Light-Emitting Fluorene Photoreactive Liquid Crystals for Organic

The first polymerizable and light-emitting small molecules [electroluminescent reactive mesogens], which are liquid crystalline at room temperature an...
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Chem. Mater. 2004, 16, 4928-4936

Light-Emitting Fluorene Photoreactive Liquid Crystals for Organic Electroluminescence Mathew P. Aldred, Amanda J. Eastwood, Stephen M. Kelly,* and Panos Vlachos Department of Chemistry, The University of Hull, Kingston Upon Hull, HU6 7RX, U.K.

Adam E. A. Contoret, Simon R. Farrar, Bassam Mansoor, Mary O’Neill,* and W. Chung Tsoi Department of Physics, The University of Hull, Kingston Upon Hull, HU6 7RX, U.K. Received November 18, 2003. Revised Manuscript Received August 17, 2004

Light-emitting liquid crystals for organic light-emitting diodes (OLEDs) require lowtemperature liquid crystal phases for room-temperature processing and a range of molecular energies for electron and hole injection, as well as tunable color and color purity for multicolor OLEDs. We report a number of light-emitting polymerizable liquid crystals (reactive mesogens) based on 2,7-disubstituted-9,9-dialkylfluorene, whose energy levels can be tuned for optimized charge injection and light emission. As a consequence of these systematic property/structure investigations small molecule reactive mesogens have been synthezised, which exhibit low melting points, even below room temperature and nematic phases above room temperature as single components. Many of the molecules retain a supercooled nematic phase on cooling to room temperature. Simple binary eutectic mixtures of reactive mesogens with identical aromatic cores form light-emitting nematic phases at room temperature with a high clearing point to generate a high order parameter. The ionization potential of sixring fluorene reactive mesogens can be tuned between 4.93 and 5.57 eV by chemical modification of the aromatic cores. Similarly the emission spectrum can be tuned from blue to green. A typical performance for an OLED using such liquid crystalline materials as a cross-linked polymer network is described.

I. Introduction (OLEDs)1-3

Organic light-emitting diodes are emerging as a genuine alternative to liquid crystal displays (LCDs) in the multibillion-dollar flat-panel-display industry.4 At the moment commercial OLEDs use either small aromatic molecules1 or mainchain, conjugated polymers2,3 as the charge transport and emission layers. However, the processing costs of OLEDs based on small molecules are high, polymer OLEDs have poor multilayer capability, and low-resolution pixellation techniques (shadow masking and printing, respectively) are used to process both material types. Consequently, there is interest in cross-linked polymer networks to form solution-processable, multilayer OLEDs. Such OLEDs can also be photolithographically patterned. Recently a red-green-blue pixellated OLED has been demonstrated based on mainchain polyfluorenes with photocrosslinkable oxetane side chains and a photoacid as a cationic initiator.5 There are a number of other reports * Authors to whom correspondence should be addressed. E-mail: [email protected] or [email protected]. (1) Chen, C. H.; Shi, J.; Tang, C. W. Coord. Chem. Rev. 1998, 171, 161. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (3) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (4) Kelly, S. M. In Flat Panel Displays; Advanced Organic Materials; Connor, J. A., Ed.; RSC Materials Monograph; Royal Society of Chemisty: Cambridge, 2000.

describing the photochemical or thermal cross-linking of organic side chain polymers to form insoluble polymer networks as charge-transport and/or emission layers in OLEDs.6-10 However, the photochemical cross-linking process often caused a substantial degree of photochemical degradation. Liquid crystalline polymer networks formed from reactive mesogens, which contain a polymerizable endgroup attached via a flexible spacer to each end of a light-emitting aromatic core, have also been developed as an alternative approach to charge-transporting and light-emitting polymer networks in OLEDs.11-22 Polymerization and cross-linking occur either by the thermal (5) Mu¨ller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.; Frohne, H.; Nuykens, O.; Becker, H.; Meerholz, K. Nature 2003, 421, 829. (6) Remmers, M.; Neher, D.; Wegner, G. Macromol. Chem. Phys. 1997, 198, 2551. (7) Li, X. C.; Yong, T. M.; Gruener, J.; Holmes, A. B.; Moratti, S. C.; Cacialli, F.; Friend, R. H. Synth. Met. 1997, 84, 437. (8) Kla¨rner, G.; Lee, J.-I.; Lee, V. Y.; Chan, E.; Chen, J. P.; Nelson, A.; Markiewicz, D.; Siemens, R.; Scott, J. C. Miller, R. D. Chem. Mater. 1999, 11, 1800. (9) Bellman, E.; Shaheen, S. E.; Thayumanavan, S.; Barlow, S.; Grubbs, R. H.; Marder, S. R.; Kippelen, B.; Peyghambarian, N. Chem. Mater. 1998, 10, 1668. (10) Davey, A. P.; Howard, R. G.; Blau, W. J. J. Mater. Chem. 1997, 7, 417. (11) Sanchez, C.; Villacampa, B.; Alcala, R.; Martinez, C.; Oriol, L.; Pinol, M. J. Appl. Phys. 2000, 87, 274. (12) Sanchez, C.; Alcala, R.; Cases, R.; Oriol, L.; Pinol, M. J. Appl. Phys. 2000, 88, 7124. (13) Hikmet, R. A. M.; Braun, D. B.; Staring, A. G. J.; Schoo, H. F. M.; Lub, J. Int. Patent Appl. WO 97/07654, 1995.

10.1021/cm0351893 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/06/2004

Fluorene Reactive Mesogens

or photoinduced generation of free radicals or by ionic photoinitiation.20 Polymer networks formed from reactive mesogens provide a unique and advantageous combination of properties compared to other approaches: they are monodisperse after standard purification procedures; they form insoluble, intractable polymer films by spin coating and subsequent polymerization; their films are photopatternable and some exhibit higher photoluminescence efficiency and improved currentvoltage characteristics in prototype OLEDs than the monomers themselves before crosslinking;16,19 they can be used to generate polarized emission;17,18 and the charge-carrier mobility can also exhibit a low field dependence.18 Photopolymerization is preferred to thermal polymerization because of the pixellation capability and also because high temperatures can reduce the order parameter of uniformly oriented reactive mesogens and lead to photodegradation.21,22 A very limited number of reactive mesogens with acrylate, methacrylate, or nonconjugated diene polymerizable end-groups, have been used so far as charge-transport or lightemitting layers in multilayer OLEDs.11-22 The large majority of these reactive mesogens emit blue or bluegreen light. The polymerizable end-groups should preferentially be polymerized by a radical mechanism to avoid the presence of ionic initiator and reaction products within the resultant cross-linked polymer network. It is suspected that these charged ionic contaminants may act as traps and potentially contribute to device failure. A potential advantage of nonconjugated diene end-groups compared to acrylates or methacrylates is the low tendency of such nonconjugated dienes to polymerize thermally.23,24 This means that unreacted monomers should not polymerize spontaneously during the fabrication and operation of an OLED. Acrylates and methacrylates may also show a greater tendency to photochemical degradation during the cross-linking process. We differentiate here between reactive mesogens and related liquid crystalline oligomers with polymerizable endgroups, which can also exhibit very low melting points and high polarization ratios of emission.25,26 The molecular core of reactive mesogens [small molecules] only incorporates a small number (n) of aromatic rings (14) Bacher, A.; Bentley, P. G.; Bradley, D. D. C.; Douglas, L. K.; Glarvey, P. A.; Grell, M.; Whitehead K. S.; Turner, M. L. J. Mater. Chem. 1999, 9, 2985. (15) Contoret, A. E. A.; Farrar, S. R.; O’Neill, M.; Nicholls, J. E.; Richards, G. J.; Kelly, S. M.; Hall, A. W. Chem. Mater. 2002, 14, 1477. (16) Farrar, S. R.; Contoret, A. E. A.; O’Neill, M.; Nicholls, J. E.; Richards, G. J.; Kelly, S. M. Phys. Rev. B 2002, 66, 125107. (17) Grell, M.; Bradley, D. D. C. Adv. Mater. 1999, 11, 895. (18) Contoret, A. E. A.; Farrar, S. R.; Jackson, P. O.; May, L.; O’Neill, M.; Nicholls, J. E.; Richards, G. J.; Kelly, S. M. Adv. Mater. 2000, 12, 971. (19) Contoret, A. E. A.; Farrar, S. R.; Khan, S. M.; O’Neill, M.; Richards, G. J.; Aldred, M. P.; Kelly, S. M. J. Appl. Phys. 2003, 93, 1465. (20) O’Neill, M.; Kelly, S. M. Adv. Mater. 2003, 15, 1135. (21) Whitehead, K. S.; Grell, M.; Bradley, D. D. C.; Jandke, M.; Strohiegl, P. Appl. Phys. Lett. 2000, 76, 2946. (22) Jandke, M.; Hanft, D.; Strohriegl, P.; Whitehead, K.; Grell, M.; Bradley, D. D. C. Proc. SPIE 2001, 4105, 338. (23) Hall, A. W.; Lacey, D.; Buxton, P. I. Macromol. Rapid Commun. 1996, 17, 417. Hall, A. W.; Lacey, D.; Buxton, P. I. Macromol. Chem. Phys. 1997, 198, 2307. (24) Butler, G. B.; Angelo, R. J. J. Am. Chem. Soc. 1957, 79, 3128. (25) Strohriegel, P.; Hanft, D.; Jandke, M.; Pfeuffer, T. Mater. Res. Soc. Symp. Proc. 2002, 709, 31. (26) Chen, J. P.; Kla¨rner, G.; Lee, J.-I.; Markiewicz, D.; Lee, V. Y.; Miller, R. D.; Scott, J. C. Synth. Met. 1999, 107, 129.

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(2 < n < 8) in order to minimize the synthetic complexity and subsequent cost for practical applications. Most importantly, reactive mesogens are monodisperse and still exhibit a low enough viscosity in the nematic phase (although it is presumed to be very high compared to that of common liquid crystals used in LCDs) to be oriented spontaneously on alignment layers, e.g., for polarized emission.17,18 Furthermore, cross-linkable oligomers produced by statistical aryl-aryl cross-coupling reactions are also polydisperse25 and their property spectrum more resembles that of polymers than that of small molecules, i.e., they represent short polymers. The 2,7-disubstituted-9,9-dialkylfluorene group combines an almost unique combination of attractive features for light-emitting organic materials. It is the presence of the two alkyl chains at the bridging benzylic position of the 9,9-dialkylfluorene moiety that is critical in generating this attractive combination of physical properties.17-22,25-31 The two alkyl chains give rise to a larger intermolecular distance, which lowers the melting point and increases the solubility in organic solvents compared to the corresponding nonsubstituted fluorenes. They also contribute to the relatively high viscosity of the 9,9-dialkylfluorenes, which results in a high tendency for glass formation. However, a further crucial property of the two alkyl chains is their tendency to suppress the formation of smectic phases, the layered structure of which induces a much higher viscosity than that of the nematic phase.20 Thus, smectic phases are much more difficult to macroscopically align, e.g., for polarized emission, and consequently the nematic phase is preferred.17,18 To realize further progress in liquid crystal polymer network OLEDs, it is essential to prepare light-emitting, reactive mesogens with low melting points so that they can be aligned and cross-linked as close as possible to room temperature in the nematic phase in order to reduce processing costs and to maximize the order parameter for high charge-transport and polarized emission.20 Higher melting points can be tolerated if glassy phases form on cooling. However, the polymerization of the endgroups of reactive mesogens is much slower in the glassy nematic state than in the nematic state due to the much higher effective viscosity in an organic glass. The energy levels of the chromophores must also be tailored for hole or electron injection and for blue, green, and red emission for full color capability. The synthetic pathways should be as short as possible to facilitate commercialization of this technology, for example see Scheme 1. It is a significant challenge to modify the molecular shape of the aromatic core and aliphatic substituents to give room-temperature nematic phases because the long molecular cores required for visible emission are associated with high melting points. Hence, most lightemitting or carrier transporting smectic and nematic (27) Geng, Y.; Chen, A. C. A.; Ou, J. J.; Chen, S. H.; Klubek, K.; Vaeth, K. M.; Tang, C. W. Chem. Mater. 2003, 15, 4352. (28) Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 1565. Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998, 74, 1400. (29) Grell, M.; Knoll, W.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H. G.; Scherf, U.; Yasuda, A. Adv. Mater. 1999, 11, 671. (30) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365. (31) Oda, M.; Nothofer, H. G.; Scherf, U.; Sunjic, V.; Richter, D.; Regenstein, W.; Neher, D. Macromolecules 2002, 35, 6792.

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gens with a terminal nonconjugated diene as the polymerizable group. We shall describe the corresponding acrylates and methacrylates at a later date. We also show here how the electronic properties of the fluorenes can be tailored for efficient hole and electron injection for OLEDs as well as for blue and green emission in practical OLEDs based on this unique approach. II. Experimental Section

a Reagents: (a) K CO , CH COC H ; (b) (i) BuLi, THF, (ii) 2 3 3 2 5 B(OCH3)3, (iii) HClaq; (c) (i) BuLi, THF, (ii) CnH2n+1Br; (d) Br2, CHCl3; (e) Pd(Ph3)4, Na2CO3, HO, DME; (f) (i) B Br3, CHCl3, (ii) HClaq; (g) pyridine, CH2Cl2.

liquid crystals reported to date are crystalline at room temperature.32-40 We have reported the electroluminescent properties of polymer networks formed from a small number of reactive mesogens containing the 2,7-disubstituted-9,9-dialkylfluorene moiety.17,20 However, the photopolymerization was carried out in the very viscous glassy nematic state, which led to long processing times for photochemical cross-linking at room temperature. We now report the first systematic study of reactive mesogens incorporating the 9,9-dialkylfluorene group, and show how molecular design can be used modify the liquid crystal phase transition temperatures to obtain room-temperature nematic phases for monodisperse, nonoligomeric materials. Here we report reactive meso(32) Funahashi, M.; Hanna, J.-I. Jpn. J. Appl. Phys. 1996, 35, L703. (33) Funahashi, M.; Hanna, J.-I. Phys. Rev. Lett. 1997, 78, 2184. (34) Funahashi, M.; Hanna, J.-I. Mol. Cryst. Liq. Cryst. 1999, 331, 516. (35) Tokuhisa, H.; Era, M.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 2639. (36) Tokuhisa, H.; Er,a M.; Tsutsui, T. Adv. Mater. 1998, 8, 2639. (37) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes, P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudhoelter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057. (38) Haristoy, D.; Mery, S.; Heinrich, B.; Mager, L.; Nicoud, J. F.; Guillon, D. Liq. Cryst. 2000, 27, 321. (39) Mochizuki, H.; Hasui, T.; Kawamoto, M.; Shiono, T.; Ikeda, T.; Adachi, C.; Taniguchi, Y.; Shirota, Y. J. C. S. Chem. Commun. 2000, 1923. (40) Haramoto, Y.; Yamada, T.; Nanasawa, M.; Funahashi, M.; Hanna, J.-I.; Ujiie, S. Liq. Cryst. 2002, 29, 1109.

Materials Synthesis. The structures of intermediates and final products were confirmed by proton (1H) nuclear magnetic resonance (NMR) spectroscopy (JEOL JMN-GX270 FT nuclear resonance spectrometer), infrared (IR) spectroscopy (PerkinElmer 783 infrared spectrophotometer), and mass spectrometry (MS) (Finnegan MAT 1020 automated GC/MS). Reaction progress and product purity wwere checked using a CHROMPACK CP 9001 capillary gas chromatograph fitted with a 10 m CP-SIL 5CB (0.12 µm, 0.25 mm) capillary column. All of the final products were more than 99.5% pure by GLC. Transition temperatures were determined using an Olympus BH-2 polarizing light microscope with a Mettler FP52 heating stage and a Mettler FP5 temperature control unit. The analysis of transition temperatures and enthalpies was carried out by a Perkin-Elmer DSC7-PC differential scanning calorimeter. The octyloxy chain was used as a protecting group as the starting material was available from another program and longer chains are easier to remove than shorter ones. We now use branched chains, such as citronenyl groups, since they act as a better protecting group in that they lower the melting point of the reaction intermediates and increase their solubility in organic solvents. 4-Octyloxybiphenyl-4′-ylboronic Acid. Butyllithium (13.30 cm3 of 2.5 M in hexanes, 0.033 mol) was added dropwise to a stirred solution of 4-bromo-4′-octyloxybiphenyl (10.00 g, 0.028 mol) in THF (250 cm3) at -78 °C. The reaction mixture was stirred at -78 °C for 2 h, then trimethyl borate (5.76 g, 0.055 mol) was added dropwise while a temperature of -78 °C was maintained. On complete addition the reaction temperature was allowed to warm to room-temperature overnight. A 20% hydrochloric acid solution (150 cm3) was added and the reaction mixture was stirred for 3 h. The crude product was extracted into diethyl ether (3 × 150 cm3) and the combined organic layers were washed with water (3 × 100 cm3), dried (MgSO4), and filtered, and the filtrate was evaporated down. The resulting white solid was washed with hexane and filtered off to yield 7.10 g (79%) of the desired product.1H NMR (CDCl3) δH: 0.85 (3H, t, J ) 7 Hz), 1.26-1.44 (10H, m), 1.71 (2H, quin), 3.99 (2H, t, J ) 7 Hz), 6.99 (2H, d, J ) 9 Hz), 7.56 (2H, d, J ) 8 Hz), 7.60 (2H, d, J ) 8 Hz), 7.82 (2H, d, J ) 8 Hz), 8.01 (2H, s). IR νmax/cm-1: 3411, 2932, 2859, 1608, 1561, 1534, 1393, 1342, 1285, 1259, 1212, 1028, 995, 819. MS m/z: No mass ion. 2,7-bis(4-Octyloxybiphenyl-4′-yl)-9,9-dipropylfluorene. Tetrakis(triphenylphosphine)palladium(0) (30.00 mg, 2.45 × 10-2 mmol) was added to a stirred solution of 4-octyloxybiphenyl4′-ylboronic acid (1.76 g, 5.39 mmol), 2,7-dibromo-9,9-dipropylfluorene (1.00 g, 2.45 mmol), and a 20% aqueous sodium carbonate solution (2.9 cm3) in DME (75 cm3) at room temperature. The reaction mixture was heated under reflux for 24 h. The cooled reaction mixture was added to water (50 cm3), and the product was extracted into DCM (3 × 100 cm3). The combined organic layers were washed with a 20% hydrochloric acid solution (50 cm3) and water (3 × 100 cm3), dried (MgSO4) and filtered, and the filtrate was evaporated down. The crude product was purified by column chromatography [silica gel; DCM/hexane 1:4] and recrystallization from toluene to yield 1.10 g (55%) of the desired product. Transition temp, °C: Cr 157 N 232 I. 1H NMR (CDCl3) δH: 0.70 (6H, t, J ) 7 Hz), 0.740.82 (4H, m), 0.90 (6H, t, J ) 7 Hz), 1.30-1.53 (20H, m), 1.82 (4H, quin, J ) 7 Hz), 2.02-2.07 (4H, m), 4.02 (4H, t, J ) 7 Hz), 7.00 (4H, d, J ) 9 Hz), 7.58 (4H, d, J ) 9 Hz), 7.62-7.64 (4H, m), 7.66 (4H, d, J ) 9 Hz), 7.74 (4H, d, J ) 9 Hz), 7.79 (2H, d, J ) 8 Hz). IR νmax/cm-1: 3454, 2932, 2859, 1610, 1503,

Fluorene Reactive Mesogens 1468, 1249, 1178, 816. MS-APCI/+ve ion: 811 (M+). Elemental analysis: expected (%) C, 87.36; H, 8.70; O, 3.94. Found (%) C, 87.52; H, 8.89; O, 3.59. 2,7-bis(4-Hydroxybiphenyl-4′-yl)-9,9-dipropylfluorene. Boron tribromide (5.90 cm3, 1 M solution in DCM, 5.92 mmol) was added dropwise to a stirred solution of 2,7-bis(4-octyloxybiphenyl-4′-yl)-9,9-dipropylfluorene (1.20 g, 1.48 mmol) in DCM (100 cm3) at 0 °C. On complete addition the reaction mixture was allowed to warm to room-temperature overnight. The reaction mixture was poured onto ice (150 g) and stirred for 30 min. The product was extracted into diethyl ether (3 × 150 cm3). The combined organic layers were washed with a 20% hydrochloric acid solution (50 cm3) and water (3 × 100 cm3), dried (MgSO4) and filtered, and the filtrate was evaporated down. The crude product was purified by column chromatography [silica gel; DCM/ethanol, 30:1] and recrystallization from toluene to yield 1.35 g (32%) of the desired product. 1H NMR (CDCl3) δH: 0.70 (6H, t, J ) 7 Hz), 0.75-0.79 (4H, m), 2.032.07 (4H, m), 6.96 (4H, d, J ) 8 Hz), 7.51 (4H, d, J ) 9 Hz), 7.62-7.66 (8H, m), 7.73 (4H, d, J ) 9 Hz), 7.79 (2H, d, J ) 8 Hz), 8.90 (2H, s). IR νmax/cm-1: 3358, 3029, 2957, 1610, 1530, 1502, 1466, 1377, 1236, 1175, 815. MS m/z: 586 (M+), 514, 488, 376, 314, 286, 264 (100), 118, 107, 69. 2,7-bis{4-[5-(1-Vinylallyloxycarbonyl)pentyloxy]-4′-biphenyl}9,9-dipropylfluorene 25. A mixture of potassium carbonate (0.24 g, 1.71 mmol), 2,7-bis(4-hydroxybiphenyl-4′-yl)-9,9-dipropylfluorene, and penta-1,4-dien-3-yl 6-bromohexanoate in DMF (50 cm3) was heated at 70 °C for 24 h. The reaction mixture was cooled to room temperature and the solvent was removed under reduced pressure. DCM (30 cm3) was added to the residue, which was filtered, and the filtrate was evaporated down. The crude product was purified by column chromatography [silica gel; hexane/DCM, 4:1] and recrystallization from toluene to yield 0.16 g (20%) of the desired product. Transition temp. (°C): Cr 143 N 166 I. 1H NMR (CDCl3) δH: 0.70 (6H, t, J ) 7 Hz), 0.75-082 (4H, m), 1.51-1.58 (4H, m), 1.75 (4H, quin, J ) 7 Hz), 1.85 (4H, quin, J ) 7 Hz), 2.03-2.07 (4H, m), 2.41 (4H, t, J ) 7 Hz), 4.02 (4H, t, J ) 6 Hz), 5.23 (2H, t, J ) 1 Hz), 5.25 (2H, t, J ) 1 Hz), 5.29 (2H, t, J ) 1 Hz), 5.33 (2H, t, J ) 1 Hz), 5.73 (2H, tt, J ) 1, 6 Hz), 5.80-5.89 (4H, m), 6.99 (4H, d, J ) 9 Hz), 7.58 (4H, d, J ) 9 Hz), 7.62-7.64 (4H, m), 7.66 (4H, d, J ) 7 Hz), 7.74 (4H, d, J ) 9 Hz), 7.79 (2H, d, J ) 8 Hz). IR νmax/cm-1: 3954, 2872, 1737, 1609, 1503, 1466, 1247, 1177, 992, 929, 814. MS-APCI/+ve ion: 947 (M+ + 2H). Elemental analysis: expected (%) C, 82.42; H, 7.45; O, 10.13. Found (%) C, 82.13; H, 7.94; O, 9.93. Experimental Methods. The thermotropic mesophases observed for the compounds 1-37 were investigated between crossed polarizers using optical microscopy. The only phase observed was the nematic phase. Nematic droplets were observed on cooling from the isotropic liquid (see Results and Discussion section) to form the Schlieren texture with two- and four-point brushes characteristic of the nematic phase along with optically extinct homeotropic areas. As the sample is cooled further the texture often formed more optically extinct homeotropic areas, which indicates that the phase is optically uniaxial. The birefringent and homeotropic areas flashed brightly on mechanical disturbance. This behavior and the simultaneous presence of both the homeotropic and the Schlieren textures confirm that the mesophase observed is indeed a nematic phase. The values for the transition temperatures were confirmed by DSC (see Results and Discussion for a typical DSC plot). Good agreement (≈1-2 °C) with those values determined by optical microscopy were obtained. These values were determined twice on heating and cooling cycles on the same sample. The values obtained on separate samples of the same compounds were reproducible and usually very little thermal degradation was observed even at relatively high temperatures. The baseline of the spectra is relatively flat and sharp transition peaks are observed for single components, although mixtures exhibited broader peaks, see below. Both liquid crystalline transitions are first order as expected. A degree of supercooling below the melting point is often observed on the cooling cycle and many of the compounds 1-37 remain

Chem. Mater., Vol. 16, No. 24, 2004 4931 nematic at room temperature for several hours, although their melting points are often much higher than room temperature. This may be attributed, at least in part, to the high viscosity of the nematic phase of these materials. The ionization potentials of the reactive mesogens were measured electrochemically by cyclic voltammetry using a computer-controlled scanning potentiostat (Solartron 1285). The compound (1 mM) was dissolved in 5 cm-3 of an electrolytic solution of 0.1 M tetrabutylammonium hexafluorophosphate in dichloromethane. The solution was placed in a standard three-electrode electrochemical cell. A glassy carbon electrode was used as the working electrode. Silver/silver chloride (3M NaCl and saturated Ag/Cl)) and a platinum wire formed the reference and counter electrodes, respectively. The electrolyte was recrystallized twice before use and 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. Two scans were performed to check the repeatability. A typical cyclic voltammetry scan is shown in Results and Discussion. The onset potential for oxidation, Eox is clearly defined by a step change in current and is obtained from the intersection of the two tangents at the current discontinuity based on the empirical relationship proposed by Bredas, IP ) [Eox + 4.4] eV.41 We were unable to measure a value for the reduction potential because of the limited working range of the electrolyte. However, EA was estimated by subtraction of the optical bandedge, taken as the energy of the onset of absorption of the compound, from the IP. Although this approximation does not include a correction for the exciton binding energy, the values obtained agree within ( 0.05 eV with those measured electrochemically in our laboratory for another class of reactive mesogen. Thin films of the materials were prepared by spin coating from a 0.5-2.0% weight solution in chloroform onto quartz substrates. The photopolymerizable films were usually polymerized in a nitrogen-filled chamber using UV light from a helium cadmium laser at 325 nm with a constant intensity of 50 mW cm-2. PL and EL were measured with the samples mounted in a chamber filled with dry nitrogen using a photodiode array (Ocean Optics S2000) with a spectral range from 200 to 850 nm and a resolution of 2 nm. A multilayer OLED was fabricated on a glass substrate (12 mm × 12 mm × 1 mm) covered with an ITO transparent anode and a polystyrene sulfonate/polyethylene dioxythiophene (PSS/PEDOT) EL grade layer (thickness 45 nm) deposited by spincoating. The PSS/PEDOT layer was baked at 165 °C for 5 min to cure the layer and remove volatile components. Compound 37 was deposited as a uniform thin film (100 nm) by spincoating (2 krpm) from a dilute solution (0.5 wt %) in chloroform followed by baking at 50 °C for 5 min and then several hours under hard vacuum to remove solvent. A cross-linked polymer network is formed by polymerizing this layer of compound 37 with a UV exposure at 325 nm with a fluence of 500 Jcm-2. A hole-blocking layer (6 nm) of commercially available (H. W. Sands) 3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) was deposited on top of the cross-linked emission layer by vacuum deposition under a vacuum of 10-6 mbar or better. A layer of lithium fluoride (1 nm) and then an aluminum layer (80 nm) were deposited by vacuum deposition as a combined cathode. All the processing was carried out in a glovebox filled with dry nitrogen (99.99% purity) to avoid oxygen and moisture contamination.

III. Results and Discussion Liquid Crystalline Behavior. In this section we use a systematic approach to show how small variations in molecular structure can be used to alter LC transition temperatures. We aim for low melting points and a wide (41) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am. Chem. Soc. 1983, 105, 6555.

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Table 1. Transition Temperatures for the Symmetrical Esters 1-8 and the Ethers 9-13

1 2 3 4 5 6 7 8 9 10 11 12 13 a

n

OR

3 3 3 3 3 8 8 8 3 3 3 3 8

OC3H6CO2CH(CHdCH2)2 OC4H8CO2CH(CHdCH2)2 OC5H10CO2CH(CHdCH2)2 OC10H20CO2CH(CHdCH2)2 OC2H4CH(CH3)C2H4CO2CH(CHdCH2)2 OC5H10CO2CH(CHdCH2)2 OC7H14CO2CH(CHdCH2)2 OC10H20CO2CH(CHdCH2)2 OC5H10OCH(CHdCH2)2 OC6H12OCH(CHdCH2)2 OC8H16OCH(CHdCH2)2 OC9H18OCH(CHdCH2)2 OC5H10OCH(CHdCH2)2

Tg

Cr

N

• • • • • • • • • • • • •

62 45 39 18 -26 -25 -27 25 19 2 -25

I

• • • (• • (• (• (• • • • • (•

92 92 92 58 96 43 41 101 92 97 93 97

• • • • • • • • • • • • •

116 120 108 82)a 87 29) 25) 32) 116 116 106 98 44)

( ) Represents a monotropic transition temperature.

nematic range so that device processing can be carried out at RT in the nematic phase and that the order parameter of the nematic phase reaches its maximum value for optimized charge-transport and polarized emission. First we investigate a set of substituted fluorenes 1-13, listed in Table 1, having the same aromatic core and different aliphatic end and side chains. The transition temperatures of several of the fluorenes 1-4 have been reported15 before and will not be commented on again here. However, they serve as a point of comparison for the new compounds listed in the table and compound 3 is discussed in Results and Discussion. The presence of a branching methyl group in the spacer between the aromatic core and the polymerizable endgroup in compound 5 leads to lower transition temperatures than those of the corresponding compound 3 with the same spacer length. This is due to steric effects increasing the intermolecular distance and lowering the van der Waals forces of attraction between adjacent molecules.4 The transition temperatures of the compounds 6-8 demonstrate the effect of the presence of two lateral substituents at position 9 of the fluorene chromophore on the transition temperature of the otherwise identical compounds. The two alkyl chains attached to position 9 of the fluorene are orthogonal to the fluorene unit.42 The presence of an octyl chain in compounds 6 and 8 leads to much lower clearing points than those of the corresponding compounds 3 and 4 with the much shorter propyl chain in place of the octyl chain. This can be attributed to weaker van der Waals forces between the molecules as the longer octyl chain increases the distance between the molecules since they act as large lateral substituents.4 The effect on the melting point is not as great, and, therefore, the clearing point of the octyl fluorenes 6-8 is monotropic in each case. However, large variations in melting point within homologous series are common.4 The melting point and clearing point of the ether 9 are higher than those (+9 °C and +8 °C, respectively) of the corresponding ester 3, where the only difference between the two compounds is the nature of the linking group (O and CO2, respectively) between the aliphatic spacer and the polymerizable diene endgroup. The melting point and clearing point of the ether 13 are also (42) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306.

Table 2. Transition Temperatures for the Hexaphenylenes 14-23 and the Fluoro-Substituted Hexaphenylenes 24-29

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 a

X

Y

n

m

H H H H H H H H H H F F F F H H

H H H H H H H H H H H H H H F F

3 4 5 6 8 8 8 8 10 10 8 8 8 8 8 8

5 5 5 5 5 7 10 11 7 10 5 7 10 11 10 11

Tg

Cr 25

-26 -20

-16

-27 -26

• • • • • • • • • • • • • • • •

N 143 126 126 137 91 52 38 58 57 53 93 63 64 70 54 58

• • • (• • • • • • • (• (• (• (• • (•

I 166 151 137 124)a 109 103 96 88 79 88 56) 52) 51) 44) 58 51)

• • • • • • • • • • • • • • • •

( ) Represents a monotropic transition temperature.

higher than those (+1 °C and +12 °C, respectively) of the corresponding ester 6, although the magnitude of the difference is not exactly the same. This may be due to the more linear structure of the ether spacer compared to that of the ester spacer. The other ethers 1012 of this short homologous series also exhibit an enantiotropic nematic phase at elevated temperatures. The glass transition temperatures are too low to be of relevance for device applications. The liquid crystalline transition temperatures of the reactive mesogens 14-29 are collated in Table 2. The melting and clearing points of the homologues 14-18 (n ) 3-6, 8; m ) 5) generally decrease with increasing number of methylene units (n) in the two alkyl substituents in position 9 on the fluorene moiety. This is probably due to steric effects as the longer alkyl chains increase the intermolecular distance, and, consequently, lower the van der Waals forces of attraction between the aromatic cores. Similar steric effects also explain the absence of smectic phases for any of the fluorene derivatives shown in Tables 1-3. Smectic phases are usually observed for apolar liquid crystals with two or more aromatic rings joined together by a single carboncarbon bond.4 The greater degree of flexibility of the

Fluorene Reactive Mesogens

Chem. Mater., Vol. 16, No. 24, 2004 4933

Table 3. Transition Temperatures for the Asymmetric Reactive Mesogens 32-36

32 33 34 35 36

n

m

3 3 6 8 8

5 10 10 7 10

Tg

Cr 11 -2 -15

alkyl spacers between the aromatic core and the polymerizable diene endgroup of the esters 19-21 with longer chains (n ) 8; m ) 7, 10, 11) is probably responsible for the lower melting and clearing points due to the presence of more nonlinear conformations. These in turn lead to a lower degree of anisotropy of molecular shape and polarizability. In both cases a degree of microphase separation and dilution effects may also play a role, as the melting point of the reactive mesogen becomes more like that of the alkyl groups and less like that of the aromatic cores. This is confirmed by comparing the nematic-isotropic transition temperatures of the di-octyl compounds 19 and 20 and those of the di-decyl compounds 22 and 23. Those of the latter are lower than those of the former as expected. However, the same trend is not observed for the melting point. The branched structure of the photopolymerizable diene-endgroups also makes a significant contribution to the low melting point, absence of smectic phases, and the high tendency to form the glassy state, cf., homologue 15. The combination of these effects results in the low melting of most of the compounds 14-23, e.g., compare the melting point (38 °C) of compound 20 which incorporates many methylene (CH2) units to that (465 °C) of the para-sexiphenyl with a rigid aromatic core of six phenylene rings. The incorporation of fluorine atoms in a lateral position of the aromatic cores of liquid crystals often reduces the melting points.4 However, the presence of a fluorine atom (X ) F) in the dioctyl esters 24-27 results in melting points similar to or higher than those of the corresponding dioctyl esters 18-21 with a hydrogen atom (X ) H) in place of the fluorine atom. However, the clearing point is much lower, which is probably due to steric effects similar to those described above.4 Therefore, the nematic phase of the homologues 24-27 is monotropic. A similar effect is also observed for the compounds 28 and 29 with the fluorine substituent attached to the phenyl ring closest to the central fluorene unit. However, the melting points are not much higher than those of the corresponding compounds 20 and 21 with a hydrogen atom (X ) H) in place of the fluorine atom. Therefore, compound 28 exhibits an enantiotropic nematic phase, but only with a narrow temperature range. Low glass transition temperatures or no glass transitions at all are observed for the fluorenes 14-29 shown in Table 2. This is a little surprising considering the high glass transition temperatures exhibited by the structurally related compounds 1-4 listed in Table 1. Perhaps the non-collinear nature of the 2,5-disubstituted thiophene bonds leads to a higher tendency for glass formation. The transition temperatures for the pyrimidines 30 and 31, designed as electron-transport materials, again

• • • • •

N 133 44 78 50 -28

I

• (•

113 75)

•

21

• • • • •

clearly demonstrate the correlation between the liquid crystalline transition temperatures and the length of the chains at position 9 of the fluorene central unit as well as the two spacers between the aromatic core and the polymerizable endgroups as already shown in Tables 1 and 2. The melting point and the nematic clearing point of pyrimidine 30 (Cr-N ) 128 °C; N-I ) 111 °C; Tg ) 20 °C) with two propyl chains in position 9 on the fluorene moiety are much higher than those of the analogue 31 (Cr-N ) 68 °C; N-I ) 55 °C) with the much longer octyl chains in the same position and a longer spacer group between the aromatic core and the polymerizable endgroup.

The materials 14 and 30, where the 2,5-disubstitued thiophene rings adjacent to the fluorene chromophore of compound 3 have been replaced by either a 1,4disubstituted phenyl ring or a 2,5-disubstitued pyrimidine ring, as the only difference in molecular structure, exhibit higher transition temperatures. The hexaphenylene compound 14 has the highest melting and clearing point as well as the broadest nematic phase. In contrast, the compound 3 containing the thiophene rings possesses the lowest melting point and clearing points. This may be due to the low length-to-breadth ratio of compound 3 caused by the nonlinear and noncoaxial nature of the bonds between the thiophene ring and the phenyl rings (bond angle 148°, i.e., a 32° deviation from linearity).43 However, the clearing point of the pyrimidine 30 is only slightly higher than that of the thiophene 3, but the melting point is much higher. The electronegative nitrogen atoms and the dipole moment associated with their nonconjugated lone pair electrons in the pyrimidine 30 may induce greater intermolecular dipole-dipole interactions, and, hence a higher melting point, than that of the analogous phenyl compound 14 with an almost identical rotation volume. Table 3 collates thermal data for the asymmetric compounds 32-36, confirming the conclusion that the combination of long side-chains and end-chains induces low melting points. The supercooled isotropic liquid phase of compounds 32 and 35 both recrystallize before a liquid crystalline phase can be observed. Compound 33 exhibits a low melting point, a high clearing point, (43) Campbell, N. L.; Duffy, W. L.; Thomas, G. I.; Wild, J. H.; Kelly, S. M.; Bartle, K.; O’Neill, M.; Minter, V.; Tuffin, R. P. J. Mater. Chem. 2002, 12, 2706.

4934

Chem. Mater., Vol. 16, No. 24, 2004

Figure 1. Absorbance spectra from a cross-linked network of the symmetrical fluorene diene ester 8 before (solid line) and after (dashed line) washing in chloroform.

and, as a consequence, a wide nematic phase. The replacement of one of the phenyl rings in the hexaphenylene 20 to produce the asymmetric thiophene derivative 36 results in significant decreases in the melting and clearing points. The clearing point is at, or just below, room temperature. Indeed, compound 36 is the first light-emitting small molecule, rather than an oligomer, to exhibit an enantiotropic nematic phase close to room temperature, cf., para-sexiphenyl (mp 465 °C) with a similar aromatic core. This is consistent with a degree of asymmetry contributing to low melting points due to packing considerations. Compound 37 (m ) 10) exhibits a relatively low melting and clearing point (Cr-N ) 52 °C; N-I ) 143 °C;) for a compound with eight aromatic rings in a long thin molecular core and can even be supercooled to room temperature. As such it is suitable for use in OLEDs, see below.

Room-Temperature Phases. Previously it was thought the glassy state was necessary to stabilize the nematic state so that on cooling to room temperature the thin films could be cross-linked in the glassy state by UV irradiation. However, it was found that many of the compounds reported here, even those with high melting points, supercool in the nematic state for many hours and often days before recrystallization occurs. For example, although compound 8 is monotropic with a clearing point of 32 °C and a glass transition temperature of -27 °C, below a melting point of 41 °C, in practice, this compound retains a fluid nematic phase at room-temperature indefinitely. A helium-cadmium laser at 325 nm with a total fluence of 100 J cm-2 was used to photopolymerize and cross-link a thin film of compound 8 at 25 °C. No photoinitiator was used. Figure 1 shows the absorption spectra of the polymer network formed from compound 8 before and after washing with chloroform, which should remove any unreacted monomer. The two absorbance spectra are almost identical, showing that a completely insoluble film is formed. Cross-linking the liquid crystal retains the nematic order above the liquid crystal clearing point and allows multilayer films to be constructed. The polymerization at room temperature of a nematic mixture 1, see below, requires half the fluence necessary to polymerize compound 3 in the glassy nematic state at the same

Aldred et al.

Figure 2. Photomicrograph at 73 °C of nematic droplets of the mixture 2 just below the nematic clearing point.

temperature. This can be explained by the higher thermal energy required to reorient adjacent polymerizing radicals and carbon-carbon double bonds in the glassy state compared to corresponding radicals and bonds in the fluid nematic state. No member of the series of compounds 14-29 with a linear hexaphenylene molecular core has a room-temperature nematic phase. However room-temperature liquid crystallinity can be realized by mixing compounds 19 and 20, which have the lowest melting points of the series. They were mixed in different proportions in order to identify a eutectic mixture with a lower melting point. The lowest melting point (22 °C) was observed for the eutectic mixture 1 consisting of 25% of compound 19 and 75% of compound 20. This was confirmed by DSC plots which contained the melting point and clearing point peaks on heating the 1:3 mixture of 19 and 20 in powder form during the first heating cycle, see below. The resultant nematic mixture 1 formed by the in situ mixing of the two components on the DSC pan above their melting points exhibits an average nematic clearing point intermediate between those of the two components. On cooling the mixture, some crystallization occurs at about -17 °C and then just below zero before melting at 22 °C on the second heating cycle. This cycle can be repeated many times with no observable change in the temperatures of the melting and clearing point. Samples of mixture 1 have so far remained in the nematic phase at room-temperature indefinitely. In a similar way a room nematic phase was obtained for asymmetric chromophore using a 1:1 mixture of the reactive mesogens 33 and 36 to form mixture 2. The reactive mesogen 36 has a very low melting point (-28 °C) and the reactive mesogen 33 exhibits a very high clearing point (113 °C). Therefore, a 1:1 binary mixture of these two components should possess a nematic phase with a low melting point and a relatively high clearing point. Nematic droplets were observed on cooling mixture 2 from the isotropic liquid, see Figure 2, to form the Schlieren texture with two- and four-point brushes characteristic of the nematic phase along with optically extinct homeotropic areas, see Figure 3. The broad nature of these liquid crystal transitions is a characteristic feature of a nematic mixture as shown in Figure 2 where nematic droplets of various sizes are seen with a range of nematic clearing points. This is ideal mesomorphic behavior for technical applications where the nematic phase exhibits a high order parameter far from the clearing point, and processing, such as photochemical cross-linking, can be carried out at room temperature. A thermogram ob-

Fluorene Reactive Mesogens

Chem. Mater., Vol. 16, No. 24, 2004 4935 Table 4. Ionization Potential and Electron Affinity of the Reactive Mesogens 3, 15, 25, 30, 34, and 37 IPa (eV) ( 0.02 Eg b (eV) ( 0.04 EAc (eV) ( 0.06 3 15 25 30 34 37

5.01 5.30 5.36 5.57 5.07 4.93 a

Figure 3. Photomicrograph at 25 °C of the nematic Schlieren texture of the mixture 2 just below the nematic clearing point.

tained by differential scanning calorimetry of the mixture 2 is shown in Figure 4 to demonstrate this fact. The enthalpy of fusion (2.6 J g-1) of the transition from the crystalline state to form the nematic phase (Cr-N ) -20 °C) of mixture 2 shown in Figure 4 is not very large compared to that normally found for the melting point of a liquid crystal. This suggests a low tendency of crystallization for the mixture 2. In comparison, the enthalpy of transition between the nematic phase and the isotropic liquid (N-I ) 74 °C) is almost an order of magnitude smaller (0.4 J g-1) than that of the transition from the solid state into the nematic state, as expected due to the much lower degree of order in the nematic phase. Both of these transitions are seen again during the cooling cycle with a degree of supercooling as could be expected considering the high viscosity of the nematic phase consisting of molecules with a large aromatic core and four alkyl chains (two of which are branched while the other two make an orthogonal angle with the aromatic core).42 Mixture 2 does not form a nematic glass down to -50 °C on cooling the nematic phase. Mixtures 1 and 2 demonstrate for the first time the ability to generate a nematic phase at room temperature using simple binary mixtures of light-emitting liquid crystalline compounds. Electronic Properties. One of the main advantages of LC-polymer networks is their multilayer capability, and, as shown above, completely insoluble polymernetwork films can be formed from these reactive mesogens. Efficient multilayer OLEDs require the matching of energy levels to minimize the barriers for carrier injection and to trap both electron and holes in the

2.68 3.11 3.10 3.01 2.65 2.45

2.33 2.19 2.26 2.56 2.42 2.48

remark reversible reversible reversible irreversible reversible reversible

From CV. b From optical absorption spectrum. c From IP - Eg.

luminescent region. The work-function of InSnO is 4.8 eV and that of Ca is 2.9 eV so that hole injection materials with low IPs and electron-injection materials with high EAs are required. The standard strategy to increase/decrease the IP of a molecule is to include an electron withdrawing/donating group in its aromatic core. The IP is insensitive to the spacer length of the aliphatic end-chains and side-chains. Table 4 shows the measured IP and Figure 5 shows the PL spectra of a range of compounds. The blue emitter 15 (spectrum Figure 5a) has reasonably high barriers (0.3 and 0.4 eV, respectively) for hole and electron injection and may be suitable for a singlelayer OLED as it would provide balanced hole and electron currents, if the mobilities for both carrier types are similar. This reactive mesogen exhibits the highest hole-transport in a nematic glass at room temperature (µh ) 1 × 10-3 cm2 V-1 s-1) reported so far for a nematic phase and for a nematic polymer network after photochemical cross-linking.16 It can also be aligned on a polyimide orientation layer in the nematic phase and then cooled into the nematic glassy state at room temperature. Plane polarized emission from the glassy nematic reactive mesogen 15 is also observed with a relatively high polarization ratio (11:1). LC oligomers and polymers exhibit higher PL and EL polarization ratios due to a higher nematic order parameter, but require a thermal annealing procedure to obtain a uniform macroscopic orientation.25-30 Two electronwithdrawing fluorine atoms are present in the aromatic hexaphenylene core of compound 25 and this increases the IP by 0.06 eV. We have shown previously that OLEDs incorporating compound 3 as a hole-transporting/luminescent layer have a low threshold voltage of >3.5 V. This is attributed here to the excellent match between its IP and the work function of InSnO/PEDOT.

Figure 4. Differential scanning thermogram as a function of temperature for the first heating and cooling cycle for mixture 2 (scan rate 10 °C min-1).

4936

Chem. Mater., Vol. 16, No. 24, 2004

Figure 5. PL spectra of compounds (a) 15, (b) mixture 2, (c) 3, and (d) 37.

The pyrimidine 30 is a candidate to form a holeblocking/electron-injection layer in a multilayer OLED since it has a high IP of 5.57 eV and an EA of 2.56 eV. Unfortunately it is a crystal at room temperature and could not be used. As shown above, compound 3 is an ideal hole injector, but shows blue-green emission as shown in Figure 5c. We designed the asymmetric materials 33 and 36, which form mixture 2, in anticipation that the retention of one electron donating thiophene ring would result in a low IP while the twisted biphenyl would provide a blue shift of emission. A low IP of 5.05 eV is measured, and although the vibronic peaks of the PL spectrum of mixture 2 (see Figure 5b) show only a small blue-shift relative to that of compound 3, the 0-0 and 0-1 transitions have a larger intensity giving a more blue spectrum. Green emission is obtained from the eightring chromophore 37 as shown in Figure 5d. This material has a low IP of 4.93 eV and is therefore suitable as a hole injection/luminescent material in a three-layer OLED. Full-color OLEDs require red reactive mesogens for use in combination with the blue and green chromophores reported here. This has been achieved by replacing the 2,7-disubstituted-9,9-dialkylfluorene central unit with a 4,7-disubstituted-benzothiadiazole moiety and will be reported elsewhere. The high polarizability of the benzothiadiazole ring induces a significant red shift compared to similar reactive mesogens with a fluorene group in place of benzothiadiazole. A multilayer liquid crystal OLED was fabricated consisting of an ITO-covered glass substrate as a transparent anode, a PSS/PEDOT layer as a holetransporting passivation layer, a polymer network formed from a thin layer (100 nm) of compound 37 as a combined hole-transport and emission layer, a hole-

Aldred et al.

Figure 6. IV characteristics of a multilayer OLED containing compound 37 as the emission layer.

blocking layer (6 nm) of TAZ, and a layer of lithium fluoride (1 nm) and an aluminum layer (80 nm) as a combined cathode. Green emission was observed with an efficiency of 1.7 cd A-1 at 100 cd m-2 at 6 V as shown in Figure 6. A brightness of over 3000 cd m-2 can be obtained using this configuration. It is found generally in our work that the process of photochemically crosslinking reactive mesogens of this kind, including the ones reported here, leads to improved charge-transport and emission properties, i.e., an increase in OLED device efficiency.16,19 IV. Conclusions We conclude that reactive mesogens with exceptionally low melting points and nematic phases can be synthesized by molecular engineering of appropriate 9,9dialkylfluorene chromophores. Smectic phases are completely suppressed. Room-temperature nematic phases can be prepared by the appropriate substitution of aliphatic side-chains and end-chains. Alternatively, binary eutectic mixtures of homologous series of compounds can provide room-temperature phases. Crosslinking in the nematic phase at room temperature gives completely insoluble thin films. The IP and emission spectra of the compounds can be modified by incorporating electron-donating and electron-withdrawing groups into the aromatic core of the mesogens. These anisotropic polymer networks can be used as hole-transporting, emission, or electron-transporting layers in multilayer OLEDs. Acknowledgment. We are grateful for the financial support provided by the EPSRC GR/N28115 in the U.K. and the European Union as part of the Photoledd program IST-2001-37181. CM0351893