r y i n g out the s p e c i f i c two e l e c t r o n o x i d a t i o n o f a wide v a r i e t y o f a l c o h o l s . Even low redox p o t e n t i a l a l c o h o l s such as b e n z y l a l c o h o l show no tendency t o be o v e r o x i d i z e d to t h e a c i d . F u r t h e r , r e a c t i v i t y does not appear t o be s i g n i f i c a n t l y i n f l u e n c e d by t h e presence o f s i t e s o f u n s a t u r a t i o n near the a l c o h o l f u n c t i o n a l i t y . Thus, as demonstrated u s i n g cinnamyl a l c o h o l c o n j u g a t e d aldehydes c a n be generated i n good y i e l d . That cyclobutanone i s produced i n h i g h y i e l d from t h e c o r r e s p o n d i n g a l c o h o l i n d i c a t e s t h e process has a "two
e l e c t r o n " c h a r a c t e r t o i t . Presumably t h i s r a d i c a l c l o c k i s s t a b i l i z e d by the r a p i d l o s s o f an e l e c t r o n t o C u , a s p e c i e s known t o be a f a s t r a d i c a l t r a p . For t h e case o f 2 - p r o p a n o l o x i d a t i o n quantum y i e l d s as h i g h as 0.15 have been o b s e r v e d f o r acetone f o r m a t i o n u s i n g 488 nm l i g h t . T u r n o v e r numbers i n excess o f 150 have been o b s e r v e d when w h i t e l i g h t i s employed w i t h o n l y a s l i g h t d e c r e a s e i n c a t a l y t i c a c t i v i t y a t the end o f the r e a c t i o n p e r i o d . C a t a l y s t s t a b i l i t y i s q u i t e good. 2 +
The r e a c t i o n i s found t o be f a i r l y s e n s i t i v e t o t h e amount o f CuC2 present. As the C u C i c o n c e n t r a t i o n i s i n c r e a s e d the turnover r a t e i s observed to increase l i n e a r l y . T h i s phenomenon appears t o s a t u r a t e a t - 1:2 r a t i o o f CuCi to [ P t C i ] " . A d d i t i o n o f an e x c e s s i v e amount o f C u C i causes the s o l u t i o n t o t u r n brown w i t h a c o n c o m i t a n t l o s s o f c a t a l y t i c a c t i v i t y . Pt NMR s t u d i e s demonstrate t h a t C u cannot o x i d i z e [ P t C i ] " . This r e s u l t can be b e s t e x p l a i n e d by assuming t h a t C u i s oxidizing a Pt(III) chloride complex t o r e g e n e r a t e [ P t C i ] ' . T h e r e f o r e the r e q u i r e m e n t o f two e q u i v a l e n t s of C u C i c a n be u n d e r s t o o d i n terms o f i t s d u a l r o l e as an o x i d a n t o f P t ( I I I ) and a r a d i c a l t r a p f o r *0CHRR'. The o v e r a l l c y c l e i s summarized i n F i g . 2. The h i g h p r o d u c t y i e l d s a l o n g w i t h the l a c k o f t y p i c a l o r g a n i c f r e e r a d i c a l p r o d u c t s c a n b e s t be e x p l a i n e d by t h e e x i s t e n c e o f a b i n u c l e a r ( p o s s i b l y c h l o r o b r i d g e d ) p l a t i n u m - c o p p e r complex as t h e a c t i v e c a t a l y t i c s p e c i e s . Such a complex c o u l d p r o v i d e the s t r o n g cage e f f e c t n e c e s s a r y t o produce the observed product y i e l d . 2
2
2
2
6
2
1 9 5
2 +
2
4
2 +
2
6
2
hv PtCI.*-
+ (n) ROH
Pt- +(n) R C B )
hv
o
H
+ ROH
o
A/hv
•Pt(lll)'
+ (n-1) [ROH] •
Cu(l) + R * °
PtCI *- + < n - 1 ) R t 4
CuCI,
PtCI,*.2 ' P t d l D
F i g u r e 2:
H
-
M e c h a n i s t i c sequence f o r t h e r e a c t i o n o f [ P t C i ] ~/C\iC& /0 w i t h p r i m a r y a l c o h o l s and [ P t C i ] " i t s e l f w i t h p r i m a r y alcohols. 2
6
z
2
2
6
Acknowledgment i s made t o t h e Donors o f The P e t r o l e u m R e s e a r c h Fund, a d m i n i s t e r e d by t h e A m e r i c a n C h e m i c a l S o c i e t y f o r Support o f t h i s R e s e a r c h . References: Cameron, RE, B o c a r s l y AB (1985) P h o t o a c t i v a t e d o x i d a t i o n o f a l c o h o l s by oxygen. J Am Chem Soc 107: 6116-6117. Cameron, RE, B o c a r s l y , AB (1986) M u l t i e l e c t r o n - p h o t o i n d u c e d r e d u c t i o n o f c h l o r o p l a t i n u m complexes: V i s i b l e l i g h t d e p o s i t i o n o f platinum metal. I n o r g Chem 25: 2910-2913. Cox, LE, P e t e r s , DG, Wehry, EL (1972) P h o t o a q u a t i o n o f H e x a c h l o r o p l a t i n a t e ( I V ) > J I n o r g N u c l Chem 34: 297-305. Meyer, K, Rocek, J . (1972) O n e - e l e c t r o n v s . t w o - e l e c t r o n o x i d a t i o n s . Ce(IV) and
cyclobutanol.
J Am Chem Soc 94:
1209-1214.
S w i h a r t , DL, and Mason, WR (1970) E l e c t r o n i c s p e c t r a o f o c t a h e d r a l P t ( I V ) complexes. I n o r g Chem 9: 1749-1757. V o g l e r , A, and H l a v a t s c h , J (1983) P h o t o c h e m i c a l f o u r - e l e c t r o n redox r e a c t i o n o f h e x a a z i d o p l a t i n a t e ( I V ) . Angew Chem I n t Ed 22: 154-155.
DYNAMIC AND STATIC OUTER-SPHERE AND INNER-SPHERE QUENCHING PROCESSES
L.Checchi, C . C h i o r b o l i ,
M.A.Rampi S c a n d o l a , and F.Scandola
D i p a r t i m e n t o d i Chimica d e l l ' U n i v e r s i t a , Centro d i Fotochimica d e l CNR, 44100 F e r r a r a , ITALY
INTRODUCTION B i m o l e c u l a r quenching v i a energy o r e l e c t r o n t r a n s f e r o f e l e c t r o n i c a l l y e x c i t e d s t a t e s o f c o o r d i n a t i o n compounds i n f l u i d s o l u t i o n s has been t h e s u b j e c t o f i n t e s i v e i n v e s t i g a t i o n s i n t h e l a s t , t e n years ( B a l z a n i 1978,Sutin 1979,Sutin 1980,Balzani 1981, S u t i n 1982,Balzani 1983). The k i n e t i c s o f such processes can be c o n v e n i e n t l y d i s c u s s e d by s e p a r a t i n g d i f f u s i v e and r e a c t i v e steps as shown i n Scheme 1: Scheme 1. + hv kd kt A + B *A + B •*A B A + B 1/T° k. A +*B The r e a c t i v e s t e p t a k i n g p l a c e i n t o t h e s o - c a l l e d p r e c u r s o r complex *A B, i s c o n s i d e r e d as a u n i m o l e c u l a r p r o c e s s . I n most cases (A and B uncharged o r w i t h p o s i t i v e charge p r o d u c t ) b o t h t h e ground and e x c i t e d s t a t e c o n c e n t r a t i o n s o f A B a r e s m a l l and t h e c o n s t a n t k t can be a t best i n d i r e c t l y i n f e r r e d from t h e second o r d e r quenching r a t e c o n s t a n t kq. I f , , on the o t h e r hand, s t r o n g e l e c t r o s t a t i c a t t r a c t i o n between A and B i s o p e r a t i n g (A and B o f h i g h and o p p o s i t e c h a r g e ) , a s u f f i c i e n t l y h i g h c o n c e n t r a t i o n of t h e ground s t a t e p r e c u r s o r complex may be present t o a l l o w d i r e c t e x c i t a t i o n o f t h i s s p e c i e s and o b s e r v a t i o n o f t h e u n i m o l e c u l a r s t e p . R e c e n t l y , s t u d i e s o f o p t i c a l l y induced energy o r e l e c t r o n t r a n s f e r processes between t r a n s i t i o n m e t a l c e n t e r s c o v a l e n t l y l i n k e d i n a supermolecular s t r u c t u r e have been performed ( V o g l e r 1985, B i g n o z z i 1985, C u r t i s 1985). These processes a r e summarized i n Scheme 2. Scheme 2 - B hv kt *A B -*B d
I f t h e supermolecule A - B forms i n s o l u t i o n upon a d d i t i o n o f A and B ( o r r e l a t e d s p e c i e s ) and t h e c o v a l e n t l i n k a g e does not change s t r o n g l y t h e p r o p e r t i e s o f t h e s u b u n i t s , t h e l o c a l l y - e x c i t e d molecule can be viewed as a c o v a l e n t l y l i n k e d p r e c u r s o r complex o f t h e t r a n s f e r p r o c e s s . From t h i s v i e w p o i n t , a g e n e r a l k i n e t i c scheme (Scheme 3) c a n r e p r e s e n t photoinduced e l e c t r o n o r energy t r a n s f e r between A and B which are capable o f e l e c t r o s t a t i c and/or c o v a l e n t ( u s u a l l y c a l l e d second sphere donor-acceptor) i n t e r a c t i o n . Scheme. 3 •A
B
•
'
K
° '
•
* A
1/T
AB
A - B
I n t h i s Scheme, *A B i s t h e p r e c u r s o r complex o f an o u t e r - s p h e r e t r a n s f e r mechanism, w h i l e *A - B can be c o n s i d e r e d as t h e p r e c u r s o r complex of an i n n e r sphere t r a n s f e r mechanism. Depending on the r e l a t i v e c o n s t a n t s o f t h e v a r i o u s ground-state s p e c i e s and on t h e r e l a t i v e r a t e s o f t h e e x c i t e d e s t a t e p r o c e s s e s , t h e complex k i n e t i c b e h a v i o r p r e d i c t e d by t h i s Scheme may g i v e r i s e t o two e x p e r i m e n t a l d i s t i n g u i s h a b l e s i t u a t i o n s ( B a l z a n i 1975, Rybak 1981, Frank 1983, White 1984): 1) i f ( i ) t h e g r o u n d - s t a t e c o n c e n t r a t i o n o f a s s o c i a t e d s p e c i e s i s n e g l e g i b l e o r ( i i ) the e x c i t e d a s s o c i a t e d * s p e c i e s undergo prompt d i s s o c i a t i o n ( k < k _ d ; k < k * _ ) , t h e system behaves as i f *A was the o n l y e x c i t e d s p e c i e s . In t h i s case both e m i s s i o n i n t e n s i t y ( I ) and l i f e t i m e (T) f o l l o w the same S t e r n Volmer b e h a v i o r : T°/T = I°/I = l+k x°[B] . I n t h i s case the quenching i s u s u a l l y c a l l e d dynamic quenching;2) i f ( i ) t h e ground s t a t e c o n c e n t r a t i o n o f o u t e r - s p h e r e and/or i n n e r - s p h e r e a s s o c i a t e d s p e c i e s i s h i g h and ( i i ) t h e i r excited states react b e f o r e d i s s o c i a t i n g ( k > k_^; k > k > v _ i ) , a complex m u l t i e x p o n e n t i a l decay i s e x p e c t e d , w i t h one o r two s h o r t components whose l i f e t i m e i s independent o f B c o n c e n t r a t i o n , f o l l o w e d by a l o n g e r one t h a t obeys Stern-Volmer law. The i n t e n s i t y quenching i s always l a r g e r t h a n t h e l i f e t i m e quenching and u s u a l l y f o l l o w s a q u a d r a t i c law o f t h e t y p e : I°/I- ( l + k x [B] )(1+KA[B] ) where K i s an " e f f e c t i v e " a s s o c i a t i o n c o n s t a n t o f a l l the p r e s e n t a s s o c i a t e d s p e c i e s . I n t h i s case t h e quenching i s u s u a l l y c a l l e d s t a t i c quenching. S t u d i e s i n t h i s f i e l d have been l i m i t e d by the use o f p o s i t i v e s e n s i t i z e r s f o r w h i c h a s m a l l number of n e g a t i v e quenchers i s a v a i l a b l e . We r e p o r t h e r e t h e r e s u l t s o f quenching s t u d i e s i n v o l v i n g r e c e n t l y s y n t h e t i z e d , n e g a t i v e l y charged p h o t o s e n s i t i z e r s , I r ( Q O S C ^ ^ ^ - ^ B a l l a r d i n i 1985) and Ru(bpy)CN^^~( B i g n o z z i 1986) and p o s i t i v e l y charged quenchers. t o
t i
o i
a
t o
ti
0
0
A
q
RESULTS and DISCUSSION 3 +
3 +
System I : I r ( Q 0 9 0 ) 3 - + C r ( b p y ) , C r ( p h e n ) ( D M F , u=0.01M TEAP) The d i f f u s i o n a l parameters have been e s t i m a t e d by n u m e r i c a l i n t e g r a t i o n o f t h e Debye-Smoluchowski, E i g e n - F u o s s e q u a t i o n s ( C h i o r b o l i 1986). They a r e r e p o r t e d i n T a b l e 1. The e m i s s i o n o f I r (QOSC^^ " i s quenched by C r ( b p y ) a c c o r d i n g t o I°/I= (1+ k x°[B])(1+K [B]). By u s i n g the e x p e r i m e n t a l v a l u e o f t h e quenching c o n s t a n t 3
3
3
3
3 +
3
q
A
obtained i n l a s e r
1 0
lifetime
1
measurements (k = 1 . 8 x l 0 M ^ s " ) the a s s o c i a t i o n 3-1 " c o n s t a n t i s o b t a i n e d as K =1.5xlO M. I n s i n g l e photon c o u n t i n g e x p e r i m e n t s , the e m i s s i o n decays a r e n o n e x p o n e n t i a l as shown i n F i g . l . A
Tt3
iu
ft
45
SfftTns
A
c : 8 x l O " M i y =0.01M TEAP.
The s h o r t e r component has a c o n s t a n t l i f e t i m e ( T * = 1 i 0.3 ns) and a r e l a t i v e weight t h a t i n c r e a s e s w i t h quencher c o n c e n t r a t i o n s . The l o n g e r component has a l i f e t i m e t h a t d e c r e a s e s w i t h quencher c o n c e n t r a t i o n s a c c o r d i n g t o a Stern-Volmer kinetic (k =1.8xl0 M ^ s " ) .The absence of a r t i f a c t s i n the appearence of the s h o r t component i s i n d i c a t e d by t h e s t r i c t l y monoexponential decay o b t a i n e d f o r s i m i l a r systems h a v i n g one o f t h e p a r t n e r s as an.uncharged s p e c i e s ( l r ( Q O S 0 ) ~ + benzoquinone, Ir(Q0) + C r ( b p y ) ) . L a s e r p h o t o l y s i s f a i l s t o g i v e any p r o o f f o r energy t r a n s f e r or k _ , a c o n d i t i o n t h a t makes i t p o s s i b l e t o d e t e c t s t a t i c quenching upon e x c i t a t i o n o f *A B ; ( i i i ) s t a t i c quenching shows up b o t h as a d e v i a t i o n o f I°/I from T°/T and as a double e x p o n e n t i a l decay. The s h o r t component o f t h e decay ( * A B = Ins) g i v e s a d i r e c t measurement o f k ( f e w examples o f d i r e c t measurements of k are a v a i l a b l e ( B a l l a r d i n i 1985)); ( i v ) the experimental v a l u e o b t a i n e d from I°/I vs T°/T i s h i g h e r than t h e c a l c u l a t e d one, presumably because o f compenetration e f f e c t s . a b
1 0
1
q
3
3
3
3 +
3
3
q
d
t o
d
t
t o
t Q
3
3 +
3 +
System I I : I r ( Q 0 S 0 ) ' + C o ( N H ) , C o ( e n ) ( H 0 , u=0.01 M NaCl) The e s t i m a t e d d i f f u s i o n a l parameters are r e p o r t e d i n Table 1. The e m i s s i o n decays i n both l a s e r and s i n g l e photon c o u n t i n g experiments a r e a p p r e c i a b l y monoexponential and g i v e k = 1.7x10 s~*. E m i s s i o n i n t e n s i t y quenching g i v e s s m a l l d e v i a t i o n of I°/l from Stern-Volmer b e h a v i o r , s m a l l e r than t h a t c a l c u l a t e d from I°/I=( l + k ° [B] ) ( 1 + K [ B ] ) w i t h the v a l u e s o f e x p e r i m e n t a l k and c a l c u l a t e d K . L a s e r p h o t o l y s i s f a i l s t o g i v e any p r o o f f o r energy or e l e c t r o n t r a n s f e r mechanism f o r t h e quenching. S i m i l a r r e s u l t s have been o b t a i n e d f o r t h e both quenchers. They can be i n t e r p r e t e d i n terms o f Scheme 3 as f o l l o w s : i ) t h e quenching mechanism i s o f t h e o u t e r - s p l i e r e t y p e ; ( i i ) t h e k v a l u e i s i n somewhat smaller than k , implying t h a t k^ *> °f same o r d e r of magnitude as k _ ( 1 x 1 0 s " ) t h i s makes i t d i f f i c u l t t o d e t e c t s t a t i c quenching e f f e c t s ; ( i i i ) i n f a c t , v e r y s m a l l s t a t i c quenching i s o b t a i n e d as d e v i a t i o n of I°/l from T°/T. 3
3
3
6
3
2
q
T
q
A
q
A
q
m a v
s
e
t n e
Q
d
1
d
;
2
3 +
System I I I :Ru(bpy)CN ~ + C r ( H 0 ) ( H 0 , y=0.6 M KN0 ) The e s t i m a t e d v a l u e s o f d i f f u s i o n a l parameters are r e p o r t e d i n T a b l e 1. Aqueous s o l u t i o n s c o n t a i n i n g Ru(bpy)CN and C r ( H 0 ) undergo slow a b s o r p t i o n s p e c t r a l changes i n d i c a t i n g c o o r d i n a t i o n of t h e complex t o the c y n i d e l i g a n d s Ru(bpy)CN " chromophore(Demas 1977, B i g n o z z i 1985, Scandola 1986) a c c o r d i n g t o : A
2
6
2
3
3 +
A
2
6
2
Zf
2
Ru(bpy)CN " A
+ Cr(H 0) 2
3 + 6
^ Ru(bpy)CN -Cr(R* 0) ++H 0 4
2
5
2
2
W i t h t h e same k i n e t i c s t h e e m i s s i o n i n t e n s i t y o f R u ( b p y ) C N ~ decreases w i t h t h e t i m e . E q u i l i b r a t e d s o l u t i o n s do not emit a p p r e c i a b l y . I t i s p o s s i b l e t o p e r f o r m quenching experiments on s o l u t i o n s where the adduct f o r m a t i o n i s n e g l e g i b l e . The e x p e r i m e n t s g i v e c o i n c i d e n t I°/I and T°/T Stern-Volmer p l o t s , y i e l d i n g k =1.6xl0 M s s m a l l e r than X j . L a s e r p h o t o l y s i s does not supply^ any e v i d e n c e f o r e l e c t r o n o r energy t r a n s f e r quenching mechanism. A c c o r d i n g t o Scheme 3, we can c o n c l u d e t h a t : ( i ) t h e system e v o l v e s w i t h the time from o u t e r - s p h e r e dynamic quenching to i n n e r - s p h e r e s t a t i c q u e n c h i n g ; ( i i ) the dynamic quenching has a k lower than t h e k v a l u e , i m p l y i n g t h a t k < k_ ( e s t i m a t e d v a l u e k = 4x10 s " ) ; ( i i i ) ^ t a k i n g t h e lower l i m i t i n g v a l u e from t h e l a c k of e m i s s i o n o f the adduct, k > 1 0 s . T h i s system c l e a r l y shows t h e l a r g e i n c r e a s e i n t r a n s f e r r a t e c o n s t a n t observed upon g o i n g from o u t e r t o i n n e r - s p h e r e mechanism. A
8
-1
_ 1
q
q
1
t o
d
t Q
ti
d
T a b l e 1: D i f f u s i o n a l parameters c a l c u l a t e d k^M'V
B
A
+
Ir(Q0S0,)2~ J Ir(QOS0 ) ~
Cr(bpy)
Ir(QOS0 )^~
Co(NH )^
Ir(QOS0 )^~
Co(en)
Ru(bpy)CN|~
Cr(H 0)^
3
3
3
3
3
3+ Cr(phen)
k. •d>
-1
K ,M
1
A
s
1.8X10
10
1 2X10
7
1.5X10
3
1.8X10
10
1 2X10
7
1.5X10
3
2.0X10
10
8 9X10
7
2.3X10
2
1.8X10
10
1 0X10
8
1.7X10
2
2 .0X10
9
A.3
3
3
2
1
f o r t h e i n v e s t i g a t e d systems.
+ 3
+
+
8.7X10
9
CONCLUSIONS The p o s s i b i l i t y o f measuring d i r e c t l y t h e r a t e c o n s t a n t o f t h e u n i m o l e c u l a r r e a c t i v e s t e p , t a k i n g advantage o f i o n i c o r d o n o r - a c c e p t o r a s s o c i a t i o n , i s r e s t r i c t e d w i t h i n a r a t h e r narrow window o f c o n d i t i o n s . I n f a c t , w i t h t h e " f a s t " C r ( l l l ) r e a c t a n t s t h e r e a c t i v e s t e p o f i o n p a i r i s a t t h e l i m i t o f e x p e r i m e n t a l d e t e c t i o n , whereas w i t h t h e C o ( I I l ) r e a c t a n t s t h e r e a c t i v e s t e p i s a l r e a d y t o o slow t o occur b e f o r e d i s s o c i a t i o n o f t h e e x c i t e d i o n p a i r . The l a r g e i n c r e a s e i n t h e r a t e o f u n i m o l e c u l a r r e a c t i v e s t e p expected i n g o i n g from t h e o u t e r - s p h e r e p r e c u r s o r complex t o t h e i n n e r - s p h e r e adduct i s c l e a r l y shown i n t h e l a s t system. REFERENCES - B a l l a r d i n i . R . ; V a r a n i , G . ; I n d e l l i , M . T . ; S c a n d o l a , F . J.Am.Chem.Soc.1986,25,3858. - B a l l a r d i n i , R . ; G a n d o l f i , M . T . ; B a l z a n i , V . Chem.Phys.Lett.1985,119,459. -Balzani,V.;Moggi,L.;Manfrin,M.F.;Bolletta,F.;Laurence,G.S. Coord.Chem.Rev.1975,15,321. - B a l z a n i , V . ; B o l l e t t a , F . ; G a n d o l f i , M . T . ; M a e s t r i , M . Top.Curr.Chem.1978,75,1. - B a l z a n i , V . ; S c a n d o l a , F . i n " P h o t o c h e m i c a l C o n v e r s i o n and Storage o f S o l a r Energy";J.S., Ed.; Academic P r e s s : New York,1981,Chapter 4,p.97. - B a l z a n i , V . ; S c a n d o l a , F . i n " E n e r g y Resources through P h o t o c h e m i s t r y and C a t a l y s i s " , Graetzel,M.,Ed.; Academic P r e s s : London,1983;Chapter1,p.1. -Bignozzi,C.A.;Roffia,S.;Scandola,F. J.Am.Chem.Soc.1985,107,1644. - B i g n o z z i , C . A . ; C h i o r b o l i , C . ; I n d e l l i , M . T . ; R a m p i Scandola,M.A.;Varani,G.;Scandola,F. J.Am.Chem.Soc.1986,108,7872. -Chiorboli,C.;Scandola,F.;Kisch,H. J.Am.Chem.Soc.1986,90,2211. -Curt i s , J . C . ; B e r g s t e i n , J . S . ; M e y e r ,T. J . Inorg.Chem.1985,24,385. -Demas,J.M.;Addington,J.W.;Peterson,S.H.;Harris,E.W. J.Phys.Chem.1977,81,1039. -Frank,R..;Rau,H. J.Phys.Chem. 1983,87,5181. -Rybak,W.;Haim,A,;Netzel,T.L.;Sutin,N. J.Phys.Chem.1981,85,2856. - S c a n d o l a , F . ; B i g n o z z i , C . A . ; B a l z a n i , V . in"Homogeneous and Heterogeneous P h o t o c a t a l y s i s " P e l l i z z e t t i , E . and Serpone,N.eds.; D.Reidel:Dordrecht,1986. - S u t i n , N . J.Photochem.1979,10,19. -Sutin,N.;Creutz,C. Pure Appl.Chem.1980,52,2717. -Sutin,N.;Creutz,C. J.Chem.Educ.1983,60,809. -Vogler,A.;Osman,A.H.;Kunkely,H. Coord.Chem.Rev.1985,64,159. -White,H.S.;Becker,W.G.;Bard,A.J. J.Phys.Chem.1984,88,1840.
PHOTOCHEMISTRY AND SPECTROSCOPY OF ION PAIR CHARGE TRANSFER COMPOUNDS
H.Hennig, D.Rehorek, and R . B i l l i n g S e k t i o n Chemie, K a r l - M a r x - U n i v e r s i t a t , L e i p z i g , GDR
Ion
pairs
charge
o f metal
transfer
Linhard
c h a r a c t e r i z e d by s p e c t r o s c o p i c i o n p a i r
(IPCT) t r a n s i t i o n s
(1944),
concerning
complexes
but until
the general
have been d e s c r i b e d
now t h e r e
behaviour
a r e no s y s t e m a t i c
of this
interesting
first
time by
investigations class
o f com-
pounds . Examples rare
o f IPCT compounds b a s e d
on m e t a l
( B a l z a n i 1 9 8 6 ) . We h a v e b e e n a b l e
complexes a r e s t i l l
t o prepare
compounds w h i c h a r e d i s t i n g u i s h e d by i n t e r e s t i n g photocatalytic ciates
behaviour.
Our i n v e s t i g a t i o n s
o fcopper(II)complexes
nylborate lates
(Hennig
1983;
with diphenyliodonium
photochemical and
concern
andc o b a l t ( I I I ) a m m i n e s
Rehorek
1979,
1980) as w e l l
c a t i o n s (Rehorek
very
some f u r t h e r I P C T
1979;
i o np a i r with
asso-
tetraphe-
as of cyanometalBilling
1985).
These compounds a r e d i s t i n g u i s h e d by s p e c t r o s c o p i c t r a n s i t i o n s visible
which
ming t h e i o np a i r s sfer
but which are t o consider
excitation
dox
r e a c t i o n s i n low-energy regions
(as
o f these
[Co(NH^) ]^
+
charge
tran-
b r u t t o quantum y i e l d
efficient
where t h e p a r e n t
photo r e -
complexes
alone
f o r i n s t a n c e ) show n o t any p h o t o
values
concerning
the formation
and Mo(V) ( 1 ) , ( 2 ) a r e due t o t h e f o r m a t i o n
tetraphenylbor radicals
bute
t overy
reactivity.
high
Co(ll)
t o overcome
>
{[Co(NH ) ] 3
6
; B
Systematic
back
2
+
2 +
3
investigations
6
4
*{Mo(CN) J 8
3
of
short-li-
which
contri-
processes.
*(co(NH ) l ; B 0 ' }
J-^—Z
of both
and d i p h e n y l i o d i n e r a d i c a l s electron transfer
4
{[Mo(CN) ] -;0 I 8
fast
0 i-ii_v:
4
ted
IPCT s t a t e s l e a d s
and [Mo(CN)gJ^~,
6
redox
ved
as i o n p a i r
for-
transitions.
The
The
i n the
c a n n o t b e e x p l a i n e d a s t h e sum o f t h e c o m p o n e n t s
• ....
;
~>
c a s e s o f BET
RuL
a 3
*
+
within
geminate
"charge of the,
radical
pair.
TMPD
If y o u f o c u s on t h e c h a r g e o f t h e m e t a l c o m p l e x , the p r o c e s s can be regarded as "charge s e p a r a t i o n " . And, i f y o u f o c u s on TMPD, the p r o c e s s c a n be? r e g a r d e d a s " c h a r g e r e c o m b i n a t i o n " . The whole? a s p e c t of t h e s e BET i s i n t e r p r e t e d t o f a l l b e t w e e n t h e t w o e x t r e m e s s o that t h e maximum o f k*, c o u l d b e o b s e r v e d a t a h i g h e r e x e r g o n i c i t y compared w i t h t h e p r e d i c t i o n a n t h e i n t r a m a 1 e c u 3. a r m o d e d e p & n d i n g F r a n c: k - C o n d o n i ntegral. It i s a pleasure to acknowledge a number of very illuminating c J i s c u s s i o n w i t h P r o f . N.Mataqa of Osaka U n i v e r s i t y and P r o f . U . S t e i n e r of Konstanz University. I thank Dr. T-Ulrich of Konstanz U n i ver s i t y f o r t r an s I at i n g t h & man usc r i p t . -
E i g e n M. ( 1 9 5 4 ) Z . p h y s . C h e m . NF 1, 1 7 6 - 2 0 0 . I n d e l l i M.T, B a l l a r d i n i R , S c a n d o l a F, ( 1 9 8 4 ) J . P h y s . C h e m . 8 8 , 2 5 4 7 - 5 1 . K a k i t a n i T, M a t a g a N, ( 1 9 8 5 ) J P h y s . Chem. 8 9 , 4 7 5 2 - 7 . Ohno T. K a t o S. Y a m a d a A, T a n n o T, ( 1 9 8 3 ) J . P h y s . C h e m . 8 7 , 7 7 5 - 8 1 .
PHOTOCHEMISTRY OF COPPER COMPLEXES AND ITS CATALYTIC ASPECTS
J.sykora Slovak T e c h n i c a l U n i v e r s i t y , Department o f I n o r g a n i c C h e m i s t r y , 812 37 B r a t i s l a v a , CSSR
Within t h e vehement development o f photochemistry o f c o o r d i n a t i o n compounds a n d i t s p h o t o c a t a l y t i c aspects i n t h e past t e n years / H e n n i g 1 9 8 5 / a p r o g r e s s w a s made a l s o i n t h e f i e l d o f c o p p e r p h o t o c h e m i s t r y , The presented c o n t r i b u t i o n summarizes and analyzes t h e c u r rent state i n t h e f i e l d o f photochemical behaviour o f copper complexes m a i n l y from t h e p o i n t o f view o f c a t a l y s i s . The h i t h e r t o known p h o t o c h e m i c a l r e a c t i o n s o f copper c o o r d i n a t i o n compounds a r e p r e s e n ted w i t h i n t h e c l a s s i f i c a t i o n suggested i n o u r laboratory. The state r e a c h e d i n some t y p e s o f c o p p e r p h o t o c h e m i c a l r e a c t i o n s i s i l l u s t r a ted b y purposefully selected representative examples; mainly r e s u l t s / a f t e r 1979/ n o t i n c l u d e d i n t h e l a t e s t review a r t i c l e i n t h e f i e l d / P e r r a u d i 1 9 8 1 / a r ep r e f e r r e d f o rd i s c u s s i o n i n t h i s l e c t u r e . C u / I / PHOTOCHEMISTRY The v a r i e t y o f p o s s i b i l i t i e s o f p h o t o c h e m i c a l C u / l / complexes b e h a v i o u r i s c o n d i t i o n e d b y t h enature o f p h o t o c h e m i c a l l y a c t i v e e x c i t e d s t a t e / s / : LIICT, LMCT, CTTS a n d i n t r a l i g a n d e x c i t e d s t a t e s . S o t h a t a many v a r i o u s r e d o x r e a c t i o n s were observed a s a consequence o f d e activation o f these excited states / e . g . photooxidation o f C u / l / met a l center accompanied b y formation o f solvated electron followed b y H2 p r o d u c t i o n , p h o t o i n d u c e d i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r etc./, A number o f p h o t o c h e m i c a l p r o c e s s e s where c o o r d i n a t i o n compounds o f C u / l / catalyze transformations o f t h eorganic substrates was reviewed r e c e n t l y b y Salomon /1983/ and Kutal /1985/. According t o K u t a l ' s c l a s s i f i c a t i o n o f o l e f i n photo-transformations i n the presence o f C u / l / complexes three types o f such phototransformation reactions can b e d i s t i n g u i s h e d Type A C u / l / ^ [ c u / I ^ - ^ [ c u / l / - o l e f i n f — * C u / l / + o l e f i n ' Type B C u / l / + o l e f i n ^ C u / l / - o l e f i n - ^ p u / l / - o l e f i n ] — • C u / I / - o l e f i n ' Type C o l e f i n ^ - * o l e f i n *[cu/I/-olefin]—*Cu/l/ + o l e f i n ' I t i so f i n t e r e s t t o note t h a t i n c o n n e c t i o n w i t h t h e p o s s i b l e a p p l i cation o f photoisomerisation o f olefins catalyzed b y C u / l / complexes
applicable rated
to the solar
a Cu/l/
le-light Cu/Il/
by
rapid
or
ground
active
state;
i s usually
observed
internal
Sakaki
state,
/1984/
working
marily
populated,
toactive
dublet
on p u b l i s h e d
chemistry
c a n be
S=solvent,
very
elabo-
under
visib-
/Sykora
I^S, L
dublet
dub-
accompanied
excited
state
states
are
state
CU[L]*
i s
state
conversion take
1982/ i n t o
pri-
t o t h e pho-
the Cu/Il/
three
ligand
photo-
place.
behaviour
=oxidized
transfer
i s often
excited
internal
excited
on photochemical
classified ligand
field
rapid
transfer
data
d-d
intraligand excited
usually
charge
ligand
charge
reaction
t o the lowest
respectively,
V/hen s p i n - a l l o w e d
spin-allowed
photoredox
conversion
inert,
where
problem
efficiently
PHOTOCHEMISTRY
excited
Based
storage
system
irradiation,
Photochemically let
energy
photocatalytic
classes
photoA, B, C
or solvent,
respecti-
ox vely. Class
A
Cu/ll/LS C U / I I / L
Cu/l/S S - ^ C U / I / L
2
— ^
2
C
Cu/Il/L
Cu/Il/L
h v
main
aspects
use
the data
nic
substrates.
a
X
—Cu/ll/
o
The
/Cervone
C l * also closing
+
Cu/l/
i n photochemical
those
S
+
was
concentrated
transfer
a
excited
by
of 02
i n the
results
laser
dioxygen often
to
orga-
oxidation
flash agent-
again
regarded i n this
of oxidation
i r r a d i a t e d under
of
state
i s of significance
the yields
cataly-
systems
by pulsed
powerful
oxidable
The p r e s e n c e
studies,
of these
evidenced
i s formed,
o f systems
the
i n order
of phototransformations
radical
to increase
upon
photochemistry
1979/. Besides
the c y c l e .
i t possible with
2
charge #
L ^ S
C u °
photosensitivity
o f C l ~t o C l
+
.
i n the field
observed
spin-allowed
compared
+
2
halogenocomplexes
of lowest
makes
L'
of our research
obtained
radical
ox
+
^ C u / I l / L Cu/I/L
of Cu/Il/
2
ox
» Cu/I/...1^
the oxidation
drawback
and
L +
attention
as C l
L
^ Cu/Il/L +
Cu/Il/L
tic
+
+ S*—^[Cu/Il/Lg, • £ ] — C u / l / L
2
2Cu/I/L Cu/I/L
thus
X
—iSU. C u / I l l / . . . L ~
Cu/l/...Lo
The
Cu/I/L * C u ° +
Class
Cu/Il/
Q
L
Cu/Il/L
-the
S
I '
B
photolysis
+
2
Cu/Il/LL;-^Cu/I/LL'+
Class
in
QX
C u / I l / L Cu/I/L
region
L
+
anaerobic
to as
case
products conditions.
T h e s e r e s u l t s l e d u s to s u g g e s t p h o t o a s s i s t e d c a t a l y t i c r e a c t i o n o f the C u / I l / - C u / I / redox cycle, which would render possible the o x i d a t i o n o f organic substrates j e . g . a l i p h a t i c alcohols /Sykora 1982/, u n s u b s t i t u t e d and a l k y l s u b s t i t u t e d phenols /Engelbrecht 1986, Sykora 1 9 8 6 / - P i g . 1 / s c h e m e suggested f o r p h o t o o x i d a t i o n o f p h e n o l s / .
o
2
I
Fig.
o
2
I
o
2
I
1
It was found t h a t s e l e c t i v i t y o f quinone f o r m a t i o n c a n be r e g u l a t e d by c o m p o s i t i o n o f t h e system / t h e c o m p o s i t i o n o f copper complexes present i n solution i s of great importance/* The p o s s i b i l i t i e s o f p r a c t i c a l u s e o f knowledge o n copper c o o r d i n a t i o n compounds photochemistry and p h o t o c a t a l y s i s i n t h e f i e l d o f p o l y m e r c h e m i s t r y , p h o t o s y n t h e s i s o f o r g a n i c a n d c o o r d i n a t i o n compounds!, s o l a r energy s t o r a g e , p h o t o e l e c t r o c h e m i s t r y and o t h e r s were reviewed r e c e n t l y / H e n n i g 19855 S y k o r a 1986j V/atanabe 1985j M c M i l l i n 1 9 8 5 / .
References Cervone E , Diomedi-Camassei P, Giannini I, Sykora J /1979/ Photoredox behaviour o f c h l o r o c o p p e r / l l / complexes i n a c e t o n i t r i l e : m e c h a n i s m a n d q u a n t u m y i e l d s • J P h o t o c h e m 11: 521-332 E n g e l b r e c h t P Thomas P h , H e n n i g H , Sykora J / 1 9 8 6 / P h o t o o x y g e n i e r u n g v o n P h e n o l i n G e g e n w a r t v o n C h l o r o c u p r a t e n . Z Chem 2 6 : 137 Perraudi G, Muralidharan S /1981/ Photochemical properties o f copper c o m p l e x e s . C o o r d Chem R e v s 36: 45-88 H e n n i g H , Rehorek D , A r c h e r RD / 1 9 8 5 / P h o t o c a t a l y t i c systems w i t h l i g h t - s e n s i t i v e c o o r d i n a t i o n compounds a n d p o s s i b i l i t i e s o f t h e i r s p e c t r a l s e n s i M z a t i o n - a n o v e r v i e w . C o o r d Chem R e v s 61: 1-53 KXrtal C /1985/ P h o t o c h e m i s t r y o f t r a n s i t i o n m e t a l - o r g a n i c systems. C o o r d Chem R e v s 6 4 : 1 9 1 - 2 0 6 M c M i l l i n E R , K i r c h h o f f J R , Goodwin KV /1985/ E x c i p l e x quenching o f p h o t o - e x c i t e d c o p p e r c o m p l e x e s . C o o r d Chem R e v s 64s 8 3 - 9 2 S a k a k i S, O k i t a k a I, Ohkubo K / 1 9 8 4 / T r a n s - c i s i s o m e r i z a t i o n o f s t i l bene photocatalyzed by c o p p e r / l / complexes. The f i r s t example o f copper/l/ photocatalysis efficient under visible-light irradiation. I n o r g C h e m 23: 1 9 8 - 2 0 3 f
Salomon RG /1983/ Homogeneous m e t a l - c a t a l y s i s i n organic p h o t o c h e m i s t r y . T e t r a h e d r o n 39s 4 8 5 - 5 7 5 Sykora J /1982/ Photochemical reactions o f copper complexes and t h e i r c a t a l y t i c a s p e c t s . Chem l i s t y 7 6 : 1 0 4 7 - 1 0 6 7 S y k o r a J , ICurekova* m , E n g e l b r e c h t P , Thomas P h , H e n n i g H / 1 9 8 6 / P h o t o o x i d a t i o n s c a t a l y z e d b y copper complexes-a new r o u t e o f p r e p a r a t i o n o f m o n o - a n d d i r a e t h y l b e n z o q u i n o n e s . P r o c Symp P o l y g r a f i a A c a demica'86 373-375 Sykora J , Jakubcovd M, Cvengrosova Z /1982/ Photooxidation effect o f cupric complexes on a l i p h a t i c alcohols i n nonaqueous s o l u t i o n s . C o l l e c t C z e c h C h e m C o m m u n 47: 2 0 6 1 - 2 0 6 8 Sykora J Sima J /1986/ Photochemistry o f coordination compounds. Veda, Bratislava #
V / a t a n a b e T , M a c h i d a IC, S u z u k i H , K o b a y a s h i M , H o n d a K / 1 9 8 5 / lectrochemistry -224
of metallochlorophylls.
C o o r d Chem R e v s 6 4 :
Photoe207-
INTRAMOLECULAR EXCITED STATE ELECTRON TRANSFER FROM NAPHTHALENE TO
A.H.Osman
COBALT(III)
and A . V o g l e r
I n s t i t u t f u r A n o r g a n i s c h e Chemie d e r U n i v e r s i t a t R e g e n s b u r g , U n i v e r s i t a t s s t r . 3 1 , 8400 R e g e n s b u r g , FRG
Introduction
The
majority
of intramolecular
photoredox processes o f metal
complexes
which
1 2) h a v e been r e p o r t e d excitation. excited
* ' takes place
As an a l t e r n a t i v e i n t r a m o l e c u l a r
state electron
u n d e r g o an e l e c t r o n intermolecular tor
t r a n s f e r . An e x c i t e d
defined
recent
electron t r a n s f e r occurs i n
environment. Although these features
make i t a t t r a c t i v e t o s t u d y
state electron transfer this
subject
largely
excited
state electron
transfer
i s asso-
a t t e m p t s t o u n d e r s t a n d t h e p r i m a r y e v e n t s o f p h o t o s y n t h e s i s and t o systems f o r t h e n a t u r a l
an e x c i t e d
state
light
into chemical
rapid
downhill
uphill
energy.
and an a r t i f i c i a l
electron transfer
photosynthesis.
i s required
In s i m p l e systems t h i s
charge recombination.
a c h i e v e d by i n t r o d u c i n g
first
linked covalently
be
attached
as e x c i t e d
t o a q u i n o n e as e l e c t r o n
acceptor.
as a d o n o r t o a c c o m p l i s h c h a r g e s e p a r a t i o n
T. J . M e y e r and h i s r e s e a r c h g r o u p h a v e i n v e s t i g a t e d separation
i n compounds w h i c h c o n t a i n I
n
t
n
e
s
e
c
a
s
e
s
i n order t o convert
step
i s followed
by a
a b a r r i e r f o r back e l e c t r o n t r a n s f e r . R e c e n t l y model
f o u n d much a t t e n t i o n c o n s i s t s o f a p o r p h y r i n is
In t h e f i r s t
In t h e p h o t o s y n t h e s i s a c h a r g e s e p a r a t i o n
p o u n d s h a v e been d e s i g n e d t o s t u d y t h e c h a r g e s e p a r a t i o n
phores 4)^
has been
a few y e a r s ago.
interest in intramolecular
d e s i g n model step
o f t h e same c o m p l e x . W h i l e i n
i s n o t known i n t r a m o l e c u l a r
excited
neglected u n t i l
ciated with
chromophoric group o f a complex can
p h o t o r e d o x p r o c e s s e s t h e s t r u c t u r a l a r r a n g e m e n t o f d o n o r and a c c e p -
intramolecular
The
c h a r g e t r a n s f e r (CT)
p h o t o r e d o x p r o c e s s e s may o c c u r by an
t r a n s f e r t o o r from another p a r t
i n the encounter pair
a better
upon d i r e c t o p t i c a l
metal
in detail.
A system
state electron
donor
is comwhich which
In a d d i t i o n , a c a r o t e n e 3) over
large
distances
the light-induced
c o m p l e x e s as i n i t i a l l y
may
excited
charge chromo-
t h e c h a r g e r e c o m b i n a t i o n r e g e n e r a t e d t h e s t a r t i n g com-
pounds. Under s u i t a b l e c o n d i t i o n s compete w i t h
the
a n o t h e r s e c o n d a r y r e a c t i o n may
c h a r g e r e c o m b i n a t i o n . As
a result
stable
be
rapid
enough
p h o t o p r o d u c t s can
to
be
formed. In
1969
ligand
Adamson e t
( I L ) e x c i t a t i o n of
carboxylate releases of
a l . studied
the
excited
i t s ligands
[Co
other complexes of the naphthyl
Excited
state electron
(NH ) TSC] 3
an
efficient
type
the
also
as
an
1
. Upon
electron
2
to
Co(III)
^.
same b e h a v i o r as
the
TSC
The A
variety
group 7,8)
complex
Co(III)
Co(II)
place.
R = aromatic
1
intra-
trans-4-stilbene
t r a n s f e r from aromatic molecules t o
intermolecular an
=
[Co** (NH^OOCR] " " w i t h
shows q u a l i t a t i v e l y t h e
assumption that
an
TSC"
type
charge recombination takes
8 place
of t h i s
with
2 +
5
TSC-ligand transfers
before
s u c h as
i n
a photoreaction
ammines
takes
9)
reaction
'
. First
energy t r a n s f e r occurs to
observations
r e a c t i v e CT
were e x p l a i n e d
states
of the
complex
9) H o w e v e r , more r e c e n t
i n v e s t i g a t i o n s h a v e shown t h a t
explained
b e s t by
an
In t h e
present
study the
with
n = 1 to
excited
complexes
5 were i n v e s t i g a t e d
requirements f o r excited
Results and
state electron
a l l results
t r a n s f e r mechanism
"
can
2
in order to
state electron
n
l e a r n more a b o u t t h e
transfer
in t h i s
be
.
[2-naphthyl-C0NH-(CH ) -C00Co
I H
(NH ) ] 3
2 +
5
structural
system.
Discussion
Synthesis
The
free
benzyl
ligands
esters
were s y n t h e s i z e d
of the
amino
by
sodium s a l t s
3
5
the
protonated
2
n
2
n
2
ligands +
2
n
2
2
3
4
3
from acetone y i e l d e d
5
and
2 +
n
were o b t a i n e d
the
sodium s a l t s
analytically
pure
as
6
6
and
the
5
compounds.
H0 2
complexes
perchlorates
of the
+
5
w h i c h w e r e c o n v e r t e d by
2 - n a p h t h y l - C 0 - N H - ( C H ) - C 0 0 ~ N a . The
[Co(NH ) H 0](C10 )
2-naphthoic acid
NH -(CH ) -C00CH -C H 2
C0NH-(CH ) -C00Co(NH ) ] 2
of
2-naphthyl-C0-NH-(CH ) -C00-CH -C H
Saponification yielded the
reaction
acids:
2-naphthyl-C00H + -
the
ligands.
by
the
NaOH t o
[2-naphthylreaction
by
of
Recrystallization
Absorption Spectra
The
electronic
s p e c t r a o f t h e sodium
thyl-C0-NH-(CH ) -C00~Na 2 0 0 nm a n d a 1-cm
3
cell.
The
absorption
bands a t x
m
a
spectrum o f [ T l ( b i p y ) I ] 2
= 374 nm (
x
i n CH CN ( F i g . 1) c o n s i s t s o f 3
+
2
3
= 4 7 0 0 ) , 302 nm ( 2 3 2 1 0 ) ,
e
and 244 nm ( 2 8 2 0 0 ) .
w a v e l e n g t h b a n d a t 374 nm may be due t o a J " -* T l ( I I I ) [T1I ]"
shows s u c h an a b s o r p t i o n
4
chemical
behavior
of [ T l ( b i p y ) I ] 2
a s s i g n m e n t . As a r e a s o n a b l e bipy
assigned
this
x
( s e e below)
+
2
= 7000)
e
to the ™ *
8
)
* \
spectral
(
LMCT t r a n s i t i o n
absorption
changes
a
type
could
x
this
[Be(bipy)1^3
shows s u c h a LLCT
a t 302 nm
+
2
of the bipy
to a J" -
should
l i g a n d which absorbs i n
also contribute to this
band
a r e expected t o appear near t h i s
a t 244 nm i s c e r t a i n l y behavior
a I
wave-
- * T 1 ( I I I ) LMCT band
of L T 1 ( b i p y ) I ] . +
2
> 2 0 0 nm) o f [ T l ( b i p y ) I l
x m
The c o m p l e x
(IL) t r a n s i t i o n
agreement w i t h t h e photochemical Irradiation
i s not consistent with
2
intraligand
The t h i r d
5
. The s e c o n d band o f [ T l ( b i p y ) I ]
low-energy absorptions' o f t h i s
length
since
a l t e r n a t i v e t h e band a t 374 nm may be a s s i g n e d
r e g i o n . An I " - T l ( I I I )
since
in
a
LMCT t r a n s i t i o n
= 397 nm ' ^ . However, t h e p h o t o 4
m
l i g a n d t o l i g a n d ( L L ) CT t r a n s i t i o n .
band a t 3 6 8 nm ( be
at x
The l o n g e s t
2
2
+
2
i n CH CN was a c c o m p a n i e d by 3
( F i g . 1) w h i c h a r e c o n s i s t e n t w i t h
a reductive elimination according
to:
[Tl
In t h e p h o t o l y z e d
(bipy) I ] 2
2
+
- T l
+
+ 2 bipy + I
J
a s u p e r i m p o s i t i o n o f a b s o r p t i o n maxima o f I
(x = 280 nm). The p r e s e n c e o f r e l e a s e d b i p y max
by b a n d s a t x
quantum y i e l d s
302
nm, a n d 2 . 9 x 1 0 The
(
=
x m
a
i s also
bands o f [ T l ( b i p y ) I 2
o f t h e r e d u c t i v e e l i m i n a t i o n (* = 0.54 a t x
photochemistry
2
x
indi-
= 2 3 5 and 243 nm.
In a c c o r d a n c e w i t h t h e a s s i g n m e n t s o f t h e a b s o r p t i o n the
2
2
nm) a n d f r e e b i p y ^
cated
H
s o l u t i o n t h e a b s o r p t i o n maximum a t 360 nm i s c a u s e d by I . The new
b a n d a t 2 8 6 nm i s a p p a r e n t l y 289
I
a t 366 nm) d e c r e a s e d w i t h
i r r
2
]
+
= 254 nm, 0 . 0 3 a t
i n c r e a s i n g wavelength o f i r r a d i a t i o n .
o f some t h a i 1 i u m ( I I I ) c a r b o x y l a t e s o f t h e t y p e
Tl(RC00)
3
where
2) R i s a larger these
aliphatic
g r o u p was s t u d i e d by K o c h i
compounds i n b e n z e n e s o l u t i o n
carboxylate
(RC00" - e" - R • + C 0 ) . 2
and B e t h e a
ledt o the formation
;
. The p h o t o l y s i s o f
o f T l ( I ) and o x i d a t i o n o f
In t h e p r e s e n t not
w o r k we i n v e s t i g a t e d t h e p h o t o l y s i s o f T 1 ( C H C 0 0 ) 3
d i s s o l v e i n benzene. In t h e s o l i d
three chelating acetate
ligands
partially
substituted
generally
less e f f i c i e n t
out
dissociation.
state Tl(III)
. I n aqueous
Since
i sessentially
solution the acetate
in acetonitrile
hexacoordinated
by
ligands are
such l i g a n d s u b s t i t u t i o n s a r e
i t is*assumed t h a t T1(CH C00) 3
The a b s o r p t i o n
which does
3
d i s s o l v e s i n CH^CN w i t h -
3
spectrum of T1(CH C00) 3
3
in acetonitrile
( F i g . 2)
abs
1.4
t
1
—
200
300
500 nm
400
Fig. 2 Absorption i n CH CN, x 3
is
spectra of 9.9x10" i
r
> 2 0 0 nm, 1-cm
r
M [ T l ( C H C O O ) ] ( a ) and i t s p h o t o l y s i s p r o d u c t ( b )
5
3
3
cell
r a t h e r f e a t u r e l e s s . The c o m p l e x s t a r t s
tion
r e g i o n . The
absorp-
i n c r e a s e s t o w a r d s s h o r t e r w a v e l e n g t h . A maximum a p p e a r s a t 2 0 3 nm (e = 1 2 4 3 0 )
while can
t o absorb i n t h e v i s i b l e
shoulders
certainly The
acetate
a t 2 4 0 nm ( e = 6 4 4 0 ) a n d 3 9 5 nm ( e = 1 6 6 0 ) . T h e s e
be a s s i g n e d
spectral
pressure
occur
t o acetate -
Tl(III)
LMCT
transitions.
changes which accompanied t h e i r r a d i a t i o n
mercury a r c ) o f T1(CH C00) 3
3
(white
light
i n CH CN i n d i c a t e d t h e f o r m a t i o n
( F i g . 2 ) w h i c h shows a b s o r p t i o n
absorptions
3
bands a t x
from a h i g h of T l ( I )
= 2 5 6 nm a n d 2 1 4 nm. A l t h o u g h max
the tion
o x i d a t i o n products takes
place
were n o t i d e n t i f i e d
according
t o the equation:
i t i s assumed t h a t t h e r e d u c t i v e e l i m i n a -
T1
I I I
(CH C00) 3
The
methyl
254
nm t h e q u a n t u m y i e l d
T1 (CH C00) + 2 C0 I
3
3
2
+ 2 CH^
r a d i c a l s may u n d e r g o d i m e r i z a t i o n o r o t h e r s e c o n d a r y
d r o p p e d t o * = 0.01 a t x.
reactions. At x
f o r t h e d i s a p p e a r a n c e o f T l ( C H ^ C O O ) ^ was * = 0 . 1 4 . I t = 395 nm.
References 1) V o g l e r , A.; Q u e t t , C ; P a u k n e r , A.; K u n k e l y , H. J . Am. Chem. S o c . 1 9 8 6 , 1 0 8 , 8263. 2 ) K o c h i , J . K.; B e t h e a , T. W. J . O r g . Chem. 1 9 6 8 , 3 3 , 7 5 . 3 ) S a g i , S. R.; P r a k a s a R a j u , G. S.; Appa Rao, K.; P r a s a d a Rao, M. S. T a l a n t a 1 9 8 2 , 2 9 , 4 1 3 and r e f e r e n c e s c i t e d t h e r e i n . 4 ) M a t t h e w s , R. W.; W a l t o n , R. A. J . Chem. S o c . A 1968, 1 6 3 9 . 5 ) Day. P.; S e a l , R. H. J . Chem. S o c . D a l t o n 1972, 2054. 6 ) B e c k , W.; F e h l h a m m e r , W. P.; P o l l m a n n , P.; S c h u i e r e r , E.; F e l d l , K. Chem. B e r . 1967, 1 0 0 , 2 3 3 5 . 7 ) Sutton7~~G. J . A u s t r . J . S c i . R e s . , A 1 9 5 1 , 4, 6 5 4 . 8) C o a t e s , G. E.; G r e e n , S. I . E. J . Chem. S o c . 1 9 6 2 , 3 3 4 0 . 9 ) F a g g i a n i , R.; B r o w n , I . D. A c t a C r y s t . B 1 9 7 8 , 3 4 , 2 8 4 5 . 10) L e e , A. G. The C h e m i s t r y o f T h a l l i u m , E l s e v i e r A m s t e r d a m 1 9 7 1 .
TOPIC 5 Organometallic Photochemistry
PHOTOPHYSICS AND
PHOTOCHEMISTRY OF
TUNGSTEN CARBYNE COMPLEXES
A . B . B o c a r s l y * , R.E.Cameron, A . M a y r * , and Department o f C h e m i s t r y , P r i n c e t o n
G.A.McDermott
University, Princeton,
NJ
08544,
USA
T r a n s i t i o n m e t a l complexes t h a t luminesce a t room temperature i n f l u i d s o l u t i o n upon e x c i t a t i o n w i t h v i s i b l e l i g h t have a t t r a c t e d much a t t e n t i o n s i n c e such s p e c i e s have l o w - l y i n g e x c i t e d s t a t e s which may a l l o w the u t i l i z a t i o n o f o p t i c a l energy i n the p r e p a r a t i o n o f u s e f u l chemical p r o d u c t s . Despite t h i s i n t e r e s t only few such s p e c i e s have been d i s c o v e r e d . I n most cases luminescence i s a s s o c i a t e d w i t h a charge t r a n s f e r t r a n s i t i o n i n v o l v i n g m e t a l d7r e l e c t r o n s and the TT* o r b i t a l of l i g a t e d a r o m a t i c d i i m i n e s . Here we d e s c r i b e some o f the p h o t o p h y s i c a l and photochemical p r o p e r t i e s of bis-donor l i g a n d - s u b s t i t u t e d tungsten arylcarbyne complexes [(W=CAryl)X(CO) L2] ( X - h a l i d e , L»donor l i g a n d ) ( F i s c h e r 1977), a new c l a s s o f l u m i n e s c e n t o r g a n o m e t a l l i c s p e c i e s ( B o c a r s l y 1985). The e m i s s i v e e x c i t e d s t a t e i s a s s o c i a t e d w i t h the lowest energy a b s o r p t i o n band, which i s a s s i g n e d to a d-metal t o 7r*(M=CAryl) t r a n s i t i o n . Quenching experiments i n d i c a t e t h a t a s i g n i f i c a n t amount o f t r i p l e t c h a r a c t e r i s a s s o c i a t e d w i t h the e m i s s i v e e x c i t e d s t a t e . B i m o l e c u l a r o x i d a t i v e and r e d u c t i v e charge t r a n s f e r quenching i s a l s o observed, d e m o n s t r a t i n g the e x c i t e d s t a t e to be s t r o n g l y r e d u c i n g and o x i d i z i n g . Photoinduced a s s o c i a t i v e l i g a n d s u b s t i t u t i o n s occur i n these m o l e c u l e s (Cameron 1986). 2
ELECTRONIC ABSORPTION SPECTRA The s p e c t r o s c o p i c p r o p e r t i e s o f t u n g s t e n carbyne complexes o f the type [(W=CR)X ( C O ) L ] are l i s t e d i n Table 1. As a c h a r a c t e r i s t i c example the spectrum o f [(W=CPh)Br(CO) (tmeda)] i s shown i n F i g . 1. The e l e c t r o n i c a b s o r p t i o n s p e c t r a f o r t u n g s t e n p h e n y l c a r b y n e complexes o f the type [ (WsCPh)X(CO) L ] , where X - C l , Br, I and L - 2 p y r i d i n e ( p y ) , t e t r a m e t h y l e t h y l e n e diamine (tmeda), and b i s d i p h e n y l phosphinoethane (dppe) , e x h i b i t a f a i r l y weak a b s o r p t i o n a t about 450 nm and a more i n t e n s e a b s o r p t i o n a t about 350 nm. The lowest energy a b s o r p t i o n s are assigned to d -• 7r(M=C)* t r a n s i t i o n s ( z - a x i s c o i n c i d i n g w i t h M^C bond a x i s ) . For p h e n y l c a r b y n e t u n g s t e n complexes the 7r(M=C)* o r b i t a l i s c o n j u g a t e d w i t h the 2
2
2
2
2
2
x y
T a b l e 1.
E l e c t r o n i c A b s o r p t i o n and Emission Data.
[(W^CR)X(CO) L ] R X L 2
2
2
A Ph Ph Ph Ph Ph Ph CMe
3
CI Br I CI Br CI CI CI
tmeda tmeda tmeda 2py 2py dppe tmeda 2py
448 450 454 460 460 435 364 475
(393) (400) (560) (1064)* (1086)* (368) (617)
*(M«C) - 7r(M=C)* [nm], ( e l M - W ] )
Emission
1
m a x
330 327
(5500) (13000)
340 (17000)+ 350*+ 360 (10000 270 (8000) 354+
*shoulder "•"solvent dependent, v a l u e s i n d i c a t e d are i n n o n p o l a r s o l v e n t s .
640 630 630 625 630 660 660
200
300
400
500
WAVELENGTH
600
700
800
550
600
(nm)
650
WAVELENGTH
tl Ik!
700
750
(nm)
F i g u r e 1. E l e c t r o n i c A b s o r p t i o n Spectrum and E m i s s i o n Spectrum o f [(WCPh)Br(C0) (tmeda)]. 2
7r-system o f the p h e n y l group. M o l e c u l a r o r b i t a l c a l c u l a t i o n s by K o s t i c and Fenske (1982) on the r e l a t e d compound [ ( C r - C P h ) C l ( C 0 ) ] i n d i c a t e t h a t t h i s c o n j u g a t i o n i s s i g n i f i c a n t . A s i m i l a r assignment has been s u g g e s t e d b y V o g l e r (1983) f o r [(Os«CPh)Cl(CO)(PPh ) ]; a l t h o u g h t h i s complex has a d i f f e r e n t geometry from the t u n g s t e n systems, t h e l o w e s t l y i n g e x c i t e d s t a t e seems t o be s i m i l a r i n nature. Replacement o f the phenyl group by a t e r t - b u t y l group r e s u l t s i n a s h i f t o f the l o w energy a b s o r p t i o n band toward the b l u e b y about 85 nm. T h i s i s c o n s i s t e n t w i t h removal o f the c o n j u g a t i o n from the LUMO o r b i t a l , TT(M=C)*. On the o t h e r hand, replacement o f the p h e n y l group w i t h a 2-naphthyl group l e a d s t o a r e d s h i f t by about 15 nm, i n d i c a t i n g an e x t e n s i o n o f t h e c o n j u g a t e d system. The n a t u r e o f the donor l i g a n d s a l s o s i g n i f i c a n t l y i n f l u e n c e s the low energy a b s o r p t i o n . R e p l a c i n g tmeda b y two monodentate p y r i d i n e l i g a n d s causes a s h i f t toward the r e d by about 10 nm, r e p l a c i n g tmeda by t h e phosphorous-based dppe l i g a n d causes a b l u e s h i f t b y about 15 nm. Assuming t h a t the energy o f TT(MKC)* remains more o r l e s s u n a f f e c t e d by these l i g a n d s u b s t i t u t i o n s , the o b s e r v e d changes i n a b s o r p t i o n e n e r g i e s may be e x p l a i n e d by a weak 7r-donor a b i l i t y o f the p y r i d i n e and a weak 7r-acceptor a b i l i t y o f dppe l i g a n d s , r e s p e c t i v e l y . R e p l a c i n g tmeda by 2 , 2 ' - b i p y r i d i n e o r by phenant h r o l i n e causes a s i g n i f i c a n t r e d s h i f t (38 nm) i n the l o w e s t a b s o r p t i o n band o f the t u n g s t e n carbyne system. However, by comparison w i t h the a b s o r p t i o n spectrum o f [ W ( C 0 ) ( b p y ) ] t h i s e l e c t r o n i c t r a n s i t i o n i s b e l i e v e d t o c o n t a i n a l a r g e component o f m e t a l d -+ bpy 7r* c h a r g e - t r a n s f e r c h a r a c t e r . V a r i a t i o n o f the h a l i d e l i g a n d t r a n s t o t h e carbyne has o n l y a minor e f f e c t on the p o s i t i o n o f the low energy abs o r p t i o n , r u l i n g out these o r b i t a l s as the HOMO and f u r t h e r s u g g e s t i n g t h a t t h e h a l i d e p - o r b i t a l s ( l o n e p a i r s ) are n o t s i g n i f i c a n t l y i n t e r a c t i n g w i t h the metal c e n t e r i n these t u n g s t e n carbyne complexes. 4
3
2
4
The more i n t e n s e a b s o r p t i o n a t about 350 nm f o r [ ( W * C P h ) X ( C 0 ) L ] i s a s s i g n e d t o 7r->7r* t r a n s i t i o n s i n the c o n j u g a t e d M*CPh systems i n analogy t o the absorption i n d i p h e n y l a c e t y l e n e (295 nm, 29000 cm" M * ) . I n cases where t h e l i g a n d s L cont a i n a 7r* system, f o r example L - p y r i d i n e , t h e r e i s a l s o a component o f the abs o r p t i v i t y i n t h i s r e g i o n a s s o c i a t e d w i t h an MLCT t r a n s i t i o n . 2
1
2
1
LUMINESCENCE IN ROOM TEMPERATURE FLUID SOLUTION S e v e r a l t u n g s t e n p h e n y l c a r b y n e complexes o f the type [(W*CPh)X(CO) L ] have been found t o l u m i n e s c e a t room temperature i n f l u i d s o l u t i o n upon e x c i t a t i o n w i t h v i s ible light. The e m i s s i o n spectrum o f [(W»CPh) B r ( C 0 ) ( t m e d a ) i s shown i n F i g . 1. I t i s t y p i c a l f o r t h i s c l a s s o f compound, b e i n g s t r u c t u r e l e s s b o t h a t room temper a t u r e and a t 77K i n a f r o z e n g l a s s . T h i s i s i n d i c a t i v e o f a l a r g e degree o f v i b r a t i o n a l c o u p l i n g s i m i l a r t o t h a t observed f o r the s o l u t i o n e m i s s i o n o f [ R u ( b p y ) ] t 2
2
2
2
3
We a s s o c i a t e the luminescence w i t h the l o w e s t energy a b s o r p t i o n band a t 450 nm based on the o b s e r v e d o v e r l a p o f the e m i s s i o n onset w i t h the low energy t a i l o f the 450 nm a b s o r p t i o n band. A l a r g e Stokes s h i f t on the o r d e r o f 180 nm i s seen by a l l t h e s e e m i s s i v e complexes. T h i s may be due t o s i g n i f i c a n t s t r e t c h i n g o f the m e t a l - c a r b o n t r i p l e bond and/or bending o f the carbyne l i g a n d i n the e x c i t e d s t a t e . The quantum y i e l d o f e m i s s i o n ( $ ) f o r the t u n g s t e n p h e n y l c a r b y n e complexes i s i n the range o f 10" i n room temperature f l u i d s o l u t i o n , Table 1. Irradiation i n t o the h i g h e r energy a b s o r p t i o n peaks a l s o l e a d s t o e m i s s i o n from the ?r(M=C)* o r b i t a l o f t h e s e complexes. A p p a r e n t l y , e f f i c i e n t n o n r a d i a t i v e c o u p l i n g between these h i g h e r energy s t a t e s and the LUMO o r b i t a l e x i s t s . Consistent with t h i s res u l t i s the f i n d i n g t h a t the r a d i a t i v e quantum y i e l d f o r t h i s system i s wavel e n g t h - i n d e p e n d e n t f o r a l l wavelengths t e s t e d . E
3
V a r i a t i o n o f t h e carbyne s u b s t i t u e n t R has a s t r o n g i n f l u e n c e on the e m i s s i o n . I f the p h e n y l group i s r e p l a c e d by a t e r t - b u t y l group, the complex i s found not to emit i n f l u i d s o l u t i o n . Absence o f f l u i d s o l u t i o n e m i s s i o n under t h e s e c o n d i t i o n s f u r t h e r s u g g e s t s t h a t c o n j u g a t i o n o f the m e t a l - c a r b o n t r i p l e bond and the p h e n y l ?r* system i s e s s e n t i a l t o o b t a i n a p p r e c i a b l e r a t e c o n s t a n t s f o r r a d i a t i v e decay. Replacement o f the p h e n y l group w i t h a 2-naphthyl group l e a d s t o an extended m e t a l - c a r b y n e 7r* system and c o r r e s p o n d i n g l y to a r e d s h i f t i n the e m i s s i o n band. The n a t u r e o f the donor l i g a n d s L has o n l y a s m a l l i n f l u e n c e on the o b s e r v e d l u minescence p r o p e r t i e s , p r o v i d e d the l i g a n d s do not c o n t a i n low l y i n g TT* o r b i t a l s themselves. As w i t h the a b s o r p t i o n s p e c t r a , v a r y i n g the h a l i d e l i g a n d t r a n s t o the carbyne has o n l y a minor e f f e c t on the e m i s s i o n band. Donor l i g a n d s w i t h low l y i n g TT* o r b i t a l s s t r o n g l y a f f e c t the e m i s s i o n . I n t r o d u c t i o n o f 2 , 2 ' - b i p y r i d i n e l e a d s t o t o t a l quenching o f the f l u i d s o l u t i o n e m i s s i o n . W i t h 1,10-phenanthrol i n e an e m i s s i v e complex i s o b t a i n e d , however, the luminescence i s s i g n i f i c a n t l y red-shifted. F o r t h e s e complexes, the l o w e s t energy e x c i t a t i o n appears t o be the x y "* * ( ^ ) c h a r g e - t r a n s f e r t r a n s i t i o n . C o n s i s t e n t w i t h t h i s assignment i s the o b s e r v a t i o n t h a t the e m i s s i o n o f the p h e n a n t h r o l i n e complex i s b l u e s h i f t e d I n more p o l a r s o l v e n t s , u n l i k e the o t h e r t u n g s t e n p h e n y l c a r b y n e e m i s s i o n s which show no s o l v e n t dependence. d
n
2
The e x c i t e d s t a t e l i f e t i m e s o f a l l l u m i n e s c e n t t u n g s t e n carbyne complexes were a n a l y z e d and a r e l i s t e d i n T a b l e 2. The r e l a t i v e l y l o n g l i f e t i m e s found f o r the e m i s s i v e e x c i t e d s t a t e suggest the t r a n s i t i o n t o the ground s t a t e i s f o r b i d d e n . T a b l e 2.
E m i s s i o n Quantum Y i e l d s , E m i s s i o n L i f e t i m e s , and R a d i a t i v e Rate C o n s t a n t s i n Toluene a t 298K.
[(Tr*)C0 and M(diT)-KTT*) R 5 t r a n s i t i o n s a r e thought to be c l o s e i n energy and may c o n t r i b u t e s u b s t a n t i a l l y to t h e e x c i t e d s t a t e c h a r a c t e r [12-14], E m i s s i o n l i f e t i m e s were o b s e r v e d t o be s h o r t e r than 1 ns f o r each complex, c o n s i s t e n t w i t h an e x c i t e d s t a t e t h a t c o n t a i n s LF c h a r a c t e r which can d e a c t i v a t e r a p i d l y i n room-temperature s o l u t i o n v i a e f f i c i e n t n o n r a d i a t i v e processes. The p h o t o p h y s i c a l c h a r a c t e r i s t i c s o f these systems a r e c u r r e n t l y b e i n g f u r t h e r i n v e s t i g a t e d . 5
c
H
ACKNOWLEDGEMENT. We a r e g r a t e f u l to the P e t r o l e u m Research Fund, a d m i n i s t e r e d by t h e American Chemical S o c i e t y , f o r t h e i r support o f t h i s r e s e a r c h .
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
A . J . Lees, Chem. Rev. (1987) i n p r e s s . S. Chun, E.E. G e t t y and A.J. Lees, I n o r g . Chem. 2 3 , 2155 (1984). A . J . Lees, J.M. Fobare and E.F. M a t t i m o r e , I n o r g . Chem. 2 3 , 2709 (1984). R.M. K o l o d z i e j and A.J. Lees, O r g a n o m e t a l l i c s 5 , 450 (1986). D.M. Manuta and A . J . Lees, I n o r g . Chem. 2 5 , 1354 (1986). P.C. S e r v a a s , H.K. v a n D i j k , T.L. S n o e c k , D.J. S t u f k e n s and A. Oskam, I n o r g . Chem. 2 1 , 4494 (1985). P . J . Giordano and M.S. W r i g h t o n , I n o r g . Chem. 16, 160 (1977). D.M. Manuta and A . J . L e e s , I n o r g . Chem. 2 5 , 3212 (1986). P.J. G i o r d a n o , S.M. F r e d e r i c k s , M.S. W r i g h t o n and D.L. M o r s e , J . Am. Chem. S o c . 100, 2257 (1978). P . J . Giordano and M.S. W r i g h t o n , J . Am. Chem. Soc. 101, 2888 (1979). J . Van Houten and R.J. Watts, J . Am. Chem. Soc. 9 8 , 4853 (1976). G.L. G e o f f r o y and M.S. Wrighton i n " O r g a n o m e t a l l i c Photochemistry," Academic P r e s s : New York, 1979. N.J. Gogan, C.-K. Chu, J . Organomet. Chem. 9 3 , 363 (1975). D.L. L i c h t e n b e r g e r and R.F. Fenske, J . Am. Chem. Soc. 9 8 , 50 (1976).
PHOTOCHEMICALLY INDUCED C-C-BOND FORMATION IN THE COORDINATION SPHERE OF TRANSITION METALS
C.G.Kreiter
and K . L e h r
F a c h b e r e i c h Chemie d e r U n i v e r s i t a t
Unsaturated when
hydrocarbons a l t e r
coordinated
possibilities potential
of
thoroughly
to
transition
f o r the readily
hydrocarbons
induced
i n the
have
shown
irradiation
in
reactivity
metals
and
hydrocarbon
interesting
Nevertheless,
the s y n t h e t i c
complexes type
i s f a r from
years
ago,
that
(A
+
related
reaction to
behaviour.
principle
is
[Cr(CO)3(n&-C7He)], 1,3-Cyclopentadiene
n6-1,3,5-cycloheptatriene
the
C5H5)(n3-C5H7 ) ] high
yields.
cycloadduct,
but
s y s t e m 2.
spiro 2
and
conjugated
dienes
6 ] - c y c l o a d d i t i o n , forming
f»6-bi-
however.
cyclic
a
ligand
[Cr(CO)3(n6-C8Hs)],
respectively
[6+2]-cycloadduct i s
not
shifts
n6-1,3,5-cycloheptatriene
complete
the
reaction
the
formed
.
complexes, a
different substitute
[Cr(CO)2(n5-
are
formed
expected
with
1
an
in
[6+4]-
uneffected
tricarbonyl-n4 : 2-tricyclo-
1,3-Cyclohexadiene y i e l d s
2
show
photochemically.
7
to the
Other
dienes
[6.3.2.0 > ]trideca-3,5,9-triene-chromium(0). tion
complexes
or 1,3,5,7-cyclooctatetraene
spiro[4.4]nona-l,3-diene
With
free
tricarbonyl-n&-1,3,5-
acyclic,
limited and
and
metals.
cyclo[4.4.1]undeca-2,4,8-triene-tricarbonyl-chromium(0)
This
being
of r e a c t i o n s i s t h e
of t r a n s i t i o n
reacts with
a smooth
fundamentally,
f o r m a t i o n between complexed
c o o r d i n a t i o n sphere several
FRG
offer
used. A p r o m i s i n g
C-C-bond
cycloheptatriene-chromium(0) upon
their
organic synthesis. available
i n v e s t i g a t e d and
photochemically
We
K a i s e r s l a u t e r n , 6750 K a i s e r s l a u t e r n ,
After
ligand,
a
metal
[4+2]-cycloaddiassisted
1,5-H
.
Tricarbonyl-n6-1,3,5-cyclooctatriene-chromium(0) n 6 - 1 , 3, 5, 7-cyclooctatetraene-chromium(0)
do
not
or
react
tricarbonylwith
dienes
at a l l .
the
complexes
to
photochemically
the
complexes v e r y
closely
homologue
substitute Only
In
[Mo(C0)3(n©-C7Ha)]
photochemically
[W(C0)3(n*-C7Ha)],
and
[Cr(C0)3(n6-C7Hs)],
conjugated
1,3,5-cycloheptatriene related
ligand.
[ C r ( C 0 ) 3 (r»6 -C7 H8 ) ] , l i k e
to
tricarbonyl-n -8,8-dimethylheptafulvene-chromium(0)
3
6
0
6 : 6-heptafulvalene-dichromium(0),
with
conjugated
dienes
(n
3 : 5
5,
with
8
-
D
show
also
[6+4]-cycloadditions
.
tricarbonyl-n -8,8-dimethylheptafulvene-chromium(0) 6
Unexpectedly, forms
5
4
hexacarbonyl-
f
hexacarbonyl-n6 : 6 - i ( 2 , 4 , 6 -
and
cycloheptatrien-1-yl)dichromium(0)
dienes
2,3-dimethyl-1,3-butadiene the
- C i 4 Hi 6 ( C H 3 ) 2 ) ] ,
w h i c h a t ambient
d i c a r b o n y l complex
[Cr(C0)2-
c o n d i t i o n s adds c a r b o n
monoxide
6 .
[ C r ( C 0 ) 3 ( n 6 - C i o H i 2 )]
+
>
C4H4(CH3)3
[ C r ( C 0 ) 2 ( 0 3 : 5-C14H16 (CH3 )2 )] [ C r ( C 0 ) 2 (r.3: 5 - C 1 4 H 1 6 (CH3 )2 )]
+
CO
+
CO
>
[ C r ( C 0 ) 3 ( n 6 - C i 4 H i e (CH3 )2 )] The
formation
reaction
with
carbon
[6+4]-cycloaddition the to
[ C r ( C O ) 2 ( n 3 : 5 - C i 4 Hi 6 ( C H 3 ) 2 ) ]
of
monoxide sheds some l i g h t i n the
c o o r d i n a t i o n sphere
p h o t o r e a c t i o n proceeds an
activated
stepwise.
complex and
vene-r>2 - d i e n e - c h r o m i u m ( 0 ) . dimethylheptaf ulvene
and
its
smooth
on
t h e mechanism o f
of
chromium.
the
diene
i s coordinated
bond f o r m a t i o n between CI o f t h e c\ - d i e n e 2
n*-8,8-
of the
l i g a n d s produces the n (
n >: (1+n > - 1 - ( b u t e n e - 1 , 2 - d i y l ) - 7 - i s o p r o p y l i d e n e c y c l o h e p t a d i e n y l (n
=0,
for
2,
this
4).
ligand
immediately Only
Different i n the
forms a
of
loose
CC
A
new
type
i n the
incident.
We
tochemically
under
quite
unusual
way.
10,
the
K
with
After
-
ligand considered
intermediate
[6+4]-cycloadduct.
like
[Cr(C0)3(n3: 3 -
conditions,
between s i m p l e , of a t r a n s i t i o n
and
unsaturated metal
was
shown, t h a t d e c a c a r b o n y l - d i m a n g a n e s e ( 0 ) 1,3-butadiene
a s h o r t time
1-diyl-enneacarbonyl-dimanganese 9,
stability
the
5
form
6.
of c y c l o a d d i t i o n
a t 253
most c a s e s yields
photochemical
c o o r d i n a t i o n sphere
have
In
bond and
moderate
CO
[ C r ( C 0 ) 2 ( n 3 ; 5-C14H16 (CH3 )2 ) ] 5,
carbons
o f c o o r d i n a t i o n have t o be
intermediate.
second
intermediates
Ci 4Hi 6 ( C H 3 ) 2 ) ]
kinds
the
Obviously,
tricarbonyl-n*-8,8-dimethylheptaful-
forms CC
CI
First,
and
i s the
of
i n n-hexane irradiation
predominating
hydro-
found
r e a c t s pho-
solution u-n3
by
:
i n an
1-2-butene-
reaction
product
[Mn2(CO)io]
+
C4H6
[Mn2(C0)9(u-n3; 1 - C 4 H 6 ) ]
formation of [Mn2(C0)9(u-n
The stepwise
attack of
to
the
yield
other
>
[Mn(C0)5-]
• 1 - C 4 H 6 ) ] c a n be r a t i o n a l i z e d
3
by a
t o CI and C4 o f 1 , 3 - b u t a d i e n e
radicals
i n t e r m e d i a t e {[(CO)5MnC4H6Mn(CO)5]},
c a r b o n y l - n i - e n y l complexes
+C0
CO and
which l o o s e s ,
[(C0)4Mn(u-n : 3
yields
like
1-C3H4-
CH2)Mn(C0)5]. When ted
the photoreaction
with conjugated
atoms,
the reaction
reaction
with
reaction
leads to
i s transferred
The tives
products
substantially
n -E-pentadienyl,
complexes.
1-C4H6)]
s h o u l d be
when
3
with
1
*>
CsHe
{[Mn2(C0)9(n = 1-CsHs)]} 3
In diene
a further
under l o s s [Mn(C0)5H] When
instead
1 , 3 - p e n t a d i e n e and i t s d e r i v a -
A
corresponding
to
[Mn2(C0)9(n
1 -
hypothetical
respect t o a (3-elimination,
{[Mn2 (C0)9 ( n : 1-CsHe )]}
>
[Mn(C0)4(n -C5H?)]
3
[Mn(C0)5H]
by w h i c h
a r e formed.
>
3
3
+
CO
+ [Mn(CO)sH]
reacts photochemically
with
free
are
used
o f CO t o [ M n ( C 0 ) 4 ( n - C s H 9 ) ] . 3
+
>
CsHe
E,E-2,4-hexadiene o r of 1,3-butadiene
nyl-manganese ligands
step,
tet-
12.
3
+
of t h e
t h e mam
and n - p e n t e n y l
[ M n ( C 0 ) 4 ( n - C 5 H 7 ) ] and p e n t a c a r b o n y l - h y d r i d o - m a n g a n e s e [Mn2(CO)io]
those
from
intermediates,
a r e assumed.
instable,
carbon
In a k i n d o f d i s p r o p o r t i o n a t i o n h y d r o -
[Mn2(CO)io] with
understood,
o r more
and Z - l , 3 - p e n t a d i e n e
3
mononuclear
reactions of
[Mn2(C0)9(u-n : C5H8)]
differ
W i t h £-,
between two d i e n e m o l e c u l e s
are easily 3
containing a chain of f i v e
1,3-butadiene.
racarbonyl-manganese gen
o f d e c a c a r b o n y l - d i m a n g a n e s e ( 0 ) i s conduc-
dienes,
from
3
+
CO
2,4-dimethyl-l,3-pentadiene
i n the
complexes were
consisting
[Mn(C0)4(n -CsH9)]
reaction with
[Mn2(CO)io],
o b t a i n e d as b y - p r o d u c t s
two d i e n e m o l e c u l e s
with
tricarbo-
hydrocarbon
w i t h one h y d r o g e n
less.
% [Mn2 (CO)io ]
+
2 CeHio
>
[ M n ( C 0 ) 3 (C12H19 )] + 2 CO + {H}
^
+
2 C7H12
>
[Mn(C0)3 (C14H23 ) ] + 2 CO + {H}
[Mn2 (CO)io ]
An sence in
X-ray n
of an
this
diffraction 3
complex
nonadien-l-yl
13
1 4
>
.
ligand
was
The
ligands
readily
suggest
:
2
the
i f l NMR
[5
+
spectrum
for
both n : 2-2,6-cyclo3
p a t t e r n s of
a formal
ligand
-4,5,8-trimethyl-2,6-cyclo-
by
substitution
manganese
formed
related It
complexes,
4]
cycloadditon
of
the
a certain
In
3 :
2
3
irradiation
CO.
The
proove
2,5-pentadien-l-yl)] [5
l-yl)]
+ 4]
and
this
formed
good
methyl
may
substituted
complexes.
h y p o t h e s i s , we
1,3-butadiene
cycloaddition,
i s formed
hereby
[ M n ( C 0 ) 3 ( n 5 - 2 , 5 - p e n t a d i e n - l - y l ) ] complexes,
-2,6-cyclononadien-l-yl)]
order to
3
[Mn(C0)4(n -£-2,4-dimethyl-2,4-pentadien-l-
and
during the
substituted
are
[Mn(CO)4(n -
amount o f
f u r t h e r with the excess of the dienes t o the
[Mn(CO)3(n
complexes
°C.
i s r e a s o n a b l e t o assume, t h a t
methyl
Such
tetracarbonyl-n ~E~2,4-pentadien-l-yl-manganese
from
c o m p l e x e s on warming t o 60
looses already
react
respectively. 3
E-2,4-hexadien-l-yl)],
mal
prooven
3
5
tricarbonyl
yl)]
an n
Similarily,
t o n 5 - 2 , 4 - h e x a d i e n - l - y l , and n - 2 , 4 - d i m e t h y l - 2 , 4 - p e n t a d i e n - l - y l
dienes
and
the pre-
• 2 - i , 3 , 5 , 5 , 7-pentamethy 1 - 2 , 6 - c y c l o n o n a d i e n - 1 - y 1
[Mn(C0)3(Ci 2H19 ) ] . nonadien- 1 - y l
[ M n ( C 0 ) 3 ( C i 4 H 2 3 ) ] shows
s t u d y of
with UV-light. [Mn(CO)3(n
the simple
[Mn(CO)3(n 5
irradiated
3: 2
In a f a s t ,
for-
-2,6-cyclononadien-
yield.
(C0) Mn 3
Again
the question
generally yl)]
applicable
complexes.
quite
has
also
to
be
answered, whether t h i s
t o o t h e r d i e n e s and
other
E~l,5-pentadiene, [Mn(C0)3(n -CsH7)] reacts 5
With
smoth r e a c t i o n
similarily.
In c o n t r a s t ,
%-l,5-pentadiene
o n l y when benzene i s p r e s e n t i n t h e r e a c t i o n m i x t u r e as a In yl )],
the f i r s t
case
i n the second A
further
dienyl)].
With
[Mn(C0)3(n
3
we
have i n v e s t i g a t e d ,
1,3-butadiene,
butadiene
is
i s obtained.
[Mn(C0)3(n -cyclohexa5
the only product i s the c h e l a t e
3
yields
a
mixture of
nona-3,8-dien-7-yl)]
and
2,4-cyclohexadiene)]. complexes
varies
with
catalyst.
2
c a s e t h e c o r r e s p o n d i n g endo i s o m e r
system,
in a
reacts
•-ej£p-5-methyl-2,6-cyclononadien-l-
[Mn(C0)2(n : -1-(3-buten-l,2-diyl)-2,4-cyclohexadiene)]. 4
reaction i s
[Mn(C0)3 (f»5 - d i e n -
complex
2-Methyl-l,3-
[Mn(CO)3 (r» : - 3 - m e t h y l - b i c y c l o [ 4 . 3 . 1 ] 3
2
[Mn(C0)2(n : - 1 - ( 3 - m e t h y l - 3 - b u t e n - l , 2 - d i y l ) 4
Interstingly, the
reaction
3
the
ratio
between
temperature.
Low
these
two
temperature
favours
the
dominantly adds
formation the
o f t h e Mn(C0)2
Mn(C0)3
complex. At
room t e m p e r a t u r e
[Mn(C0)3 ( n • 2 - 3 , 4 -
[Mn(C0)3 (r»5 - c y c l o h e x a d i e n y l ) ] and
c l e a n l y to
dimethyl-bicyclo[4.3.l]nona-3,8-dien-7-yl)]
[Mn(C0)3 (n5-CeH7 )]
+
is
3
obtained.
> [Mn( C 0 ) 3 ( n : - C H 3 Cs Hi 2 ) ] 3
i-CsHs
2
[Mn(C0)2 ( n 4 . 3 - C n H i 5 )] [Mn(C0)3 (H5-C6H7 ) ]
(CH3)2C4H4
+
pre-
2,3-Dimethyl-l,3-butadiene
complex i s o b t a i n e d .
+
CO
> [Mn(C0)3 ( 0 3 : 2 - ( C H 3 ) 2 C 9 H l l ) ]
These
r e a c t i o n s show c l e a r l y , bicyclo[4.3.1]nonane
chemically chances, ferent
to extend
from the
There tion
are
reaction
a first
step,
{[Mn(C0)3(n -dienyl)]}
with
dienes
bond
between
principle
a l s o some i n f o r m a t i o n s In
3
good
t o systems d i s t i n c t l y
dif-
complex.
about t h e mechanism o f t h e
photochemically
{[Mn(C0)3(n -dienyl)(n2-diene)]}. the
3
n -dienyl
2
and
n -diene
Formation of
2 , 6-cyclononadien-l-yl)],
a second
reacts
F o r m a t i o n o f one
ligands
yields
C-C
unstable
{[Mn(C0)3(n : - 1 - ( e n d i y l ) 2
or C-C
reac-
a c o o r d i n a t i v e l y un-
i s formed, w h i c h
3
to
photo-
There are
intermediate
{[Mn(C0)3(n4: 1 - l - ( e n d i y l ) - 2 , 4 - d i e n e ) ] > 2,4-diene)]}.
a t manganese.
Mn(C0)3(n5-pentadienyl>
parent
available.
saturated
the
that i t i s possible to b u i l d
systems
bond l e a d s t o
photochemically
induced
CO
3
[Mn(C0)3(n :
loss to
3
2
[Mn(C0)2-
(n4:3endiyl)-2,4-diene)]. [Mn(C0)3(n5-dienyl)]
>
{[Mn(C0)3 ( r » - d i e n y l ) ] } 3
+
{[Mn(C0)3(n -dienyl)]} 3
diene
>
{[Mn(C0)3(n -dienyl)(n -diene]} 3
3
2
{[Mn(CO)3(n - d i e n y l ) ( n - d i e n e ] }
2
>
{[Mn(C0)3(n4: 1 - ( e n d i y l ) d i e n e ) ] } {[Mn(C0)3 (r»4: 1 - ( e n d i y l ) d i e n e ) ] }
>
[Mn(C0)3(n
3 :
2
-2,6-cyclononadien-l-yl)]
[Mn(C0)2(n4: - ( e n d i y l ) d i e n e ) ] 3
-
+
CO
1
S. O z k a r , metal.
2
H. K u r z ,
D. Neugebauer, and C. G. K r e i t e r ,
J . Organo-
Chem., 160, 115 (1978).
C. G. K r e i t e r ,
E. M i c h e l s ,
and H. K u r z ,
J . Organometal.
Chem.,
232, 249 (1982). 3
J . A. S. H o w e l l ,
B. F. G. Johnson,
and J . L e w i s ,
J . Chem. S o c . ,
D a l t o n T r a n s , p. 293 (1974). 4
J . D. Munro, and P. L. Pauson, J . Chem. S o c , (1961) 3484.
5
E. M i c h e l s ,
and C. G. K r e i t e r ,
J . Organometal.
Chem., 252, CI
(1983) . 6
E. M i c h e l s , 964
7
W. S. S h e l d r i c k ,
and C. G. K r e i t e r ,
Chem. B e r . , 118,
(1985).
C. G. K r e i t e r ,
and E. M i c h e l s ,
J . Organometal.
Chem., 312, 59
(1986). 8
C. G. K r e i t e r ,
E. M i c h e l s ,
and J . Kaub, J . O r g a n o m e t a l .
Chem.,
312, 221 (1986). 9
C. G. K r e i t e r , Chem.,
Int.
and W. L i p p s ,
Ed. E n g l . ,
10
C. G. K r e i t e r ,
11
M. L e y e n d e c k e r , C31
12
13
20, 201 (1981).
and W. L i p p s ,
Chem. B e r . , 115, 973 ( 1 9 8 2 ) .
and C. G. K r e i t e r ,
J . Organometal.
Chem., 249,
and M. L e y e n d e c k e r ,
J . Organometal.
Chem., 280,
(1983).
C. G. K r e i t e r , 225
Angew. Chem., 93, 191 (1981); Angew.
(1985).
C. G. K r e i t e r ,
14 C. G. K r e i t e r ,
M. L e y e n d e c k e r , Adv. O r g a n o m e t a l .
and W. S. S h e l d r i c k ,
unpublished.
Chem., 26, 297 (1986).
T R I P L E T QUENCHING BY METAL CARBONYLS
M.Kucharska-Zon
and
A.J.Poe
The D e p a r t m e n t o f C h e m i s t r y and E r i n d a l e C o l l e g e , U n i v e r s i t y o f M i s s i s s a u g a , O n t a r i o , L 5 L 1C6, CANADA
Toronto,
The q u e n c h i n g o f t r i p l e t s t a t e s by o r g a n o m e t a l l i c compounds i s w e l l e s t a b l i s h e d though not w i d e l y s t u d i e d . V o g l e r (1970, 1975) reported b e n z o p h e n o n e - p h o t o s e n s i t i z e d s u b s t i t u t i o n r e a c t i o n s o f M ( C O ) (M = g
Cr, Mo, W) i n benzene. I t was p o s t u l a t e d t h a t t r i p l e t e n e r g y was t r a n s f e r r e d t o t h e m e t a l c a r b o n y l , l e a d i n g to CO d i s s o c i a t i o n . F e r r o c e n e quenches t r i p l e t s t a t e s o f many o r g a n i c m o l e c u l e s i n benzene ( F r y , 1966; K i k u c h i , 1974; F a r m i l o , 1975) and i t was c o n c l u d e d ( F a r m i l o , 1975) t h a t t r i p l e t e n e r g y t r a n s f e r o c c u r s to a d i s t o r t e d e x c i t e d s t a t e of f e r r o c e n e . T r a v e r s o et a l . (1978) showed t h a t M(/7~CgHg)2 (M = Fe, Ru, Os) quench t r i p l e t u r a n y l i o n i n a c e t o n e by e l e c t r o n t r a n s f e r as do s e v e r a l mononuclear c a r b o n y l s and one d i n u c l e a r one, Mn^iCO)(Sostero, 1979). I t has a l s o been shown (Fox,
1982)
that
that
triplet
CCl^
and
CCl^
to form M n ( C 0 ) g C l .
this
biacetyl,
BA,
i s quenched by M n g C C O ) ^ i n
s e n s i t i z e s the Mn2(C0)^g towards r e a c t i o n The
simplest
explanation
i s that
with
triplet
e n e r g y t r a n s f e r l e a d s t o h o m o l y s i s o f the Mn-Mn bond and r e a c t i o n o f the Mn(C0)g r a d i c a l s w i t h the s o l v e n t . R e g f C O ) ^ d i d not quench BA but
Re^(CO)g(PPhg)^
Re(CO)^(PPhg)CI.
d i d and We
was
s e n s i t i z e d to formation
r e p o r t here data
of
f o r phosphorescence
o f BA by a wide r a n g e o f m e t a l c a r b o n y l s , t r i p l e t b e n z i l , BZ, q u e n c h i n g .
and
a few
quenching
examples
of
RESULTS
Phosphorescence kinetics.
intensities
Values of k
showed good agreement w i t h
f o r BA
i n benzene a r e
given
Stern-Volmer
in Table
1
q together
with
estimated
by
values
f o r the
means o f equ. k
where AE(T) the rate This the
q
triplet
(1)
energies,
(Herkstroeter,
= k / { l + exp. m
i s a measure o f the
quencher exceeds that
o f BA
o f the
t o w h i c h the
700
cm
1
) , and
k
triplet ffl
controlled rate
energy
of
i s the maximum 1 0
x 10*** M~*s~* f o r a d i f f u s i o n
1964) (1)
c o n s t a n t f o r t r i p l e t e n e r g y t r a n s f e r t a k e n as 1 x 1 0 i s c l o s e t o t h e h i g h e s t v a l u e o f k^ o b s e r v e d h e r e but 1.6
quenchers
Sandros,
(n -C H )Ir(CO)(S) 5
•
5
C
5
(n -C -H )Ir(CO)(H)(C .H -) c
s
f
+
c
S
(3)
H
6 6
Scheme 2
5
(n -C H )Ir(CO) * 5
5
2
«
3
» k
(n -C H )Ir(CO) (S) 5
3
(n -C H )Ir(CO) (S) 5
5
5
(4)
2
2
•
2
C
5
(n -C H )Ir(CO)(H)(C H ) 5
5
6
+
5
CO
+
S
(5)
H
6 6
Quantum y i e l d s have been measured f o r C-H a c t i v a t i o n o f benzene i n p e r f l u o r o benzene s o l u t i o n s s a t u r a t e d w i t h CO ( c a . 1 0 ~ M [ 1 5 ] ) , and the d a t a a r e shown i n T a b l e 2. The r e s u l t s i l l u s t r a t e t h a t the e f f e c t of added CO on i s n e g l i g i b l e a t any o f the benzene c o n c e n t r a t i o n s measured. T h i s o b s e r v a t i o n argues a g a i n s t the mechanism i n v o l v i n g i n i t i a l CO d i s s o c i a t i o n from the p h o t o e x c i t e d complex (Scheme 1). On the o t h e r hand, a h a p t i c i t y change (n^-Mri ) mechanism (Scheme 2) would not be expected to be a f f e c t e d by the CO c o n c e n t r a t i o n . Low-temperature experiments p r e v i o u s l y c a r r i e d out on the c l o s e l y r e l a t e d ( n ^ - C M e ) I r ( C O ) complex have prov i d e d a d d i t i o n a l support f o r a h a p t i c i t y change mechanism; even a f t e r p r o l o n g e d p h o t o l y s i s o f t h i s complex i n Ar o r N m a t r i c e s a t 12 K no more than t r a c e q u a n t i t i e s o f (n -C -Me -)Ir(CO) have been observed [ 1 6 ] , 2
3
5
5
2
2
5
t
c
Quantum y i e l d s (c|>) f o r the c o n v e r s i o n o f (n - C c H ) I r ( C O ) ( C O ) ( H ) ( C H ) i n p e r f luorobenzene a t 293 K.
T a b l e 2.
5
to (n - C c H c ) I r -
2
a
6
[C H ], 6
6
5
M
4
0.01 0.025 0.05 0.10 0.25 0.50 11.3
b
4>
0.013 0.017 0.018 0.021 0.027 0.029 0.030
d
a
b
0.015 0.020 0.028
R e p o r t e d v a l u e s e s t i m a t e d to be a c c u r a t e to ±10%; e x c i t a t i o n wavelength i s 366 N ~purged solutions. C O - s a t u r a t e d s o l u t i o n s (-10 M i n d i s s o l v e d CO c o n c e n t r a t i o n , see r e f . 15). N e a t benzene s o l v e n t .
nm.
2
c
d
2
E x c i t a t i o n o f 3.8 x 1 0 ~ M ( n - C H c ) I r ( C O ) i n N -purged benzene a t 20°C w i t h the t h i r d harmonic l i n e o f a Korad Nd g l a s s l a s e r (353 nm, 20 ns p u l s e w i d t h ) produces an absorbance change immediately f o l l o w i n g the l a s e r p u l s e t h a t i s d e p i c t e d i n F i g . 2. A p p r o x i m a t e l y 1.1 x 10 M (n - C H ) I r ( C 0 ) undergoes p h o t o d i s s o c i a t i o n f o l l o w i n g t h i s e x c i t a t i o n . T h i s v a l u e has been determined from the l a s e r p u l s e energy, the i r r a d i a t e d volume, absorbance, and quantum y i e l d o f ( n - C H ) I r ( C 0 ) i n the t r a n s i e n t a b s o r p t i o n experiment. No f u r t h e r absorbance changes were observed. These a b s o r p t i o n f e a t u r e s c l o s e l y match those o b t a i n e d i n the s t e a d y - s t a t e p h o t o l y s i s experiment and i t i s c o n c l u d e d t h a t ( n - C H ) I r ( C 0 ) ( H ) ( C H ) has been formed. T h i s r e s u l t demonstrates t h a t the i n t e r m o l e c u l a r C - H a c t i v a t i o n o f benzene by the p h o t o e x c i t e d m e t a l complex takes p l a c e on a v e r y f a s t t i m e s c a l e ( w i t h i n the 20 ns l a s e r p u l s e ) ; t h u s , a lower l i m i t f o r k = 4.4 (±0.9) x 1 0 M" S " (k k / [ C H ] ) can be e s t i m a t e d f o r the C - H a c t i v a t i o n process. Any t r a n s i e n t produced f o l l o w i n g p h o t o e x c i t a t i o n i s v e r y s h o r t l i v e d and r a p i d l y scavenged by the s o l v e n t m o l e c u l e s p r e s e n t (neat benzene a t 20°C i s 11.3 M). 3
5
5
2
5
2
5
2
5
5
5
6
5
b
Q b s
6
6
5
5
1
1
5
2
_J 400
I 500
I 600
1 700
nm F i g u r e 2.
Photoproduct spectrum o b t a i n e d i n t r a n s i e n t a b s o r p t i o n experiment f o l l o w i n g 353 nm e x c i t a t i o n o f 3.8 x 1 0 ~ M (n -C -H -)Ir(CO) i n benzene at 20°C. 3
5
c
t
2
S i m i l a r experiments have been c a r r i e d o u t f o r (n - C ^ H ^ I K C O ) ^ i n N^-purged p e r f l u o r o b e n z e n e c o n t a i n i n g 5 x 1 0 M benzene. Immediately f o l l o w i n g e x c i t a t i o n a t r a n s i e n t w i t h a broad absorbance ( c e n t e r e d a t 625 nm) was r e c o r d e d , which subs e q u e n t l y decayed w i t h k = 4.8(±1.0) x 10° s ~ ( t h u s k = 9.6(±1.9) x 10° M s t o form t h e product ( n ~ C H ^ ) I r ( C O ) ( H ) ( C H ) . T h i s t r a n s i e n t a b s o r p t i o n f e a t u r e i s a s s i g n e d to t h e ( r - C ^ ) I r ( C 0 ) ( S ) s p e c i e s i n accordance w i t h t h e above a n a l y s i s . Moreover, we were u n a b l e to o b s e r v e absorbance changes r e p r e s e n t i n g product f o r m a t i o n f o l l o w i n g l a s e r p h o t o e x c i t a t i o n o f ( n ^ - C ^ H ^ ) I r ( C 0 ) i n neat p e r f l u o r o b e n z e n e ; t h i s r e s u l t i s c o n s i s t e n t w i t h the n back r e a c t i o n (see e q u a t i o n 4) t a k i n g p l a c e i n the absence o f s c a v e n g i n g benzene m o l e c u l e s . 3
1
Q b s
1
1
5
5
6
5
2
2
ACKNOWLEDGMENT. We a r e g r a t e f u l t o the donors o f the P e t r o l e u m Research Fund, a d m i n i s t e r e d by the American C h e m i c a l S o c i e t y , f o r support o f t h i s r e s e a r c h . We thank Ms. A. S. Lee f o r a s s i s t a n c e w i t h i n i t i a l experiments and P r o f . A. W. Adamson f o r use o f the Nd g l a s s l a s e r and t r a n s i e n t a b s o r p t i o n equipment.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
G.W. P a r s h a l l , A c c . Chem. Res. 8, 1 13 (1975). A.E. S h i l o v and A.S. Shteinman, Coord. Chem. Rev. 2 4 , 97 (1 977). A.E. S h i l o v , Pure A p p l i e d Chem. 50, 725 (1978). G.W. P a r s h a l l , C a t a l y s i s 1, 335 (1977). D.E. W e b s t e r , Adv. Organomet. Chem. 1 5, 147 (1977). J.P. Collman and L.S. Hegedus, " P r i n c i p l e s and A p p l i c a t i o n s o f O r g a n o t r a n s i t i o n M e t a l Chemistry," U n i v e r s i t y S c i e n c e B o o k s : M i l l V a l l e y , CA, 1980. G.W. P a r s h a l l , "Homogeneous C a t a l y s i s , " Wiley:New York, 1980. A.H. J a n o w i c z and R.G. Bergman, J . Am. Chem. Soc. ^0^, 352 (1982). W.D. Jones and F . J . Feher, O r g a n o m e t a l l i c s 2, 686 (1983). J.K. Hoyano and W.A.G. Graham, J . Am. Chem. Soc. 1 04, 3723 (1982). J.K. Hoyano, A.D. M c M a s t e r a n d W.A.G. Graham, J . Am. Chem. Soc. 105, 7 1 9 0 (1983). E.O. F i s c h e r and K.S. B r e n n e r , Z. N a t u r f o r s h 16B, 774 (1962). A . J . Lees and A.W. Adamson, I n o r g . Chem. 20, 4381 (1982). R. Fukuda, R.T. W a l t e r s , H. Macke and A.W. Adamson, J . Phys. Chem. 83, 2097 (1979). D.D. Lawson, A p p l . Energy 6, 241 (1980). A.J. R e s t , I . W h i t w e l l , W.A.G. Graham, J.K. Hoyano, a n d A. M c M a s t e r , J . Chem. Soc. Chem. Commun. 624 (1 984).
I D E N T I F I C A T I O N OF H - , D -, N - BONDED INTERMEDIATES IN THE PHOTOCATALYZED HYDROGENATION REACTIONS 2
2
2
A.Oskam, R . R . A n d r e a ,
D . J . S t u f k e n s , a n d M.A.Vuurman
A n o r g a n i s c h Chemisch L a b o r a t o r i u m , U n i v e r s i t y o f Amsterdam, N i e u w e A c h t e r g r a c h t 1 6 6 , 1 0 1 8 WV A m s t e r d a m , THE NETHERLANDS
The
mechanisms
complexes
of photochemical
have
been
the
attempts
have
been
performed
by
studying
inert
matrices
characterized to
use
This
followed
under
the it
have
A
special
matrix
combination
and
noble
reactions,
site
regions,
sufficient
stabilize long
enough
symmetry
combination
ESR
and N M R - s p e c t r o s c p i c
In
order
and
(Andrea
to stabilize
photochemical
solutions useful
hydrogenation
with
of H , D
FTIR.
primary
Xenon
reactions
i |
presence
The
2
several
of
methods
of
i s t h e use o f
c a n be
i n the
used
of
to
study
(photochemical)
a n d do n o t show a n y
o f most
o f t h e compounds i n
i n t h e sample
cell
i s large
r e p o r t s about
i n solutions
IR-
at various
t h e components
identification
Several
studies
photoproducts we
have
enough
IR, U V - v i s i b l e ,
of liquid
i n LXe
and N 2
This
mechanisms
of dienes. has
and s t u d y
extended
(LXe, 170-240K).
at the reaction
Cr ( C O ) ( n o r b o r n a d i e n e )
because
noble
gases
1986).
reactions
i n liquid
to look
methods.
photoproducts
reactions
s o l v e n t s do n o t a b s o r b
solubility
s p e c t r o s c o p i c measurements.
appeared
spec-
techniques
consuming
of primary
intermediates
pathlength
for
have
possible
valuble technique f o r
ongoing
are inert,
reactive
a long
in
c a n be
t h a t r e a c t i o n s c a n be
a n d money
mobility
f o rspectral
splittings.The
with
of
as s o l v e n t s . T h e s e
spectral
be
o f t h e components.
the advantages
show
can
resolved
time-resolved
i s a very
i n studying
and m o b i l i t y
of
UV-visible
as time
Many
photoproducts
fast
advantage
spectroscopy
This
I t i s also
with
This
metal
papers.
intermediates
spectroscopy.
and c o n f o r m a t i o n a l a n a l y s i s
gases
temperatures,
reactions
restrictions
limitations
of diffusion
liquid
these
i n combination
isolation
identification
and p r i m a r y
Usually
conditions.
transition
recent
intermediates.
molecules
has t h e g r e a t
normal
several
identify
glasses.
technique
has severe
lack
parent
photolysis
troscopy.
Although
to
in
by IR a n d U V - v i s i b l e
flash
however
made
or s o l i d
reactions involving
subject
of
the
purpose
irradiated
The p h o t o p r o d u c t s
CO-frequencies
their
Were
various
thermal
experiments
solvent
of Cr(CO)
For this been
our
to
i s especially photocata1yzed
6
a at
solution 183K
mainly
of
i n the
identified
photoproducts
were
assigned
with
experiments The
the
in
use
and
infrared
of
the
data
obtained
from
irradiation
CH^-matrices.
spectrum
of
Cr(C0) H c
as
o
an
intermediate
of
the
d
D
irradiation o f C r ( C O ) ^ and in liquid Xenon has been p u b l i s h e d b e f o r e s h o w i n g a C r ( C O ) - p a t t e r n and an H - s t r e t c h i n g vibration at -1 3 0 3 0 cm (Upmacis 1985 ) . W i t h D instead of H t h e same spectra -1 a p p e a r e d b u t w i t h t h e D ^ - s t r e t c h i n g v i b r a t i o n a t 2241 cm , slightly higher than could be calculated f o r an uncoupled vibration. The L-experiment r e s u l t s i n C r ( C O ) N . w i t h a N«
^
at
2237
trans
d
cm"
and
with
after
two
prolonged
different
irradiation
sets
(a
frequencies
(Turner
Irradiation
Cr(CO)^(norbornadiene)
a
possible
rection resp.
2
a
intermediates Because
ligand
were
photocatalyzed
i.e.
H^be
too
changes
could
be 2
Cr(CO)(N)
e q
and
and
a^)
of
N^-stretching
and
or
the
D^-
i n order catalyzed
stretching The
to
identify
hydrogenation
vibration
of
c o n c e n t r a t i o n of
these
low. the
CO-stretching
reaction N^.
we
Here,
repeated three
region this
during
this
experiment
eventually
four
with photo-
identified;
a X
Cr(CO)^(N )
d
2
or
of
detected.
in
hydrogenation
Cr(CO)^(norbornadiene) products
no
could
s i m p l y was
there
b^
with
intermediate
norbornadiene
as
+
< 1
d
in C r ( C O ) ^ ( ) , cis
1 98 3).
H^-bonded
of
D.
o
c
(norbornadiene),
(norbornadiene), 2
Cr(C0) and
l j
(N )(n
Identification the
bands
the
Cr (CO)
Cr(C0) (=
1 |
norbornadiene),
2
C r ( CO ) ^ (
) ^ ( rj
of these
i n the (N
2
N «
photoproducts
as
2
norbornadiene).
well
as
)(nor bornadiene) 2
(N )(rj 2
norbornadiene)
were
i n the
4)
on
the assignments
CO-stretching region.
products (=
based
Figure)
(=
and
3)
grow
in
C r ( CO ) ^ ( N
2
and
At
of
first
later
on
) ( n-norbornadiene ) 2
2).
The stead
same
experiments
of
propose
N
2
with
show
the
the
same
that
Complete
proof of
same
H
2
or
D
patterns
type
of
and
2
in
the assignments
radiation
in
intensity
decrease
very
end
presence
and
of
of
the
with
Cr(CO)^(norbornadiene ) with the
(Cr(CO)^(norbornadiene ) i n CO-region.
products arise 13
progress.
the
(see
free
CO H
2
in
labeling and
D
2
norbornadiene
the
parent
bands
large
bands
which
of
Therefore,
these
we
experiments.
experiments
is in
after
prolonged i r -
shows
no
detectable
Cr(CO)^(norbornadiene)
evidently
belong
to
at free
norbornane. Further
experiments
catalyzed
in
hydrogenation
liquid
xenon
reaction
of
to
unraffle
the
norbornadiene
mechanism are
in
of
the
progress.
CO-region
a-.
3 6
I)
V uu
Sir?
z5s5
Hii
iioi
Hal
2T92 iTii
01 ZO B7 2699 1393 198.5 T5si
STTe i&as
Wi^V£NU|1BERS
UAVFMf IMBrH« PRODUCTS;
6=Cr(
1 2
4=Cr(
1 2
3=Cr(
1 2
Figure
6
2
C0) (77 -NBD)(N ) 4
2
4
C0) (77 -NBD)(N 3
3'=Cr( 2-Cr(
C0)
1 2
1 2
a x 2
4
C0) (77 -NBD)(N 3
C0) (7 3
, i 7
e q 2
-NBD)(N )
P h o t o l y s i s of Cr(CO),(norbornadiene) with
2
N
) )
1 &37 1&89 t623
REFERENCES Andrea
RR,
Applied Turner
H,
Spectroscopy
J J , Simpson
Inorg. Upmacis
Luyten
MB,
Vuurman
MA,
Stufkens
DJ
4 0 : 1184 a n d r e f e r e n c e s Poliakoff
M,
and
Oskam
A
(1986)
and Graham
MA
( 1 983 )
therein
Maier
I I WB
Simpson
MB,
Chem. 2 2 : 911 RK,
Simpson
Gadd
GE,
Poliakoff
AF ( 1 9 8 5 ) J.Chem.
M,
Turner
S o c . Chem. Commun. 27
J J , Whyman
R,
SPECTROSCOPY AND PHOTOCHEMISTRY
OF N i ( C O ) ( a - D I I M I N E )
COMPLEXES
2
P . C . S e r v a a s , D . J . S t u f k e n s , and A.Oskam A n o r g a n i s c h Chemisch L a b o r a t o r i u r n , U n i v e r s i t y o f Amsterdam, N i e u w e A c h t e r g r a c h t 1 6 6 , 1018 WV A m s t e r d a m , THE NETHERLANDS
INTRODUCTION During
the
laboratory
last to
photophysics most
of low-valence
of these
charge
t e n years
complexes
transfer
d -M(COK(a-diimine) 8
1984),
d -Fe(CO)
a reaction
quantum
yields
Because
of these
close-lying from
these
states In
of
close-lying
and
especially
the s t e r i c
a-diimine
study
t h e most
states, very
and because
cases, v i z . 1985) and
published;
MLCT
Kokkes
although the low
(cJXO.01 ) .
of the presence
reactions normally
a t room
For
to a-diimine
Dijk
always
photochemical
cases
temperature
of
occur
when t h e s e L F
occupied.
LF s t a t e s
photochemical
represents
be
i n our
complexes.
few
van
to
these
paid i n some
i s metal
in a
1980;
were
yields
LF s t a t e s ,
states,
state
(van Dijk, from
been and
a-diimine
Only
(Balk
reactions
l o w quantum
to investigate
photochemistry from
these
reactive latter
excited
(M=Cr,Mo,W)
was o b s e r v e d
has
metal
i n character.
become t h e r m a l l y
order
transition
(a-diimine)
of
attention
photochemistry
the lowest
(MLCT)
6
recently
much
the spectroscopy,
complexes taking
of
simple
and e l e c t r o n i c without
place,
we
the Ni(C0)
a-diimine
(= R-DAB; R-N=CH-CH=N~R). The r e s u l t s
effects
disturbing
started
(R-DAB)
ligand
of this
on t h e MLCT reactions
a spectroscopic
complexes.
R-DAB
1,4-diaza-1,3-butadiene study
are presented
here.
R E S U L T S AND D I S C U S S I O N Ni(CO)^(R-DAB) complexes bulky
therefore Both
confined
complexes
region, at
with
maxima
MLCT t r a n s i t i o n
an
be p r e p a r e d
a
transition.
2
configuration CNDO/S
i n benzene 2
and
f o r R-DAB l i g a n d s w i t h
2
The
using
band
2
in
the
visible
2
the c h a r a c t e r i s t i c
H e ( I ) and
He(II)
by A n d r e a orbitals.
known
study i s
a t 19-34 kK f o r N i ( C O ) ( t - B u - D A B ) a n d 2
reported
This
Ni(CO) (2,6-iPr ~Ph-DAB).
absorption
(2 , 6 - i P r - P h - D A B ) . T h i s
of- t h e m e t a l - d
method,
(t-Bu-DAB)
intensive
and i t shows
Ni(CO) (t-Bu-DAB),
the
t o Ni(CO)
show
1 8 . 4 8 kK f o r N i ( CO)
such
could only
s u b s t i t u e n t s R a t t h e c o o r d i n a t i n g n i t r o g e n atoms.
bond
(1985) From
band
i s assigned
solvent
dependence o f
photoelec trori pointed MO
to a
to a
spectra
of
tetrahedral
calculations
d i s t a n c e s - and a n g l e s
following of
other
tetrahedral
Ni(CO)^(a-diimine)
1 9 8 5 ),
scheme
a MO
shown i n F i g u r e
has been
complexes composed;
( v o n Hausen
the relevant
1972;
part
Sieler
of which i s
1 .
1
(DAB)
d 2. 2 X
dy d
Z
Z
X2
Figure
MO-diagram
-+—
-4—
The
d
t >
b
transition
overlap
between
possible tions.
in
the
measured
both
and
CT
the
metal
transition
will
absorption d^
z
(b^)
in detail
NJ ( C O ) ^ ( R - D A B )
some
be
(t-Bu-
due
ligand Raman
the character complexes
characteristic
t h e most
spectrum and
t h e use o f the resonance
to determine For
M L
2^ 2*
allowed
With
of Ni(CO)
-DAB)
y 2
z-polarized
involved.
1.
ones
to
strongly the
TT*
strong
orbitals
(rR) technique of these
these are
MLCT
spectra shown
i n
i ti s transi-
have
been
F i g . 2.
These
spectra
indicate
a
The
appearance
in
the
rR
of
with
found
of
had
then
been
C0-7T*
orbitals. -1
773
the
cm
that
character
therefore The
rR
that
N i (CO)
) and
the
transition
of
quasi
the
pseudo
shown
effects
The
also
for v
as
the
(see s y m
at
metal
S y m
rR
The cm"
two
D
bands
modes
of
for
these
bonds
we
the
2^ 2*
is
can
rR
D
the
say
spectra
transition
solvatochromism
and
at
effect
There
has
to
Ni(R-DAB)
be
order
to
anti-
cannot
arise
out
These
how
change
for
the
have
19 81)
latter
The
a
measured
complexes
44.5°.
no
structural we
(Svoboda
1 9 81), of
transition
prepared
complexes
Ni(c-Hex-DAB)
ligands
to
structural
find
(torn D i e c k
2
a-diimine
321
Unfortunately, be
complexes.
2
a
at
characterizes
2^°2*
could
Ni-a-diimine
tetrahedral
b
from
strong
bonding
(Ni-C)
the
complex.
2
In
cm" )
S y m
of
(Ni-C)
1
(IT*) v
S y m
v
1475
with
(2,6-iPr -Ph-DAB)
of
and
for
line
appreciably
appearance 1
)- l i g a n d
2
two
of
differs
206
(CN)
(d
laser
complex.
rR-spectra
the
was and
bending
from
large
4).
Ni(CO) (t-Bu-DAB)
from
(C0)
a-diimine-
for
summary
z-polarized
Fig.
(Ni-N)
strong
Ni(CO)
none In
in
1980)
observed
derived
a
S y m
(Balk
to
a
reflected
between
is
that
ligand with
CT-character.
2
to
having
results
are
3. show
strong the
with pseudo
shows s t r o n g
means
sight
transition. R-DAB
observed
effect
showing
Ni(2,6-Me ~Ph-DAB)
a
for
transition DAB).
a
similar
(R-DAB) c o m p l e x e s shows
of
exciting
of
between
the
is
overlap
CT-transition.
determination.
spectra
Ni(CO) DAB)
of
the
in Fig.
These
rR
first
accordance
effect
also
2
the
varying
A rR
Ni(CO) (2,6-iPr^-Ph-DAB)
the
to
planar
angle
that
such
spectra
of
in
This
space
transition
strongly
of
affect
rR
any
this
this
tetrahedral
structures
an
as
structure
changes
(CN)
, r e s p e c t i v e l y , belonging
(t-Bu-DAB)
crystals
at
electronic
is
e f f e c t s are
ligand vibrations (v
respect
X-ray
through
rR
amount o f
However,
resonance
single
2006 cm" .
MO-diagram,
of
2
weak
from a
1
at
a
vibrations (v
cm"
with
(t-Bu-DAB)
Hardly
with
a large
metal-ligand
bonding.
cm
of
the
spectrum of
by
during
accordance from
s y m
v
CT-character.
(C0)
which
main
for
s t r e t c h i n g modes w h i c h
affected
in
the
band
N i (CO)
Strong -1
913
severely
is
S y m
moiety.
metal-ligand
derived
of
explained
and
NiNCCN
the
intensive
of
M ( C O ) ^ ( R - D A B ) [M=Cr,Mo,W] c o m p l e x e s
and at
an
v
f o r the
differences
CT-character
considerable
observation
also
striking
in
spectrum
transition the
exhibit
change
rR
rR
M-L strong planar
features
i n F i g . 2. effect
stretching CT-character complex
e f f e c t s f o r the
The
for
v
as
the
tetrahedral S y m
(CN)
modes.
at
This
just
as 2
complex
1 496 result
found
Ni(2,6-Me -Ph-DAB) M-L
spectra
for 0
on
s t r e t c h i n g modes and
cm"
1
of
Ni(c-Hexand
points Ni(CO) the
the
other
a weak
weak to
a
(t-Buhand effect
for
s y m
v
(CN)
which
is in
Ni(CO)^(2,6-iPr^-Ph-DAB). spectrum Fig.
good
agreement
For
of the t e t r a h e d r a l
reasons
complex
with
of
the
rR
comparison
Ni(4-Me-Ph-DAB)
spectrum also
the
of rR
i s presented i n
3.
Ni(cHex-DAB)„
u Ni{4-Me-Ph-DAB).
Figure rR 1200
This
spectrum
spectra
complexes
— * cm
shows
3-
similar
which
rR
N i ( c-Hex-DAB ) and
of
determined scheme
by
has
distorted
t h e change
been
that
of
tetrahedral
the
differences
(Fig.
Based
on
a
4).
yz
a b
a a
0 #
#
+n
TT n
* — • — •—#
-
+7T
*(co)
Figure
D A 8
+ d
xz n + +d x y d 2 z
M
0
4.
d i a g r 6a m
of Ni(CO), 2
(2 , 6-iPr -Ph-DAB) . 2
the
primarilyMO
d -TT^
yz *DAB)
between
a
2
these
analogue
(2,6-iPr ~Ph-DAB ) assuming
"2(00)
d
structural
N i ( 2 , 6-Me^-Ph-DAB ) ^ a r e
f o r ' Ni(CO)
geometry
as
the s t r i k i n g
structure.
calculated
Ni(R-DAB),
( i n C^H^).
effects
Ni(c-Hex-DAB)^, spectra
means
rR
of
results
It
is
obvious
CT-transition of
the
bond
that
orbital
character of
during
the N i - C O
TT*-CO
a
photochemistry
primary The
of
of Ni-CO
the
Fig. 4
in particular
mixing
change in
from
bonds
in
during
this
the
main
a r e weakened
both
the
the main
compound
b+b*
HOMO
z-polarized
as
and
transition
v i z . release
the
result This
LUMO.
is
reflected
CO
of
as
the
photoprocess.
photochemical
schematically
behaviour
of both
Ni(CO)
(R-DAB)
complexes
is
shown
i n Scheme I .
Ni(CO) (t-Bu-DAB) 2
1.
+ PR
2.
+ R'-DAB (toluene)
(toluene)
3.
C H m a t r i x 10 K
4.
pentane 168 K
3
1 0 0
-
K
Ni(C0)(PR )(t-Bu-DAB) 3
—Ni(CO) (R*-DAB) 2
- ( no CO dissociation)
4
•
(high concentration)
dinuclear complex
^ ^
B r e a k i n g of a N i - N
bond
Ni(C0) (2,6- i P r - P h - D AB) 2
1. + P R
3
2
(toluene)
m
2. + R'-DAB (toluene) 3.
pentane 168 K
(high
concentration)
lfl6
K
- Ni(C0)(PR )(2,6-iPr -Ph-DAB)
r
Ni(R'-DAB)(2,6~iPr -Ph-DAB)
3
2
no reaction ^ ^
B r e a k i n g of a N i - C O
Scheme
shows
to
Ni-N
rather
high
pentane. whereas
In
the bond
case
i s
i t c a n be
leading of
the
the
to
the
complex
breaking
2
mostly said
of
behaviour.
affected
the
upon
structural
strongly
of
irradiated
2
that
Ni(CO)
formation are
Ni(CO) (2,6-iPr ~Ph-DAB)
complexes which
photochemical
complex
2
causes of
(R-DAB).
(2,6-iPr ~Ph-DAB)
breaking
which
conclusion 2
Ni(CO)
Irradiation i n the
broken
of Ni(CO)
concentrations
Ni(CO) (R-DAB) the
bond
I. Photochemistry
Contrary
is
2
when K in
bond
excitation
both
168
(Ni-N)
particular
differences
affect
dimers at
weakest that
(t-Bu-DAB)
(Ni-CO).
exist
t h e rR
bond
between
spectra
and
REFERENCES Andrea
RR,
He(I)
Louwen
and
He(II)
Complexes
RW,
Kokkes
a
T,
o f Chromium, Raman S p e c t r a
Chem.
3015-3021.
19: H,
Svoboda
Aromatischen van
Dijk
HK,
Electronic in
Spectra
DJ,
Oskam
Molybdenum and
Dijk
von
Hausen
Oskam
Transition
M,
Greiser
T
N-Substituenten, Servaas
Effects
PC, on
the
HK,
e t . a l . , t o be
HD,
Krogmann
K
and
A
A,
Metal
ligand.
(1980)
J.
(1985) Carbonyl
Organomet.
(1981) Z.
(Diimine)Carbony1
Tungsten: Relationship
Photosubstitution
Quantum
DJ ,
Quantum
Yield
Oskam
Inorg.
between
Yields.
Inorg.
Nickel(0)-bis(chelate)
Naturforsch.
Stufkens
W(CO)^(a-diimine) Complexes.
van
of
DJ,
1,4-diaza- 1,3-butadiene
Stufkens
Resonance
Dieck
Stufkens
273-289.
Snoeck
Complexes
torn
MW,
Photoelectron
containing
Chem. 2 8 1 : Balk
JN,
of
A
36b:
( 1 985 )
Steric
Photosubstitution
Chim.
Acta,
mit
823-832.
1 04:
and of
CO
1 7 9 - 1 83-
published.
(1972)
Die
Krista11struktur
diacetylbis(dimethy1 hydrazon)-Nickel(0).
Z.
Anorg.
von Allg.
DicarbonylChem.
389:
247-253. Kokkes
MW,
Stufkens
(a-diimine) Sieler
J,
struktur Anorg. Svoboda
iron
Than von
Allg. M,
(chelate) 814-822.
DJ,
Oskam
A
( 1 984 )
Photochemistry
c o m p l e x e s . J . Chem. S o c . , D a l t o n
NN,
Benedix
R,
Dinjus
E,
Walther
of
Tricarbonyl
Trans.: D
(1985)
1005-1017. Kristall-
4 , 6 -d im e t hy 1 - 2 , 2 *-d ip yr id y 1 -d ic a r bo ny 1n ic k e 1 (0 ) . Chem.
torn mit
Dieck
522: H,
Z.
131-136. Kruger
Aliphatischen
C,
Tsay
YH
N-Substituenten.
(1981) Z.
Nickel
(O)-bis-
Naturforsch.
36b:
Fe(CO) (R-DAB), A
COMPLEX WITH TWO
3
D . J . S t u f k e n s , H.K.van D i j k ,
and
CLOSE-LYING REACTIVE EXCITED STATES
A.Oskam
A n o r g a n i s c h Chemisch L a b o r a t o r i u m , U n i v e r s i t y o f Amsterdam, N i e u w e A c h t e r g r a c h t 166, 1018 WV A m s t e r d a m , THE NETHERLANDS
INTRODUCTION The
complexes
=N-R),
spectra,
the
the
band
and
accompanied
by
from
Although
the
therefore (cj)-0.2)
are
found the
for this
higher
energy
Figure
1.
into modes
has
distortion
in
Fig.
1,
2).
The
(Fig.
excitation deformation
excited
expected
to
nm
no
this
of
the
only
1983).
character
complex.
for
state
to the
be
assumption
called fairly
photosubs t i t u t i o n
presence
excitation
(still
reactive,
of
comes
into
the
a
intensive Raman
show
RR
This
means
but
Similar
(RR)
effects
that
that
i t
results
of
'MLCT' high CO
in
from 500
the nm
increase
by
of
Fig.
quantum a
close-lying reactive
is were
3)
Figure
nucleophile. LF
u p o n
state going
2
Molecular
Structure
of
(R-DAB)
Fe(CO)
spectrum
is
yields
band.
Absorption
Fe(CO)
an
Resonance
band,
(Kokkes
MLCT
possess
calculations.
lowest
not
points
support
a
m.o.
shown
500
involved
by
obtained
about
FeCO
transition
This
structure
at
obtained
R-DAB
=1,4-diaza-1,3-butadiene;R-N=CH-CH-
3
having
absorption
for
Fe(CO) (R-DAB)(R-DAB
of
(c-Hex-DAB) i n
toluene
and to
The
transition
responsible tentative the
the lowest
LF
f o r the absorption
to
band
energy
level
photochemical
state,
which
however, place
more
i s shown
will
i s , at
a t about
than
take
^MLCT.
will
n o t be o c c u p i e d
side
o f t h e 5 0 0 nm b a n d .
may
occur
state. these
Thus,
lead
from
t h e ^MLCT
to occupation of
two d i f f e r e n t
complexes
state.
when
,
reactions which
the
irradiation case
energy
may t a k e
will
be
LF
temperature,
In that
Higher
part,
temperature
from
LF and t o a r e a c t i o n
photochemical
a t low temperatures
place
At lower
LF s t a t e
however,
in
( F i g .2). A
i n F i g . 3. A t room
preferably
reactive
least
380 nm
a t t h e low energy
reaction will,
i s much 3
the
diagram
reaction
state
takes only
a
excitation from
that
placef o r
discussed
i n
detail.
LF LF
MLCT
MLCT
>0 Figure of
3. T e n t a t i v e
energy
level
diagram
Fe(CO) (R-DAB) 3
ROOM TEMPERATURE PHOTOCHEMISTRY Irradiation
o f Fe(CO)
substitution reaction mechanisms
(R-DAB) i n t h e p r e s e n c e
o f a CO l i g a n d .
i s observed. have
been
WITH P H O S P H I N E S
I n the absence
o f PR^ c a u s e s
of a subtituting
For the photosubstitution
proposed
( F i g . 4) ( K o k k e s
the photoligand
reaction
1984; T r o g l e r
no two
1986). I n
the
first
second
mechanism
one
between
a solvent
molecule
to
energy
will
cause
(excimer
o=c-
these
two
(mechanism
and
X
=
308
i s broken
mechanisms 1) w i l l
breaking
the disappearance laser,
photoprocess latter
photoprocess
a m e t a l - n i t r o g e n bond
discriminate
lower
the primary
of
a
show
f o r a l l complexes
cause
band.
that
i— N
-
The
o f CO,
o f t h e 500 band
f o r Fe(CO)
CO
i s the
(t-Bu-DAB).
i s broken.
hv
CH \ CH
0 =
C-
i=
-Fe.
N = \ R
\ R
CH \ H
CO
C
R
MECHANISM 1
x
\ Q = C — F e .
=
L
CH -
CH
H
V
\
hi/
= C H
°*
c
S
A
R
O L
c O = C
—
Fe v
^
^ C
O*
MECHANISM
H C
V
N | R
H
j-CO
2
R
i \ O EEC — Fe C - Fe ^ C
Figure CO
4.
Possible
i n Fe(CO)
\ ^ N = C H
^
V
\ R
mechanisms
(R-(DAB)
f o r the p h o t o s u b s t i t u t i o n s
of
may
CO
nm
by band
(mechanism
flashphotolysis of
i n the
Flashphoto1ysis
substitution
a shift
release
except
c o m p l e x a m e t a l - n i t r o g e n bond
-Fe:
first. since
meta1-nitrogen
of t h i s
nm)
i s release
of
2)
results primary In
the
By
irradiation
in
n-pentane
Fe ( CO ) ^ ( t - B u - D A B ) c a n Bu-DAB)(P(c-Hex)^), t-Bu-DAB
type
IN
of
with
except
for
formation this
a
with
X>500nm
the of
500
nm
band
region.
At
the
stretching
The
shift
of
concomitant whether frequency
decrease
in
unstable
in
the at
A
with
of
s
(LXe)
further
i n IR,
Vis
region
UV.
which
photolysis in
in
the
the
the
in
CO
visible the
is
modes
In
a
with
observed.
moiety.
points
result
order is
to
to
a
from
a
find
reflected
the
reaction inert
is
IR
be
out in
very can
CO-
observed.
Fe(CO)^
solvent
of
in
is
only
is
shows
of
yield
intermediates this
The
substituents
the
ligand,
5
react.
irradiation
free
R-DAB
LXe
Excita-
complexes
Fe(CO)^(R-DAB)-
up
can
R-DAB. to
the
dependence
appear
no
on
bulky
CO-stretching
of
as
show
contains
170K.
Figure
Just
not
Upon
bands
while
to
and
upon
nm.
the
Fe.(CO)_(R-DAB)_, c. b d
too
bands
this
at
advantage
and
X = 565
the
a l l
CO-frequency
quantum
IR
still
photoproducts
CO-stretching 170K
v (CN)
low
not
low
new
of
does
(R-DAB).
new
used.
for
photoproduct
ir-backbond i n g
Xenon
which
depends
wavelength
a
with
TT-backbonding of
Fe ( CO ) ^ ( o~-N-1
(P=20BAR,T=170K)
150K
presumably
Tr-backbonding
of
liquid
1986).
transparency
LXe
of
of
frequencies
frequencies
increase
in
no
photoproduct
metal-CO
performed
(Andrea
higher
decrease
this
which
three
higher the
Fe(CO) complex
and
P(c-Hex)^,
in
concentration
primary
disappears
at
the
reaction with
time
the
complex
from
same
the
on
at
photoproduct
complex,
compound
2
XENON
i n n-pentane
which
the
(R-DAB)
different
that to
decrease
of
of
into
mechanism
LIQUID
appearance
reaction
region
means
the
This
Fe(CO)
IN
and
evident
region.
parent
an
a completely
This
is
from
by
in
dimeric
(t-Bu-DAB)
and
formed
ligand a
presence
iron.
reaction
gives
dimer
to
the
quantitatively
(T=150K)AND
R-DAB
nm
carbonyl
be
The
the
Fe(CO )
reaction
will
R,
of
of
bridging
(S)
R
in
converted
bonded
N-PENTANE
A and R e l a t i v e R a t i o o f t h e P r o c e s s e s ( 1 ) and ( 2 ) .
*1
*1
L
k
8
io
8
2 X
io
8
2 X
io
8
9 x
P(OEt)
3
4 x
q
P(0Ph)
P(0i-Pr)
Q
o
5
10
CO
k
PEt3
9 X
10
8
P(n-Pr)
5 X
10
8
10
8
io
7
4 X
( 9 X 10
7 )
6
io
5
4 X io
3
3 X
(3 X
5
10 )
. *2 Ratio (D/(2)
e*
1.1
95
2.2
109
1.0
130
1.8
130
io
4
3 X io
4
7 X io
3
0.8
132
7 X io
3
1.7
132
7 X io
3
(9 X 1 0 )
1.1
132
2 X
io
3
(2 X 1 0 )
1.4
143
io
2
(4 X 10
4 )
3
0
P(n-Bu) P(i-Bu) PPh
3
2 X
3
7 X
3
(1 X 1 0 ) 7 ( 2 X 10 ) (1 X i o ) 8
3
3
144
7
Q
P(i-Pr)
3 X
3
io
6
(4 X
6
10 )
2 X
(1 X
V a l u e s i n p a r e n t h e s i s ( W a l k e r and H e r r i c k *1 m o l - 1 dm3 s " ; *2 Of t h e s y s t e m s b y 3 5 5 *3 L i g a n d s t e r i c p a r a m e t e r s ( T o l m a n 1977) 1
Effect
on
the R a t i o of the Processes
2
io )
1.3
160
1984) nm e x c i t a t i o n ;
(l)/(2)
In a d d i t i o n t o t h e above o b s e r v a t i o n s , the r a t i o i n t h e o c c u r r e n c e of t h e p r o c e s s e s ( l ) and ( 2 ) c o u l d be d e t e r m i n e d f o r e a c h L ( T a b l e 1 ) . The v a l u e s a r e a l i t t l e s c a t t e r e d i n t h e r a n g e o f 0.8-2.0, h o w e v e r i t c a n be s a f e l y n o t e d t h a t i n t h e p h o t o c h e m i c a l e v e n t , t h e p h o s p h o r u s l i g a n d may n o t a f f e c t on t h e e x c i t e d s t a t e n a t u r e o f t h e m e t a l - m e t a l b o n d ing. T h i s i s i n q u i t e c o n t r a s t w i t h the ground s t a t e events mentioned above. F o r e x a m p l e , t h e compound o f L = P ( i - P r ) , w h i c h u n d e r g o e s v e r y f a c i l e m e t a l - m e t a l b o n d c l e a v a g e i n t h e d a r k , s h o w s a l m o s t t h e same i n the r a t i o of the p r o c e s s e s ( l ) / ( 2 ) t o those of the t h e r m a l l y very s t a b l e analogs. 3
Occurrence
of
a Third
Process
B i s i d e s a f o r e m e n t i o n e d l i g a n d e f f e c t s , a new p h e n o m e n o n h a s b e e n o b s e r v e d i n the p h o t o l y s i s of L=P(n-Bu)3Another t r a n s i e n t a b s o r p t i o n w i t h X a r o u n d 4 7 0 nm g r o w s i n 100 u s a n d d i s a p p e a r s w i t h i n 2 ms ( F i g . 3 ) . M
A
X
T h e a b s o r b a n c e a t 3 8 0 nm r e c o v e r s w i t h t h r e e s t e p s a n d t h e k i n e t i c s o f each s t e p c o r r e s p o n d s t o t h e p r o c e s s ( 5 ) , t h e d i s a p p e a r a n c e o f t h e a b s o r p t i o n a t 4 7 0 nm, a n d t h e p r o c e s s ( 6 ) u n d e r CO a t o m o s p h e r e s h o w i n g t h a t t h e t h i r d compornent d e c a y s a l s o back t h e s t a r t i n g compound. Both t h e g r o w t h a n d d e c a y o f t h e a b s o r b a n c e a t 4 7 0 nm f o l l o w t h e f i r s t o r d e r k i n e t i c s a n d t h e c o v e r i n g g a s ( A r o r CO) h a s n o e f f e c t o n t h e k i n e t i c s . Solvent (cyclohexane, benzene, o r t e t r a h y d r o f u r a n ) does n o t almost a f f e c t it's rate. A d d i t i o n o f C C I 4 quenches the occurrence of t h e t h i r d compernent. These f a c t s s t r o n g l y suggest t h e p r e s e n c e o f a t h i r d p r o c e s s t o g i v e an u n k n o w n i n t e r m e d i a t e w h i c h may s e c o n d ary .give t h e compornent w i t h X x a t 4 7 0 nm. I t d e s e r v e s f u r t h e r s t u d y on t h e n a t u r e o f t h e new p r o c e s s , b u t n o t e worthy i s t h a t t h e t h i r d compornents a p p e a r o n l y i n t h e c a s e s o f L = P R , R= n-alkyl. When R=Me, i - P r , o r P h , t h e primary processes a r e s i m p l y (1) and (2) . m
a
3
F i g u r e 3. Growth and decay o f t h e a b s o r b a n c e a t 4 7 0 nm: 1 d i v . = 100 u s .
References Y e s a k a H, K o b a y a s h i T, Y a s u f u k u K, N a g a k u r a S ( 1 9 8 1 ) L a s e r P h o t o l y s i s o f D i m a n g a n e s e D e c a c a r b o n y l . R e z a - K a g a k u K e n k y u 3: 9 7 - 9 9 Y e s a k a H, k o b a y a s h i T, Y a s u f u k u K, N a g a k u r a S ( 1 9 8 3 ) L a s e r P h o t o l y s i s S t u d y o f t h e P h o t o s u b s t i t u t i o n i n D i m a n g a n e s e D e c a c a r b o n y l . J Am Chem Soc 1 0 5 : 6249-6252 Rothberg L J , Cooper N J , P e t e r s KS, V a i d a V (1982) P i c o s e c o n d Dynamics of S o l u t i o n - P h a s e P h o t o f r a g m e n t a t i o n of [Mn2(CO)iol• Chem S o c 104: 3 5 3 6 - 3 5 3 7 H e p p A F , W r i g h t o n MS ( 1 9 8 3 ) R e l a t i v e I m p o r t a n c e o f M e t a l - M e t a l B o n d S c i s s i o n and Loss o f Carbon Monoxide from P h o t o e x c i t e d Dimanganese D e c a c a r b o n y l : U n s a t u r a t e d , C O - B r i d g e d D i n u c l e a r S p e c i e s i n Low-Temp e r a t u r e A l k a n e M a t r i c e s . J Am Chem S o c 1 0 5 : 5 9 3 4 - 5 9 3 5 C h u r c h S P , H e r m a n n H, G r e v e l s F-W, S c h a f f n e r K ( 1 9 8 4 ) T h e P r i m a r y Photop r o d u c t s o f M n 2 ( C O ) i o : D i r e c t I.R. O b s e r v a t i o n a n d Decay K i n e t i c s o f Mn(C0) and Mn (C0)9 i n Hydrocarbon S o l u t i o n a t Room T e m p e r a t u r e . J Chem S o c Chem Commun 7 8 5 - 7 8 6 Y a s u f u k u K, N o d a H, I w a i J , O h t a n i H, H o s h i n o M, K o b a y a s h i T ( 1 9 8 5 ) L a s e r P h o t o l y s i s S t u d y o f D i r h e n i u m D e c a c a r b o n y l : E v i d e n c e f o r a Nonr a d i c a l P r i m a r y P r o c e s s . O r g a n o m e t a l 4: 2 1 7 4 - 2 1 7 6 K o b a y a s h i T, O h t a n i H, N o d a H, T e r a t a n i S, Y a m a z a k i H, Y a s u f u k u K ( 1 9 8 6 ) E x c i t a t i o n Wavelength Dependence o f P h o t o d i s s o c i a t i o n and t h e Secondary L a s e r P u l s e P h o t o l y s i s of Dimanganese D e c a c a r b o n y l . Organometal 5: 1 1 0 - 1 1 3 Y a s u f u k u K, N o d a H, O n a k a S, T e r a t a n i S, K o b a y a s h i T ( t o b e p u b l i s h e d ) Laser F l a s h P h o t o l y s i s Study o f Dimethylbis[pentacarbonylmanganese]t i n ( I I ) , 2(Mn-Sn). W a l k e r HW, H e r r i c k R S , O l s e n R J , B r o w n T L ( 1 9 8 4 ) F l a s h P h o t o l y s i s S t u d i e s o f D i n u c l e a r M a n g a n e s e C a r b o n y l C o m p o u n d s . I n o r g Chem 2 3 : 3 7 4 8 - 3 7 5 2 H e r r i c k RS, Brown TL (1984) F l a s h P h o t o l y t i c I n v e s t i g a t i o n o f P h o t o m d u ced Carbon Monoxide D i s s o c i a t i o n from D i n u c l e a r Manganese C a r b o n y l C o m p o u n d s . I n o r g Chem 2 3 : 4 5 5 0 - 4 5 5 3 T o l m a n CA ( 1 9 7 7 ) S t e r i c E f f e c t s o f P h o s p h o r u s L i g a n d s i n O r g a n o m e t a l l i c C h e m i s t r y a n d H o m o g e n e o u s C a t a l y s i s . Chem R e v 7 7 : 3 1 3 - 3 4 8 J
5
2
A
m
TOPIC 6 Methods, A p p l i c a t i o n s , and Other Aspects
ELECTRON TRAPPING IN COLLOIDAL T i 0 2 20 ps TO 10 ns K I N E T I C S
C . A r b o u r , D.K.Sharma, a n d
PHOTOCATALYSTS:
C.H.Langford
Canadian Centre f o r Picosecond Laser Flash P h o t o l y s i s , Concordia 1455 W e s t , de M a i s o n n e u v e , M o n t r e a l ( Q u e b e c ) H3G 1M8, CANADA
University,
ABSTRACT The t r a p p i n g of excess electrons i n surface s i t e s i n < 0.05 um c o l l o i d a l p a r t i c l e s of Ti02 i n a c i d media has a time constant near 2 ns, independently of the excess e l e c t r o n population being produced by i n j e c t i o n from an e x c i t e d dye or hole scavenging. I t i s suggested t h a t t h i s r e l a t i v e l y long l i v e d state i s one involved i n e l e c t r o n t r a n s f e r t o s o l u t i o n acceptors i n p h o t o c a t a l y t i c processes. INTRODUCTION Titanium dioxide photocatalysis has been studied since the 1930's. The e f f o r t t o avoid degradation of organic matrices c o n t a i ning the oxide as a white pigment stimulated an a c t i v e program of " a n t i - p h o t o c a t a l y t i c " research. The r e s u l t s were methods to d e a c t i vate Ti02 surfaces. Since the s t r i k i n g report of the photoassisted e l e c t r o l y s i s of water by Fujishima and Honda i n 1972, the d i r e c t i o n of research has reversed. Beyond p h o t o l y s i s of water, major i n i t i a t i v e s have included the Krautler-Bard (1977) photo-Kolbe r e a c t i o n and photochemical degradation of r e f r a c t o r y c h l o r i n a t e d aromatics introduced by Carey et a l . (1976) and Carey and O l i v e r (1980). The development of techniques f o r working with c o l l o i d a l p h o t o c a t a l y t i c Ti02 by Duonghong et a l . (1981) has made i t p o s s i b l e t o analyze the elementary events which occur i n p h o t o c a t a l y t i c r e a c t i o n s using the t o o l s of f l a s h p h o t o l y s i s . There are two e s s e n t i a l questions which must be addressed i n a n a l y s i s of e i t h e r photoxidation or photoreduction: what i s the rate of i n t e r f a c i a l e l e c t r o n t r a n s f e r , and what i s the f a t e of c a r r i e r s generated i n Ti02?
E a r l y work i n d i c a t e d that f a s t e l e c t r o n t r a n s f e r occurs with adsorbed partners. E a r l y subnanosecond studies confirmed e l e c t r o n i n j e c t i o n from E r y t h r o s i n B i n CH3CN i n l e s s than 250 ps (Kamat and Fox, 1983) and i n j e c t i o n from adsorbed e x c i t e d t e t r a s u l f o n a t o copper phthalocyanine i n l e s s than 100 ps (Kirk et a l . 1984). I t w i l l be reported below that hexasulfonated t r i s (phenanthroline)ruthenium(II) captures holes from e x c i t e d Ti02 i n l e s s than 50 ps. The f a t e of c a r r i e r s i n Ti02 was f i r s t e l u c i d a t e d by f l a s h p h o t o l y s i s studies by Henglein (1982) and Bahnemann et a l . (1984). An e l e c t r o n spectrum i n Ti02 (pH = 1.5) was e s t a b l i s h e d by e l e c t r o n i n j e c t i o n from r a d i o l y t i c a l l y generated organic r a d i c a l s . This spectrum has a peak near 625 nm and a shoulder near 500 nm. The exact shape of the spectrum v a r i e s with d e t a i l s of the preparation of the c o l l o i d s . This spectrum was a l s o generated by adsorption of p o l y v i n y l a l c o h o l (PVA) on the Ti02 followed by d i r e c t i r r a d i a t i o n of the c o l l o i d a t 347 nm. PVA y i e l d s the e l e c t r o n spectrum by scaveng i n g holes, h* , to allow an e l e c t r o n build~up i n preference to recombination. The r e a c t i o n with PVA i s " r a p i d " on the time scale of 15 ns. The decay of the e l e c t r o n s i g n a l i s slow on microsecond time s c a l e unless an e l e c t r o n acceptor i s present i n the s o l u t i o n . Henglein et a l . assumed t h a t e l e c t r o n s are trapped near the surface. The nature of a trapped e l e c t r o n remaining a f t e r steady-state i r r a d i a t i o n i s e l u c i d a t e d by ESR recorded at 77 K (Howe and G r a e t z e l ,
3 +
1985). Two s i g n a l s are observed which may be assigned t o T i . A small s i g n a l i s a s s o c i a t e d with i n t e r s t i t i a l ions i n anatase. The l a r g e r s i g n a l resembles T i 3 i n s i l i c a t e g l a s s e s and shows an environmental s e n s i t i v i t y (e.g. pH response) suggestive of a s u r f a c e location. As Bahnemann et a l . (1984) summarized the matter: "photoc a t a l y t i c r e a c t i o n s i n Ti02 s o l s can be achieved with s i z a b l e y i e l d s only i f two scavengers are present (one f o r e~, one f o r h ) and a t l e a s t one of them i s adsorbed on the c o l l o i d a l p a r t i c l e s . I t i s the adsorbed scavenger which determines the y i e l d of the r e a c t i o n of the non-adsorbed one". Recombination can be f a s t , scavenging by adsorbed molecules can be f a s t , and trapped excess c a r r i e r s can be long l i v e d . Recently, Rothenberger e t a l . (1985) have provided d i r e c t picosecond information on the recombination process i n the absence of scavengers. Using a 25 ps pulse of about 2.5 mw power a t 355 nm (Nd/YAG t h i r d harmonic), a t r a n s i e n t i s produced which decays w i t h a time constant of l e s s than 500 ps. The decay i s a second order process depending on the square of the number of photons i n i t i a l l y absorbed. The decay i s a t t r i b u t e d t o hole e l e c t r o n recombination. The i n t e r e s t i n g p o i n t i s t h a t the spectrum does not agree with the spectrum of the surface trapped e l e c t r o n reported before o r the spectrum of e l e c t r o n s seen at pH = 3 a f t e r steady s t a t e or long p u l s e i r r a d i a t i o n by K o l l e , Moser, and G r a e t z e l (1985). The t r a n s i e n t which a r i s e s i n the absence of scavenging has a maximum a t an energy near but t o the b l u e of 600 nm and does not drop s h a r p l y toward the blue. The spectrum obtained i n the absence of scavengers appears promptly a t a 20 ps probe p u l s e delay and decays' i n a second order process. I t i s a f l a t band with a maximum somewhat t o the blue o f the other s p e c t r a obtained a t longer times. I t seems probable t h a t there are two forms of t r a n s i e n t s a s s o c i a t e d w i t h c a r r i e r s i n T i 0 2 . One i s produced promptly on e x c i t a t i o n w i t h greater than band gap photons. The other a r i s e s a f t e r development of an excess e l e c t r o n population and has a long l i f e t i m e . +
+
E X P E R I M E N T A L
Ti02 c o l l o i d s were prepared by the slow a d d i t i o n of T i C l 4 to c o l d water. Concentrations were v a r i e d between 1.0 and 16 g/L. The pH of s o l u t i o n s was near 1 and were not r a i s e d . The s i z e of the p a r t i c l e s v a r i e d as a f u n c t i o n of both the c o n c e n t r a t i o n and the d e t a i l s of p r e p a r a t i o n . The p a r t i c l e s i z e estimation was based on the s h i f t t o the blue o f the absorption edge as the p a r t i c l e s i z e becomes small as d e s c r i b e d by Nozik (13). The procedure was c a l i b r a ted by dynamic l i g h t s c a t t e r i n g s t u d i e s of p a r a l l e l p r e p a r a t i o n s . (We thank Prof. Janos F e n d l e r f o r a s s i s t a n c e with these measurements and access t o h i s equipment.) The dyes were the t e t r a s u l f o n a t e d copper phthalocyanine used p r e v i o u s l y ( K i r k e t a l . . 1984), a hexasulfonate d e r i v a t i v e of t r i s p h e n a n t h r o l i n e r u t h e n i u m ( I I ) which i s t e t r a a n i o n i c and was the generous g i f t of Dr. Ann E n g l i s h , and commercial e r y t h r o s i n . Picosecond f l a s h p h o t o l y s i s experiments were c a r r i e d out u s i n g a Nd/YAG mode locked l a s e r i n the second and t h i r d harmonic modes (562 nm and 355 nm). The p u l s e width i s 30 ps and the zero of time i s adjusted a t the maximum when 50% of the p u l s e has passed. A f r a c t i o n of the fundamental of the e x c i t a t i o n pulse i s passed through a c e l l c o n t a i n i n g D2O t o generate a continuum p u l s e u s e f u l f o r p r o b i n g t h e absorption spectrum from 425 t o 675 nm. The probe pulse, which has the same time p r o f i l e as the e x c i t a t i o n pulse, i s delayed from 20 ps to 10 ns from the c e n t r e of the e x c i t a t i o n pulse. Each spectrum i s the r e s u l t of averaging ten shots.
RESULTS a. Absorption spectra. The p a r t i c l e s i z e of the Ti02 may be monitored by r e c o r d i n g the absorption spectrum of T i 0 2 . Up t o a p a r t i c l e s i z e of 0.025 urn, the observed band edge s h i f t s toward the red. T h i s i s i l l u s t r a t e d by the spectra shown i n F i g . 1 f o r samples of Ti02 a t concentrations of 1.0 t o 16 g/L. In a d d i t i o n t o the increase of concentration, t h i s s e r i e s r e f l e c t s an increase of p a r t i c l e s i z e and the red s h i f t i s observable. The p a r t i c l e s i z e range i n our experiments was c a l i b r a t e d by the study of independently prepared samples which were examined by dynamic l i g h t s c a t t e r i n g .
WAVELENGTH
F i g u r e 1. The absorption spectra of Ti02 given i n absorbance/gm.
colloids.
Absorptivity i s
b. Picosecond f l a s h p h o t o l y s i s . Figure 2 e x h i b i t s the spectrum obtained on f l a s h i n g Ti02 alone. The t r a n s i e n t observed i s present promptly 20 ps a f t e r the 355 nm pulse. I t s decay i s nearly complete w i t h i n 1 ns. These observations are e n t i r e l y concordant with those of Rothberger e t a l . (1985). F i g u r e 3 shows a r e p r e s e n t a t i v e t r a n s i e n t spectrum f o r a sample of Ti02 of 1.3 g/L with 6.7 X 10~ M t e t r a s u l f o n a t e d copper phthalocyanine ( C u P c S ) adsorbed on the oxide (as demonstrated by the f a c t t h a t the c o l o u r of the dye may be removed along with the c o l l o i d by f i l t r a t i o n or c e n t r i f u g a t i o n ) . I n i t i a l l y (20 p s ) , b l e a c h i n g of the dye and e x c i t e d s t a t e absorption centred a t 520 nm are observable. At 100 ps, b l e a c h i n g and new absorbances are i n approximate balance and the absorbance change l i e s near zero a t a l l wavelengths. Subsequently, the strong absorption i n the red grows i n . 5
4-
F i g u r e 2 . Subnanosecond t r a n s i e n t s i n Ti02 a f t e r i r r a d i a t i o n with 2.5 mJ, 20 ps h a l f width, Nd/YAG t h i r d harmonic pulses a t 355 nm. The p a r t i c l e s i z e i s near 500 A. The c o n c e n t r a t i o n i s 18 g/L.
WAVELENGTH (nm)
F i g u r e 3. Transient s p e c t r a under spectrometer c o n d i t i o n s of previous f i g u r e f o r Ti02 - CuPcS*- system.
F i g u r e 4 presents a r e p r e s e n t a t i v e experiment using the a n i o n i c Ru(II) complex adsorbed on T i 0 2 . In the p a r t i c u l a r case, the s o l u t i o n c o n t a i n s 2.5 X 10-5 M hexasulfonated t r i s ( p h e n a n t h r o l i n e ) r u thenium(II) (Ru(phen)3S*~) adsorbed onto 4.0 g/L of Ti02 of approximately 50 A p a r t i c l e s i z e . In t h i s system, the c o l l o i d has a band gap absorbance a t 355 nm of 0.20. The Ru(II) complex has an absorbance of 0.08. At 20 ps, absorbance due t o d i r e c t e x c i t a t i o n of Ti02 i s observable. I t decays r a p i d l y . The red band observed p r e v i o u s l y has grown c o n s i d e r a b l y a t 5ns.
4
F i g u r e 4. T r a n s i e n t spectra f o r Ru(phen)3S ~ - Ti02 system under spectrometer c o n d i t i o n s of Figure 2. F i g u r e 5 shows a f i n a l s e t of t r a n s i e n t s f o r 1.2 X 10~ M e r y t h r o s i n adsorbed onto 4.0 g/L T i 0 2 . Bleaching a t the center of the spectrum due t o the l o s s of the strong absorption of the dye depresses the spectrum a t a l l times recorded, but the growth of red t r a n s i e n t i s again observable. The growth of the s i g n a l i n the red has been evaluated k i n e t i c a l l y u s i n g 625 nm as the monitoring wavelength. In e i g h t s o l u t i o n s of the CuPcS*- over the p a r t i c l e s i z e range the r a t e constant was 8 6 X 10 s " i . For four samples of c o l l o i d s s e n s i t i z e d with the Ru(II) complex, the r a t e constant was 6.5 I X 10* s~i . For the s i n g l e p a r t i c l e s i z e used with e r y t h r o s i n the r a t e constant was 5.5 I X 10 s ~ i . The r a t e constants are i n d i s t i n g u i s h a b l e as the s e n s i t i z e r varies. In c o n t r a s t , there may be some r e a l v a r i a t i o n with p a r t i c l e p r e p a r a t i o n , although t h i s i s not e s t a b l i s h e d . Although i t i s d i f f i c u l t t o c o r r e c t the spectra f o r the spectra of the s e n s i t i z e r s , the e x t i n c t i o n c o e f f i c i e n t of the red band i s approximately 2 X 10 M~ cm~ . I t i s a l s o important t o observe t h a t the k i n e t i c s are the same whether the primary absorber i s the s e n s i t i z e r or the Ti02 c o l l o i d itself. 6
8
8
4
1
1
140
. 100
g
.060 1
. 0 2 0 -f
, 020 400.
450.
500.
550.
600.
650.
50 ps 1
700.
WAVELENGTH (nm)
F i g u r e 5.
T r a n s i e n t s p e c t r a f o r the e r y t h r o s i n - Ti02 system.
DISCUSSION There are three aspects of the present experiments which can be analyzed t o lead t o a reasonable i n t e r p r e t a t i o n . These are the k i n e t i c s of the growth of t h e s i g n a l i n the red, the e x t i n c t i o n c o e f f i c i e n t of the r e d band, and the s p e c t r a l shape of the absorbance. The f i r s t p o i n t i s t h e k i n e t i c behaviour. In a l l three cases, the choice of s e n s i t i z e r i s unimportant t o the r a t e of growth of the r e d band. Nevertheless, a s e n s i t i z e r i s important t o the growth of the r e d band. The r o l e of the s e n s i t i z e r i s t o ensure an excess o f e l e c t r o n s . A n a l y s i s of the e n e r g e t i c s of each of the s e n s i t i z e r s i n d i c a t e s t h a t they can p a r t i c i p a t e i n two processes. The e x c i t e d dyes are capable of i n j e c t i n g e l e c t r o n s i n t o the conduction band of Ti02. S i m i l a r l y , a hole i n the Ti02 valence band can o x i d i z e the dyes. Thus, e i t h e r o r both of the e x c i t a t i o n processes t a k i n g p l a c e can lead t o development of excess e l e c t r o n s i n the c o l l o i d . There are two p o s s i b i l i t i e s f o r the o r i g i n of the r e d t r a n s i e n t with a "grow i n " time of ~2 ns which are c o n s i s t e n t with the independence of the s e n s i t i z e r . E i t h e r the process i s a t r a p p i n g process i n the c o l l o i d o r i t i s the formation of a s o l v a t e d e l e c t r o n i n the solution. The i n t e n s i t y of t h e red t r a n s i e n t i s greater than the t r a n s i e n t produced i n i t i a l l y o r when Ti02 alone i s e x c i t e d . The observed value i s very c l o s e t o t h a t reported f o r a s o l v a t e d e l e c t r o n (14). However, the a b s o r p t i o n maximum i s not i d e n t i c a l t o s o l v a t e d e l e c t r o n s p e c t r a i n water. Moreover, the r a t e constant f o r proton quenching of a hydrated e l e c t r o n i s 2.1 X 10*0 M~* s ~ i (Hart, 1965). In a s o l u t i o n a t pH ~ 1, t h e l i f e t i m e of a s o l v a t e d e l e c t r o n should be near one ns. The s o l v a t e d e l e c t r o n explanation seems t o be excluded. In c o n t r a s t , an e x t i n c t i o n c o e f f i c i e n t near t h a t of the s o l v a t e d e l e c t r o n and a spectrum with peaks s i m i l a r t o those f i r s t d e s c r i b e d by Bahnemann e t a l . (1984) a t 500 nm and 625 nm i s c o n s i s t e n t with the ESR evidence f o r a Ti3+ s p e c i e s i n the surface of the p a r t i c l e . The two bands are reasonably those of a Ti06 d-d band and a broadened red s h i f t e d band corresponding t o d e l o c a l i z a t i o n o f the acceptor
orbital. I t appears that the band which grows i n with a time constant of ~2 ns i s r e l a t e d t o bands associated with e l e c t r o n t r a n s f e r from T i 0 2 . I t i s important t o r e a l i z e that "surface" states of hydrous oxides c o l l o i d s may not be e n t i r e l y l i k e surface s t a t e s of c r y s t a l s . Hydrous oxide c o l l o i d s may be "water swollen g e l s " with the e n t i r e s t r u c t u r e of a small p a r t i c l e i n contact with "solvent" and a c c e s s i b l e as a s i t e f o r adsorption of a s e n s i t i z e . Thus, the s t a t e which i s described may not correspond t o a s t a t e i n c r y s t a l l i n e T i 0 2 . A f i n a l question presents i t s e l f . What i s the nature of the r a p i d l y decaying s i g n a l seen when e l e c t r o n s and holes are generated i n equal numbers? This i s not a question t o which a d e f i n i t i v e answer can be given from the a v a i l a b l e evidence. However, one p o s s i b i l i t y does suggest i t s e l f . A spectrum f o r excess holes was presented e a r l y on (Bahnemann e t a l . . 1984). I t includes a band i n the blue.A combination of that band envelope and a gently r i s i n g absorbance t o the r e d could produce the broad envelope of the s o r t seen i n a s i t u a t i o n where a sum of an e l e c t r o n and a hole spectrum should be expected.
REFERENCES Bahnemann, D., Henglein, A., L i l i e , J . , and Spanhel, L., (1984), J . Phys. Chem., 88, 207. Carey, J.H. , Lawrence, J . , and Tosine, H.M., Contam. T o x i c , 16, 697.
(1976), B u l l . E n v i r .
Carey, J.H. and O l i v e r , B.G. , (1980), Water P o l l . Res. J . Canada, 15 157. Duonghong, D. , B o r g a r e l l o , E., and G r a e t z e l , M., (1981), J . Amer. Chem. Soc, 103, 4685. Fujishima, A. and Honda, K., (1972), Nature, 238, 37. Hart, E.J., (1965), "The Solvated E l e c t r o n " , ACS Advances i n Chemistry, American Chemical Society, Washington, D C , p. 51. Henglein, A., (1982), Ber. Bunsenges., 86, 241. Howe, R.F. and G r a e t z e l , M., (1985), J . Phys. Chem., 89, 4495. Kamat, P.V. and Fox M.A.,
(1983), Chem. Phys. L e t t s . , 102, 379.
K i r k , A.D., Langford, C.H., S a i n t - J o l y , C., Sharma, D.K., and LeSage R., (1984), JCS Chem Comms., 961. K o l l e , U., Moser, J . , and G r a e t z e l , M., (1985), Inorg. Chem., 24, 2253. K r a u t l e r , B. and Bard, A.J., (1977), J . Amer. Chem. S o c , 99, 7729. Nozik, A. e t a l . .
(1985), J . Phys. Chem., 89, 397.
Rothenberger, G., Moser, J . , Graetzel, M., Serpone, N., and Sharma, D.K., (1985), J . Amer. Chem. S o c , 107, 8054.
THE RADIATION S E N S I T I V I T Y OF SELECT METAL CHELATE MECHANISTIC CHANGES AT HIGHER ENERGIES
R.D.Archer, C . J . H a r d i m a n , and Department
POLYMERS:
A.Y.Lee
o f C h e m i s t r y , U n i v e r s i t y o f M a s s a c h u s e t t s , A m h e r s t , MA
01003,
USA
INTRODUCTION Our
interest
first
circuit and
i n the radiation
sensitivity
s p a r k e d by t h e g o a l o f enhanced (IC) chip
closer
niques
features.
o f heavy
As t h e f e a t u r e s
together a t t h e submicron
must use h i g h e r energy
level,
photons
metal polymers
miniaturization
o f IC chins
get closer
microlithographic
(orelectrons,
was
of integrated tech-
e t c . ) than a r e
i currently pass
used.
cuits.
The t h i n
lithographic higher the
Otherwise, d i f f r a c t i o n
through adjacent features
energy
blur
o r g a n i c polymer
resists,
suffer
radiation.
oxide layer which
effects
the features
films,
which
from an i n a b i l i t y
The u n a b s o r b e d
i s n o r m a l l y between
by t h e photons and a l l o w
short
which cir-
a r e n o r m a l l y used as t o absorb
radiation
a l lo f the
backscatters
the r e s i s t
from
and t h e semi-
conductor . Heavy
atom-containing polymer
ty
to applied
of
the photoelectric
which
i s Z
radiation
films
because
effect
s h o u l d have an enhanced
o f t h e a t o m i c number
which
sensitivi-
(Z) d e p e n d e n c e
h a s an a t o m i c a b s o r p t i o n
coefficient
atoms o n l y
o r g r e a t e r (Evans 1972) w h e r e a s p h o t o n s c a t t e r i n g by h e a v y 2 2 has a Z dependence. To t e s t t h i s c o n c e p t , we s y n t h e s i z e d
and
a number o f u r a n y l
tested
dicarboxylate
polymers
f o r gamma-ray
1 V i s i b l e a n d u l t r a v i o l e t r a d i a t i o n p a s s e d t h r o u g h masks o r o r o j e c t e d o n t o t h e s u r f a c e p r o v i d e t h e mass p r o d u c t i o n n e c e s s a r y f o r t h e l o w c o s t d e v i c e s c u r r e n t l y on t h e m a r k e t . A l t h o u g h e l e c t r o n beam l i t h o graphy can p r o v i d e h i g h e r r e s o l u t i o n , t h e throughput r a t e and backs c a t t e r i n g p r o b l e m s s u g g e s t t h a t x - r a y o r l o w - e n e r g y gamma-ray m e t h odology i s needed. F o r more d e t a i l s s e e e i t h e r o f two r e c e n t r e v i e w volumes (Thompson 1 9 8 3 , 1984) 2 P o l y m e r s w i t h m a i n g r o u p h e a v y e l e m e n t s h a v e shown i n c r e a s e d s e n s i t i v i t y u s i n g t h e e m p i r i c a l method o f d e t e r m i n i n g s o l u b i l i t i e s b e f o r e a n d a f t e r r a d i a t i o n (Webb 1 9 7 9 ; H a l l e r 1 9 7 9 ) . Such s e n s i t i v i t i e s a r e based on t h e dose p e r u n i t a r e a r e q u i r e d t o d i s s o l v e t h e i r r a d i a t e d p o l y m e r w i t h 1/2 o f t h e u n i r r a d i a t e d p o l y m e r s t i l l o n t h e s u b s t r a t e (for a p o s i t i v e r e s i s t and v i c e v e r s a f o r a n e g a t i v e r e s i s t ) . They are s o l v e n t a n d l a b o r a t o r y d e p e n d e n t , c f . Thompson (1983, 1 9 8 4 ) , a n d are n o t as fundamental as G v a l u e s .
sensitivity. chemistry,
c f . Burrows
mixed w i t h hydrogen a
sizable
o n u r a n y l c a r b o x y l a t e Dhoto
From t h e e x t e n s i v e l i t e r a t u r e
( 1 9 7 4 ) , we a n t i c i p a t e d
abstraction
(Rehorek
number o f t h e u r a n y l p o l y m e r s
irradiation,
b u t they
crosslink
C0 evolution, 2
1982).
s h o w n o CO^ l o s s
with very
possibly
Much t o o u r s u r p r i s e , 137
high efficiency
during
Cs
instead.
METHODOLOGY The
irradiation
s t u d i e s were
conducted
with
a c e s i u m - 1 3 7 gamma
photon
source
(662 k e V ) w i t h d o s e r a t e s o f 0.022 t o 0.073 M r a d / h r b a s e d
Fricke
dosimetry
(Hine
1956; G e t o f f
1962).
mass a b s o r p t i o n c r o s s s e c t i o n s b e t w e e n polymers
w e r e made s u b s e q u e n t l y
t e r p o l a t e d when n e c e s s a r y . sealed
Pyrex
mers b e f o r e (or s i z e
o r quartz and a f t e r
on U l t r a s t y r a g e l
polystyrene
standards
(GCMS), e l e c t r o n netic
resonance
( i n NMP).
spin
resonance
were used
1964)
and r e i t e r a t e d
1/M '
= 1/M ° + n
where M ^
0
1
M G
g
G^
1/M '
s
x
and
G
the
i sthe radiation i sAvogadro's t o Megarads
dose
s
products. (1954, 1960,
scissions
dose
after
i nelectron
irradiation, irradiation,
p e r 100 e l e c t r o n
p e r 100 e l e c t r o n
(Mrad) t h e e q u a t i o n
volts
volts,
volts,
p e r gram, and
becomes:
6
x 10" R)
x
[2]
on random s c i s s i o n
molecular weight
independent.
before
number,
a r e based
possible
dose dependence
(UVV)
[1]
i s t h e number o f c h e m i c a l
n
x
spectroscopy
F o u r i e r t r a n s f o r m n u c l e a r mag-
(IR), and u l t r a v i o l e t - v i s i b l e
i s t h e number o f c r o s s l i n k s
r
guide
(NMP)
with
a
J
best
Gas c h r o m a t o g r a p h y - m a s s (ESR),
i s t h e number-average m o l e c u l a r weight
random i n i t i a l
permeation
had been c a l i b r a t e d
i s t h e number-average m o l e c u l a r weight
These e q u a t i o n s
of the poly-
(1973):
= 1 / M ° 4- ( G - G ) ( 1 . 0 4
n
via gel
i n -
using
(G -G ) (r/100N )
Na Converted
i n vacuo
u s i n g t h e method o f C h a r l e s b y
by Dole
1960)
(GPC) i n N - m e t h y l p y r r o l i d o n e
to help elucidate
G v a l u e s were d e t e r m i n e d
(Mann
analysis
conducted
columns, which
(NMR) a n d i n f r a r e d
spectroscopies
n
were
on
different
and t h e u r a n y l
data
irradiated
Molecular weight
irradiation
e x c l u s i o n ) chromatography
solutions
the dosimeter
using literature
Samples were
tubes.
Corrections for
Even
and c r o s s l i n k i n g
(H /M = 2), and w i t h G w' n ' s f o r n o n - i d e a l systems, they provide t h e distributions
t o t h e magnitude o f t h e r a d i a t i o n
i s apparent
processes
from
plots
o f 1/M
n
vs dose
effects, i n Mrad.
and t h e
The
synthesis and c h a r a c t e r i z a t i o n o f the uranyl dicarboxylates i s de-
tailed
elsewhere
(Archer
1987;
Hardiman
1 9 8 7 ) . The e s s e n t i a l
synthetic
reaction i s U0 (0 CCH ) .(H 0) 2
2
3
2
2
+ H0 CRC0 H-5-5-5^.
2
2
[U0 (0 CRC0 )2
2
2
2
2
n
[3 J
+ CB^CO^
2 6 The
molecular
weights
h a v e b e e n d e t e r m i n e d b y GPC, v i s c o s i t y , 13
end-group a n a l y s i s . to
confirm
uranyl
or
t h e c o o r d i n a t i o n modes o f t h e c a r b o x y l a t e
ion.
19 8 7 ) ; occur
S o l i d - s t a t e C-NMR h a s a l s o been u s e d
The r e s u l t s
t h a t i s , both such
that
monodentate and b i d e n t a t e predominates
two moles o f s o l v e n t coordinated
carboxylate
coordination
and
one a r o m a t i c
with the
no evidence entire
4.09
of C0
absorption
barns/atom
type,
The d a t a
CROSSLINK
f o r four branched
dicarboxylates which
alkyl,
sensitive to one a l k e n e ,
appear t o e x c l u s i v e l y
e v o l u t i o n , a r e summarized
2
for the lighter
e i t h e r one
ion.
a n t i c i p a t e d , t h e u r a n y l d i c a r b o x y l a t e polymers a r e very
gamma-ray i r r a d i a t i o n .
(Hardiman
i n polymers with
to theuranyl
URANYL POLYMERS WHICH A P P E A R TO E X C L U S I V E L Y As
ligands t o the
a r e as a n t i c i p a t e d from IR r e s u l t s
7-coordination
a n d NMR
( A r c h e r 1987)
elements
i n Table
(0.257,
crosslink, 1. W h e r e a s
1.54, 2 . 0 4 ,
and
f o r H, C, 0, a n d S, r e s p e c t i v e l y ) i s o f t h e C o m p t o n
f o r uranium 50% i sp h o t o e l e c t r i c absorption
(47.7
barns/atom
total).
T h u s , t h e u r a n i u m a t o m c o n s t i t u t e s f r o m 50 t o 5 8 % o f t h e a b -
sorption
i n theunit at this
mass a b s o r p t i o n
coefficient
to water,
t h e absorber
typically
lower
than
2
0.083 c m / g u s i n g polymers. sorb
from
of t h e Fricke dosimeter.
water;
G values
as h i g h
i nTable
poly(methyl
lower
links.
Extra crosslinks
G
values—a
Organic
polymers are
than
PMMA, i s the uranyl
-G . x
curvature
was o b s e r v e d high
value
With
until
above
1934).
for scission,
i n c r e a s e d dose
thus,
levels, a l l
consequence o f fewer e f f e c t i v e
The v a l u e s
do n o t i n c r e a s e t h e
i n t h e t a b l e a r e f o r 2 Mrad
and phthalate 3 Mrad
cross-
d e r i v a t i v e s a s no
f o r these
two s p e c i e s ) .
The
f o r t h e t e t r a p i m e l a t e . d e r i v a t i v e must be r e -
t o them o b i l i t y of the longer radicals
(Thompson
b e t w e e n t h e same c h a i n s
for the succinate
extremely
a s PNMA (G = 1.3) a n d o r g a n i c
a s 10 a r e k n o w n
logical
weight any f u r t h e r .
( o r 3 Mrad
tertiary
13 t o 1 6 % r e l a t i v e
methacrylate),
1 show a n y t e n d e n c y
i se s s e n t i a l l y
exhibit
lated
e.g.,
6 0 0 t o 7 0 0 % a s much r a d i a t i o n
G -G v a l u e s x
doses
but only modifies the
t h e same n u m b e r s , o r 17 t o 2 0 % l o w e r
None o f t h e s p e c i e s
molecular
(66 2 k e V ) ,
However, on a r e p e a t i n g u n i t b a s i s , t h e u r a n y l p o l y m e r s a b -
polymers with
the
energy
f o r t h e polymer by about
coupled
with
alkyl
efficient
chain energy
coupled
with
transfer.
stabilized
Table
1.
Gamma-ray
Empirical
Formula U n i t
(bridging
U0
2
Sensitivity
of Negative
a
M
Resist Uranyl
b
G
R
Polymers
G
G
s~ x°
G
s~ x
G
carboxylate)
0 CC(CH ) CH C0 2
3
2
2
2
(C H SO) 2
9,000
6
g
~14
g
0.100
-12
g
(2,2-dimethylsuccinate) U0
2
0 CC(CH ) CH CH C0 2
3
2
2
2
(C^SO)
2
48,OOO
f
-3.1
f
0.100
-2.1"
(2,2-dimethylglutarate) U0
2
0 CCH C(CH ) CH C0 2
2
3
2
2
2
(C H SO) 2
6
12,000
g
-8.0
12,000
g
-43
g
0.100
-6.9
0.099
-37
g
(3,3-dimethylglutarate) U0
2
0 CC(CH ) (CH ) C(CH ) C0 2
3
2
2
3
3
2
2
(C H SO) 2
6
g
g
(2,2,6,6-tetramethylpimelate) U0 (Z-0 CH-CHC0 )(C H S0) 2
2
2
2
6
1
7
9,300
5
-3.3
0.099
-2.9
(maleate) U0 (o-0 CC H C0 )(C H SO) 2
2
6
4
2
2
6
17,800
2
g
-3.7
g
0.097
-3.3
g
(phthalate) a
Repeating
kfl
unit
r
e
chain
o f samples n o t i r r a d i a t e d ;
standards, d
o f polymer
Measured
o r b y NMR
including
based
o n GPC
i n NMP
end-group a n a l y s i s i f so
net G value
using
equation
2 a n d GPC
with
polystyrene
indicated 137 after Cs
irradiation
2
M a s s a b s o r p t i o n c o e f f i c i e n t s i n cm / g f o r 6 6 2 k e V i r r a d i a t i o n . 1 , . . .. ^ __2 137 Corrected G values r e l a t i v e t o F r i c k e dosimeter(0.086 cm / g f o r Cs)
"^Polymer h a s p o o r M d i s t r i b u t i o n g
solvation
M
values
URANYL
On
for this
POLYMERS
the other
ligands noted
2.
a l l show l a r g e G
a t r u e measure o f G .
for both
scission
g
shown.
reliable
g
toward
three values
different thio-bridged upon i r r a d i a t i o n
crosslinking;
uranyl
U n f o r t u n a t e l y , they
thus
G
g
i s larger
a r e so s e n s i t i v e
show
molecular
weights
were used
S
"
G X
i s
evidence G^-G^
to radiation
that
These
high
f o r t h e t h i o - b r i d g e d p o l y m e r s w o u l d be e v e n h i g h e r
polystyrene-equivalent
G
the
are impossible to obtain.
than
as
thiodigly-
that i s ,
The GPC's o f t h e o t h e r s
and c r o s s l i n k i n g ;
independent values
Gg-G^values
detn
NMR
The c l e a n e s t one i s t h e p o l y m e r i c
shows no t e n d e n c y
probably
value
by
end-group
WHICH UNDERGO S C I S S I O N WITH GAMMA R A D I A T I O N
and fumarate
which
agreement w i t h
polymer--end-group c a l i b r a t e d
hand, u r a n y l polymers w i t h
i n Table
colate,
and poor
i f the
i n s t e a d o f t h e NMR
Table
2.
Gamma-ray S e n s i t i v i t y
of Positive
a
E m p i r i c a l Formula U n i t /,.-,. (bridging carboxylate)
M n
U0 (0 CCH SCH C0 )(C H SO) 2
2
2
2
2
2
6
b
c
G -G s x
n
14,000
2
Resist Uranyl
g
56
u/o r'y
g
Polymers
f
d e
G -G s x
0.097
50
g
(thiodiglycolate) U0 (0 CCH SSCH C0 ) ( C ^ S O ) ^ 2
2
2
2
g
j
300 ~
g
h
103 '
h
i
12,000 ~
2
g
j
0.097
280 ~
g
j
g
h
0.096
93 '
g
h
i
0.098
55 '
h
1
(dithiodiglycolate) U0 (0 CCH SCH SCH C0 )(C H S0) 2
2
2
2
2
(methylenebis
2
2
6
thioglycolate )
U0 (E-0 CCH=CHC0 )(C H SO) 2
15,000 '
2
2
2
2
6
26,OOO '
2
h
63 '
(fumarate)
a
e
Same as Table
^G v a l u e s g
M
1
based on f i r s t
adjusted
M
distribution
-'polymer v e r y distribution
poor;
scission
results
diglycolate,
the
have G - G g
very
about
and methylenebis x
values
5 t o 7 times
(such with
tremely level,
o f about
thioglycolate
high
results
sensitivity
to this
these
GCMS r e s u l t s
a n d S-S s c i s s i o n
no evidence
above r e s u l t s
balt(III)
with
polymers
These polymers
absorb
w e i g h t o f 100 a t t h i s
small
from
change
occurs.
Even
A t low dose
i nmolecular modified
i n d i c a t e S-C s c i s s i o n
s p i n d e n s i t y on s u l f u r
weight molecular
f o r methylene-
i ndithiodiglycolate.
uranyl polymers
poly-
energy.
atoms p e r r e p e a t i n g u n i t , e x -
have d r a s t i c a l l y
f o r uranium(V),
And ESR
a t o m s f o r t h e same
e v e n a t -77°K.
l e d us t o synthesize
c o n t a i n i n g two 6-diketones
- S 0 - , &-S0 -) f o r b r i d g e s 2
dithio-
were o r i g i n a l l y c a l c u l a t e d
gamma r a d i a t i o n
a very
uranyl species
indicate appreciable
polymers with
-SS-,
a molecular
w h e r e PMMA s h o w s o n l y
thioglycolate
thiodiglycolate,
100, 400, and 160, r e s p e c t i v e l y , u s i n g
t h e l i g a n d s t h a t have two s u l f u r
distribution,
The
Specifically,
a s much e n e r g y p e r p o l y m e r u n i t a s a n o r g a n i c
a s PMMA) w i t h
weight d i s t r i b u t i o n . bis
approximate
polystyrene-equivalent calibrations.
mer so,
and c r o s s l i n k i n g
s e n s i t i v e t o r a d i a t i o n - - n o n i r r a d i a t e d samples change M s i g n i f i c a n t l y d u r i n g i r r a d i a t i o n o f other samples
end-group determined weights.
to
only
t o a g r e e w i t h NMR e n d - g r o u p a n a l y s i s
^Polymer undergoes both 1
0.5 t o 0.6 M r a d
bridged
with
linear coS a t o m s (-S-,
and l e u c i n e as a nonpolymerizable
third
bidentate polymers. atoms and
l i g a n d , which
allows
These and s i m i l a r
(Archer
(III)
VO
of linear
fl-diketonates
gamma i r r a d i a t i o n
polymers.
Space
F o r example, produces
limitations
coordination
bridged
1986) show r a d i a t i o n e f f e c t s w h i c h
s t a t e / s o l v e n t dependent.
balt(II),
the synthesis 2+
with
are both
sulfur
wavelength
instead of reduction
facile
precludes
C-S s c i s s i o n further
t o co-
i nthe cobalt-
details.
ACKNOWLEDGEMENTS The
financial
interactions Getoff
(Univ.
support with
o f t h e U.S. O f f i c e o f N a v a l
Professors
J.C.W. C h i e n
Wien) a r e g r a t e f u l l y
Research
and h e l p f u l
(Univ. Massachusetts)
a n d N.
acknowledged.
REFERENCES A r c h e r RD, T r a m o n t a n o V J , L e e A Y , G r y b o s R ( 1 9 8 6 ) T h i o - , d i t h i o - a n d sulfoxo-bis-13-diketonate metal c h e l a t e polymers. I n t e r n a t i o n a l Confe r e n c e on C o o r d i n a t i o n Compounds, A t h e n s , Greece A r c h e r RD, D i c k e n s o n L C , L e e A Y , O c h a y a VO, C h i e n JCW ( 1 9 8 7 ) M o n o d e n tate and bidentate carbonyl c o o r d i n a t i o n i n c o o r d i n a t i o n polymers o f u r a n i u m ( t o be p u b l i s h e d ) B u r r o w s HD, Kemp T J ( 1 9 7 4 ) T h e p h o t o c h e m i s t r y o f t h e u r a n y l i o n . Chem S o c R e v 3: 1 3 9 - 1 6 5 C h a r l e s b y A (1954) M o l e c u l a r w e i g h t c h a n g e s i n d e g r a d a t i o n on l o n g c h a i n p o l y m e r s . P r o c R o y a l S o c L o n d o n , S e r A 224: 120 C h a r l e s b y A (1960) A t o m i c r a d i a t i o n a n d p o l y m e r s . Pergamon, O x f o r d C h a r l e s b y A , M o o r e N ( 1 9 6 4 ) C o m p a r i s o n o f gamma a n d u l t r a - v i o l e t r a d i a tion e f f e c t s i n polymethylmethacrylate a t higher temperature. I n t J A p p l R a d i a t I s o t 15: 703-708 D o l e M ( 1 9 7 3 ) T h e r a d i a t i o n c h e m i s t r y o f m a c r o m o l e c u l e s , V o l 2. A c a d e m i c P r e s s , New Y o r k E v a n s RD (1 9 7 2 ) A m e r i n s t p h y s h d b k , 3 r d ed. M c G r a w - H i l l , N Y , s e c 3 , p 2 0 9 G e t o f f N (1 962) F o r t s c h r i t t e d e r S t r a h l e n c h e m i e wa.6riger L o s u n g e n . 5 s t e r r Chem Z t g 6 3 : 9 1 - 9 8 H a l l e r I , F e d e r R, H a t z a k i s M, S p i l l e r E A ( 1 9 7 9 ) C o p o l y m e r s o f m e t h y l methacrylate and m e t h a c r y l i c a c i d and t h e i r metal s a l t s as r a d i a t i o n sensitive resists. J- E l e c t r o c h e m S o c 1 2 6 : 154-161 H a r d i m a n C J , A r c h e r RD ( 1 9 8 7 ) D i o x o u r a n i u m ( V I ) c a r b o x y l a t e p o l y m e r s : synthesis and c h a r a c t e r i z a t i o n o f t r a c t a b l e coordination polymers and evidence f o r r i g i d r o d conformation. Macromol ( i n press) H i n e G J , B r o w n e l l GL ( e d s ) ( 1 9 5 6 ) R a d i a t i o n d o s i m e t r y . A c a d e m i c P r e s s , New Y o r k M a n n RA ( 1 9 6 0 ) Gamma r a y c r o s s s e c t i o n d a t a . G e n e r a l E l e c t r i c A i r c r a f t P r o p u l s i o n Dept, C i n c i n a t t i R e h o r e k D, P u a u x J P ( 1 9 8 2 ) F o r m a t i o n o f r a d i c a l s d u r i n g t h e p h o t o l v s i s of u r a n y l s a l t s i n a l i p h a t i c c a r b o x y l i c a c i d s . Radiochem. R a d i o a n a l . L e t t 52: 29-35 Thompson L F , W i l l s o n CG, Bowden M J (eds) (1983) I n t r o d u c t i o n t o m i c r o l i t h o g r a p h y : t h e o r y , m a t e r i a l s & p r o c e s s i n g , ACS S y m p o s i u m S e r i e s No. 2 1 9 . A m e r i c a n C h e m i c a l S o c i e t y , W a s h i n g t o n , DC Thompson L F , W i l l s o n CG, F r e c h e t J M J (eds) (1984) M i c r o l i t h o g r a p h y : r a d i a t i o n s e n s i t i v e p o l y m e r s , ACS S y m p o s i u m S e r i e s No. 2 6 6 . A m e r i c a n C h e m i c a l S o c i e t y , W a s h i n g t o n DC Webb D J , H a t z a k i s M ( 1 9 7 9 ) M e t a l m e t h a c r y l a t e s a s s e n s i t i z e r s f o r p o l y (methyl methacrylate) e l e c t r o n r e s i s t s . J V a c S c i T e c h n o l 16: 20082013
TEMPERATURE DEPENDENT EMISSION OF
COPPER PORPHYRINS IN
M o t o k o A s a n o , Osamu Ohno, Y o u k o h K a i z u ,
and
Hiroshi
LIQUID SOLUTION
Kobayashi
D e p a r t m e n t o f C h e m i s t r y , Tokyo I n s t i t u t e o f T e c h n o l o g y , 0-okayama, M e g u r o - k u , T o k y o 152, JAPAN
INTRODUCTION Copper p o r p h y r i n s e x h i b i t a s h o r t - l i v e d phosphorescence at ambient temperature. F l u o r e s c e n c e i s quenched f o r a prompt i n t e r s y s t e m c r o s s i n g t o t h e l o w e s t e x c i t e d t r i p l e t s t a t e c a u s e d by t h e i n t e r a c t i o n s o f 13 * an u n p a i r e d e l e c t r o n i n t h e h i g h e s t c o p p e r da o r b i t a l and t h e ' (TTR TT ) e x c i t e d c o n f i g u r a t i o n s . The i n t e r a c t i o n m a k e s a l l s i n g l e t s b e c o m e d o u b l e t s and t r i p l e t s s p l i t i n t o d o u b l e t s ( " t r i p - d o u b l e t " ) and quart e t s ( " t r i p - q u a r t e t " ) (Ake 1 9 6 9 ; S m i t h 1 9 6 8 ) . An e n e r g y g a p b e t w e e n t h e l o w e s t t r i p - q u a r t e t a n d t r i p - d o u b l e t was est i m a t e d a s 2 6 0 - 3 3 0 cm" f o r PCu (P: p o r p h i n ) by h i g h r e s o l u t i o n a l e m i s s i o n and a b s o r p t i o n measurements a t low t e m p e r a t u r e ( N o o r t 1976 ; Bohandy 1980). T h e m e a s u r e m e n t s o f z e r o - f i e l d a n d Zeeman s p l i t t i n g s o f t h e t r i p - q u a r t e t s t a t e o f PCu p r o v i d e d k n o w l e d g e on t h e s t r u c t u r e o f t h e t r i p - q u a r t e t (Van D o r p 1 9 7 5 ; V a n D i j k 1 9 7 9 , 1981). 1
The r e l a x a t i o n p r o c e s s e s a t room t e m p e r a t u r e o f e x c i t e d c o p p e r p o r p h y r i n h a v e b e e n s t u d i e d by use o f p i c o - s e c o n d f l a s h p h o t o l y s i s t e c h n i q u e ( K o b a y a s h i 1979; Kim 1 9 8 4 ) . Copper p r o t o p o r p h y r i n i n the 2 l o w e s t e x c i t e d " s i n g - d o u b l e t " ( S,) s t a t e r e l a x e s i n t o t h e l o w e s t 2 "trip-doublet" ( T ) s t a t e w i t h i n 8 ps and t h e n t o a t h e r m a l e q u i 2 4 librium state of T^ a n d t h e l o w e s t " t r i p - q u a r t e t " ( T^) s t a t e w i t h a t i m e c o n s t a n t o f 450-460 P S (Kobayashi 1979). The p h o s p h o r e s c e n c e o b 2 s e r v e d a t room t e m p e r a t u r e i s a s c r i b e d t o a d e l a y e d T emission 4 2 c a u s e d by a t h e r m a l a c t i v a t i o n f r o m T state to state. T h e e m i s s i o n b a n d o f O E P C u (OEP: 2,3,7,8,12,13,17,18-octaethylporphin) i n b o t h t o l u e n e and p o l y ( m e t h y l m e t h a c r y l a t e ) f i l m i n c r e a s e s p r o m i n e n c e b u t shows o n l y a s l i g h t s h i f t w i t h d e c r e a s i n g t e m p e r a t u r e . The e m i s s i o n s p e c t r u m o f TPPCu (TPP: 5 , 1 0 , 1 5 , 2 0 - t e t r a p h e n y l p o r p h i n ) in polymer f i l m i s r a t h e r i n v a r i a n t with temperature ( 3 0 0 — 7 7 K) a n d w e l l c o r r e s p o n d s t o t h e s p e c t r u m i n t o l u e n e r i g i d g l a s s a t 77 K. On t h e o t h e r hand, a b e l l - s h a p e d e m i s s i o n i s observed i n t o l u e n e s o l u t i o n a t room t e m p e r a t u r e and t h e band s h i f t s t o t h e r e d w i t h d e c r e a s i n g temp e r a t u r e d o w n t o 200 K. n
1
I n t h i s p a p e r , we d i s c u s s t h e s t r u c t u r e o f t h e t r i p - d o u b l e t a n d tripq u a r t e t s t a t e s i n f o u r c o p p e r p o r p h y r i n s i n t o l u e n e l i q u i d s o l u t i o n on t h e b a s i s o f t h e d e c a y l i f e t i m e s and e m i s s i o n y i e l d s m e a s u r e d w i t h d e c r e a s i n g t e m p e r a t u r e n o t so as t o f r e e z e t h e medium. S-T
A B S O R P T I O N S P E C T R A OF
COPPER(II)
PORPHYRINS
Copper p o r p h y r i n s e x h i b i t a b s o r p t i o n bands v e r y s i m i l a r t o t h o s e observed w i t h the corresponding diamagnetic porphyrins. However t h e s e p a r t i c u l a r p a r a m a g n e t i c p o r p h y r i n s do e x h i b i t a w e a k a d d i t i o n a l a b s o r p t i o n b a n d (OD c a . 0.1 i s o b s e r v e d w i t h a n a l m o s t s a t u r a t e d s o l u -
t i o n o f 10 M) t o t h e r e d o f t h e Q b a n d . The weak band f o r m s a m i r r o r image o f t h e e m i s s i o n o b s e r v e d a t room t e m p e r a t u r e . The a b s o r p t i o n b a n d i s a s s i g n e d t o a S-T (s_ing-doublet-to-£i:ip-doublet) transition. TEMPERATURE
DEPENDENCE
OF E M I S S I O N
I N O E P C u AND
TFPPCu
The t r i p - d o u b l e t e m i s s i o n o f OEPCu a n d T F P P C u (TFPP: 5,10,15,20-tetra( p e n t a f l u o r o p h e n y l ) p o r p h i n ) i n l i q u i d m e d i a 10 t i m e s i n c r e a s e s i t s i n t e n s i t y p e r s i s t i n g t h e s p e c t r a l p r o f i l e u p o n t e m p e r a t u r e down f r o m 300 K t o 2 0 0 K. L i f e t i m e i n c r e a s e s f r o m 1 0 5 n s ( O E P C u ) , 68 n s ( T F P P C u ) a t 300 K t o 1 2 0 0 n s ( O E P C u ) , 1 2 5 0 n s ( T F P P C u ) a t 2 0 0 K, r e s p e c t i v e l y . A scheme i s g i v e n f o r t h e r e l a x a t i o n p r o c e s s o f t h e e x c i t e d s p e c i e s a s i n f i g u r e 1. Assuming t h a t a p h o t o s t a t i o n a r y s t a t e i s o b t a i n e d , t h e y i e l d o f e m i s s i o n from t h e t r i p - d o u b l e t i s figure 1 K+k /k (1) a E *2 2r- 'isc'k K+k (k /k +l) 1 k 4
3
X
= k
2
4
1 AE K= exp(-j^)
where
2
3
-3 k.
:
2
isc
k
>
! K+k^/k^=K,
>
k
>
>
and
k
thus
2 4 the equation
l+k /k =l 2
(1)
2r
K
k K+k 2
=
j{k
2
+
k
3
+
k
4
+
k_ ±J{(k 3
rate
2
+
k^»—k~i~ k i 2 2r 2nr
4
+
k_ )} 3
2 +
4k k_ 3
k =k A
4
A
+k
A
4r
4nr
3
The o b s e r v e d d e c a y c o r r e s p o n d s t o t h e s l o w c o m p o n e n t >> k , k , t h e l i f e t i m e i s g i v e n b y 2
4nr
o f e m i s s i o n i s d e s c r i b e d as
k )-(k 3
4r
S;
4
On t h e o t h e r h a n d , t h e d e c a y
(J)
2nr k
(2) isc
k
i s rewritten 2
2
2r
and
3
as Y
3
k^+k^j^ v
Since
Ti
g.
Since
k ,k_ 3
3
4
k
K+k
3
.
From t h e e q u a t i o n s T/ = k - < j > 2 r
i s c
(3) (2) a n d ( 3 ) , i t f o l l o w s
(l+K)/K.
that
(4)
H e r e t h e t e m p e r a t u r e v a r i a t i o n o f t h e r a t i o r/cf) i s a t t r i b u t a b l e o n l y t o t h e e q u i l i b r i u m c o n s t a n t K, s i n c e k^ and ^ s c l dependent a
r
e
e s s
r
on t e m p e r a t u r e . U s i n g t h e l e a s t - s q u a r e s method, t h e v a l u e s o f K a r e e v a l u a t e d as a f u n c t i o n o f t e m p e r a t u r e , from w h i c h t h e energy gap 2 4 (AE) b e t w e e n t h e T., a n d T^ s t a t e s c a n b e d e t e r m i n e d a c c o r d i n g t o 1 -i 1 _ i -1 the r e l a t i o n s h i p K = y e x p ( - A E / k T ) : O E P C u , 3 1 0 cm ; T F P P C u , 3 9 0 cm . TEMPERATURE
DEPENDENCE
OF E M I S S I O N
I N T P P C u AND
T(EtO)PPCu
TPPCu shows a b e l l - s h a p e d e m i s s i o n band a t room t e m p e r a t u r e , however the e m i s s i o n p r o f i l e v a r i e s and t h e band peak s h i f t s t o t h e r e d w i t h reducing temperature. The l i f e t i m e , on t h e o t h e r hand, i n c r e a s e s w i t h
d e c r e a s i n g t e m p e r a t u r e down t o 2 5 0 K w h i l e i t d e c r e a s e s u p o n f u r t h e r c o o l i n g : 3 0 0 K, 29 n s ; 2 5 0 K, 39 n s ; 2 0 0 K, 27 n s . T h e e m i s s i o n o f T(EtO)PPCu(T(EtO)PP: 5,10,15,20-tetra(p-ethoxyphenyl)porphin) also shows a s m a l l r e d s h i f t a t l o w t e m p e r a t u r e . The l i f e t i m e o f t h i s c o p p e r p o r p h y r i n s i m p l y d e c r e a s e s w i t h d e c r e a s i n g t e m p e r a t u r e : 3 0 0 K, 17 n s ; 2 0 0 K, 1 3 . 5 n s . I n t h e s e t w o c o p p e r p o r p h y r i n s , t h e e m i s s i o n i n t e n s i t y i n t o l u e n e s o l u t i o n i s much l e s s t h a n t h a t i n p o l y m e r f i l m but v a r i e s only t o a l e s s e r extent with temperature i n c o n t r a s t w i t h OEPCu. Temperature dependence o f e m i s s i o n s p e c t r a and decay l i f e t i m e s o f TPPCu a n d T ( E t O ) P P C u c a n n o t be d e s c r i b e d b y means o f t h e s i m p l e d i s s i p a t i o n k i n e t i c s a s i n f i g u r e 1.
STRUCTURE OF THE T R I P - D O U B L E T AND T R I P - Q U A R T E T S T A T E S I N COPPER PORPHYRINS The
absorption
a ^
and
bands o f a t y p i c a l
(IT, IT ) t r a n s i t i o n s
lowest u
orbitals
degenerate
which
metalloporphyrin arise
areascribed to the
from t h e h i g h e s t
i n a c c i d e n t a l degeneracy
occupied
t o t h e lowest
a
2
u
vacant
e
orbital pair. The l o w e s t e x c i t e d s i n g l e t s t a t e s a r e a s 1 1 of (a, e ) and (a^e ), while t h e lowest e x c i t e d lu g 2u g ^ states a r eapproximately a single configuration ^ i ^ g
50-50 a d m i x t u r e s
a
triplet 3
(a
o
C
r
T
e ).
0
2u In
e
u
g
t h e case o f paramagnetic
electron
i n copper
nitrogens. phyrin
d^2_y2
Exchange
copper porphyrins,
^k-^ ) o r b i t a l
interactions
LUMO's a n d HOMO'S g i v e
rise
however,
migrates
into
of theunpaired t o a mixing
orbitals, g , 1
= 2 0 0 - 2 5 0 cm , since
t h e a.^
HOMO a
n
2u
( b ^ j a ^ )
orbital
and a
orbitals l u _ n
unpaired
the porphyrin
electron with
between
l o w e s t (TT,TT ) e x c i t e d s i n g l e t s a n d t r i p l e t s . The e x c h a n g e i n t e r a c t i o n s b e t w e e n c o p p e r b, o r b i t a l LUMO e
an
porphyrin
and p o r p h y r i n
were e v a l u a t e d :
1
( b ^ - J a ^ )
x
= 3 0 0 - 3 5 0 cm , w h i l e
h a s no p o p u l a t i o n
por-
on t h e n i t r o g e n s
( b , e eb, ) 1 1 - 0
(Asano
unpublished). In
OEPCu a n d T F P P C u , t h e l o w e s t t r i p - q u a r t e t a n d t r i p - d o u b l e t a r e i 4 2 3 m a i n l y o f | ' (b, ( a , e ) ) > o r i g i n , whereas i n TPPCu a n d T ( E t O ) P P C u 4 2 3 1 1 , 4 2 3 I ' (b, (a~e))> t u r n s o u t t o be lower than | ' (b, (a,e))>. There
l
^
z
2
1
i s o n l y a s m a l l e n e r g y g a p b e t w e e n t h e T.. a n d T -1 -1 ( 3 3 0 cm ) a n d T F P P C u ( 3 5 0 cm ), while a qreater -1 -1 n
cm is
) and T(EtO)PPCu
(—800 cm
).
Since
observed decay
a b o u t — 3 0 n s i n TPPCu a n d — 2 0 n s i n T ( E t O ) P P C u ,
librium other state close
between t h e lowest
1
s t a t e s i n OEPCu g a p i n T P P C u (~7 00 2 rate
from
a Boltzmann
equi-
e x c i t e d s t a t e s i s n o t e s t a b l i s h e d . On t h e 4 2 2 T and T a r e f a rfrom t h e o f TPPCu a n d T ( E t O ) P P C u a r e
hand, t h e second e x c i t e d s t a t e s i n OEPCu a n d T F P P C u , w h i l e t h o s e 2 t o T^ s t a t e .
2
2
CONCLUSION The
lowest excited
t r i p - d o u b l e t and t r i p - q u a r t e t c o n f i g u r a t i o n s o f i 2 4 3 OEPCu a n d TFPPCu a r e m a i n l y | ' ( b ( a , e ) ) > , w h i l e t h o s e o f TPPCu i 2 4 3 2 4 T ( E t O ) P P C u a r e | ' {b (a e))>. An e n e r g y gap b e t w e e n T and 1
1
2
±
and was
1
o b t a i n e d 3 0 0 — 4 00 c m " f o r OEPCu and T F P P C u by k i n e t i c s s t u d y and t h e 2 4 T^ a n d T^ s t a t e s a r e i n a B o l t z m a n n e q u i l i b r i u m . I n c a s e o f TPPCu and T(EtO)PPCu, t e m p e r a t u r e dependence o f e m i s s i o n s p e c t r u m and lifetime i s v e r y anomalous i n t o l u e n e s o l u t i o n . A p o s s i b l e i n t e r p r e t a t i o n i s as f o l l o w s . Rotation of peripheral phenyl groups can cause d e l o c a l i z a t i o n o f TT-electrons o f the p o r p h y r i n macrocycle to the s u b s t i t u e n t s . I t follows that the t r a n s i t i o n energ i e s v a r y w i t h t e m p e r a t u r e i n l i q u i d s o l u t i o n s i n c e t h e r o t a t i o n depends on t e m p e r a t u r e . i t i s n o t e d t h a t a.^ o r b i t a l h a s no p o p u l a t i o n on a 2
u
t h e meso-carbons to which phenyl substituents o r b i t a l does have. I n TPPCu and T ( E t O ) P P C u ,
are bonding, while anomalous temperature
dependence o f e m i s s i o n s p e c t r u m and decay l i f e t i m e i s a t t r i b u t a b l e t o the v a r i a t i o n of phenyl groups r o t a t i o n . In f a c t , spectrum of these copper p o r p h y r i n s i n polymer f i l m , which i s c o i n c i d e n t w i t h that i n t o l u e n e r i g i d g l a s s a t 77 K, p e r s i s t s i t s p r o f i l e a n d s h o w s n o s h i f t with decreasing temperature. However, i n case o f TFPPCu, t h e l o w e s t ,24 3 e x c i t e d t r i p - d o u b l e t and t r i p - q u a r t e t c o n f i g u r a t i o n s | ' (b^ ( a e ) ) > are l e s s influenced by t h e r o t a t i o n a n d ortho-fluoro substituents of p h e n y l g r o u p s may i n h i b i t it. 1
References Ake RL, G o u t e r m a n M (1969) P o r p h y r i n s XIV. Theory f o r the luminescent s t a t e i n VO, C o , Cu c o m p l e x e s . T h e o r e t Chim A c t a 15: 20-42. A s a n o M, Ohno 0, K a i z u Y, K o b a y a s h i H ( t o b e p u b l i s h e d ) T h e l o w e s t e x c i t e d states of copper porphyrins. B o h a n d y J , K i m BF ( 1 9 8 0) T e m p e r a t u r e d e p e n d e n c e o f Mg p o r p h i n , C u p o r p h i n , and Pd p o r p h i n l u m i n e s c e n c e . J Chem P h y s 7 3 : 5 4 7 7 - 5 4 8 1 . K i m D, H o l t e n D, G o u t e r m a n M ( 1 9 8 4 ) E v i d e n c e f r o m p i c o s e c o n d t r a n s i e n t a b s o r p t i o n and k i n e t i c s s t u d i e s o f c h a r g e - t r a n s f e r s t a t e s i n c o p p e r ( I I ) p o r p h y r i n s . J Am Chem S o c 1 0 6 : 2 7 9 3 - 2 7 9 8 . K o b a y a s h i T, H u p p e r t D, S t r a u b KD, R e n t z e p i s PM ( 1 9 7 9 ) P i c o s e c o n d k i n e t i c s o f copper and s i l v e r p r o t o p o r p h y r i n s . J Chem P h y s 7 0 : 1720-1726. N o o r t M, J a n s e n G, C a n t e r s GW, v a n d e r W a a l s J H ( 1 9 7 6 ) H i g h r e s o l u t i o n s p e c t r a o f p a l l a d i u m - , p l a t i n u m and c o p p e r p o r p h y r i n s i n n - o c t a n e crystals. S p e c t r o c h i m A c t a 32: 1 3 7 1 - 1 3 7 5 . S m i t h BE, G o u t e r m a n M (1968) Q u a r t e t l u m i n e s c e n c e f r o m c o p p e r porphyrins. Chem P h y s L e t t 2: 5 1 7 - 5 1 9 . V a n D i j k N, v a n d e r W a a l s J H ( 1 9 7 9 ) R a d i a t i v e d e c a y o f m e t a s t a b l e quartet state of copperporphin. M o l P h y s 38: 1 2 1 1 - 1 2 2 3 . V a n D i j k N, N o o r t M, v a n d e r W a a l s J H ( 1 9 8 1 ) Z e e m a n s p e c t r o s c o p y o f 4 2 the B-j^ p h o s p h o r e s c e n c e o f c o p p e r p o r p h i n i n an n - a l k a n e s i n g l e c r y s t a l . I. T h e e n e r g y p r o b l e m , I I . T h e i n t e n s i t y p r o b l e m . Mol Phys 44: 8 9 1 - 9 1 1 , 9 1 3 - 9 2 3 . V a n D o r p WG, C a n t e r s GW, v a n d e r W a a l s J H ( 1 9 7 5 ) T h e l o w e s t q u a r t e t state of copper p o r p h i n : Z e e m a n e x p e r i m e n t s a t 4.2 K. Chem P h y s L e t t 35: 450-456.
PRESSURE EFFECTS ON NONRADIATIVE DEACTIVATION FROM METAL COMPLEX E X C I T E D STATES IN SOLUTION
P.C.Ford
and
Department of
J.DiBenedetto Chemistry, University
o f C a l i f o r n i a , S a n t a B a r b a r a , CA
93106,
USA
T h i s a r t i c l e s u m m a r i z e s some o b s e r v a t i o n s i n t h i s a n d o t h e r l a b o r a t o r i e s r e g a r d i n g t h e e f f e c t o f p r e s s u r e on r a t e s o f n o n r a d i a t i v e d e a c t i v a t i o n f r o m m e t a l c o m p l e x e x c i t e d s t a t e s (ES) i n f l u i d solutions. S t u d i e s here ( p a r t i a l l y i n c o l l a b o r a t i o n w i t h Henry Offen a t UCSB a n d w i t h R u d i v a n E l d i k a t t h e U n i v e r s i t y o f F r a n k f u r t , FRG) have been concerned w i t h u s i n g pressure e f f e c t s t o probe p h o t o r e a c t i o n mechanisms and o t h e r p r o p e r t i e s of m e t a l complex e x c i t e d s t a t e s ( F o r d 1986, D i B e n e d e t t o and F o r d 1985). Nonradiative deactivation often i s the p r i n c i p a l pathway f o r e x c i t e d s t a t e decay; thus u n d e r s t a n d i n g o r , a t l e a s t , d e t e r m i n i n g the s e n s i t i v i t y of t h i s pathway t o such a systemic p e r t u r b a t i o n i s c r u c i a l to i n t e r p r e t i n g the pressure e f f e c t s on o t h e r e x c i t e d s t a t e p r o c e s s e s . A C T I V A T I O N VOLUMES The a p p l i c a t i o n o f h y d r o s t a t i c p r e s s u r e c h a n g e s a number o f p a r a m e t e r s f o r a s o l u t i o n p h a s e r e a c t i o n w h i c h w i l l be r e f l e c t e d i n t h e r e a c t i o n dynamics. A p p l i c a t i o n o f t r a n s i t i o n s t a t e t h e o r y l e a d s t o an a c t i v a t i o n volume d e f i n e d according t o . (1)
AV* 1
d(ln
k.)
= -RT( 1
dP w h e r e k. i s t h e r a t e c o n s t a n t f o r a p r o c e s s o f i n t e r e s t . D e s c r i b i n g t h e p r e s s u r e dependence of t h e r e a c t i o n r a t e as an a c t i v a t i o n "volume" p r o v i d e s a c o n v e n i e n t , q u a l i t a t i v e p e r c e p t i o n of t h e d i s t o r t i o n s t h a t may be o c c u r r i n g a l o n g t h e r e a c t i o n c o o r d i n a t e o f a molecular process. H o w e v e r , t h i s v i e w may be m i s l e a d i n g s i n c e p r e s s u r e may p e r t u r b o t h e r s y s t e m i c p a r a m e t e r s i n c l u d i n g s o l v e n t d e n s i t y , v i s c o s i t y a n d d i e l e c t r i c c o n s t a n t , w h i c h may profoundly i n f l u e n c e the r e a c t i o n dynamics. D e s p i t e t h i s q u a l i f i c a t i o n , t h e use o f m e a s u r e d p a r t i a l m o l a r volumes and a c t i v a t i o n v o l u m e s as d e f i n e d a c c o r d i n g t o eq. 1 a l l o w s the c o n s t r u c t i o n o f r e a c t i o n volume p r o f i l e s which have p r o v i d e d v a l u a b l e i n s i g h t s i n t o t h e natures of the mechanisms o f numerous t h e r m a l r e a c t i o n s (van E l d i k 1986). The s i t u a t i o n w i t h e x c i t e d s t a t e s i s somewhat more c o m p l i c a t e d . A n a l y s i s o f ES d y n a m i c s a c c o r d i n g t o t r a n s i t i o n s t a t e t h e o r y c a n o n l y be v a l i d f o r s l o w e r p r o c e s s e s where t h e r e l e v a n t s t a t e s have a c h i e v e d v i b r a t i o n a l e q u i l i b r a t i o n w i t h t h e medium. In a d d i t i o n , the p a r t i a l m o l a r v o l u m e s o f s h o r t - l i v e d e l e c t r o n i c s t a t e s c a n n o t b e m e a s u r e d by the conventional methodologies, adding a f u r t h e r element of uncertainty i n t o the i n t e r p r e t a t i o n . The u n i m o l e c u l a r d e c a y o f a s i n g l e ES may b e s u m m a r i z e d i n t e r m s o f t h r e e t y p e s o f p r o c e s s e s , r e a c t i o n t o p r o d u c t s ( r a t e c o n s t a n t k ), r a d i a t i v e d e a c t i v a t i o n ( k ) and n o n r a d i a t i v e d e a c t i v a t i o n ( k ) , e a c h o f w h i c h p o s s i b l y a c o m p o s i t e of s e v e r a l competing mechanisms. The sum o f t h e d e c a y r a t e c o n s t a n t s k = £k. i s e q u a l t o t h e i n v e r s e o f t h e l i f e t i m e > w h i l e t h e q u a n t u m y i e l d 0. of a p a r t i c u l a r p r o c e s s i s d e s c r i b e d by t h e p r o d u c t o f 0^ r k. a n d T * , w h e r e 0. i s the e f f i c i e n c y of forming the r e l e v a n t s t a t e v i a i n t e r n a l c o n v e r s i o n / i n t e r s y s t e m c r o s s i n g . V a l u e s o f k^ t h u s c a n be d e t e r m i n e d as f u n c t i o n s of p r e s s u r e f r o m t h e c o r r e s p o n d i n g values of 0±, 0 i c and r . r
n
H
1
c
1
PHOTOREACTION
AV
's
T h i s a p p r o a c h was u s e d s u c c e s s f u l l y t o ^ d e t e r m i n e AV* v a l u e s f o r t h e l i g a n d l a b i l i z a t i o n pathways from t h e E l i g a n d f i e l d (LF) s t a t e of t h e h a l o p e n t a a m m i n e r h o d i u m ( I I I ) i o n s ( e q . 2, A = NH-, o r ND^ X = CI o r B r ) (Weber 1 9 8 3 , 1 9 8 4 ) . f
6
y—» R h A ( H 0 ) 5
(2)
[RhA X
2 +
5
]*
+
3 +
6
+ X
2
H 0 2
2 +
^—» R h A ( H 0 ) X 4
2
+ A
A volume p r o f i l e f o r t h e c h l o r i d e complex i s represented i n F i g . 1 w h e r e i t i s s e e n t h a t , w h i l e AV f o r NH^ l o s s f r o m t h e L F e x c i t e d s t a t e i s l a r g e and p o s i t i v e , c o n s i s t e n t with a l i m i t i n g d i s s o c i a t i v e mode f o r t h i s e x c i t e d s t a t e s u b s t i t u t i o n m e c h a n i s m , A V f o r C l ~ loss f r o m t h e same s p e c i e s i s - n e g a t i v e . This seemingly contradictory r e s u l t i s e a s i l y e x p l a i n e d i n terms of s o l v e n t c o n t r a c t i o n around t h e t r a n s i t i o n s t a t e of__the l a t t e r pathway o w i n g t o t h e c r e a t i o n o f c h a r g e ( d i s s o c i a t i o n of CI l e a v e ^ a +3 r h o d i u m ( I I I ) fragment). The volume d i f f e r e n c e s b e t w e e n t h e A V 's o f t h e t w o p a t h w a y s a r e i n d e e d c l o s e t o the d i f f e r e n c e s i n t h e p a r t i a l molar volumes of t h e r e a c t i o n products. T h u s , i t was. c o n c l u d e d t h a t t h e p r e s s u r e e f f e c t s o n t h e L F e x c i t e d s t a t e s u b s t i t u t i o n r a t e s f o r t h i s and r e l a t e d R h ( I I I ) complexes were c o n s i s t e n t w i t h t h e proposed d i s s o c i a t i v e mechanism.
Figure 1 Volume p r o f i l e diagram f o r the competitive p h o t o a q u a t i o n o f NH^ and o f CI ^ f r o m Rh(NH ) Cl i n aqueous s o l u t i o n v i a a proposed l i m i t i n g dissociative mechanism. 3
5
RXN COORD PHOTOPHYSICAL
AV*'s
P r e s s u r e e f f e c t s on r a d i a t i v e d e a c t i v a t i o n have n o t been w i d e l y explored f o r metal complexes i n s o l u t i o n , i n part because emission quantum y i e l d s a r e o f t e n ( n o t a l w a y s ! ) s m a l l . Limited studies i n s o l u t i o n a r e c o n s i s t e n t w i t h t h e changes i n k correlating with pressure induced changes i n t h e square of t h e solvent r e f r a c t i v e index as p r e d i c t e d b y t h e S t r i c k l e r - B e r g e q u a t i o n (Drickamer 1982). r
With regard t o n o n r a d i a t i v e d e a c t i v a t i o n , pressure e f f e c t s on k would depend on t h e d e t a i l e d mechanism. D e a c t i v a t i o n f r o m a p a r t i c u l a r ES may o c c u r d i r e c t l y t o t h e g r o u n d s t a t e o r b y c r o s s i n g o v e r t o a n o t h e r ES ( e . g . , o n e o f d i f f e r e n t o r b i t a l p a r e n t a g e ) w h i c h d e a c t i v a t e s more efficiently. F o r an i n d i v i d u a l s t a t e , k i s d e t e r m i n e d by v i b r a t i o n a l and e l e c t r o n i c f a c t o r s . V i b r o n i c c o u p l i n g has been a n a l y z e d i n terms o f t h e "weak" a n d " s t r o n g " c o u p l i n g l i m i t s . The f o r m e r c a s e h a s a r e l a t i v e l y s m a l l d i s p l a c e m e n t o f t h e ES p o t e n t i a l e n e r g y s u r f a c e , t h u s n
n
k
i s
d
o
m
i
n
a
t
e
d
b
t n
n Y e h i g h f r e q u e n c y m o l e c u l a r modes and i s p r e d i c t e d t o i n c r e a s e e x p o n e n t i a l l y as A E b e t w e e n t h e g r o u n d and e x c i t e d s t a t e s d e c r e a s e s ( t h e " e n e r g y gap l a w " ) . The s t r o n g - c o u p l i n g l i m i t i n v o l v e s a l a r g e r d i s p l a c e m e n t o f t h e ES p o t e n t i a l e n e r g y s u r f a c e ( a t l e a s t o n e n o r m a l mode) r e l a t i v e t o t h e a c c e p t o r s t a t e s u c h t h a t t h e surfaces c r o s s n o t f a r f r o m t h e minimum of the h i g h e r s t a t e . According to t h e o r y , s t r o n g - c o u p l i n g s h o u l d show an A r r h e n i u s t y p e t e m p e r a t u r e d e p e n d e n c e w h i l e w e a k - c o u p l i n g s h o u l d be e s s e n t i a l l y t e m p e r a t u r e independent. S t r o n g c o u p l i n g c a n a l s o be a s s o c i a t e d w i t h c h e m i c a l d e a c t i v a t i o n p a t h w a y s , t h u s may c o n t r i b u t e t o k i n those cases where unimolecular r e a c t i o n s o f t h e ES a r e common. n
I n g e n e r a l one m i g h t e x p e c t t h a t weak c o u p l i n g s h o u l d show a r a t h e r small s e n s i t i v i t y to pressure. Changes i n h y d r o s t a t i c p r e s s u r e may p e r t u r b t h e ES e n e r g y by c o m p r e s s i o n o f t h e c o m p l e x o r c h a n g i n g t h e s o l v e n t d i e l e c t r i c c o n s t a n t , and k should respond to - A E exponentially. F o r e x a m p l e , S a l m a n and D r i c k a m e r (1982) i n v e s t i g a t e d t h e p r e s s u r e e f f e c t s on t h e m e t a l t o l i g a n d c h a r g e t r a n s f e r (MLCT) p h o s p h o r e s c e n c e s p e c t r a , l i f e t i m e s and quantum y i e l d s f o r t h e c o m p l e x e s R e C l ( C O ) ^ ( p h e n ) and R e C l ( C O ) ( 4 , 7 - P h p h e n ) as f u n c t i o n s o f pressure i n several solvents. By s y s t e m a t i c v a r i a t i o n o f s o l v e n t p r o p e r t i e s with pressure, they demonstrated a l i n e a r r e l a t i o n s h i p b e t w e e n l n ( k ) and A E i n d i c a t i v e of a weaj^-coupling deactivation mechanism f o r each complex. V a l u e s of AV c a n be c a l c u l a t e d f o r e a c h c o m p l e x i n t h e v a r i o u s s o l v e n t s , and f o r tRe p o l a r s o l v e n t s dimethyl f o r m a m i d e a n d a c e t o i j i i - t r i l e , t h e s e a r e q u i t e s m a l l , 0 t o +1 cm / m o l . , but i n m-xylene A V 's a r e l a r g e r r e f l e c t i n g t h e greater c o m p r e s s i b i l i t y of ?hat solvent. 3
2
2+ A s i m i l a r o b s e r v a t i o n h a s b e e n made f o r t h e R u ( b p y ) - . cation in polar s o l v e n t s at ambient temperature. Early studies o v e r t h e r a n g e 0.1 to 230 MPa i n d i c a t e d t h a t AV i s s m a l l and n e g a t i v e (Kirk' 1980). S u b s e q u e n t e x p e r i m e n t s ( F e S t e r o l f 1985) confirmed the small ^ under these c o n d i t i o n s but a l s o demonstrated a d r a m a t i c temperature sensitivity for AV (see b e l o w ) . n v
n
u
Given t h a t the s t r o n g - c o u p l i n g mechanism i s o f t e n c l o s e l y a s s o c i a t e d w i t h u n i m o l e c u l a r r e a c t i o n s o f t h e ES, one m i g h t e x p e c t a c o r r e l a t i o n b e t w e e n p r e s s u r e e f f e c t s e x p e r i e n c e d by t h e r a t e s o f c h e m i c a l d e a c t i v a t i o n and t h e a s s o c i a t e d c o n t r i b u t i o n s t o n o n r a d i a t i v e deactivation rates. T h i s a p p e a r s t o be t h e c a s e f o r t h e Rh(III) ammine s y s t e m s d i s c u s s e d a b o v e . P r e s s u r e e f f e c t s on k values f o r the l o w e s t LF s t a t e s g i v e m e a s u r a b l e AV values which p a r a l l e l those of t h e p r e d o m i n a n t p h o t o s u b s t i t u t i o n r e a c t i o n s b o t h i n m a g n i t u d e and sign (Weber 1 9 8 3 ) . S u c h a n o b s e r v a t i o n may indicate a substantial s t r o n g - c o u p l i n g component t o the k m e c h a n i s m as p r o p o s e d e a r l i e r as an e x p l a n a t i o n o f t e m p e r a t u r e e f f e c t s on a p p a r e n t n o n r a d i a t i v e d e a c t i v a t i o n r a t e s of hexaamminerhgdium(III^ (Petersen 1974). We h a v e found t h a t a p l o t of AV vs 0 • A V + 0 •A V (the o r d i n a t e r e p r e s e n t i n g t h e sum o f ? h e ES r e a c t i o n a c t i v a t i o n v o l u m e s w e i g h t e d by t h e i r r e s p e c t i v e a m b i e n t p r e s s u r e quantum y i e l d s ) i s l i n e a r f o r the halopentaammine complexes. A l t h o u g h the q u a n t i t a t i v e s i g n i f i c a n c e of t h i s p l o t has y e t t o be d e l i n e a t e d , t h e c o r r e l a t i o n b e t w e e n A V and t h e w e i g h t e d c o n t r i b u t i o n s of t h e p h o t o s u b s t u t i o n pathways i s c l e a r l y s u g g e s t i v e of the c o n t r i b u t i o n s of s t r o n g - c o u p l i n g pathways t o the d e a c t i v a t i o n modes. n
N
n
a
A R h ( I I I ) complex which does not f i t the above c o r r e l a t i o n i s trans-Rh(cyclam)(CN) (cyclam = 1,4,8,11-tetraazacyclotetradecane), w h i c h i s i n a c t i v e t o w a r d p h o t o s u b s t i t u t i o n b u t has a r e m a r k a b l y l o n g l i v e d LF e m i s s i o n i n a m b i e n t a q u e o u s s o l u t i o n (8.1 u s e e ) ( M i l l e r 2
F I G U R E 2: P l o t o f
AV
vsthe n
summed p r o d u c t s AV*0 A
for
Rh(III)
+ AV*0 A
X. A.
complexes
. 2 +i n D 0 a: R h ( N D ) c C i ; R M N H ^ C l ^ i n HCONH. b: c : R h ( N H ^ ) ^ C l ^ i n DMSO d: R h ( N H ^ ) ^ C l ^ i n DMF e: R h ( N H ^ ) ^ B r ^ i n H 0 RMND^Br f in d i a m o n d : Rn (bpy) Clt> in H 0 2
3
1
9
2
2
AVJ
1983). T h e l a c k o f p h o t ^ o r e a c t i v i t y may b e a t t r i b u t e d t o t h e l o w e s t e n e r g y L F ES b e i n g t h e A . For this state, ligand lab^lization, expected t o be e q u a t o r i a l i n analogy t o t h e Rh(NH^) CN i o n (Skibsted 1983), i s b l o c k e d by t h e c y c l a m ^ m a c r o c y c l e * We h a v e f o u n d t h a t u n d e r p r e s s u r e > i s s h o r t e n e d a n d A V = - 4 . 4 cm / m o l , a n d we i n t e r p r e t t h i s |s s u g g e s t i n g t h a t t h e modes n e c e s s a r y f o r s t r o n g - c o u p l i n g from t h e A state also involve d i s t o r t i o n along the macrocycle^constrained equatorial metal-ligand bonds. Thus, t h e n e g a t i v e AV v a l u e may i n d i c a t e t h e r o l e o f an a l t e r n a t i v e ^ d e a c t i v a t i o n pathway, t h e c r o s s i n g o f t h e s y s t e m t o t h e h i g h e r e n e r g y _E s t a t e f r o m w h i c h nonradiative deactivation occurs. A negative AV f o r t h e A —> E transition might be e x p l a i n e d i n terms of s o l v a t i o n c o n t r i b u t i o n s t o V o f t h e l a t t e r s t a t e o w i n g t o t h e more i o n i c n a t u r e o f t h e Rh-CN b o n d i n t h e latter state. 2
5
d
2
2
A s t r o n g c o u p l i n g mechanism w i t h s i g n i f i c a n t volume changes would c e r t a i n l y b e e x p e c t e d i f t h e t h e r m a l l y r e l a x e d ES h a s a c o n s i d e r a b l y d i f f e r e n t s t r u c t u r e than does t h e ground s t a t e . An example i s t h e four coordinate N i ( I I ) complex N i ( d p p e ) C l (dppe = 1 , 2 - b i s ( d i p h e n y l p h o s p h i n o ) e t h a n e ) f o r which p r e s s u r e e f f e c t ^ on n o n r a d i a t i v e d e c a y ( A m i r - E b r a h i m i 1 9 8 4 ) g a v e A V ^ = - 1 0 cm / m o l . T h i s was i n t e r p r e t e d a s i n d i c a t i n g t h a t t h e t r a n s i t i o n s t a t e f o r g d e a c t i v a t i o n more c l o s e l y r e s e m b l e s t h e s q u a r e p l a n a r d i a m a g n e t i c d ground s t a t e than t h e l e s s t i g h t l y solvated t e t r a h e d r a l , t r i p l e t E S . The h i g h s p i n / l o w s p i n r e l a x a t i o n o f F e ( I I ) c h e l a t e s , e . g . , F e ( p y i m ) (pyim = 2 - ( 2 - p y r i d y l ) i m i d a z o l e ) i s somewhat a n a l o g o u s , a l t h o u g h i n t h i s case r a d i a l c o n t r a c t i o n rather than coordination sphere t w i s t i n g was a r g u e d t o b e r e s p o n s i b l e f o r t h e n e g a t i v e A V 's ( D i B e n e d e t t o , A r k l e e t c . 1985). 2
2 +
3
n
A s s u g g e s t e d a b o v e , n o n r a d i a t i v e d e a c t i v a t i o n may o c c u r v i a c r o s s i n g t o a n o t h e r ES f r o m w h i c h d e a c t i v a t i o n i s much more r a p i d . I n such a c a s e , t h e r e may a l s o b e v o l u m e d i f f e r e n c e s b e t w e e n t h e t w o E S . This i s d r a m a t i c a l l y d e m o n s t r a t e d b y the_ e m i s s i o n s p e c t r u m o f t h e iridium(III) cation Ir(Mephen) C1 i n DMF ( F i g . 3 ) w h i c h e m i t s f r o m two s t a t e s ( F i g . 4 ) . F o r t h i s s y s t e m , i t i s e v i d e n t t h a t increased p r e s s u r e l e a d s t o e n h a n c e d c h a r g e t r a n s f e r e m i s s i o n ( 5 5 0 nm) a t t h e e x p e n s e o f L F e m i s s i o n ( 7 2 0 nm) w i t h l i t t l e o r n o s h i f t s a p p a r e n t i n the peak maxima. The s p e c t r a l c h a n g e s were a t t r i b u t e d t o s h i f t s i n t h e r e l a t i v e p o p u l a t i o n s o f t h e t w o ES o w i n g t o p a r t i a l m o l a r v o l u m e d i f f e r e n c e s b e t w e e n t h e L F a n d MLCT s t a t e s ( D i B e n e d e t t o 1 9 8 4 ) . A plot of t h e l o g o f t h e CT/LF i n t e n s i t y r a t i o v s P p r o v e d l i n e a r . From t h e slope and t h e assumption that t h e r a t i o of t h e r a d i a t i v e rate c o n s t a n t s i s p r e s s u r e independent, an apparent volume d i f f e r e n c e o f 2
2
4.2 cm / m o l b e t w e e n t h e s t a t e s w a s c a l c u l a t e d a c c o r d i n g t o e q . 3, t h e L F ES b e i n g t h e l a r g e r . That t h e LF s t a t e i s t h e l a r g e r i s c o n s i s t e n t with the population of antibonding o r b i t a l s i n t h a t ES l e a d i n g t o e x t e n s i o n s o f t h e M-L b o n d s . f
(3)
AV
^ app
=
d(ln(kf/ki )) -RT( ) + d
AV eq
p
NANOMETERS
F I G U R E 3 ( l e f t ) : E m i s s i o n s p e c t r u m o f I r ( M e p h e n ) JZ1~ i n DMF a t 0.1 MPa ( a ) a n d a t 3 0 0 M P a ( b ) s h o w i n g t h e i n c r e a s e I n t h e MLCT e m i s s i o n i n t e n s i t y a t t h e expense o f t h e longer wavelength LF e m i s s i o n a t t h e higher pressure. FIGURE 4: different
Scheme f o r l u m i n e s c e n c e o r b i t a l parentages. K
from two e m i t t i n g defined as equal
states of t o [MLCT]/[LF]
F o r I r ( M e p h e n ) C 1 2 / t h e MLCT i s t h e l o w e r e n e r g y s t a t e y e t t h e t w o ES a r e i n t h e r m a l e q u i l i b r i u m a s e v i d e n c e d by t h e w a v e l e n g t h i n d e p e n d e n t emission lifetimes. L i f e t i m e m e a s u r e m e n t s i n DMF s o l u t i o n s h o w t h a t the a p p l i c a t i o n of pressure decreases the d e a c t i v a t i o n rates with t h e l n ( k . ) v s P p l o t g i v i n g t h e A V v a l u e +4.0 + 0.2 cm / m o l . The u n i m o l e c u l a r d e a c t i v a t i o n r a t e c o n s t a n t k^ i s r e l a t e d t o t h e v a r i o u s c o n s t a n t s n o t e d i n F i g . 4 a c c o r d i n g t o e q . 4 . ^However, s i n c e t h e * ' s a r e m u c h s m a l l e r t h a n t h e k 's a n d , f o r s u c h d complexes, n o n r a d i a t i v e d e a c t i v a t i o n i s g e n e r a l l y much f a s t e r f r o m L F s t a t e s t h a n 2
d
r
f r o m MLCT s t a t e s , case K is
>>
e q
1,
consistent
k
simplifies to k* (1+ K f
d
eq. 5 would hold, with
this
thus
the
AV
l
)~ . n
For the limiting
v a l u e o f +4.0
3
cm /mol
model.
(4)
(5)
A
vJ
= -RT[d(ln
f
k* )/dP]
-
AV
e q
A two s t a t e model has a l s o been proposed t o e x p l a i i j p r e s s u r e e f f e c t s on t h e d e a c t i v a t i o n r a t e s o f R u ( b p y ) . S m a l l A V "s w e r e f o u n d a t l o w t e m p e r a t u r e s , b u t a v a l u e o f +7.5 cm / m o l w a s t h e c a s e f o r 7 0 aqueous s o l u t i o n s ( F e t t e r o l f 1985). T h i s was e x p l a i n e d i n t e r m s o f 3
3
two c o m p e t i n g n o n r a d i a t i v e p r o c e s s e s , slow w e a k - c o u p l i n g d i r e c t l y t h e e m i s s i v e MLCT s t a t e and c o m p e t i t i v e t h e r m a l p r o m o t i o n t o a n o n l u m i n e s c e n t , h i g h e r e n e r g y LF s t a t e f r o m w h i c h n o n r a d i a t i v e d e a c t i v a t i o n i s extremely r a p i d .
from
CONCLUDING REMARKS The above has been c o n c e r n e d w i t h t h e p r e s s u r e e f f e c t s e x p e r i e n c e d by t h e n o n r a d i a t i v e d e a c t i v a t i o n f r o m m e t a l complex e x c i t e d s t a t e s . S u b s t a n t i a l e f f e c t s j f t e n r e s u l t f r o m t h e i n v o l v e m e n t o f s e v e r a l ES, so t h a t a p p a r e n t AV ' S may r e f l e c t d i f f e r e n c e s between two ES r a t h e r t h a n t h e d i s t o r t i o n s i n h e r e n t t o a s p e c i f i c d e a c t i v a t i o n mode. N o n e t h e l e s s , i t a p p e a r s t o be a s a f e c o n c l u s i o n t h a t t h o s e s y s t e m s f o r w h i c h a w e a k - c o u p l i n g n o n r a d i a t i v e d e a c t i v a t i o n mechanism i s d o m i n a n t , p r e s s u r e e f f e c t s on k a r e s m a l l ( a s a r e t e m p e r a t u r e e f f e c t s ) . In c o n t r a s t , d e a c t i v a t i o n by s t r o n g - c o u p l i n g p a t h s n o t o n l y i s t e m p e r a t u r e d e p e n d e n t , b u t may a l s o be much more p r e s s u r e d e p e n d e n t , s i n c e t h e s e a p p e a r t o i n v o l v e much more d i s t o r t i o n p r i o r t o t h e d e a c t i v a t i o n event. R e c e n t l y , we have e x t e n d e d o u r i n v e s t i g a t i o n s t o d i n u c l e a r complexes. G i v e n t h a t l o w e s t e n e r g y ES f o r many o f t h e s e d e r i v e f r o m e l e c t r o n p r o m o t i o n between o r b i t a l s d e l o c a l i z e d between t h e two c e n t e r s , s u b s t a n t i a l d i s t o r t i o n s o f t h e m e t a l - m e t a l bonds often result. An i n t r i g u i n g s u b s e t i s t h a t where b o t h m e t a l c e n t e r s have t h e d c o n f i g u r a t i o n s u c h t h a t t h e h i g h e s t o c c u p i e d MO i s M-M a n t i b o n d i n g b u t t h e LUMO i s M-M b o n d i n g , t h u s t h e l o w e s t ES h a s a s i g n i f i c a n t l y s h o r t e r M-M bond t h a n t h e g r o u n d s t a t e ( M a r s h a l l 1986). P r e l i m i n a r y p r e s s u r e s t u d i e s ( F e t t e r o l f 1987) have shown t h a t t h e phosphorescence l i f e t i m e s i n ^ a c e t o n i t r i l e of the t e t r a b r i d g e d i o n Pt~ (^-pop) ^ (§°P ^ 2 ° 5 2 ' ^^tle a f f e c t e d by p r e s s u r e ( Z V l = -176 cm /mol) b u t trie d i b r i d g e d complex I r ^ (ju-pz ) (COD) ( p j H i s p y r a ^ o l e , COD i s c y c l o o c t a d i e n e ) has a s u b s t a n t i a l l y p o s i t i v e AV" (4.6 cm / m o l ) . These o b s e r v a t i o n s s u g g e s t t h a t P t ( P O P ) . d e a c t i v a t e s by a w e a k - c o u p l e d pathway w h i l e t h e i r i d i u m a i m e r i n v o l v e s s t r o n g - c o u p l i n g d e a c t i v a t i o n . The much g r e a t e r t e m p e r a t u r e d e p e n d e n c e of k^ f o r t h e l a t t e r s p e c i e s i s f u l l y c o n s i s t e n t w i t h t h i s m o d e l . n
s
P
H
a
r
e
2
2
n
2
Acknowledgement: T h i s r e s e a r c h was s u p p o r t e d by t h e US N a t i o n a l S c i e n c e F o u n d a t i o n (CHE84-19283 and INT83-04030) REFERENCES: A m i r - E b r a h i m i V, McGarvey J (1984) I n o r g Chem A c t a 89: L39 D i B e n e d e t t o J , A r k l e V, Goodwin H, F o r d PC (1985) I n o r g Chem 24: 455 D i B e n e d e t t o J , F o r d PC (1985) C o o r d Chem Rev 64: 361-382 D i B e n e d e t t o J , W a t t s R J , F o r d PC (1984) I n o r g Chem 23: 3039-3040 D r i c k a m e r HG (1982) Ann Rev Phys Chem 33: 25-47 F o r d PC (1986) Chpt 6 i n I n o r g a n i c H i g h P r e s s u r e C h e m i s t r y . van E l d i k R ( e d ) , E l s e v i e r , Amsterdam F e t t e r o l f ML, O f f e n HW (1985) J Phys Chem 89: 3320-3323 F e t t e r o l f ML, Yang YY, O f f e n HW, F r e i d m a n A, F o r d PC (1987) manuscript i n preparation K i r k AD, P o r t e r GB (1980) J . Phys Chem 84: 2998-2999 M a r s h a l l AL, S t i e g m a n AE, G r a y HB (1986) ACS Symp S e r 307: 166-176 M i l l e r DB, M i l l e r PK, K a n e - M a g u i r e NAP (1983) I n o r g Chem 22: 3832 P e t e r s e n JD, F o r d PC (1974) J Phys Chem 78: 1144-1149 Salman OA, D r i c k a m e r HG (1982) J Phys Chem 77: 3337-3343 S k i b s t e d LH, F o r d PC (1983) I n o r g Chem 22: 2749-2453 van E l d i k R ( e d , 1986) I n o r g a n i c H i g h P r e s s u r e C h e m i s t r y , K i n e t i c s a n d M e c h a n i s m s . E l s e v i e r , Amsterdam Weber W, v a n E l d i k R, Kelm H, D i B e n e d e t t o J , Ducommun Y, O f f e n H, F o r d PC (1983) I n o r g Chem 22: 623-628 Weber W, D i B e n e d e t t o J , O f f e n H, v a n E l d i k R, F o r d P (1984) I n o r g Chem 23: 2033-2038
HETEROGENEOUS PHOTOCATALYSIS
H.Kisch,
W.Hetterich,
BY METAL SULFIDE SEMICONDUCTORS
and G.Twardzik
I n s t i t u t f u r Anorganische Chemie der U n i v e r s i t a t E g e r l a n d s t r . 1 8520 E r l a n g e n , FRG
Erlangen-Nurnberg
f
Metallized fide
may
semiconductor
photocatalyze
compounds. except
the reduction
In t h e l e t t e r
(Bucheler
1982;
2,5-dihydrofuran
hitherto the
case
unknown
Zeug
no
compounds
1 - 3
formation
i s coupled
a noble
metal
catalyst.
of
2,5-DHF
zinc
d e p e n d s on of water,
sulfide
Competition
and
the
nature
The
sulfides
fate
and
reactions
were
by
Yanagida
1985).
This
2 , 3-dihydrofuran
traces
evolution
organic
(Fox
1983)
(2,3-DHF)
water
and
which
report
zinc
reaction affords the
from
THF
of the 2,3'-isomer.
I n t h e f o l l o w i n g we
sul-
photoexcited
amounts,
from
2
of
obtained
ethers
in preparative
cadmium
Their
occurs
how
with-
the r e a c t i o n
3
1
centration of
and
materials
(4) w i t h o u t
to hydiugen
dioxide
of c y c l i c
and
1
Scheme
titanium of water
new
1985;
(2,5-DHF)
2,2'-bitetrahydrofury1
out
like
i n the dehydrodimerization
sulfide from
powders
by
t h e method 2,5-DHF
cadmium
inhibition
of i n t e r f a c i a l
a t room
were
prepared
temperature
and
of c a t a l y s t zinc
sulfide
sulfide,
and
o r homogeneous
experiments
give
electron transfer
by
preparation,
some
the
the
substitution
s o l u t i o n s of basic
con-
ZnS/CdS.
insights
into
processes.
a d d i t i o n of sodium
( Z n S - A ) o r by
on
on
sulfide
the r e a c t i o n of
to zinc
zinc
sul-
sulfate
with
thiourea
samples of
(Kurian
were
cubic
1972)
obtained
zinc
as
sulfide.
in alkaline
amorphous
The
solution
powders
sulfides
ZnS-B
a t 80°C
containing have
(ZnS-B) .
various
surface
A l l
amounts
areas
o f 10
-
15
2 m
/g
( B E T - m e t h o d ) and
that
the r a t i o
/0.68 Zn
2 +
when
/0H"
tent
All
the s u l f i d e
of zinc
pared
sulfides
originate
from and
which
sulfides
19
ml
from
1.00/0.83
a solution
with
respectively.
the higher
zinc
the reaction
absorption 2 +
and
Zn (0)
i s equal
activity
induces
ZnS-A^_
a t 354
The
rate
shows
to
1.00
the r a t i o higher
to sulfur
temperature
from
impurity changes
4
by
con-
ratio,
50% as
com-
luminescence
spectra
the l a t t e r of Mn(II).
a t 430, and
No
i n the emission
are p r e c i p i t a t e d o f ammonia, ratios
reflectance
nm,
(Kurian
which
1972).
of ZnS-A^_
on
as
, ZnS-A
9
and
4
upon
670
traces
spectra
nm
of
obvious
rates
of ZnS-A^_
the presence
corand
o f ZnS-A^,
When
thus
1/20
of
a t 336 The
respectively.
t h e ammonia
ca-
con-
medium
1.00/0.83
Induction
nm
inter-
the lower
the a l k a l i n e
and
the
intersti-
i n h i b i t s the reaction;
1.00/0.55
and
contain
3
sulfide.
and
cona r e 7,
1/7,
absorption
zinc
explainable.
which
solutions
= 1/0,
3
has t h i s
of pure
i s 1.00/0.87, ZnS-A.,
/NH
recombination
becomes
oxide
2 +
spectra
ZnS-A
i n the case
of zinc
the surface 1
3
Zn
from
the i n i t i a l
indicates
promote e l e c t r o n - h o l e
i s t o o low*
of ZnS-A
an
slight
Diffuse
the formation
Zn/S
nm.from
nm
to the l i t e r a t u r e value
talytic
centration
a t 500
f o r the molar
2
respectively.
Zn
a t room
i n the emission
concentrations
H /h
atoms
case
by
EDAX-analysis
activity.
stitial
ratio
(ZnS-B^),
exhibit
Maxima
between
different
and
bandgap tial
from
decreases
sulfide,
a t 600
i s found
the four
taining
1/30,
and
prepared
zinc
photocatalytic
7,
1/10
indicated
illumination.
oxide,
relation
0,
increases
is precipitated
1
as
photocorrosion.
the surface
( Z n S - B ) and
oxide,
strong
t o ZnS-B,. .
front-face
When
on
photocorrosion
zinc
zinc
o f Zn/S
= 1/5
increases
exhibit
times
the i n the are
longer
with
interstitial action. by
the less atoms
The r a t e
ZnS-B^
active
samples
and s u r f a c e
induced
and i t s
oxide
by Z n S - A ^
catalytic
pointing at t h i s
i s about
activity
to a possible
removal
initial
of ther e -
three
persists
stage
times
much
that
of
obtained
longer.
This
may
be 2
due
to the larger
However,
other
important
properties
and t h e r e
photocatalytic
Homogeneous initial be
specific
rate
of the s u l f i d e s
of the surface
i s no s i m p l e
correlation
solutions when
of p l a t i n i z e d
t h e amount
ZnS/CdS
o f cadmium
decrease
between
evolution
mmol
of catalyst
suspended i n
4 >
2: Z n
Q
2
Cd
Q
induce
( 1 0 0 - 130 m /g)
seem
t o be
surface
also
area
gS/Pt,
120 3:
a decrease
i s increased
as f u n c t i o n
of hydrogen
ZnS-A
porosity
of the reducing
Rate
1:
like
ZnS-A
and
activity.
due t o t h e c o n c o m i t a n t
Fig.l:
surface
(Fig.l).
of i r r a d i a t i o n
2
Q
Cd
Q
g
This
and o x i d i z i n g
ml o f 2,5-DHF/H 0 Zn
of the
S/Pt,
4:
power
time;
= 1/14 CdS/Pt
may
0.3 (v/v);
of
the photogenerated
positions bandgap the
of zinc
energies
and cadmium a t 3.65
reaction occurs
Competition catalyst
Since
THF,
dehydrodimers
when
THF
to
specific
hydrodimer of
zinc
when
2,5-DHF
When
the ratio
DHF/ZnS formed 2,5-DHF action
nor in
i s present
that
zinc
lower
i n case 2,5-DHF
THF
when
a l l 2,5-DHF
substrate
better
to
THF
a slower
sites
o r 1 - 3.
Contrary,
when
b) t h e r e
seems
has r e a c t e d .
continues
These
introduced
by
some
free
proper
t h e de-
(case a ) .
o f 2,5are
a after a l l
rate,
due t o r e by t h e a s -
present
in
excess
f o r adsorption 2,5-DHF
i s present
surface
available
oxidation of
demonstrate
selection
amounts
and 1 - 3
through
experiments
these
predominantly
be r a t i o n a l i z e d when
of
points
of equal
i n case
use these
(case
300/1,
product
sites
that
100/1,
and t h a t
cannot
and t h e r e a c t i o n
c a n be
adsorption
after
with
that
formed
the sulfide
stops
the
that
of the r a t i o
of
catalyst.
dependence
water;
t h e main
a l l adsorption
2,5-DHF
o f THF
chemoselectivity
type
covers
i s about
over
be
The f a c t
i s at least
t o 1000/1
reaction
should
are formed
excess
becomes
b ) . The
CdS/Pt
with
i s ten times
i n the case
1 - 3
b . T h e d i f f e r e n c e may
concentrations
adsorption
product
but continues
and t h a t
adsorbed
for
The
consumed
sulfide,
replace
(case
of
conducted
2,5-DHF.
the ratio
i s increased 4
were
o f 2,5-DHF
THF/2,5-DHF
contrary,
t o 0.3,
in traces
a n d THF
over
in a threefold
T H F / 2 ,5-DHF
o f THF,
excess
When
(pH=7), t h e
In the case
and c r o s s - p r o d u c t s
i s t h e major
and 2,5-DHF;
has been
sumption over
, 4,
i s decreased only
effects.
V
light.
2,5-DHF
i f the ratio
the flatband
a n d -0.7
respectively.
visible
between
respectively;
a r e a t -1.6
the reaction rate
adsorption
sulfide
with
eV,
in a tenfold
only
o f THF
sulfide
of the l a t t e r
i s present appear
and h o l e s ,
a n d 2.4
also
experiments
ZnS-A^.
products
electrons
an
o f 2,5-DHF
of the rate initial
i s further
supported
on t h e c o n c e n t r a t i o n s
linear
increase
a plateau
by t h e
o f 2,5-DHF, o f maximum
LangmuirTHF a n d rate i s
reached plot
at concentrations
of reciprocal
constants THF
of adsorption
and w a t e r ,
From
these
processes
f o r t h e sake
RH
versus
are obtained
+
D
results
2
Z
n
0H ' a
a n d 0.6
^
v
M
equilibrium
_ 1
f o r 2,5-DHF
7
of water
wherein a r e o-
symbolizes
q
an
(1)
g d
+
•>
ZnS(h )
+
+ 1/2
(2) H
g
+ 0H
(3)
a d
OH' »
RH
+
R
RH ' ad
—
d
—
ad R
2); D
ZnS(e~,h )
+ ZnS(h )
1/2
i s proposed
equilibrium
•
d
n;
scheme
(Scheme
RH
+
g d
a mechanistic
S
+ ZnS(e",h )
RH
a s 1 0 , 0.5
of s i m p l i f i c a t i o n
+
g c |
concentrations,
and t h e a d s o r p t i o n
ZnS H 0
reciprocal
r e s p e c t i v e l y . From t h e
respectively.
experimental
photophysical mitted
rates
o f 1.5, 6 a n d 15 M,
/
+ ZnS
a d
+
ad
H
: 1
(4)
H
(5)
6
ad
+
(
2
Rp
7) (8)
ad «
9
m
1/2
R
(9)
ad R
R"
a d
(10)
1/2
Scheme
empty
R
(11)
2
2
adsorption
site,
The
photogenerated
(3)
and t h e r e m a i n i n g
radical
cation.
and s o l v a t e d
electron-hole holes
Deprotonation
species
pair
oxidize should
(step
a r e shown 2) r e d u c e s
the adsorbed be f a s t
without
ether
water
an to
index. hydrogen
(1,5) t o t h e
due t o t h e h i g h
hydration
energy (8,9) fact
of
the
and/or that
amounts (3,5) both
proton fully
the
to
hydrogen
solvated
to
the
be
strongly
and N^O
conduction
band
electrons
nary
concentration.
second
hole,
inhibition
of
or
preventing
occurs
at
(10,11)
the
as
formation
the
This
surface
dimerize the
final
in
induce
site
A
by
the
more
scavenger
may
are
no
in
The equal
transfer
inhibition of
efficient
further
adsorbed
electron
presence
should
the
formed
the
of
electron
removal
of
increase
the
RH *
+
or
R'
possibility
is
that
oxidation
where
in
products.
Interfacial
indicated
dimerization.
a
to
c h l o r i d e . The
by
may
dehydrodimers
coupled
zinc
R"
possibility.
dehydrodimer
like
radical
state
latter
scavengers
hole
The
diastereomers
points
seems
(6,7).
ether
of
species
is
statioby
a the
adsorbed.
References Bucheler
J,
genen Fox
Zeug
(1983)
Sensitized A,
of
zinc
2:
223
Organic by
Cyclic
II
1985:
C-C
N,
Angew.Chem.
/1972)
for
Katalysator
94:
Studies
luminescent
Chemical
Acc.Chem.Res. on
a
der
hetero-
792
Photocatalysis:
Semiconductors.
CV
als
new
16:
method
phosphors.
of
Conversions 314
-
321
preparation
J.Appl.Electrochem.
229 Azuma
T,
1487
on
Midori
Part
Ethers -
Bucheler
Bonds
Zinksulfid
Heterogeneous
useful
Photocatalysis. of
(1982) Wasser.
Irradiated
sulfide
S,
H
von
Suryanarayana
-
Yanagida
Zeug
Kisch
Photoreduktion
MA
Kurian
N,
4.
Pac
Hydrogen
Catalyzed
by
Ch,
Sakurai
Evolution
Zinc
Sulfide.
H
and
(1985)
Semiconductor
Photoredox
Reactions
J . Chem.Soc.Perkin
Trans
1493 J,
Kisch
Illuminated
J.Am.Chem.Soc.
Y,
107:
1459
H
(1985) Zinc -1465
Catalytic
Sulfide
Formation
Generated
from
of
Hydrogen
Zinc
and
Dithiolenes.
INORGANIC PHOTOINITIATORS FOR PHOTOLITHOGRAPHIC
C.Kutal*
APPLICATIONS
a n d C.G.Wi11 s o n * *
* D e p a r t m e n t o f C h e m i s t r y , U n i v e r s i t y o f G e o r g i a , A t h e n s , GA 3 0 6 0 2 , USA **IBM A l m a d e n R e s e a r c h C e n t e r , 650 H a r r y R d . , San J o s e , CA 9 5 1 2 0 , USA
INTRODUCTION Photolithography i s used e x t e n s i v e l y i n t h e m i c r o e l e c t r o n i c s i n d u s t r y to generate three-dimensional patterns i n a solid substrate such as single crystal silicon ( W i l l s o n 1 9 8 3 ; Bowden 1 9 8 4 ) . Figure 1 illustrates t h e sequence of steps that comprise the typical photolithographic process. The s u b s t r a t e initially i s coated with a thin layer of a photosensitive m a t e r i a l , termed a r e s i s t , and then exposed to light t h r o u g h a mask c o n t a i n i n g transparent and opaque a r e a s t h a t d e f i n e t h e d e s i r e d p a t t e r n . The t r a n s p a r e n t areas t r a n s m i t l i g h t which causes a photochemical r e a c t i o n i n t h e r e s i s t , t h e r e b y a f f o r d i n g a means o f d i f f e r e n t i a t i n g t h e e x p o s e d a n d u n e x p o s e d regions. P r e f e r e n t i a l d i s s o l u t i o n o f t h e unexposed resist in a suitable developing s o l v e n t , f o r example, creates a negative tone i m a g e o f t h e mask on t h e s u b s t r a t e s u r f a c e ( a p o s i t i v e tone image results i f t h e exposed resist dissolves preferentially i n the developer). This image, i n turn, c a n be t r a n s f e r r e d into the substrate v i a a chemical and/or p h y s i c a l e t c h i n g process i n which t h e a r e a s n o t p r o t e c t e d by t h e r e s i s t a r e a t t a c k e d by t h e e t c h a n t . The f i n a l s t e p i n v o l v e s s t r i p p i n g a w a y t h e r e s i s t t o y i e l d a n e g a t i v e tone r e l i e f image i n t h e s u b s t r a t e t h a t r e p l i c a t e s t h e p a t t e r n o f t h e mask.
Light
Develop
Expose
Strip
Figure
1. S e q u e n c e o f s t e p s
i n the photolithographic
process.
Resists composed o f a f u n c t i o n a l i z e d p o l y m e r and a photoinitiator have been a m a i n s t a y o f the p h o t o l i t h o g r a p h i c p r o c e s s f o r a number of years. The g e n e r a l i z e d response of t h i s type of system to l i g h t can be s u m m a r i z e d by e q . 1 and 2. Absorption o f a p h o t o n by the P I
Ji!L—
+ polymer
I
[1]
modified
polymer
[2]
photoinitiator P r e s u l t s in i t s conversion t o one o r more r e a c t i v e species I. Subsequent thermal r e a c t i o n of I w i t h the polymer causes the change i n s o l u b i l i t y or other p r o p e r t i e s that form the basis for d i s t i n g u i s h i n g b e t w e e n e x p o s e d and unexposed areas. Since the photoinitiator and the polymer serve different functions, i t is possible to optimize the p r o p e r t i e s of one without affecting the desirable features of the other. This inherent flexibility of a two-component system greatly simplifies the task of designing radiation-sensitive materials. The m a j o r i t y o f c o m m e r c i a l l y - i m p o r t a n t p h o t o i n i t i a t o r s are nonmetallic compounds which generate radicals and/or strong acids upon irradiation. The l a t t e r s p e c i e s p l a y the r o l e of I i n eq. 1 and 2 and react with the f u n c t i o n a l i z e d polymer via well-precedented radical or cationic pathways. Examples of commonly-used nonmetal initiators include benzoin and benzoin ethers, benzyl ketals, benzophenones p l u s h y d r o g e n atom d o n o r s , and "onium" s a l t s belonging to the a r y l d i a z o n i u m , t r i a r y l s u l f o n i u m , and diaryliodonium families (Gatechair 1983; Vesley 1986). In contrast, little information currently e x i s t s concerning the use of t r a n s i t i o n m e t a l complexes as photoinitiators for the reactions of f u n c t i o n a l i z e d polymers. This oversight i s surprising, since several classes of complexes a p p e a r t o be w e l l - s u i t e d f o r t h i s r o l e . Exemplary i n t h i s regard are the c l a s s i c a l acidopentamminecobalt ( I I I ) c o m p l e x e s , CoiNH^) t^X 2 where X i s C I , B r , I or o t h e r u n i n e g a t i v e group. In a d d i t i o n to their ease of s y n t h e s i s , these complexes resist decomposition by a i r , m o i s t u r e , and h e a t (>100°C), a b s o r b s t r o n g l y i n t h e ultraviolet spectral region, and undergo quantum efficient photodecomposition from a l i g a n d - t o - m e t a l (X t o Co) charge t r a n s f e r e x c i t e d s t a t e . As described by e q . 3> this i n t r a m o l e c u l a r redox r e a c t i o n i n aqueous solution generates the aquated cobalt(II) cation (a Lewis acid), +
}
Co(NH ) X 3
5
+ 2
— —
C
o
(
+
a
2
q )
+
5 N H
3
+
X
*
t h e X" r a d i c a l , a n d f i v e m o l e c u l e s o f a m m o n i a ( a L e w i s b a s e ) ( B a l z a n i 1970; Endicott 1975). One or more o f these chemically distinct species could play the r o l e of I i n eq. 2 and initiate useful chemistry i n a f u n c t i o n a l i z e d polymer. To test this possibility, we selected [ C o ( N H 3 ) B r ] ( C I O 4 ) 2 as the p h o t o i n i t i a t o r owing to i t s s t r o n g a b s o r p t i o n i n the d e e p - u l t r a v i o l e t r e g i o n , w h i l e t h e c o m m e r c i a l l y - a v a i l a b l e r e s i n , COP ( F i g . 2), served as t h e f u n c t i o n a l i z e d p o l y m e r . The l a t t e r i s a c o p o l y m e r o f g l y c i d y l methacrylate and e t h y l a c r y l a t e w h i c h has f o u n d use as a negative electron-beam resist (Thompson 1974, 1975). The epoxide group s i t u a t e d on t h e g l y c i d y l s i d e c h a i n i s s u s c e p t i b l e t o r i n g opening b o t h by a c i d i c a n d basic reagents and thus i s a l o g i c a l site for crosslink formation (Lee 1967). As d i s c u s s e d i n the next s e c t i o n , the COP-[Co(NH3) Br](0104)2 system proved to be of considerable i n t e r e s t as a p h o t o r e s i s t . 5
5
r
f ir H
i
-j-CH — C — H C H — C 2
0
cr
o
0
'
N
n/m
»
0.74
o
CH2
CH2
Mn = 0.6x
10
(by
gpc)
H
Figure
2.
S t r u c t u r e and
R E S U L T S AND
properties
o f COP
sample used
in this
study.
DISCUSSION
W h i l e r e s i s t s o f v a r y i n g c o m p o s i t i o n have been i n v e s t i g a t e d (Kutal 1987), t h e r e s u l t s summarized h e r e r e f e r s p e c i f i c a l l y t o a f o r m u l a t i o n c o n t a i n i n g 9 wt % o f COP a n d 1 wt % o f [Co (NH3) B r ] ( C I O 4 ) dissolved in a 1:3 (v:v) solvent mixture of N-methyl-2-pyrrolidinone/ chlorobenzene. F o l l o w i n g f i l t r a t i o n t h r o u g h a 0.2 ym f i l t e r , this solution was s p i n - c o a t e d onto q u a r t z and silicon wafers and the solvent removed by heating at 62° C for minutes. Films of COP-[Co(NH3)5Br](CIO4)2 prepared by this procedure were 0.5 pm t h i c k a n d , when e x a m i n e d u n d e r a m i c r o s c o p e a t 1500x m a g n i f i c a t i o n , showed no e v i d e n c e o f a g g r e g a t i o n o f t h e c o b a l t s a l t . The e l e c t r o n i c a b s o r p t i o n spectrum o f a f i l m a b o v e 250 nm i s e s s e n t i a l l y t h a t o f Co ( N H j ) ^Br" "^ ^ s i n c e COP a b s o r b s v e r y w e a k l y i n t h i s w a v e l e n g t h r e g i o n . E x p o s i n g a f i l m t o 25^ nm r a d i a t i o n c a u s e s a b l e a c h i n g o f t h e i n t e n s e Br-to-Co c h a r g e t r a n s f e r a b s o r p t i o n band ( X a x 58 nm) i n the complex. This behavior i s consistent with the occurrence of a photoredox process which, in analogy to the aqueous solution photochemistry of the complex (eq. 3), generates one or more weakly-absorbing cobalt(II) species. While a detailed characterization of the photoproducts has yet t o be undertaken, we w o u l d e x p e c t t h e r e d u c e d m e t a l t o r e m a i n c o o r d i n a t e d t o a n u m b e r o f ammonia m o l e c u l e s s i n c e s u c h c o m p l e x e s a r e known t o p e r s i s t i n t h e s o l i d s t a t e a t room t e m p e r a t u r e (Wendlandt 1967). Dissociation of a m m o n i a may o c c u r , however, e s p e c i a l l y when t h e p o l y m e r film i s heated. 5
2
1
=
2
m
2
A f i l m e x p o s e d t o >60 m J / c m o f 25^ nm r a d i a t i o n d i s s o l v e s c o m p l e t e l y away f r o m t h e u n d e r l y i n g w a f e r u p o n b e i n g s p r a y e d f o r 15 seconds with a 5:3 mixture of 2-butanone/ethanol. In contrast, heating a comparably irradiated film a t 68°C f o r 6.5 minutes renders i t insoluble in this solution. S i n c e h e a t i n g , by itself, does not cause insolubilization, i t appears that one or more of the photodecomposition products of [Co(NH3) Br](CIO4)2 react(s) with COP in a thermally-activated process which c r o s s l i n k s - the epoxy resin. Experiments designed to elucidate the mechanism of c r o s s l i n k i n g s u g g e s t t h a t one o r p e r h a p s b o t h o f t h e f o l l o w i n g paths ( F i g . 3) p l a y ( s ) a n i m p o r t a n t r o l e : ( i ) r e a c t i o n o f t h e b a s i c ammonia 5
molecule with pendant epoxide groups on adjacent polymer chains, ( i l ) c o o r d i n a t i o n of a c a t i o n i c cobalt species to the oxygen atom of an epoxide r i n g followed by n u c l e o p h i l i c attack of a second epoxide.
Figure 3. Mechanisms f o r the c r o s s l i n k i n g of COP: ( i i ) cation i n i t i a t e d .
( i ) base i n i t i a t e d ,
Dose-response measurements on C0P-[Co(NH3) ^Br ] (CIO4) 2 f i l m s r e v e a l that 20-25 mJ/cm of 254 nm r a d i a t i o n i s sufficient to cause detectable g e l formation, while one-half of the ultimate film thickness can be achieved with 35~40 mJ/cm . Figure 4 d i s p l a y s the l i n e - s p a c e p a t t e r n obtained upon exposing a f i l m i n contact with a chrome mask to a 40 mJ/cm dose, baking a t 68° C f o r 6.5 minutes, and then developing with 2-butanone/ethanol. Despite some s w e l l i n g of the negative tone images by the developer, 1-2 micron r e s o l u t i o n can be obtained. 2
2
2
Films of pure COP are i n s e n s i t i v e to 254 nm r a d i a t i o n s i n c e , as noted earlier, the r e s i n e x h i b i t s n e g l i g i b l e absorption a t t h i s wavelength. Photochemistry does occur upon prolonged irradiation at shorter wavelengths, but quite i n t e r e s t i n g l y , i t results i n degradation of the polymer chains. Consequently, upon development, the exposed areas of the f i l m d i s s o l v e away. Thus, quite apart from extending the e f f e c t i v e p h o t o s e n s i t i v i t y of the system to a convenient wavelength, the i n c o r p o r a t i o n of [ C o ( N H 3 ) ^ B r ] ( C I O 4 ) 2 i n t o a f i l m of COP changes the b a s i c response of the m a t e r i a l from p o s i t i v e tone to negative tone. Moreover, the a b i l i t y to image a COP-[.C6(NH3)5Br](C10i|)2 f i l m with both photons and e l e c t r o n s ( r e c a l l that COP, i t s e l f , i s a s e n s i t i v e e-beam r e s i s t ) o f f e r s the prospect
of "hybrid" lithography (Imamura 1984) whereby relatively large p a t t e r n s a r e f i r s t p r i n t e d w i t h uv l i g h t a n d t h e n v e r y f i n e p a t t e r n s a r e d e l i n e a t e d w i t h a f o c u s e d e l e c t r o n beam. The interesting chemical and lithographic properties of the C0P-[Co(NH3)^Br](CIO4)2 system provide a promising indication that new classes of radiation sensitive materials can r e s u l t from the proper combination of an inorganic initiator and a functionalized polymer. The design of such m a t e r i a l s , t h e i r fundamental chemistry, and t h e i r p o t e n t i a l a p p l i c a t i o n are t o p i c s t h a t we are continuing to pursue.
REFERENCES Balzani V, Carassiti V compounds. Academic P r e s s ,
(1970) Photochemistry L o n d o n New York, p 193
of
coordination
B o w d e n MJ (1984) A p e r s p e c t i v e on resist m a t e r i a l s *for fine-line lithography. In: Thompson LF, Willson CG, Frechet JMJ (eds) Materials for microlithography. American Chemical Society, W a s h i n g t o n , D.C., p 39 Endicott JF (1975) Charge-transfer photochemistry. Fleischauer PD (eds) Concepts of inorganic W i l e y - I n t e r s c i e n c e , New York London, p 8 l
I n : Adamson AW, photochemistry.
Gatechair LR, Wostratzky D (1983) P h o t o i n i t i a t o r s : an mechanisms and a p p l i c a t i o n s . J R a d i a t C u r i n g 10:4-18 I m a m u r a S, T a m a m u r a methacrylate) as 391-400
T, K o g u r e 0 ( 1 9 8 4 ) electron beam and
overview
of
Chloromethylated poly(napthyl photoresist. Polymer J 16:
Kutal C, Willson CG (1987) P h o t o i n i t i a t e d c r o s s l i n k i n g and formation in thin polymer films containing a transition compound. J E l e c t r o c h e m Soc i n p r e s s
image metal
L e e H, York,
Neville Chap 5
K
(1967)
Handbook
o f epoxy
T h o m p s o n L F , F e i t E D , H e i d e n r e i c h RD ( 1 9 7 4 ) chemistry o f epoxy containing negative
resins.
McGraw-Hill,
New
Lithography and r a d i a t i o n electron resists. Polym
E n g S c i 1 4 : 529~533 Thompson LF, Ballantyne JP, Feit ED (1975) Molecular parameters and l i t h o g r a p h i c p e r f o r m a n c e o f p o l y ( g l y c i d y l m e t h a c r y l a t e - c o - e t h y l acrylate): a negative electron resist. J Vac S c i Technol 12:
1280-1283
V e s l e y GF (1986) M e c h a n i s m s J R a d i a t C u r i n g 13:4-10 Wendlandt WW, transition-metal
of the photodecomposition
Smith JP (1967) The ammine c o m p l e x e s . E l s e v i e r ,
thermal New Y o r k ,
of
initiators.
properties Chap 4
of
Willson CG (1983) Organic resist m a t e r i a l s - t h e o r y and chemistry. In: Thompson LF, Willson CG, B o w d e n M J (eds) Introduction to microlithography. American Chemical Society, Washington, D.C., p 88
PHOTOCHEMICAL BEHAVIOUR OF
R.Reisfeld*,
LUMINESCENT DYES IN SOL-GEL AND
M.Eyal*, R.Gvishi*,
and
BORIC ACID GLASSES
C.K.J0rgensen**
* D e p a r t m e n t o f I n o r g a n i c C h e m i s t r y , Hebrew U n i v e r s i t y , J e r u s a l e m , ISRAEL * * S e c t i o n ' de C h i m i e , U n i v e r s i t e de G e n e v e , 1211 Geneva 4, SWITZERLAND
Fluorescence of
the
the
of
organic
surrounding
excited state,and
unsymmetrical
vibrational
the
is
much m o r e e f f e c t i v e
macroscopic
are
dyes
here
like
studying
various
cases,their strong is
and
H
with
pH
*
F
r
o
m
t
1,but
process"
triplet
of
first
the
e
point 4 36
triplet from
nm
excitation
spectroscopic
the
luminescence
energy i s the
absorption
bands at
stationary
concentration
states.
650
category
(Martin
of
xanthene
other
to
CH^)
process"
d i f f e r e n c e 3 0 0 0 cm
of
the
(in strong
and
505
nm
due
of
the
lowest
triplet
by
two
at
"beta
increasing
since
state
to
agreeing
i l l u m i n a t i o n ) of
to
of
exceptionally
energy
transitions
acid
energy.
.Another complication
appearance
close
decay
3 seconds
fluorescence"
activation
by
below,the
at higher
long-lived triplet
1
formed
slow
state,enhanced
"delayed
to
very
t o aqueous
a very
emission
"alpha
Arrhenius
two
their
discussed
a l . (1941) d i s t i n g u i s h
the
of
1975).
r e a c h i n g xy-9-phenyl-fluoron
large
should
i n 7 solvents
tion
plays
shifts
t o 6.9 i n t h e e x c i t e d
t o 4 3 1 nm i n 7.4 m o l a r p e r c h l o r i c a c i d
maximum o c c u r s
t o many
o f h e t e r o c y c l i c compounds s t u d i e d by F o r s t e r ,
p K = 6.7 o n l y
state.The molar
i s p r e v a i l i n g f o r a l l pH a b o v e 8 . C o n t r a r y
singlet states
third
C->"N-*A->D i n w a t e r
1 9 5 8 ; L e o n h a r d t e t a l . 1 9 7 1 ) a r e 2 . 2 , 4.4 a n d 6.7.
Hence,the d i - a n i o n excited
f o r thedeprotonation
£ has
3
life-time
than a percent
i s 4.5 ms i n
of thevalue
f o rboric
a n d H u g 1 9 8 6 ) . C o r r e s p o n d i n g l y , V| = 0.31
(N) a n d 0 . 9 3 f o r t h e d i - a n i o n
(D) ( W e b e r a n d T e a l e
Y j = 0.23 a t pH = 1 ( c a t i o n C) s h o w i n g
4 3 7 nm, Yj = 0.92 a t pH = 6 i n w a t e r
theabsorption
(mixture
f o r 1958).
maximum
o f A a n d D,479
nm)and
Y| = 0.92 a t p H = 13 ( D , 4 9 0 nm) . Heat-sensitive
colorants
c a n be i n c o r p o r a t e d
al.
1983) a n d i n g e l g l a s s e s
for
instance,the
rhodamine
silica
i nVycor glass
(Mack e t
( R e i s f e l d 1984;Levy e t a l . 1984) w h e r e ,
cage m o d i f i e s
6 G and s t r o n g l y enhances
the spectra
of
trapped
i t s p h o t o s t a b i l i t y (Avnir e t a l .
1 984 ).Recently,we have incorporated fluorescein derivatives (in methanolic solution) in a glass prepared by controlled hydrolysis of Si(OCH ) at 60°C for 48 hours.The glasses obtained are of good optical quality,transparent above 340 nm,and of densities 1.15 to 1.2 g/ml (Reisfeld et a l . 1987).Also thin sol-gel glass films prepared by the method of Avnir,Kaufman and Reisfeld (1985) show r] = 0.9 after light absorption in a maximum at 455 nm.If heated to 200°C,the absorption shifts to 490 nm,resembling the effect of deprotonation.Fluorescein substituted by two mercury acetate groups,flu(MA)in the scheme above, shows rapid transformation of the f i r s t excited singlet to the lowest t r i p l e t state,as a typical r e l a t i v i s t i c "heavy-atom" effect.The timeevolution of the emissibn spectrum of flu(MA) in boric acid glass is roughly invariant intensity of the 570 nm emission for a few m i l l i s e conds , followed by a slow decay (at room temperature) with T=2.8 :t0 .5 seconds,the T v a l u e Lewis et al.(1941) found for 570 nm emission of fluorescein in boric acid glass at 60 K,to be compared with about 1 s at 300 K.These measurements are rendered d i f f i c u l t by the photochemical dissociation of the two "HAgroups.Nevertheless,the observed 490 nm emission from flu(MA) has less than a-quarter of the intensity at 570 nm after at least 0.01 s has elapsed since the illumination stopped. 3
4
The luminescence of fluorescein in glasses prepared at moderate temperatures may have several applications.lt seems rather probable that flat-plate luminescent solar concentrators (transporting emitted light by a series of total reflections out to a rim covered with a photovoltaic material) are only a competitive alternative to flat-plate silicon coverage,if combined with some organic materials (Reisfeld and J$6rgensen 1 982 ; Reisfeld et a l . 1 983;Neuroth and Haspel 1 986).Lasers involving 4f and 3d group ions may also be combined with thin films containing organic luminophores (Reisfeld 1983 and 1985).Fluoresceindoped boric acid glass shows non-linear optical properties applicable to phase conjugation devices (Kramer et a l . 1986).
Acknowledgements: This work was supported by the National Council of Research and Development,Israel.
Avnir
D,Levy D , R e i s f e l d
R
(1984) J.Phys.Chem.
A v n i r D,Kaufman V R , R e i s f e l d Carmichael organic
T,Hug G L
R
(1985)
Jablonski
A
(1935)
J
Mfl-l-O-Mn-1 I
I
0
0
+ 2 H
+
The complex ( b p y ) 2 ( H 2 0 ) R u I - 0 - R u ( H 0 ) ( b p y ) 2 II
III
2
4+
was recently studied in detail by
Gilbert et. al. (1985) and found to be a precursor to a water oxidation catalyst, indeed. Unfortunately, this compound has a low turnover number. We found that by using the 2,2-bipyridyl-5,5-dicarboxylic acid ligand ( H 2 U
instead of
2,2 -bipyridine (bpy), a more efficient water oxidation catalyst is obtained.
1 Invited professor, on leave of absence from: a) The Chemical Research and Development Center of the United States Army, Edgewood, Maryland 21010. b) The Department of Chemistry, Oregon Graduate Center, Beaverton, Oregon 97006.
SYNTHESIS AND CHARACTERIZATION Attempts to prepare the (HL)2Ru(H20)2 (HL: monodeprotonated H2L
ligand) directly
from "RUCI33 H2O" and the ligand failed, presumably because the desired product polymerizes via displacement of coordinated water by carboxylate. ( H L ) 2 R u ^ ( H 2 0 ) 2 in x
0.5 M H2SO4 (1 < x ^ 2) was prepared as follows:
"RuCl • 3 H 0 " 3
2
L'ltOH -» RUL2CI2• H2O
(L : 2,2'-bipyridyl-5,5-dicarboxylic acid diethyl ester) 1
NEt
3
RuL 2CI2 • H2O
R u ( H L ) C l 2 • 2,5 H2O 2
2
H 0/Et0H 2
+
Ag R u ( H L ) C l - 2 . 5 H2O -> (H L)2RuII(H 0)2 05MH S0 2
2
2
x
2
2
4
( H L ) 2 R u ( H 2 0 ) 2 forms R ^ ^ L ^ S O ^ 4 H2O on standing in 0.5 M H S 0 n
x
2
4
for a couple
of weeks. The IR spectrum shows that sulfate acts as a bidentate chelating ligand. From this behavior and the CV (exhibiting E j / = 0 7 8 V for the diaqua complex and a small 2
wave at E j / - 0.61 V for the sulfato complex) we conclude that sulfato complexes must 2
also exist in solution. Electrolysis of ( H L ) 2 R u ( H 2 0 ) 2 n
x
at
1.1
V (SCE)
produced the corresponding
Ru
1 1 1
complex which then was dimerized by heating to 40°C and keeping the potential at 1.1 V. The primarily formed dimers are presumably sulfato complexes exhibiting E j / - 0.68 2
and 0.87 V. The sulfato dimers were subsequently reduced at 0.65 V which lead to the diaqua complex ( H L ) ( H 0 ) R u x
the corresponding R u
2
I i r
2
-Ru
I I I
-0-Ru
I I I
(H 0)(H D x
2
which is oxidized reversibly to
2
dimer at 0.98 V. This complex slowly oxidizes water to
I V
molecular oxygen. The sulfato complexes of the R u ' ^ - R u ^ dimer are reformed, if the solution is again electrolyzed at 1.1 V. The thus obtained R u ^ - R u * 1
1
and R u ^ - R u '
UV-vis spectroscopy. The former complex M^cm"
1
whereas the latter has *
m
a
x
v
dimers were further characterized by
exhibits X
m
a
x
at 654
at 500 nm with e -
nm with e -
18000
17000 M ' ^ c m " . These 1
maximas are red-shifted compared to those of the corresponding bpy analogs (Gilbert et. al. 1985). The Raman spectra of the R u ^ - R u * ' * and R u ^ ^ - R u ' ^ dimers show v
s
at 375 and 381
c r n ^ , respectively. These vibrations are resonance enhanced and typical to v ( M - 0 - M ) g
(Plowman et. al 1984, Burke et. al. 1978, San Filippo et. al. 1976 and Campbell et. al. 1980). Further evidence for the proposed structures and oxidation states were obtained
from chemical reductions of ( H L ) 2 ( H 0 ) R u - 0 - R u ( O H ) ( H L ) x
U A
2
i V
x
with ascorbic acid.
2
Addition of one equivalent of reductant to the R u * ^ - R u * dimer lead to the R u ' ^ - R u v
dimer which was converted to two moles of ( H D R u ( H 0 ) x
equivalent.of reductant. At pH * 3 the R u
-Ru
n i
n
2
2
2
1 1 1
upon addition of two
dimer precipitates after addition of
n i
acetone. The solid analyzes as Ba[(HL)(L)(H 0)Ru -0-Ru (H 0)(L)(HL)]-13 H 0 . III
2
The trans isomers of the ( b p y ) R u 2
I I / i n
(H 0) 2
2
2 + /
III
3
2
2
complexes are thermodynamically
+
unstable, but they can be generated by irradiation of the corresponding cis (Durham et. al 1980). Since our preparations have been carried out in the monomers are expected to be present in the cis configuration. Dimerization complex, however, can still lead to two isomers, the racemate and the meso
complexes dark, the of the cis form. The
crystal
is
structure of
[(bpy) (H 0)Ru -0-Ru (H 0)(bpy) KC10 ) -2 2
III
2
III
2
2
4
4
H 0 2
known
(Gilbert et. al. 1985). and this compound is racemic. The solution of ( H D ( H 0 ) R u x
2
2
I I I
-0-Ru
I I I
(H 0)(H L) 2
x
in 0.5 M H S 0
2
2
4
prepared by the
procedure described above exhibits, in addition to the prominent wave at 0.98 V, an additional small wave at * 0.9 V (SCE) in the CV. We assume that the wave at ~ 0.9 V is due to small amounts of another isomer (eventually the meso form). Also, the isomer produced in large amounts is less soluble in acetone/H 0 at pH * 3. 2
PROPOSED WATER OXIDATION MECHANISMS a) Mechanism proposed by Gilbert et. al. (1985): Based on CV using a glassy carbon electrode Gilbert et. al. (1.985) postulate a three electron
oxidation
of
the
(bpy) (H 0)Ru -0-Ru (H 0)(bpy) 2
i n
2
n i
2
2
complex
to
the
corresponding R u ^ - O - R u ^ dimer which restores the R u I H - 0 - R u W dimer by reductive elimination of molecular oxygen. We assume that this mechanism is not established, because we were unable to obtain the three electron wave using In doped S n 0 electrode. 2
(bpy)2RuIH-0-Ru (bpy)2 m
I
I
0H2
OH2
«
» ±e,±H
(bpy) RuIII-0-RuIV(bpy) 2
|
+
OH
(bpy) RuV-0-RuV(bpy) 2
2
| 2
OH
2
b) Alternative Mechanism: Only the ( H L ) 2 ( H 2 0 ) R u - 0 - R u H ( H 2 0 ) ( H L ) 2 complex and the corresponding R u 1II
I
x
x
n i
-0-
R u ^ dimer can be observed by UV-vis spectroscopy and CV. Therefore, we propose that 1
formation of the R u ! V - 0 - R u
dimer is slow and/or thermodynamically unfavorable, but
I V
this presumably very reactive species forms a n-oxo, ^.-peroxo complex which would quickly form molecular oxygen upon oxidation and reform the R u ' ^ - O - R u ^ ' dimer.
(H L) Runi-0-Runi(H L)2 x
2
x
I
I
OH2
OH2
i
> ±e,±H
(H L) Ruin-0-RuIV(H L)2 x
2
+
x
|
|
OH2
OH
-2e ±e, ± H
^2H20
slow and/or
+
unfavorable
-02
m
(H L) Runi-0-Ru (H L)2 x
2
x
(H L) RuIV-0-R IV(H L)2
+•
x
2
U
x
-2H+ OH
0-
OH
2
(O2 -)
REFERENCES Burke J J, Kincaid J R, Spiro T G (1978) J A m Chem Soc 100: 6077. Campbell J R, Clark R J H (1980) J Chem Soc Faraday Trans II 76: 1103. Durham B, Wilson S R, Hodgson D J, Meyer T J (1980) J A m Chem Soc 102: 600. Gilbert J A, Eggleston D S, M u r p h y W R, Geselowitz D A, Gersten S W, Hodgson D J, Meyer T J (1985) J A m Chem Soc 107:3855. Plowman J E, Loehr T M, Schauer C K, Anderson 0 P (1984) Inorg Chem 23: 3553. San Filippo J Jr, Grayson R L, Sniadock H T (1976) Inorg Chem 15: 1103.
THE A P P L I C A T I O N OF D I F F U S E R E F L E C T A N C E L A S E R F L A S H METAL P H T H A L O C Y A N I N E S IN AN OPAQUE ENVIRONMENT
F.Wilkinson
and
Department
PHOTOLYSIS
TO
C.J.Willsher
of Chemistry,
University
o f Technology, Loughborough,
LE11
3TU,
UK
S i n c e i t s i n i t i a l d i s c o v e r y , t h e e l e g a n t t e c h n i q u e o f f l a s h p h o t o l y s i s has been c o n t i n u a l l y r e f i n e d for A
i n response to developments
i n the instrumentation
capturing very f a s t s i g n a l s and i n the shortening of the e x c i t a t i o n pulse. recent
development
has
been our
work
to
extend
flash
opaque and h i g h l y - s c a t t e r i n g s u b s t a n c e s ; f o r such samples
photolysis
to
the t r a n s i e n t
s p e c i e s generated by an e x c i t i n g pulse i s interrogated by means of monitoring light f
which
has been d i f f u s e l y r e f l e c t e d
D i f f u s e Reflectance F l a s h P h o t o l y s i s
from
the t r i p l e t
1
from t h e sample.
We
(DRFP) t o d e t e c t t r a n s i e n t
have used absorption
s t a t e o f a number of chromophores i n a wide v a r i e t y of
d i f f e r e n t opaque environments, such as h y d r o c a r b o n s adsorbed on ^ - a l u m i n a , (Kessler,
W i l k i n s o n , 1981)
ketones i n m i c r o c r y s t a l l i n e form
(Wilkinson,
W i l l s h e r , 1984) and i n t e r c a l a t e d i n t h e c h a n n e l s of t h e s y n t h e t i c 'silicalite'
zeolite
(Wilkinson, W i l l s h e r , Casal, Johnston, Scaiano, 1986), and i n the
d y e s t u f f Rose Bengal c h e m i c a l l y attached t o a polymer
substrate
(Wilkinson,
W i l l s h e r , P r i t c h a r d , 1984). We have a l s o shown t h a t p u l s e r a d i o l y s i s can be c a r r i e d o u t on opaque s u b s t a n c e s by u s i n g t h e d i f f u s e r e f l e c t i o n f r o m t h e sample
to
monitor
transients
formed
by
ionising
radiation
(Wilkinson,
W i l l s h e r , Warwick, Land, Rushton, 1984), and r e c e n t l y we have extended DRFP to f o l l o w p r o c e s s e s i n i t i a t e d by a p i c o s e c o n d e x c i t a t i o n s o u r c e ( W i l k i n s o n , W i l l s h e r , L e i c e s t e r , Barr, Smith, 1986).
Aluminium Sulphonated Phthalocyanine (ALPCS)
o I-H -a
