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Jul 12, 2011 - A number of nematic spiro[cyclopentyl-1,9 ]fluorene reactive mesogens with polymerisable oxetane or non-con- jugated diene end-groups ...
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Liquid crystalline organic semiconductors: nematic spiro[cyclopentyl-1,9′]fluorenes a

a

b

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Muralidhar Reddy Billa , Krishna Kassireddy , Marta Haro , Manea S. Al-Kalifah , a

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Stephen M. Kelly , Stuart P. Kitney & Mary O'Neill a

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Department of Chemistry , University of Hull , Hull, UK

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Department of Physics , University of Hull , Hull, UK Published online: 12 Jul 2011.

To cite this article: Muralidhar Reddy Billa , Krishna Kassireddy , Marta Haro , Manea S. Al-Kalifah , Stephen M. Kelly , Stuart P. Kitney & Mary O'Neill (2011) Liquid crystalline organic semiconductors: nematic spiro[cyclopentyl-1,9′]fluorenes, Liquid Crystals, 38:7, 813-829, DOI: 10.1080/02678292.2011.577913 To link to this article: http://dx.doi.org/10.1080/02678292.2011.577913

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Liquid Crystals, Vol. 38, No. 7, July 2011, 813–829

Liquid crystalline organic semiconductors: nematic spiro[cyclopentyl-1,9 ]fluorenes Muralidhar Reddy Billaa , Krishna Kassireddya , Marta Harob , Manea S. Al-Kalifahb , Stephen M. Kellya *, Stuart P. Kitneya and Mary O’Neilla * a

Department of Chemistry, University of Hull, Hull, UK; b Department of Physics, University of Hull, Hull, UK

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(Received 10 February 2011; final version received 31 March 2011) A number of nematic spiro[cyclopentyl-1,9 ]fluorene reactive mesogens with polymerisable oxetane or non-conjugated diene end-groups have been successfully synthesised. The melting and clearing points of these new spiro[cyclopentyl-1,9 ]fluorenes are much higher than those of the corresponding 9,9-diethyl- or 9,9-dipropyl– substituted fluorenes, which are present in many light-emitting polymers currently used in polymer light-emitting diodes. The significant difference in transition temperature may be attributed to the lower intermolecular separation induced by the more rigid and less bulky cyclopentyl group, confirmed by molecular modelling. The presence of either these short but flexible aliphatic chains, or a rigid alicyclic group of similar size and shape, results in only small differences in the photoluminescence spectra and efficiency of these model liquid crystalline organic semiconductors, and in their improved thermal stability. Keywords: spiro-fluorenes; nematics; organic semiconductors

1.

Introduction

Liquid crystals have been developed as a new class of organic semiconductors offering a combination of high stability, high charge transport and efficient photoluminescence and electroluminescence [1–8]. Liquid crystalline organic semiconductors can be deposited from solution in common organic solvents, for example by spin coating or inkjet printing, which makes them suitable for large scale roll-to-roll manufacture of electronic components for the emerging plastic electronics industry. Especially attractive for such applications are liquid crystal monomers with a polymerisable group at the end of two terminal spacer groups attached to a molecular core (reactive mesogens), since they can be photochemically polymerised and patterned to form an insoluble polymer network that facilitates the stepwise fabrication of multilayer devices [3–6]. Reactive mesogens [9–16] and liquid crystalline oligomers [17–19], and liquid crystalline conjugated main-chain polymers [20–25] incorporating 2,7-disubstituted fluorene moieties, have been used to fabricate organic lightemitting diodes (OLEDs) [9–12, 17, 18, 20–25] and organic photovoltaics [13, 14]. These liquid crystalline semiconductors usually incorporate 2,7disubstituted fluorene moieties with two bulky substituents in the 9-position, most often straight-chain or branched aliphatic chains [9–25]. The substitutents control the intermolecular distance by steric effects and can, for example, induce a nematic phase with low melting point to allow room temperature ∗

processing of homogeneously aligned monodomains. The magnitude of the charge transport and the quantum efficiency of the photoluminescence (PL) and electroluminescence (EL) emission is also dependent on the intermolecular distance determined by the nature of these lateral substituents [15, 16]. Many liquid crystalline fluorenes show efficient blue PL or EL in thin films [20–23] and also exhibit good hole-transport properties with high charge transport mobility [15, 16, 19, 24, 25]. A problem with some conjugated main-chain polymers, oligomers and reactive mesogens incorporating the 2,7-disubstituted-9,9-dialkyl-fluorene moiety is the appearance of a tail of green emission, accompanied by colour instability and lower efficiency [26–29]. This phenomenon has been attributed to the formation of aggregates or eximers [26, 27], as well as the formation of keto defects [21, 28, 29], to form fluorenone derivatives by oxidative degradation of the 9,9-dialkyl-fluorene moiety. An approach to resolving the tendency of 2,7-disubstituted-9,9dialkyl-fluorenes to form fluorenones is to block the 9-position with a spiro- or cycloalkyl-group [30–33]. We wished to study the mesomorphic behaviour, and other physical properties such as photoluminescence spectra and efficiency (QEPL ), both in solution and as thin films. Also of interest are thermal stability, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, and their dependence on the size and shape of lateral substituents in the 9-position of

Corresponding authors. Email: [email protected]; M.O’[email protected]

ISSN 0267-8292 print/ISSN 1366-5855 online c 2011 Taylor & Francis  DOI: 10.1080/02678292.2011.577913 http://www.informaworld.com

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model 2,7-disubstituted fluorene reactive mesogens, and hence on the intermolecular distance. The presence of a spiro-cyclopentyl substituent in these compounds may prevent oxidation at the 9-position and induce higher electrochemical stability, and hence give longer OLED lifetimes in practical applications [1–8]. The model fluorene-based reactive mesogens chosen for our studies consisted of a central 2,7disubstituted fluorene ring with short straight-chain (ethyl or propyl), or spiro (cyclopentyl), aliphatic groups as lateral substituents in the 9-position of the fluorene moiety (Figures 1 and 2). They also contained 2,5-disubstituted thiophene and 1,4disubstituted phenyl aromatic rings attached to the fluorene core in the 2,7-positions, with a variety of alkoxy end-groups attached to the phenyl rings. The terminal end-chains possessed either a polymerisable non-conjugated diene or an oxetane end-group. The end-groups can be polymerised to form highly crosslinked, intractable and insoluble polymer networks. Studies of similar non-liquid crystalline model compounds, shown in Figure 3, indicate that it is the shape and volume occupied by the cyclopentyl group that determines the intermolecular distance and the degree of aggregation in the solid state, and strongly influences photoluminescence efficiency [34, 35].

(a)

(b)

Figure 1. MM2 optimised geometry of 2,7-disubstituted9,9-diethylfluorene in (a) side-view, and (b) end-view (colour version online).

(a)

(b)

Figure 2. MM2 optimised geometry of 2,7-disubstituted9-cyclopentylfluorene in (a) side-view, and (b) end-view (colour version online).

S S

Figure 3. MM2 optimised geometry of 2 ,7 -bis(5thiophen-2-yl)spiro[cyclopentane-1,9 - fluorene].

2. Experimental 2.1 Instrumentation All commercially available starting materials and reaction intermediates, reagents and solvents were obtained from Aldrich, Strem Chemicals, Acros Organics or Lancaster Synthesis and were used as supplied, unless otherwise stated. Tetrahydrofuran was pre-dried using sodium wire and distilled over this under nitrogen as required, with a benzophenone indicator. Unless water was present as solvent or reagent, all reactions were carried out in an anhydrous nitrogen atmosphere, and temperatures were measured externally. Mass spectra were recorded using a gas chromatography mass spectrometer (GC/MS), Shimadzu QP5050A, with electron impact (EI), at a source temperature of 200◦ C for compounds with relative molecular mass (RMM) < 800 g mol–1 , and for compounds with RMM above this using a Bruker time of flight mass spectrometer, Reflex IV, matrix-assisted laser desorption/ionisation (MALDI), with a 384-well microlitre plate format and a scout target. Samples were dissolved in dichloromethane (DCM) with a 2(4-hydroxyphenylazo)benzoic acid matrix (1:10). IR spectra were recorded using a Perkin– Elmer Paragon 1000 Fourier transform–infrared (FT–IR) spectrometer, and proton nuclear magnetic resonance (1 H NMR) spectra were recorded on a JEOL Lambda 400 spectrometer with tetramethylsilane (TMS) as internal standard. Thin-layer chromatography (TLC) using aluminium-backed TLC plates coated with silica gel (60 F254 Merck), and gas–liquid chromatography (GLC) using a Chromopack CP3800 gas chromatograph equipped with a 10 m CP–SIL 5CB column, were employed to measure the progress of reactions. Purification of intermediates and final products was carried out by gravity column chromatography, using silica gel (40–63 µm, 60 Å) obtained from Fluorochem. A combination of TLC, GLC and elemental analysis, using a Fisons CHN elemental analyser, EA 1108, was used to determine the purity of the reaction intermediates and the final compounds, 11, 12, 15 and 17–19. The structure of the intermediates and final products was confirmed by 1 H NMR

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Liquid Crystals spectroscopy (JOEL JMN–GX270 FT nuclear resonance spectrometer), infrared (IR) spectroscopy (Perkin–Elmer 783) and mass spectrometry (MS; Finnegan MAT 1020 automated GC/MS). Reaction progress and product purity were checked using a Chrompack CP 9001 capillary gas chromatograph provided with a 10 m CP–SIL 5CB (0.12 µm, 0.25 mm) capillary column. All the final products were more than 99.5% pure by GLC. Melting points were determined using an Olympus BH–2 polarising light microscope together with Mettler FP52 heating stage and FP5 temperature control unit. Molecular modelling was carried out using ChemDraw pro 3D MOPAC 2000 (CambridgeSoft Corporation, USA) and MM1 minimisation. The chemical bonds in the compounds shown in Figures 1–6 define the molecular geometry of the aromatic core

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of 2,7-disubstituted-9,9-diethylfluorene, 2,7-disubstituted-9-cyclopentylfluorene, 2 ,7 -bis(5-thiophenand the 2-yl)spiro[cyclopentane-1,9 -fluorene] compounds 15 (m = 2), 19 and 17 (m = 2), and no assumptions about their conformation were required. The disposition of the phenyl and thiophene rings of the compounds shown in Figures 1–6 was chosen to give the most linear structure to the modelling, and was based upon the assumption the preferred molecular conformation will be that giving rise to the highest length-to-breadth (aspect) ratio and greatest degree of molecular polarisability consistent with current understanding of the nematic phase of thermotropic calamitic liquid crystals. The preferred conformation of the aliphatic chains in the lateral and terminal positions in 15 (m = 2), 19 and 17 (m = 2), shown in Figures 4–6, was assumed

Figure 4. MM2 optimised conformation of 15 (m = 2) (colour version online).

Figure 5. MM2 optimised conformation of 19 (colour version online).

Figure 6. MM2 optimised conformation of the 9,9-diethyl-substituted oxetane, 17 (m = 2) (colour version online).

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in each case to be the all-trans, antiperiplanar conformation, for the reasons given above. 2.2 Synthesis The reactive mesogens, 11 and 12 (n = 5 and 10), where n is the number of methylene units (–CH2 –) in the aliphatic spacer between the aromatic core and the polymerisable end-group, were synthesised as shown in Scheme 1. The reaction intermediates, 2, 4, 5, 7 and 14, were synthesised as previously reported [10, 36]. Alkylation of commercially available 4-bromophenol, 1, using 1-bromooctane in a Williamson alkylation reaction, gave 1-bromo-4octyloxybenzene, 2 [36], which was reacted with 2-thiopheneboronic acid, 3, in a Suzuki aryl–aryl cross-coupling reaction [37] to give 1-octyloxy-4(thiophen-2-yl)benzene, 4 [36]. The thiophene, 4, was lithiated using n-BuLi in THF at –78◦ C and quenched with tributyl tin chloride, resulting in the stannyl thiophene, 5. This was not purified further, as silica gel can protonate the 2-position of the stannyl thiophene, 5, and remove the tin moiety [38]. 2-Bromo-7-iodofluorene, 7, was synthesised by iodination of 2-bromofluorene, 6, by the standard procedure using a combination of iodine and periodic acid [39]. This was followed by alkylation in the 9-position of the fluorene, 7, with 1,4-dibromobutane to give 2 -bromo-7 iodospiro[cyclopentane-1,9 -fluorene], 8. A Stille aryl–aryl cross-coupling reaction [40] between the 9-spirofluorene, 8, and the stannyl thiophene, 5, gave 2 ,7 -bis{5-[4-(octyloxy)phenyl]thiophen-2-yl}spiro[cyclopentane-1,9 -fluorene], 9. Commercially available boron tribromide was used in the usual way [41] to remove the octyloxy groups, giving the bisphenol, 10. The oxetane diether, 11, was prepared using the Williamson ether reaction [36] between the bisphenol, 10, and 3-(6-bromohexyloxymethylenyl)3-methyloxetane, which was synthesised in the usual way from 1,6-dibromohexane and (3-methyloxetan3-yl)methanol [42]. The diene diethers, 12 (n = 5 and 10), where n is the number of carbon atoms in the alkyl chains attached in the 9-position of the 2,7-disubstituted fluorene ring, were prepared similarly using bisphenol, 10, and penta-1,4-dien3-yl 6-bromohexanoate or penta-1,4-dien-3-yl 11-bromoundecanoate, respectively, synthesised from the corresponding ω-bromoalkanoic acids and 1,4-pentadien-3-ol in the usual way [43]. The reactive mesogens, 17 and 18 (m = 5 and 10), were synthesised as shown in Scheme 2. 2,7-Dibromo-9,9-diethylfluorene, 14, was synthesised from 2,7-dibromofluorene, 13, and ethyl bromide using the standard alkylation procedure

[44]. A Suzuki aryl–aryl cross-coupling [37] reaction between 2,7-dibromo-9,9-diethylfluorene, 14, and the stannyl thiophene, 5, resulted in the formation of 2,7-bis[5-(4-octyloxyphenyl)thiophen-2-yl]9,9-diethylfluorene 15, which was dealkylated using boron tribromide in the usual way [41] to yield the corresponding bisphenol, 16. The bisphenol, 16, was reacted with 3-(6-bromohexyloxymethylenyl)-3methyloxetane [42] by the standard Williamson ether reaction to produce the oxetane reactive mesogen, 17. The diene reactive mesogens, 18 (m = 5 and 10), were prepared similarly using the bisphenol, 16, and penta-1,4-dien-3-yl 6-bromohexanoate and penta-1,4-dien-3-yl 11-bromoundecanoate, respectively [43].

2.2.1 2 -Bromo-7 -iodo-spiro[cyclopentyl-1,9 ]fluorene, 8 Powdered KOH (4.75 g, 0.0846 mol) was added in small portions over 30 min to a mixture of 2-bromo-7-iodofluorene, 7 (7.50 g, 0.020 mol), 1,4-dibromobutane (4.79 g, 0.022 mol), potassium iodide (0.33 g, 0.0020 mol) and dimethyl sulphoxide (DMSO; 100 cm3 ) at room temperature. The mixture was stirred for 2 h and then poured into water (300 cm3 ). The crude product was extracted into DCM (3 × 200 cm3 ). The combined organic extracts were washed with aqueous hydrochloric acid (10%, 200 cm3 ), brine (2 × 200 cm3 ), and dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, hexane] and recrystallisation from ethanol, to yield white needles (6.00 g, 68%). Melting point: 138–139◦ C; purity: > 99% (GC). 1 H NMR (CDCl3 ) δ H : 2.00 (4H, m), 2.11, (4H, m), 7.45–7.52 (4H, m), 7.65–7.73 (2H, m). IR (ν max , cm–1 ): 2960, 1584, 1449, 1401, 1271, 1230, 1111, 1007, 1006, 969, 870, 790, 749. MS m/z (EI): 424 (M+ ), 414, 386, 364, 320, 259 (M 100), 219, 186, 154, 115, 84, 50.

2.2.2 2 ,7 -bis{5-[4-(Octyloxy)phenyl]thiophen-2yl}-spiro[cyclopentyl-1,9 ]-fluorene, 9 Pd(PPh3 )4 (0.27 g, 2.3 × 10–4 mol) was added to a solution of 2 -bromo-7 -iodo-spiro[cyclopentyl-1,9] fluorene, 8, (2.00 g, 0.0047 mol), tributyl{5-[4(octyloxy)phenyl]thiophen-2-yl}stannane, 5, (8.15 g, 0.0141 mol) and dimethylformamide (DMF; 50 cm3 ). The mixture was heated at 80◦ C overnight, allowed to cool to room temperature and then poured into water (100 cm3 ) while stirring vigorously. The crude product was extracted into DCM (2 × 100 cm3 ) and the combined organic fractions washed with

Liquid Crystals

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Br

HO 1

Br

(i)

6 (iv)

Br

C8H17O 2

HO B HO

(ii)

S

Br

I 7

3

S

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C8H17O

(v) 4 (iii) S

Sn(C4H9)3 Br

C8H17O

I 8

5

(vi)

C8H17O

S

S

OC8H17

9 (vii)

HO

S

S

OH

10 (viii)

RO

11; RO = O

S

S

CH2OC6H12O–

12; RO =

OR

O2CCnH2nO–

Scheme 1. Synthesis of reactive mesogens, 11 and 12 (n = 5 and 10). Notes: Reagents and conditions: (i) K2 CO3 , CH3 COCH2 CH3 , 80◦ C; (ii) Pd(PPh3 )4 , Na2 CO3 (20% aq. soln.) DME, 80◦ C; (iii) (a) n-BuLi, –78◦ C, (b) ClSn(Bu)3 , –78◦ C; (iv) H5 IO6 , I2 , H2 SO4 , H2 0, CH3 COOH, 75◦ C; (v) BrC4 H8 Br, KOH, KI, DMSO, 20◦ C; (vi) Pd(PPh3 )4 , DMF, 80◦ C; (vii) (a) BBr3 , DCM, 0◦ C, (b) H3 O+ ; (viii) RBr, K2 CO3 , DMF, 80◦ C.

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Br

Br 13 (i) CmH2m+1

CmH2m+1 Br

Br 14

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S

Sn(C4H9)3

C8H17O

(ii) 5 CmH2m+1

CmH2m+1 C8H17O

S

S

OC8H17

15; m = 2 and 3

(iii)

CmH2m+1

CmH2m+1 HO

S

S

OH

16

(iv) CmH2m+1 RO

17; RO = O

CmH2m+1 S

S

CH2OC6H12O–

18; RO =

OR

O2CCnH2nO–

Scheme 2. Synthesis of reactive mesogens, 17 and 18 (n = 5 and 10). Notes: Reagents and conditions: (i) KOH, KI, DMSO, Cm H2m+1 Br, 80◦ C; (ii) Pd2 dba3 , PPh3 , DMF; (iii) (a) BBr3 , DCM, 0◦ C, (b) H3 O+ ; (iv) RBr, K2 CO3 , DMF, 80◦ C.

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Liquid Crystals brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, DCM : hexane, 1 : 1] and recrystallisation from DCM/EtOH to yield a yellow crystalline solid (2.50 g, 67%). Transition temp. (◦ C): Cr 173 N 256 I. 1 H NMR (CDCl3 ) δ H : 0.88 (6H, t, J J = 6.9 Hz), 1.24–1.56 (20H, m), 1.79 (4H, quint), 1.94 (4H, m), 2.07 (4H, m), 4.01 (4H, t, = J = 6.6 Hz), 6.93 (4H, d, J = 8.8 Hz), 7.20 (2H, d, J = 3.7 Hz), 7.31 (2H, d, J = 3.7 Hz), 7.59–7.62 (6H, m), 7.69 (4H, d, J = 8.2 Hz). IR ν max (cm–1 ): 2922, 2853, 2361, 1605, 1564, 1492, 1469, 1384, 1296, 1242, 1173, 1123, 1049, 867, 804. MS m/z (EI): 793 (M+ ). Combustion analysis: Expected: C, 80.05%; H, 7.88%; S, 8.06%. Obtained: C, 80.18%; H, 8.02%; S 8.09%. 2.2.3 2 ,7 -bis[5-(4-Hydroxyphenyl)thiophen-2-yl]spiro[cyclopentyl-1,9 ]-fluorene, 10 A 1.0 M solution of boron tribromide (7.6 cm3 , 0.0076 mol) was added dropwise to a cold (0◦ C) solution of 2 ,7 -bis{5-(4-[octyloxy)phenyl]thiophen-2yl}-spiro[cyclopentyl-1,9 ]-fluorene, 9, (1.50 g, 0.0019 mol) in DCM (40 cm3 ). The reaction mixture was stirred overnight at room temperature and then poured into cold (0◦ C) water (200 cm3 ) with vigorous stirring, stirred for 30 min, and DCM (100 cm3 ) was then added. The aqueous layer was separated and washed with DCM (2 × 50 cm3 ). The combined organic layers were washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, DCM] to yield a yellow crystalline solid (1.26 g, 83%). Melting point: > 300◦ C. 1 H NMR (CDCl3 ) δ H : 2.00 (4H, m), 2.10 (4H, m), 4.81 (2H, s), 6.80 (4H, d, J = 7.7 Hz), 7.18 (2H, d, J = 3.9 Hz), 7.32 (2H, d, J = 4.0 Hz), 7.50–7.56 (6H, m), 7.70 (4H, d, J = 8.0 Hz). IR ν max /cm–1 : 3306, 2952, 1603, 1577, 1454, 1289, 1239, 1161, 1005, 966, 862, 654. MS m/z (EI): 528 (M+ ). 2.2.4 2 ,7 -bis[5-(4-{6-[(3-Methyloxetan-3-yl) methoxy]hexyloxy}phenyl)thiophen-2-yl]spiro[cyclopentyl-1,9 ]-fluorene, 11 A mixture of 2 ,7 -bis[5-(4-hydroxyphenyl)thiophen-2-yl]-spiro[cyclopentyl-1,9 ]-fluorene, 10, (0.30

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g, 0.0006 mol), potassium carbonate (0.15 g, 0.0014 mol), 3-(6-bromohexyloxymethyl)-3-methyl-oxetane (0.29 g, 0.0014 mol) [42] and DMF (12 cm3 ) was heated (80◦ C) overnight. The mixture was allowed to cool to room temperature and poured into water (100 cm3 ). The crude product was extracted into DCM (3 × 50 cm3 ) and the combined organic extracts washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, DCM, 4 : 3] and recrystallisation from DCM/EtOH to yield a yellow crystalline solid (0.37 g, 66%). Transition temp. (◦ C): Cr 126 N 190 I. 1 H NMR (CDCl3 ) δ H : 1.31 (6H, s), 1.43–1.53 (4H, m), 1.64 (4H, quint), 1.82 (4H, quint), 1.96 (4H, m), 2.01 (4H, t, J = 4.56 Hz), 2.08 (4H, m), 3.48 (8H, s), 3.99 (4H, t, J = 6.44 Hz), 4.36 (4H, d, J = 5.7 Hz), 4.52 (4H, d, J = 5.5 Hz), 6.92 (4H, d, J = 8.6 Hz), 7.20 (2H, d, J = 3.8 Hz), 7.34 (2H, d, J = 3.7 Hz), 7.56–7.62 (6H, m), 7.67 (4H, d, J = 7.9 Hz). IR ν max /cm–1 : 2952, 2870, 1604, 1565, 1478, 1471, 1370, 1282, 1241, 1145, 980, 931, 877. MS m/z (MALDI): 937 (M+ ), 801. Combustion analysis: Expected: C, 75.44%; H, 7.51%; S, 6.83%. Obtained: C, 75.22%; H, 7.56%; S, 6.95%. 2.2.5 Dipenta-1,4-dien-3-yl diester, 12 (n = 5) A mixture of 2 ,7 -bis[5-(4-hydroxyphenyl)thiophen2-yl]-spiro[cyclopentyl-1,9 ]-fluorene, 10 (0.22 g, 0.0004 mol), potassium carbonate (0.12 g, 0.0009 mol), penta-1,4-dien-3-yl 6-bromohexanoate (0.22 g, 0.0009 mol) [43] and DMF (10 cm3 ) was heated (80◦ C) overnight. The mixture was allowed to cool to room temperature and then poured into water (100 cm3 ). The crude product was extracted into DCM (3 × 50 cm3 ) and the combined organic extracts washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, DCM, 1 : 1] and recrystallisation from DCM/EtOH to yield a yellow crystalline solid (0.28 g, 76%). Transition temp. (◦ C): Tg 31 Cr 151 N 196 I. 1 H NMR (CDCl3 ) δ H : 1.24–1.39 (4H, m), 1.59–1.67 (4H, m), 1.82 (4H, quint), 1.91 (4H, m), 2.08 (4H, m), 2.35 (4H, t), 4.02 (4H, t), 5.21–5.33 (8H, m), 5.72–5.74 (2H, m), 5.80–5.88 (4H, m), 6.90 (4H, d, J = 8.8 Hz), 7.20 (2H, d, J = 3.8 Hz), 7.32 (2H, d, J = 3.9 Hz), 7.56–7.61 (6H, m), 7.69 (4H, d, J = 8.8Hz).

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IR ν max (cm–1 ): 2951, 2867, 1736, 1604, 1566, 1500, 1471, 1379, 1296, 1241, 1169, 983, 930, 875, 801. MS m/z (MALDI): 929 (M+ ). Combustion analysis: Expected: C, 76.09%; H, 6.71%; S, 6.89%. Obtained: C, 76.19%; H, 6.67%; S, 6.95%.

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2.2.6 Dipenta-1,4-dien-3-yl diester, 12 (n = 10) A mixture of 2 ,7 -bis[5-(4-hydroxyphenyl)thiophen2-yl]-spiro[cyclopentyl-1,9 ]-fluorene, 10 (0.30 g, 0.0006 mol), potassium carbonate (0.15 g, 0.0014 mol), penta-1,4-dien-3-yl 11-bromoundecanoate (0.47 g, 0.0014 mol) [43] and DMF (10 cm3 ) was heated (80◦ C) overnight. The mixture was allowed to cool to room temperature then poured into water (100 cm3 ). The crude product was extracted into DCM (3 × 50 cm3 ) and the combined organic extracts washed with brine (100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, DCM, 1 : 1] and recrystallisation from DCM/EtOH to yield a yellow crystalline solid (0.30 g, 52%). Transition temp. (◦ C): Cr 111 N 169 I. 1 H NMR (CDCl3 ) δ H : 1.24–1.39 (24H, m), 1.59– 1.67 (4H, m), 1.82 (4H, quint), 1.91 (4H, m), 2.08 (4H, m), 2.35 (4H, t), 4.02 (4H, t), 5.21–5.33 (8H, m), 5.72–5.74 (2H, m), 5.80–5.88 (4H, m), 6.90 (4H, d, J = 8.8 Hz), 7.20 (2H, d, J = 3.8 Hz), 7.32 (2H, d, J = 3.9 Hz), 7.56–7.61 (6H, m), 7.69 (4H, d, J = 8.8Hz). IR ν max /cm–1 : 2951, 2867, 1736, 1604, 1566, 1500, 1471, 1379, 1296, 1241, 1169, 983, 930, 875. MS m/z (MALDI): 1070 (M+ ). Combustion analysis: Expected: C, 77.49%; H, 7.54%; S, 6.00%. Obtained: C, 77.29%; H, 7.76%; S, 6.08%.

2.2.7 2,7-bis{5-[4-(Octyloxy)phenyl]thiophen-2yl}-9,9-diethylfluorene, 15 A mixture of 2,7-dibromo-9,9-diethylfluorene 14 (1.0 g, 0.0026 mol) [10] and tributyl{5-[4(octyloxy)phenyl]thiophen-2-yl}stannane, 5, (7.50 g, 0.013 mol) was dissolved in DMF (10 cm3 ). Tris(dibenzylideneacetone)dipalladium (0.06 g, 6.5 × 10–5 mol ) and triphenyl phosphine (0.07 g, 2.6 × 10–4 mol ) were added and the mixture heated overnight at 90◦ C, allowed to cool, poured into water (200 cm3 ) and the crude product extracted into ethyl acetate (3 × 200 cm3 ). The combined organic extracts were washed with brine (2 × 200 cm3 ), dried (MgSO4 ) and the solvent removed under reduced

pressure. Purification was carried out by column chromatography [silica gel, ethyl acetate : hexane] to yield a green solid (1.00 g, 48.5%). Transition temp. (◦ C): Cr 144 N 212 I. 1 H NMR (CDCl3 ) δ H : 0.40 (6H, t, J = 7.16 Hz), 0.90 (6H, m), 1.35 (20H, m), 1.790 (4H, quint, J = 8 Hz), 2.10 (4H, dd), 3.99 (4H, t, J = 6.6 Hz), 6.93 (4H, d, J = 8.76 Hz), 7.20 (2H, d, 3.64 Hz), 7.33 (2H, d, 3.68 Hz), 7.57 (6H, d, J = 8.8 Hz), 7.61 (2H, dd), 7.69 (2H, d, J = 7.88 Hz). IR ν max /cm–1 : 2921, 2850, 2274, 1605, 1569, 1543, 1509, 1471, 1390, 1287, 1175, 1127, 1111, 1067, 1045, 1028, 1001, 951, 876, 827, 797, 725, 664, 634, 608. MS m/z (EI): 900, 815, 797, 794 (M+ , M100), 693, 590, 551, 508, 459, 345. Combustion analysis: Expected: C, 80.05%; H, 7.86%; S, 8.06%. Obtained: C, 79.98%, H, 8.00%, S, 8.21%. 2.2.8 2,7-bis[5-(4-Hydroxyphenyl)thiophen-2-yl]9,9-diethylfluorene, 16 A 1.0 M solution of boron tribromide (4 cm3 , 0.0037 mol) was added dropwise to a cold (0◦ C) solution of 2,7-bis{5-[4-(octyloxy)phenyl]thiophen-2-yl}-9,9diethylfluorene, 15, (0.75 g, 0.00094 mol) in DCM (40 cm3 ). The reaction mixture was allowed to warm to room temperature and stirred overnight. The solution was poured into cold (0◦ C) water (100 cm3 ) with vigorous stirring, stirred for 30 min, and then DCM (100 cm3 ) was added. The aqueous layer was separated and washed with DCM (2 × 50 cm3 ). The combined organic layers were washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, ethyl acetate in hexane] to yield a yellow crystalline solid (0.50 g, 94.3%). Melting point: > 300◦ C. 1 H NMR (CDCl3 ) δ H : 0.36 (6H, t, J = 7.3 Hz), 1.22 (4H, s), 2.07 (4H, q, 7.95), 6.85 (4H, d, J = 8.64 Hz), 7.14 (2H, d, J = 3.64 Hz), 7.30 (2H, d, J = 3.64 Hz), 7.45 (6H, d, J = 8.6 Hz), 7.57 (2H, d, J = 7.84 Hz), 7.65 (2H, d, J = 7.84 Hz). IR ν max (cm–1 ): 2961, 2917, 1699, 1606, 1541, 1508, 1472, 1373, 1273, 1231, 1173, 1106, 951, 876, 832, 798, 736, 717, 669, 656. MS m/z (EI): 572, 570 (M+ , M100), 507. 2.2.9 2,7-bis[5-(4-{6-[(3-Methyloxetan-3-yl) methyl]hexyloxy}phenyl)thiophen-2-yl]-9,9diethylfluorene, 17 A mixture of 2,7-bis[5-(4-hydroxyphenyl)thiophen2-yl]-9,9-diethylfluorene, 16, (0.15 g, 0.0026 mol),

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Liquid Crystals potassium carbonate (0.1 g, 0.0007 mol), 3-(6bromohexyloxymethylenyl)-3-methyloxetane (0.20 g, 0.00078 mol) [42] and DMF (5 cm3 ) was held overnight at 80◦ C. The mixture was allowed to cool to room temperature, then poured into water (200 cm3 ). The crude product was extracted into DCM (3 × 50 cm3 ) and the combined organic extracts washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, DCM] and recrystallisation from MeOH to yield a yellow crystalline solid (0.13 g, 54.16%). Transition temp.(◦ C): Tg 16 Cr 92 N 146 I. 1 H NMR (CDCl3 ) δ H : 0.40 (8H, t, J = 7.95 Hz), 1.82 (12H, t), 2.10 (4H, d, J = 8.1 Hz), 4.0 (4H, t, J = 6.6 Hz), 4.35–4.36 (2H, m), 4.50 (4H, t, J = 5.5 Hz), 6.90 (4H, d, J = 8.61 Hz), 7.19 (2H, d, 3.67 Hz), 7.33 (2H, d, 3.67 Hz), 7.57 (6H, d, J = 8.43 Hz), 7.63 (2H, d), 7.68 (2H, d, J = 8.06 Hz). IR ν max (cm–1 ): 2931, 2855, 2282, 1605, 1570, 1541, 1509, 1472, 1376, 1277, 1177, 1112, 1027, 978, 875, 830, 797, 735, 664, 610. MS m/z (EI): 938 (M+ , M100), 926, 908. Combustion analysis: Expected: C, 75.44%; H, 7.51%; S, 6.83%. Obtained: C, 83.54%; H, 7.67%; S 6.74%. 2.2.10 Diester, 18 (m = 5) A mixture of 2,7-bis[5-(4-hydroxyphenyl)thiophen2-yl]-9,9-diethylfluorene, 16, (0.15 g, 0.00026 mol), potassium carbonate (0.1 g, 0.0007 mol), penta-1,4dien-3-yl 6-bromohexanoate (0.20 g, 0.00078 mol) [43] and DMF (5 cm3 ) was held overnight at 80◦ C. The mixture was allowed to cool to room temperature and then poured into water (100 cm3 ). The crude product was extracted into ethyl acetate (2 × 100 cm3 ) and the combined organic extracts washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, ethyl acetate : hexane] and recrystallisation from methanol to yield a yellow crystalline solid (0.14 g, 58.33%). Transition temp. (◦ C): Tg 20 Cr 76 N 137 I. 1 H NMR (CDCl3 ) δ H : 0.40 (4H, t, J = 7.24 Hz), 1.80 (10H, m), 2.09 (4H), 2.35 (4H, t), 4.0 (4H, t, J = 6.32 Hz), 4.16 (4H, t), 5.22 (4H, quint), 5.72 (2H, t), 5.80 (4H, t), 6.91 (4H, d, J = 8.84 Hz), 7.20(2H, d, 3.84 Hz), 7.33 (2H, d, 3.84 Hz), 7.56 (6H, d), 7.61 (2H, d, J = 7.6 Hz), 7.69 (2H, d, J = 8.04 Hz). IR ν max (cm–1 ): 2920, 2288, 1730, 1639, 1604, 1570, 1540, 1509, 1472, 1417, 1375, 1250, 829, 796, 737, 662, 608.

821

MS m/z (EI): 930(M+ , M100), 815, 578, 287. Combustion analysis: Expected: C, 84.72%; H, 9.44%; S, 3.9%. Obtained: C, 86.11%; H, 9.78%; S 4.1%. 2.2.11 Diester, 18 (m = 10) A mixture of 2,7-bis[5-(4-hydroxyphenyl)thiophen2-yl]-9,9-diethylfluorene, 16, (0.15 g, 0.00026 mol), potassium carbonate (0.1 g, 0.0007 mol), penta-1,4dien-3-yl 11-bromoundecanoate (0.26 g, 0.0007 mol) and DMF (5 cm3 ) was held at 80◦ C overnight. The mixture was allowed to cool to room temperature, then poured into water (100 cm3 ). The crude product was extracted into ethyl acetate (2 × 100 cm3 ) and the combined organic extracts washed with brine (2 × 100 cm3 ), dried (MgSO4 ), filtered and concentrated under reduced pressure. Purification was carried out by column chromatography [silica gel, ethyl acetate] and recrystallisation from MeOH to yield a yellow solid (0.20 g, 71.4%). Transition temp. (◦ C): Tg 70 Cr 76 N 125 I. 1 H NMR (CDCl3 ) δ H : J = 1.22–1.30 (26H, m), 1.65 (4H, m), 1.82 (4H, quint), 1.90 (4H, m), 2.34– 2.36 (4H, m), 3.98 (4H, t, J = 8.4 Hz), 5.30 (4H, t, J = 8.05 Hz), 5.71 (2H, t), 5.83 (4H, m), 6.92 (4H, d, J = 8.01 Hz), 7.20 (2H, d, 3.67 Hz), 7.33 (2H, d, 3.67 Hz), 7.57 (6H, d. J = 8.43 Hz), 7.61–7.64 (2H, m), 7.68 (2H, d, J = 8.01 Hz). IR ν max (cm–1 ): 2910, 2282, 1736, 1639, 1605, 1570, 1540, 1509, 1471, 1417, 1373, 1277, 1247, 1176, 1112, 984, 929, 874, 829, 796, 735, 720, 664, 631, 608. MS m/z (EI): 1070 (M+ , M100), 1004, 815, 551, 287. Combustion analysis: Expected: C, 77.34%; H, 7.71%; S, 5.99%. Obtained: C, 75.69%; H, 7.99%; S, 5.91%. 2.3 Mesomorphic properties The mesomorphic behaviour of compounds 11, 12, 15 and 17–19 was investigated and the liquid crystal transition temperatures determined using polarising optical microscopy with a Linkam 350 hot-stage and control unit in conjunction with a Nikon E400 polarising microscope. The only liquid crystalline phase observed was the nematic phase. Nematic droplets were seen on cooling the sample from the isotropic liquid to form the Schlieren texture, with two- and four-point brushes characteristic of the nematic phase, along with some optically extinct homeotropic areas (Figure 7). As the sample was cooled the texture sometimes formed further optically extinct homeotropic areas, indicative of the fact that the phase was optically

822

M.R. Billa et al.

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attributed, at least in part, to the high viscosity of the nematic phase and the presence of a glass transition in many of these compounds. Thermal gravimetric analysis (TGA) of the 9-cyclopentyl-substituted fluorenes 12 (n = 5 and 10) and the reference 9,9-dialkylsubstituted fluorenes, 17 (m = 2 and 18; m = 2, n = 5) was performed on a Netzsch TGA TG209 thermobalance interfaced with a Balzers ThermoStar GSD 300T mass spectrometer.

Figure 7. Part Schlieren texture and part nematic droplet texture of 15 (m = 2) formed from cooling of isotropic liquid to a temperature just below 213◦ C (colour version online).

uniaxial. These birefringent and homeotropic areas flashed brightly when mechanically disturbed, which, combined with the simultaneous presence of both the homeotropic and the Schlieren texture, confirmed that the mesophase observed was the nematic phase. The values for the liquid crystal transition temperatures of the 9-cyclopentyl-substituted fluorenes, 9, 11 and 12 (n = 5 and 10) and the reference 9,9-diethyl-substituted fluorenes, 15 (m = 2 and 3), 17 (m = 2 and 3) and 18 (m = 2 and 3, n = 5 and 10) were confirmed by DSC analysis using a Perkin–Elmer DSC-7 and in conjunction with a TAC 7/3 instrument controller, using the peak measurement for the reported value of the transition temperatures. A half Cp extrapolation was used to determine the value of the glass transition temperature (T g ) and good agreement (≈ 1–2◦ C) was obtained with the values determined using polarising optical microscopy. The DSC values determined twice on heating and cooling cycles on the same sample, and also on separate samples of the same compounds, were reproducible, and very little thermal degradation of the samples was observed, even at relatively high temperatures. The base line of the DSC traces was relatively flat, and sharp transition peaks were observed. The melting point (Cr–N and Cr–I) and the clearing point (N–I) transitions were both first-order as expected. A degree of supercooling below the melting point was often observed during the cooling cycle of several of the 9-cyclopentyl-substituted fluorenes, 9, 11, 12 (n = 5 and 10) and the reference 9,9dialkyl-substituted fluorenes, 15 (m = 2 and 3), 17 (m = 2 and 3) and 18 (m = 2; n = 5 and 10) and 18 (m = 3, n = 5 and 10). Supercooled samples of a number of these compounds may remain nematic at room temperature over several hours, which may be

2.4 Physical properties PL and optical quantum efficiency (μ) measurements were carried out with the samples inside a barium sulphate-coated integrating sphere filled with nitrogen [45]. A GaN laser diode (emission wavelength 406 nm) was used as excitation source for the sample, detected using an Ocean Optics S2000 photodiode array. UV–Vis optical absorbance measurements for these samples were recorded using a Unicam UV–Vis spectrometer. Thin films of the liquid crystals were prepared by spin coating from a solution in toluene (1.5 wt%) on to quartz substrates in a nitrogen-filled glove box. The samples were subsequently baked at 120◦ C for 10 min, and then cooled to room temperature to form the nematic phase or a nematic glass. The ionisation potentials (IP) of the compounds were measured electrochemically by cyclic voltammetry using a computer-controlled scanning potentiostat (Solartron 1285), which functions as a wave generator, potentiostat and current to voltage converter. Corrware and Corrview software packages were used to control and record the cyclic voltammetry experiments, respectively. 1 mM of the compound was dissolved in an electrolytic solution (5 cm3 ) of 0.3M tetrabutylammonium hexafluorophosphate in dichloromethane. The solution was placed in a standard three-electrode electrochemical cell and a glassy carbon electrode was used as the working electrode. Silver/silver chloride (3M NaCl and saturated AgCl) and a platinum wire were used as the reference and counter electrodes, respectively. The electrolyte was recrystallised twice before use. Oxygen contamination was avoided by purging the solution with dry argon before each measurement. The measured potentials were corrected to an internal ferrocene reference added at the end of each measurement. A typical scan rate of 20 mV s–1 was used and two scans were performed to check reproducibility. A typical cyclic voltammetry scan is shown in Figure 8. The sample material was deposited by spincoating on to a transparent or nearly transparent quartz substrate as a thin solid film. The onset potential for oxidation, E ox was clearly defined by a step change in current and was obtained from

Liquid Crystals 3.0x10–5

200

1.0x10–5 0.0

–1.0x10–5

0.0

(a)

Compound 31 (Calibrated with FC/FC+)

1.4

0.7

Compound 19 (Calibrated with FC/FC+)

2.0x10–5 Current (A)

Absorbance (a.u.)

Compound 19 Compound 31 Compound 32

823

400 600 Wavelength (nm)

800

0.0 (b)

0.5 1.0 Voltage (V)

1.5

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Figure 8. UV-Vis absorption and PL spectra of the spiro-substituted compound, 12 (n = 5), in solution and as a thin solid film (colour version online).

the intersection of the two tangents at the current discontinuity based on an empirical relationship proposed by Bredas, IP = [E ox + 4.4] eV [46]. A value for the reduction potential could not be determined because of the limited working range of the electrolyte. However, the electron affinity (EA) was estimated by subtraction of the optical band edge, taken as the energy of the onset of absorption of the compound from the IP. Although this approximation did not include a correction for the exciton binding energy, the values obtained agree well (within ± 0.05 eV) with those measured electrochemically in our laboratory for other classes of reactive mesogen. A UV–Vis spectrometer (Lambda 40, Perkin Elmer) was used to measure the optical absorption spectrum in the range 200–800 nm, to give the optical energy gap, E g , The absorption spectra were measured in the solid state, as described above. 3. Results and discussion 3.1 Mesomorphic properties The liquid crystal transition temperatures of the 9,9diethyl-substituted fluorenes 15 (m = 2) (m is the number of carbon atoms in the alkyl chains attached in the 9-position of the 2,7-disubstituted fluorene ring) and 19 [43] are collated in Table 1. The chemical structure of 15 differs from that of 19 only in that a 2,5-disubstituted thiophene ring is situated on each side of the fluorene moiety in place of the corresponding phenyl ring. Both of the compounds, 15 (m = 2) and 19, shown in Table 1 exhibited an enantiotropic nematic phase (Figure 7). The smectic phase or a glass transition could not be observed, despite substantial cooling below the melting point. The biphenyl-fluorene, 19, exhibited a higher melting and clearing point, as well as a broader nematic phase, than the analogous

thiophene-containing fluorene, 15 (m = 2). This may have been due to the fact that 19 had a greater lengthto-breadth ratio than 15 (m = 2), as indicated by molecular modelling, see Figures 4 and 5. The liquid crystal transition temperatures of the 9,9-diethyl-substituted fluorene, 15 (m = 2), the 9,9dipropyl-substituted fluorene, 15 (m = 3) and the analogous 9-cyclopentyl-substituted fluorene, 9, are collated in Table 2. Compounds 15 (m = 2 and 3) and Compound 9 incorporate an octyloxy end-group attached to the aromatic core of the molecules and are not polymerisable. They exhibited a nematic phase, and no smectic phases could be observed despite cooling substantially below the melting point. Although the melting point (144◦ C) of the 9,9-diethyl-substituted fluorene, 15 (m = 2) was somewhat lower than that (166◦ C) of the 9,9-dipropyl-substituted analogue, 15 (m = 3), the clearing point (212◦ C) of the former was significantly higher than that (173◦ C) of the latter, see Table 2. This may be attributable to steric effects: the two propyl groups attached at the 9-position on the fluorene in 15 (m = 3) will project further from the plane of the conjugated molecular core and lead to greater intermolecular separation. This will in turn lead to weaker van der Waals forces of attraction between the aromatic core of adjacent molecules. The melting and clearing points (173◦ C and 256◦ C, respectively) of the 9-cyclopentyl-substituted fluorene, 9, were in each case higher than those of the 9,9-diethyl-substituted fluorene, 15 (m = 2) and the 9,9-dipropyl-substituted fluorene, 15 (m = 3), see Table 2. This is most probably attributable to the rigid shape of the cyclopentyl ring, where the sides of the ring are constrained, see Figure 2(a) and 2(b). In contrast, the two methyl groups at the end of the ethyl groups in the 9,9-diethyl-substituted fluorene, 15 (m = 2), are free to rotate out of the plane of the

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M.R. Billa et al. Table 1. Liquid crystalline transition temperatures (◦ C) of the 9,9-diethyl-substituted fluorenes, 15 (m = 2 and 19) [43].

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Compound

R

Cr

N

I

15 (m = 2)



144



212



19



146



290



Table 2. Liquid crystalline transition temperatures (◦ C) of the 9-cyclopentylsubstituted fluorene, 9, the 9,9-diethyl-substituted fluorene, 15 (m = 2) and the 9,9-dipropyl-substituted fluorene, 15 (m = 3).

Compound

A

Cr

9

C2 H 5

C2 H 5

C3H7

C3H7

15 (m = 2)

15 (m = 3)

molecule, see Figure 1(a) and 1(b). This should lead to greater intermolecular separation and broadening of the molecular rotation volume and hence weaker van der Waals forces of attraction between neighbouring molecules, as well as lower anisotropy of molecular polarisability. The liquid crystal transition temperatures of the 9,9-diethyl-substituted fluorene, 17 (m = 2), the 9,9dipropyl-substituted fluorene, 17 (m = 3), and the analogous 9-cyclopentyl-substituted fluorene, 11, all incorporating an oxetane group at the end of the flexible spacer attached to the aromatic core of the molecule, are collated in Table 3. The liquid crystal transition temperatures of the oxetanes, 11 and 17 (m = 2 and 3), showed very similar relationships to those of the ethers, 9 and 15

N

I



173



256





144



212





166



173



(m = 2 and 3) shown in Table 2. The 9,9-diethyl fluorene, 17 (m = 2) exhibited an enantiotropic nematic phase and a glassy transition below room temperature. The clearing point (146◦ C) of 17 (m = 2), was much higher than that (94◦ C) of the 9,9dipropyl-substituted fluorene 17 (m = 3). The melting point (92◦ C) of 17 (m = 2) was somewhat lower than that (113◦ C) of 17 (m = 3). The melting and clearing points (126◦ C and 190◦ C, respectively) of the 9-cyclopentyl-substituted fluorene, 11, were each higher than those of the 9,9-diethyl-substituted fluorene, 17 (m = 2), and the 9,9-dipropyl-substituted fluorene, 17 (m = 3). The reasons for these differences will probably be similar to those suggested for the octyloxy-substituted ethers, 9 and 15 (m = 2 and 3), shown in Table 2. The nematic clearing points of the

Liquid Crystals

825

Table 3. Liquid crystalline transition temperatures (◦ C) of the 9-cyclopentyl-substituted fluorene, 11, the 9,9-diethyl-substituted fluorene, 17 (m = 2) and the 9,9-dipropylsubstituted fluorene, 17 (m = 3). S O

CH 2 C 6 H 12 O

Compound

S A

Tg

A

Cr

-

11

C2 H 5

C3H7

O

N

I

126



190



C2 H 5

17 (m = 2)

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OC 6 H 12 CH 2



16



92



146





17



113

(•

94)



C3H7

17 (m = 3)

Note: Figures in brackets indicate a monotropic transition.

oxetanes given in Table 3 are lower than those of the corresponding ethers with the same fluorene central unit in Table 2. This is probably due to steric effects, attributable to the bulky nature of the methyl oxetane group at the end of the terminal aliphatic groups of oxetanes, 11 and 17 (m = 2 and 3), see Figure 6. The presence of the bulky methyl oxetane will lead to broadening of the molecular rotation volume and a lower length-to-breadth ratio, which will in turn lead to weaker van der Waals forces of attraction between neighbouring molecules of the oxetanes, 11 and 17 (m = 2 and 3), shown in Table 3 compared to those between the octyloxy-substituted ethers, 9 and 15 (m = 2 and 3), shown in Table 2. The length of the terminal chains of the oxetanes, 11 and 17 (m = 2 and 3) is not the same as that of the octyloxysubstituted ethers, 9 and 15 (m = 2 and 3) and so caution is required when making a direct comparison of transition temperatures. The liquid crystal transition temperatures of the 9,9-diethyl-substituted fluorene, 18 (n = 5 and 10; m = 2), the 9,9-dipropyl-substituted fluorene, 18 (n = 5 and 10; m = 3), and the analogous 9cyclopentyl-substituted fluorene, 12 (n = 5 and 10), where n is the number of methylene units (–CH2– ) in the aliphatic spacer between the aromatic core and the polymerisable end-group and m is the number of carbon atoms in the alkyl chains attached in the 9-position of the 2,7-disubstituted fluorene ring, are summarised in Table 4. All the compounds shown in Table 4 incorporated a photopolymerisable non-conjugated

1,4-pentadiene residue at the end of the flexible spacer attached to the aromatic core of the molecule. The liquid crystal transition temperatures of the dienes listed in Table 4 showed very similar trends to those of the octyloxy-substituted ethers, 9 and 15 (m = 2 and 3) shown in Table 2 and the oxetanes, 11 and 17 (m = 2 and 3), in Table 3. It seems reasonable to assume that the reasons for this mesomorphic behaviour will be similar. The 9,9-diethyl-substituted fluorenes, 18 (n = 5 and 10; m = 2), each exhibited an enantiotropic nematic phase, relatively high melting point and a glass transition temperature near to or above room temperature, see Table 4. The clearing points (196◦ C, 137◦ C and 108◦ C, respectively) of the dienes, 12 (n = 5) and 18 (n = 5; m = 2 and 3), with five methylene groups in the aliphatic spacer between the aromatic core and the terminal ester/diene groups, were higher than those (169◦ C, 125◦ C and 82◦ C, respectively) of the corresponding dienes, 12 (n = 10 and 18; n = 10; m = 2 and 3), with 10 methylene groups in the spacer. This may be due to dilution effects, as the longer spacers reduce the van der Waals forces of attraction between neighbouring molecules. The longer spacers also have a greater probability of forming non-all-antiperiplanar conformations of adjacent methylene groups. Only the melting points (151◦ C and 111◦ C, respectively) of the homologous 9-cyclopentyl-substituted fluorenes, 12 (n = 5 and 10), differed significantly from those in Table 4. The nematic clearing points of the two series of dienes in Table 4 were lower those of the

826

M.R. Billa et al. Table 4. Liquid crystalline transition temperatures (◦ C) of the 9-cyclopentyl-substituted fluorenes, 12 (n = 5 and 10), the 9,9-diethyl-substituted fluorenes, 18 (n = 5 and 10, m = 2) and the 9,9-dipropyl-substituted fluorenes, 18 (n = 5 and 10, m = 3). S

S

O2CCnH2nO

Compound

A

A

Tg

12 (n = 5)

C2 H 5

C2 H 5

C3H7

C3H7

18 (n = 5, m = 2)

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OCnH2nCO2

18 (n = 5, m = 3)

C2 H 5

C3H7

C3H7

18 (n = 10, m = 2)

18 (n = 10, m = 3)

N

I



31



151



196





20



76



137





39



92



108



-



111



169





70



76



125





18



92

(•

82)



12 (n = 10)

C2 H 5

Cr

Note: Figures in brackets indicate a monotropic transition.

corresponding oxetanes shown in Table 3, which is probably attributable to steric effects associated with the non-conjugated diene end-groups.

4.

Physical properties

There were only small differences in the UV–Vis absorption and PL spectra of 12 (n = 5) and 18 (n = 5, m = 2), shown in Figures 9 and 10, respectively. The second emission peak in the PL spectrum of the ethyl-substituted compound, 18 (n = 5, m = 2) is more clearly resolved than that of spiro-substituted compound 12 (n = 5). The quantum efficiency (QE) of emission is higher for compounds 12 (n = 5) and 18 (n = 5; m = 2) in solution (QEPL = 68% and 53%, respectively) than as a thin film (QEPL = 9.5% and 9%, respectively) as expected, due to quenching of the excited state in the solid films. However, the differences in efficiency for the ethyl substituted compounds, 18 (n = 5, m = 2), and the 9-cyclopentylsubstituted fluorene, 12 (n = 5), are probably not

significant, although the broader tail of the UV–Vis absorption spectrum of the spiro-substituted compound, 12 (n = 5), in the solid state than that of the corresponding ethyl-substituted compound, 18 (n = 5, m = 2), may indicate a higher degree of molecular aggregation. Very similar values for the quantum efficiency of emission were found (QEPL = 67% and 62%, respectively, in solution and QEPL = 9% for both as a thin film) for the related compounds, 11 and 17 (m = 2), with an oxetane polymerisable group in place of the diene group. The data in Table 5 reveal small differences in the values of the IP, EA and the band gap (E g ) of the 9-cyclopentyl-substituted fluorenes, 9, 11, 12 (n = 5 and 10), and the 9,9-dipropylsubstituted fluorene, 18 (n = 5, m = 3). This indicates that the nature of the aliphatic substituent in the 9position (cyclic or linear) and that of the aliphatic terminal chains (alkoxy-chain or alkoxy-chain plus polymerisable diene or oxetane end-group) have little effect on the HOMO and LUMO energy levels of the molecular orbital of the highly conjugated aromatic core.

Liquid Crystals 1.0 Abs solution Abs film PL solution PL film

Table 5. The ionisation potential (IP), electron affinity (EA) and the band gap (E g ) of the 9-cyclopentylsubstituted fluorenes, 9, 11 and 12 (n = 5 and 10) and the 9,9-dipropyl-substituted fluorine, 18 (n = 5, m = 3).

0.8

Compound

0.6

0.6

0.4

0.4

0.2

0.2

0 300

0 400

500 600 Wavelength (nm)

700

Figure 9. UV-Vis absorption and PL spectra of the 9,9diethyl-substituted fluorene, 18 (n = 5, m = 2), in solution and as a thin solid film (colour version online). 1.0

1.0 Abs. solution Abs. film PL solution PL film

Normalised absorption (a.u.)

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0 300

Normalised PL (a.u.)

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1.0

Normalised PL (a.u.)

Normalised absorption (a.u.)

0.8

0.2

0 400

500 600 Wavelength (nm)

827

700

Figure 10. (a) Normalised absorption spectrum of the 9-cyclopentyl-substituted fluorene, 12, n = 10, and the 9,9-dipropyl-substituted fluorenes, 18 (n = 5, m = 3) and 18 (n = 10, m = 3); (b) Cyclic voltammogram of the 9-cyclopentyl-substituted fluorene, 12 (n = 10) and the 9,9-dipropyl-substituted fluorene, 18 (n = 5, m = 3). The potential, E, is plotted with respect to that of the saturated calomel electrode (colour version online).

IPa (eV) E g b (eV) EAc (eV)

9 11 12 (n = 5) 12 (n = 10) 18 (n = 5, m = 3)

5.56 5.54 5.54 5.53 5.53

2.73 2.72 2.72 2.71 2.71

2.83 2.82 2.82 2.82 2.82

Note Irreversible Irreversible Irreversible Irreversible Irreversible

Notes: a Determined by CV; b obtained from optical absorption spectrum; c calculated from IP – E . g

The absorption spectra of 9-cyclopentylsubstituted fluorene, 12 (n = 10), and the 9,9-dipropyl-substituted fluorenes, 18 (n = 5, m = 3) and 18 (n = 10, m = 3) are shown in Figure 8(a). These values reveal a significant variation in the absorption wavelength edge, which may possibly indicate different effects of linear propyl side-chains on the nature of the molecular packing and morphology of thin films of these materials, compared to those of the spiro-group. The presence of the spirogroup in the 9-position does appear to have an effect on the calibrated potential onset of the oxidation onset as shown in Figure 8(b). The TGA process Eox of the 9-cyclopentyl-substituted fluorenes, 12 (n = 5 and n = 10) and the reference 9,9-dialkyl-substituted fluorenes, 17 (m = 2) and 18 (m = 2, n = 5), shown in Table 6 indicates that the 9-cyclopentyl-substituted fluorenes, 12 (n = 5 and n = 10) are only marginally more thermally stable than the corresponding 9,9-dialkyl-substituted fluorenes, 17 (m = 2) and 18 (m = 2, n = 5). 5.

Conclusions

A study of the effect of spiro-groups and alkyl chains on the mesomorphism number of liquid crystalline 2,7-disubstituted fluorenes with ethyl, propyl and cyclopentyl groups in the 9-position of the Table 6. Thermal gravimetric analysis (TGA) of the 9cyclopentyl-substituted fluorenes, 12 (n = 5 and n = 10), and the reference 9,9-diethyl-substituted fluorenes, 17 (m = 2) and 18 (m = 2, n = 5). Compound 12 (n = 5) 12 (n = 10) 17 (m = 2) 18 (n = 5, m = 2) Note: a 5% mass loss.

Decomposition temperature (◦ C)a 421 426 406 410

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fluorene shows that the smaller, more rigid spirocyclopentyl group leads to much higher melting and clearing points. This observation, which is supported indirectly by UV–Vis absorption spectra and cyclic voltammetry measurements, indicates a smaller intermolecular distance for the spiro-compounds. This interpretation needs to be confirmed by Xray diffraction measurements. The nature of the aliphatic substituents in the 9-position (cyclic or linear) and aliphatic terminal chains (alkoxy-chain or alkoxy-chain plus polymerisable diene or oxetane end-group) had a limited effect on the HOMO and LUMO energy levels. Only marginal differences due to the presence of either short but flexible aliphatic chains, or a rigid alicyclic group of similar size and shape, on the photoluminescence spectra and efficiency are observed. The spiro-substituted compounds manifest a marginally higher thermal stability than that of the analogous compounds with linear aliphatic chains in the same position.

Acknowledgements We thank the EPSRC for the award of a postdoctoral fellowship to Dr S.P. Kitney, the Saudi Arabian Government for the award of a PhD studentship to Dr M.S. Al-Kalifah and the Spanish Government for a postdoctoral fellowship to Dr M. Haro. We are grateful to Dr R. Lewis and Dr K. Welham, respectively, for the 1 H NMR and MS spectroscopic measurements.

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