On the Role of Microcrystal Formation in

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(acceptor) was studied in various rigid glasses at 77 K. It is shown that energy transfer is much more efficient ... cyclohexane/n-pentane is opaque, and the phos-.
On the Role of Microcrystal Formation in Intermolecular Triplet-Triplet Energy Transfer in Rigid Glasses M. Z a n d e r Laboratory of Riitgerswerke AG, Castrop-Rauxel Z. Naturforsch. 38 a, 1146 - 1148 (1983); received July 20. 1983 The intermolecular triplet-triplet energy transfer system benzophenone (donor)/naphthalene (acceptor) was studied in various rigid glasses at 77 K. It is shown that energy transfer is much more efficient if the donor is present in the form of microcrystals than in the case where both donor and acceptor compound are in true solution.

Intermolecular singlet-singlet energy t r a n s f e r f r o m 5-acetyl-5H-benzo[b]-carbazole ( d o n o r , 1 0 - 3 M ) to 5 H - b e n z o [ b ] c a r b a z o l e (acceptor, 2 - 1 0 - 5 M ) in a rigid matrix at 77 K has previously b e e n s h o w n to occur in microcrystals of the d o n o r c o n t a i n i n g small a m o u n t s of the acceptor [1]. R e l a t e d results h a v e now been o b t a i n e d with the i n t e r m o l e c u l a r triplettriplet energy transfer system b e n z o p h e n o n e ( d o n o r ) / n a p h t h a l e n e (acceptor) [2], All e x p e r i m e n t s were p e r f o r m e d at 77 K using / e x = 380 nm as the excitation w a v e l e n g t h . Since t h e uv a b s o r p t i o n s p e c t r u m of n a p h t h a l e n e lies at w a v e lengths < 380 n m , the n a p h t h a l e n e p h o s p h o r e s c e n c e cannot be directly excited with 380 n m light. T h e p h o s p h o r e s c e n c e excitation spectra of b e n z o p h e n o n e are almost identical in the solvents used (ethanol, methylcyclohexane/n-pentane (4:1, vol/vol) and EPA). Curve c in Fig. 1 shows t h e p h o s p h o r e s c e n c e spectrum of the b e n z o p h e n o n e / n a p h t h a l e n e system (concentration of each c o m p o u n d = 10" 1 M) in ethanol. T h e s p e c t r u m is identical with that of benzophenone in the absence of n a p h t h a l e n e (curve a in Fig. 1, 1 0 - 1 M ) a n d does not indicate any trace of n a p h t h a l e n e p h o s p h o r e s c e n c e (curve b in Fig. 1, 10 _ 1 M. /. ex = 320 n m ) . A d e c r e a s e in b e n z o p h e n o n e p h o s p h o r e s c e n c e intensity of a p p r o x . 8% is o b s e r v e d in the b e n z o p h e n o n e / n a p h t h a l e n e system c o m p a r e d to p u r e b e n z o p h e n o n e in ethanol. A n a l o g o u s results were o b t a i n e d w h e n either the b e n z o p h e n o n e or n a p h t h a l e n e or b o t h c o n c e n t r a t i o n s were r e d u c e d . Obviously energy transfer is w e a k u n d e r the con-

Reprint requests to Professor Dr. M. Zander. Riitgerswerke AG. D-4620 Castrop-Rauxel. FRG.

X [ nm ] Fig. 1. Phosphorescence spectra of 1 0 " ' M solutions in ethanol at 77 K of a) benzophenone, b) naphthalene and c) benzophenone/naphthalene 1:1. Spectra are normalized to the intensity of the most intensive band.

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ditions used. Both d o n o r a n d a c c e p t o r molecules can be a s s u m e d to be p r e s e n t in t r u e solution. Q u i t e a d i f f e r e n t b e h a v i o u r was o b s e r v e d w h e n methylcyclohexane /n-pentane (4:1, vol/vol) was used as a solvent. In contrast to the ethanol solution the 10" 1 M solution of b e n z o p h e n o n e in m e t h y l c y c l o h e x a n e / n - p e n t a n e is o p a q u e , a n d t h e phosphorescence s p e c t r u m ( c u r v e a in Fig. 2) is slightly d i f f e r e n t f r o m t h a t in e t h a n o l . Microcrystal f o r m a tion has a l r e a d y a c c o u n t e d for the p h o s p h o r e s c e n c e b e h a v i o u r of b e n z o p h e n o n e in h y d r o c a r b o n glasses at low t e m p e r a t u r e [3], A d d i t i o n of n a p h t h a l e n e

( 1 0 " ' M) causes a d e c r e a s e in b e n z o p h e n o n e p h o s phorescence intensity of a p p r o x . 90% c o m p a r e d to p u r e b e n z o p h e n o n e in m e t h y l c y c l o h e x a n e / n - p e n tane, and s i m u l t a n e o u s l y t h e n a p h t h a l e n e p h o s phorescence a p p e a r s in t h e s p e c t r u m ( c u r v e c in Fig. 2, for comparison see n a p h t h a l e n e s p e c t r u m b in Fig. 2, 1 0 " ' M . /. ex = 320 n m ) . T h e r e is clear evidence that it is virtually only t h e p h o s p h o r e s cence of b e n z o p h e n o n e microcrystals t h a t is q u e n c h e d in the presence of n a p h t h a l e n e , w h i l e m u c h less energy transfer, or n o n e at all, t a k e s p l a c e f r o m the b e n z o p h e n o n e m o l e c u l e s p r e s e n t in t r u e solution: (i) T h e p h o s p h o r e s c e n c e l i f e t i m e of t h e b e n z o p h e n o n e m e a s u r e d at t h e 415 n m b a n d in spectrum c (Fig. 2) is identical w i t h t h a t of b e n z o p h e n o n e in the a b s e n c e of n a p h t h a l e n e in d i l u t e solution (5 msec), (ii) the b e n z o p h e n o n e p a r t of s p e c t r u m c (Fig. 2) is very s i m i l a r to t h a t of dissolved b e n z o p h e n o n e . Although a drastic d e c r e a s e in b e n z o p h e n o n e phosphorescence intensity in t h e p r e s e n c e of n a p h thalene was o b s e r v e d in t h e h y d r o c a r b o n solvent, the intensity of sensitized n a p h t h a l e n e p h o s p h o r e s cence was r a t h e r w e a k in t h e 10" 1 M s o l u t i o n s (curve c in F i g u r e 2). It was a s s u m e d that, a l t h o u g h the energy transfer f r o m b e n z o p h e n o n e m i c r o c r y s tals to n a p h t h a l e n e is highly effective, t h e n a p h thalene triplet state is strongly q u e n c h e d by intermolecular radiationless i n t e r a c t i o n d u e to t h e h i g h n a p h t h a l e n e c o n c e n t r a t i o n . T h i s a s s u m p t i o n is s u p ported by the o b s e r v a t i o n t h a t t h e intensity of sensitized n a p h t h a l e n e p h o s p h o r e s c e n c e i n c r e a s e s with decreasing n a p h t h a l e n e c o n c e n t r a t i o n (see curve d in Fig. 2, b e n z o p h e n o n e : 1 0 " ' M , n a p h thalene: 1 0 " 2 M ) .

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Fig. 2. Phosphorescence spectra in methylcyclohexane/ n-pentane (4:1, vol/vol) at 77 K of a) benzophenone (10"' M), b) naphthalene (10" 1 M), c) benzophenone/naphthalene (each 10 -1 M) and d) benzophenone ( 1 0 - 1 M ) / naphthalene ( 1 0 - 2 M ) . Spectra are normalized to the intensity of the most intensive band.

EPA solutions of benzophenone/naphthalene (each 1 0 " ' M ) s h o w a d e c r e a s e of a p p r o x . 26% in b e n z o p h e n o n e p h o s p h o r e s c e n c e intensity c o m p a r e d to p u r e b e n z o p h e n o n e in EPA, a n d a w e a k n a p h thalene p h o s p h o r e s c e n c e is o b s e r v e d (see c u r v e c in Fig. 3, for c o m p a r i s o n c u r v e a: benzophenone, 1 0 " ' M and b: n a p h t h a l e n e , 10" 1 M). Since E P A has a lower solvent p o w e r t h a n e t h a n o l , a g a i n it c a n be a s s u m e d that the effects o b s e r v e d a r e d u e to microcrystal f o r m a t i o n . In contrast to t h e h y d r o carbon solution t h e E P A s o l u t i o n is t r a n s p a r e n t . However, it is well k n o w n t h a t in s o m e cases s m a l l crystallites are not visually o b s e r v a b l e in rigid matrices [3], T o give an e x a m p l e , a 10" 3 M s o l u t i o n of b e n z o p h e n o n e in m e t h y l c y c l o h e x a n e / n - p e n t a n e

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forms a transparent glass at 77 K. In the presence of naphthalene ( 1 0 - 1 M) the b e n z o p h e n o n e phosphorescence is effectively q u e n c h e d and sensitized naphthalene phosphorescence a p p e a r s . " Since this has not been observed at these concentrations in solvents of higher solvent power e.g. ethanol, the effect most likely has to be explained by t h e f o r m a t i o n of invisible benzophenone microcrystals in the h y d r o carbon solvent. The findings presented in this N o t e do not stand in contradiction to the result r e a c h e d by Siegel and Judeikis [4] that the probability of intermolecular triplet energy transfer in rigid glasses is essentially independent of the solvent used, because microcrystal formation is rather unlikely in the systems studied by these authors. Although we used high donor a n d acceptor concentrations in our experiments, it h a s to be a s s u m e d that the concentration of electronically excited d o n o r molecules was rather low d u e to the low light intensity of the excitation source (xenon l a m p , 380 nm). This proved favourable to study the d i f f e r ent luminescence b e h a v i o u r of t r u e solutions a n d those containing microcrystals. Although it has been recognized in the literature [5] that formation of microcrystals can o b s c u r e "solution" luminescence spectroscopy at low t e m perature. the p h e n o m e n o n is possibly m u c h m o r e widespread than is normally realized. Experimental £00

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Fig. 3. Phosphorescence spectra of 1 0 " ' M solutions in EPA at 77 K of a) benzophenone, b) naphthalene and c) benzophenone/naphthalene 1:1. Spectra are normalized to the intensity of the most intensive band.

[1] M. Zander, Z. Naturforsch. 35 a, 779 (1980). [2] A. Terenin and V. Ermolaev, Trans. Faraday Soc. 52,1042 (1956). [3] R. A. Keller and D. E. Breen, J. Chem. Phys. 43, 2562 (1965). [4] S. Siegel and H. Judeikis, J. Chem. Phys. 41, 648 (1964).

Benzophenone was purified by extensive zone melting. Measurements were p e r f o r m e d as described in I.e. [1]. I thank Mr. K. Bullik for v a l u a b l e e x p e r i m e n t a l assistance.

[5] M. M. Moodie and C. Reid, J. Chem. Phys. 19, 986 (1951): ibid. 20, 1510 (1952); ibid. 22, 1126 (1954); G. von Foerster, J. Chem. Phys. 40, 2059 (1964); R. J. McDonald and B. K. Selinger, Aust. J. Chem. 24, 249 (1971); R. J. McDonald. CM. Logan, I. G. Ross, and B. K. Selinger. J. Mol. Spectr. 40, 137 (1971); T. S. Spencer and C. M. O'Donnell, J. Amer. Chem. Soc. 94, 4846 (1972).

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