Grazing Incidence X-ray Diffraction of a Photoaligned Nematic

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Grazing Incidence X-ray Diffraction of a Photoaligned Nematic Semiconductor Stefan Dro¨ge,§ Manea S. Al Khalifah,§ Mary O’Neill,*,§ Huw E. Thomas,† Henje S. Simmonds,† J. Emyr Macdonald,† Matthew P. Aldred,‡ Panos Vlachos,‡ Stuart P. Kitney,‡ Andreas Lo¨bbert,‡ and Stephen M. Kelly‡ Department of Physics, UniVersity of Hull, Cottingham Rd., Hull, HU6 7RX, U.K.; School of Physics and Astronomy, Cardiff UniVersity, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, U.K.; and Department of Chemistry, UniVersity of Hull, Cottingham Rd., Hull, HU6 7RX, U.K. ReceiVed: April 18, 2008; ReVised Manuscript ReceiVed: October 2, 2008

Grazing incidence X-ray diffraction is used to find the thin film morphology of an extended molecule with an irregular alternating fluorene-thiophene structure, which is used to obtain linearly polarized electroluminescence and the photovoltaic effect. The material has a room temperature nematic glassy phase and is uniaxially aligned in the plane of the film using photoalignment techniques. Two distinct intermolecular separations of 0.45 and 1.5 nm are identified showing that the molecules are lamellar. The lamellae stack with only local order and the two short axes of the lamellae have no preferred orientation at the surface or bulk of the film. Neighboring molecules show a wide range of longitudinal displacements along the axis of the director, as expected for a nematic liquid crystal with no positional order. There is, however, a dominant feature corresponding to a longitudinal offset of 0.51 nm. Unlike some other fluorene-containing semiconductors where microphase separation of the side chains inhibits close packing of neighboring molecules, the lamellar structure and 0.45 intermolecular spacing found here allows π-π intermolecular interactions for efficient carrier transport. We obtain a room temperature hole mobility up to 3.4 × 10-3 cm2 V-1 s-1 using a timeof-flight technique. Conjugated polymers and oligomers are widely studied as organic semiconductors for a wide range of applications, such as light-emitting diodes,1,2 solar cells,3-5 and thin-film transistors.6,7 A key factor determining performance is the morphology of the film. For example, the carrier mobility of regioregular headto-tail (HT) poly(3-alkylthiophene) in field effect transistors is often >10-1 cm2 V-1 s-1 as a result of π-π interactions between closely spaced neighbors.6,8 However, values can vary by orders of magnitude depending on the microcrystalline morphology.9,10 This may be due to poor interconnectivity between microcrystalline domains. Traps may also develop at grain boundaries. Nematic liquid crystals have been used as organic semiconductors to a lesser extent,11-13 but have one key advantage: they can be aligned macroscopically to obtain a monodomain on suitable alignment layers. The organic field-effect mobility (OFET) performance of a polyfluorene copolymer in the nematic phase was substantially improved by uniaxial rubbing.13 The OFET mobility was higher when the molecular cores were aligned along the channel between the source and drain, because fewer of the rate-determining intermolecular hops were required for carriers to cross the channel. However, conjugated polymers are highly viscous so that high-temperature annealing (>200 °C) is required to obtain the desired macroscopic order. Nematic oligomers or extended molecules are aligned using less-extreme processing conditions and they have also been successfully applied to OLEDs,11,12 photovoltaics,5,14 and thin-film transistors.15 In particular, they are easily photoaligned, whereby a photoinduced anisotropy in an alignment film provides a template for their in-plane uniaxial orientation.16 Photoaligned * Corresponding author. E-mail: [email protected]. † University of Cardiff. ‡ Department of Chemistry, University of Hull. § Department of Physics, University of Hull.

light-emitting nematic semiconductors have been used to obtain polarized electroluminescence with the added advantage that the alignment direction can be spatially patterned.11,17-19 A key property of organic semiconductors is good carrier transport, which requires small intermolecular separations so that carrier can hop from molecule to molecule. Many rod-shaped organic semiconductors, both polymers and oligomers, contain 9,9dialkyl-2,7-disubstituted-fluorene groups to aid solubility and induce lower melting points.20 However, their long side chains or so-called hairy rods can promote microphase separation,21 so that the aromatic cores of the rods are widely separated impeding charge transport. In this paper, we examine the morphology of a photoaligned organic semiconductor which forms a nematic glassy phase at room temperature. The extended molecule and its homologues have an irregular alternating 9,9dialkyl-2,7-disubstituted-fluorene-2,5-disubstituted-thiophene structure, show bright green electroluminescence, and extremely high birefringence (∆n f 1.4).19,22,23 We are primarily interested in determining whether its thin-film morphology promotes good carrier transport. We use grazing incidence X-ray diffraction (GIXRD) which is a powerful tool to examine the morphology of polymers thin films,24,25 in particular, organic semiconductors.6,26-28 The X-ray penetration depth can be tuned by varying the angle of incidence of the incoming X-ray beam to examine either the bulk or surface structure of the thin film. The inplane and out-of plane morphology of the thin film can be separately probed. Experimental Section The chemical structure of the photaligning polyimide 1 is shown in Chart 1. The corresponding polyamic acid precursor was synthesized in house by the condensation of cyclobutane1,2,3,4-tetracarboxylic acid dianhydride (CBDA) and 1,3-

10.1021/jp803379a CCC: $40.75  2009 American Chemical Society Published on Web 12/12/2008

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Dro¨ge et al.

CHART 1: Chemical Structures of Compounds 1-3 Used in This Study

diaminobenzene (DB) in a condensation polymerization reaction. This preparation will be reported in more detail elsewhere. A thin film of 1 was prepared by imidization (thermal annealing in nitrogen at 200 °C) of a film of the precursor, which was spin cast from a solution of 2% by weight in n-methylpyrrolidone. A series of quartz plates at Brewster’s angle was used to linearly polarize the output from a Hg lamp. The polyimide sample was irradiated at room temperature at normal incidence using a fluence of 0.75 J cm-2 at 254 nm. The chemical structure of the nematic semiconductor 2 is shown in Chart 1. Compounds 2 and 3 were synthesized according to a literature procedure.14 Both 2 and 3 have low glass transition temperatures (Tg ) 55 and 26 °C, respectively) and high nematic-isotropic transition temperatures respectively (N-I ) 235 and 188 °C). A melting point could not be determined for either material even using differential scanning calorimetry (DSC) at low temperatures. Despite the low glass transition temperature, both compounds remain in the glassy phase for many weeks at room temperature. 2 was deposited onto the photoalignment polyimide film 1 by spin coating from a solution of 7% by weight in toluene, heated to 70 °C at 10°/min and cooled to room temperature after 10 min. Polarized absorbance spectra were recorded using a Unicam 5625 UV spectrophotometer and a Coherent sheet UV polarizer (Polacoat 105UV). Grazing incidence X-ray diffraction measurements were taken on the XMaS beamline at the European Sychrotron Radiation Facility (ESRF), Grenoble, France. Full details of the beam setup are provided elsewhere.28 The sample was mounted on an 11-axis, 8-circle diffractometer in a heliumfilled chamber to minimize scatter by air. The angle of incidence was varied between 0.05° and 0.32°, above and below the critical angle of ≈0.16°. The scattered X-rays were detected with an area detector with 2048 × 2048 pixels placed about 40 cm from the sample. A lead beam stop was used to protect the area detector from the specularly reflected X-ray beam. Intensity profiles were obtained as a function of the scattering vector, q, from the image data, on correction for the aberration caused by recording a spherical wave with a plane detector; q ) (4π/λ)sin θ, where θ is half the scattering angle and λ is the wavelength of the incident radiation. The d-spacing of a peak is given by 2π/q and the coherence length is estimated from the DebyeScherrer equation. The resolution of the area detector is limited by the finite sample length of 0.8 cm giving an error in q of 2%. The photocurrent time-of-flight method was used to obtain the hole mobility. Two types of sample configurations were used. Films of thickness l ) 3 - 4 µm were prepared by spincasting from concentrated solutions (∼100 mg mL-1) of the compounds in toluene. For compound 2 the films were sandwiched between semitransparent gold and indium tin oxide electrodes, whereas aluminum electrodes were used for sample 3. (We will show elsewhere that the electrodes chosen do not

influence the transit time but affect the photocurrent amplitude at early times.) The nematic glassy films were not annealed to avoid thickness fluctuations, which were ∼3%. Instead, the solvent was removed by placing the film overnight in vacuo at room temperature. A second sample of compound 3 was tested in a glass/ITO/film/ITO/glass cell configuration. The cell was filled by vacuum-assisted movement of the compound in its isotropic phase and then slowly cooled. An optical pulse from N2 laser (Laser Science VSL-337ND) incident on the thin film creates a thin sheet of electron-hole pairs next to the contact. A uniform electric field, E, was applied across the organic layer and the transit time, τ, is obtained from the intercept of the photocurrent plateau and tail, plotted on a logarithmic scale. The carrier mobility, µ, was obtained from the equation µ)l/ (Eτ).29 Results Figure 1 shows the polarized absorbance spectrum of the photoaligned thin film of the nematic organic semiconductor 2. The absorbance spectrum of an unaligned film is shown in the Supporting Information for comparison with the aligned spectra. A| refers to the absorbance measured parallel to the polarization direction of the ultraviolet beam incident to the photoaligning polyimide 1, and A⊥ refers to the perpendicular orientation. The sample is a well-aligned monodomain with a maximum polarization ratio of 11:1. This corresponds to an order parameter of 0.78. The alignment is homogeneous (in-plane) with the director (molecular direction) perpendicular to the polarization direction of the UV beam, since A⊥> A|.. This agrees with the expected alignment mechanism: homogeneous photoalignment of main-chain polyimides proceeds via anisotropic photodegradation and the alignment direction of the overlying liquid crystalline phase corresponds to that of minimal photodecomposition.16,30 Figure 2 shows the area detectors images from the scattered X-rays for two orientations of the sample. The X-rays

Figure 1. Polarized absorbance spectrum of photoaligned liquid crystal organic semiconductor 2.

GIXRD of a Photoaligned Nematic Semiconductor

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Figure 4. Illustration of conformation of semiconductor 1 obtained using MM2 optimization. Figure 2. Diffraction image from photoaligned 1 for different orientations of the sample. The angle of incidence of the beam to the sample was 0.25°. (a) φ ) 0°, the director lies in the plane of incidence. (b) φ ) 90°, the director is orthogonal to the plane of incidence.

Figure 3. Intensity of scattered X-rays as a function of qx for the outof-plane scan. For the in-plane scans, the intensity is plotted as a function of qy for φ ) 0° and qz for φ ) 90°.

are scattered by the semiconductor 2 rather than by the underlying photoalignment layer, which shows no well-defined features when irradiated separately. In the GIXRD studies, we define the x axis as normal to the surface of the film and the y and z axes are parallel to the surface, with the director lying in the z direction. When φ ) 0°, the director lies in the plane of incidence of the X-rays, so that reflections are seen in the (xy0) plane, as shown in Figure 2a. When φ ) 90°, the plane of incidence of the X-rays is normal to the director, so that reflections are seen in the (x0z) plane (Figure 2b). In-plane and out-of-plane line scans were extracted from the area detectors images for the two orthogonal orientations of the sample. They are shown in Figure 3. The in-plane scans correspond to scattered intensity as a function of qy when φ ) 0° and as a function of qz when φ ) 90°. The out-of-plane scans were measured at qy(z) ) 0.15 nm-1 rather than qy(z) ) 0 to avoid overlap with the specular reflectivity signal. Therefore, the line scans are not exactly along (00z). However, the displacement is much smaller than the width of the spots in q space and so is considered insignificant. For φ ) 0°, two rings are observed on the area detector image, with maxima at 13.9 and 4.19 nm-1, corresponding to spacings of 0.45 and 1.50 nm, respectively. For φ ) 90°, the two rings are replaced by two diffraction spots out of plane peaking at the same q value as for φ ) 0°. Hence the two features are not observed along the director (z) direction and so represent intermolecular spacings. By analogy to the XRD of oligofluorenes and polyfluorenes, we assign the larger spacing of 1.5 nm to layering due to microphase separation of the aromatic core of the molecule and alkyl side chains.21,26,31,32 The two linear aliphatic chains attached at position 9 of the fluorene moiety are orthogonal to the fluorene unit and the alkyl chains extend upward. The spacing is approximately twice the extended length ()0.77 nm) of the n-hexyl (C6H13) side chains. The n-octyl (C8H17) side chains

have an extended length of 1.02 nm and so must overlap or are tilted. The smaller spacing of 0.45 nm is a typical intermolecular spacing for nematic liquid crystals. The simultaneous presence of two distinct intermolecular spacing suggests that the molecule must have a planar or near-planar lamellar shape, as represented in Figure 4. Indeed, an interannular twist of only 20° is predicted between the thiophene and fluorene groups in the molecular modeling of oligomers and polymers.33,34 It is also well established that molecules adopt a less twisted conformation in a thin film. The liquid crystal 2 has three similar 2,7-disubstituted fluorene units and the twist direction must not be additive across the molecule, because its neighbor would not be able to follow a cumulative twist to retain a 0.45 nm intermolecular separation, given the nematic order, macroscopic uniaxial alignment, and the irregular arrangement of the molecular fluorene and thiophene groups in the molecule. The structure depicted in Figure 4 is calculated by minimization using the MM2 method on Chemdraw. It may not represent the structure corresponding to minimum energy as a global energy minimum is not guaranteed. The calculations were carried out in the gas phase so that intermolecular interactions were ignored. More sophisticated calculations are planned. Bulk XRD measurements of a series of related shorter molecules showed nematic order with lamellar molecules. These have only a single fluorene unit as well as thiophene and phenyl groups so that significant twisting would not be expected.35 The intermolecular correlation length of 2.9 and 1.4 nm are obtained for 1.5 and 0.45 nm, respectively. These lateral correlation lengths are typical for liquid crystals.36 Their low values and the circular intensity curves show that the lamellae stack with only local order and in no preferred direction. A sharp peak sitting on an extremely broad asymmetric structure is observed in-plane along z in the φ ) 90° scan in Figure 3. Its maximum corresponds to a spacing of 0.51 nm. We estimate the correlation length to be 4.7 nm from the halfwidth at half-maximum. The origin of this peak is unclear: we suggest that it originates from density correlations between longitudinally displaced neighboring molecules, which have nematic order. Similarly, correlation lengths of 8-10 nm were obtained along the molecular direction by X-ray diffraction of aligned polyfluorene samples in the nematic phase.37,32 The underlying broad structure extends upward from 7.5 nm-1 with a shoulder at 13.7 nm-1. Its presence implies that the neighboring molecules show a wide range of axial offsets, in addition to the dominant one of 0.51 nm. This is an expected result for a nematic liquid crystal, which has orientational, but no positional, order. The GIXRD experiment was carried out as a function of angle of incidence above and below the critical angle. Below the critical angle, the X-ray penetrates to only a depth of ≈10 nm so that surface properties can be probed.24,27,28 There is no significant difference in the line widths and relative intensities of the various peaks in the scans taken at the surface and bulk, suggesting that the lamellae have random biaxial orientation across the film.

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Figure 5. Hole mobility of compounds 2 and 3 as a function of electric field at room temperature.

Figure 5 shows the room temperature mobility of spin-cast film of 2 as well as the reactive mesogen 3, which has the same fully congugated aromatic core and fluorene side chains as compound 2, but incorporates polymerizable, noncongugated diene groups at the end of its terminal aliphatic chains.22 The experiment measures the mobility from the top to bottom of the film and so depends on intermolecular hopping of carriers between the molecules, which lie in the plane of the film. An example of a typical transient is shown as Supporting Information. The hole mobility of a spin-cast film of 2 has a small field dependence reaching 5.6 × 10-4 cm2 V-1 s-1 for a field of 1.9 × 107 V m-1. The spin-cast sample of 3 has a somewhat smaller mobility, up to 5.1 × 10-4 cm2 V-1 s-1. This is consistent with previous work where bulkier terminal aliphatic chains are shown to reduce mobility.35 Interestingly, the hole mobility of 3 in an enclosed cell configuration is significantly higher, up to 3.4 × 10-3 cm2 V-1 s-1. This improved performance can be attributed to the different processing conditions of the two samples; in the cell 3 was slowly cooled from the isotropic phase so it is more highly ordered than the spin-cast sample which was not annealed to avoid thickness fluctuations. Discussion This class of compound with an irregular alternating [A]-[B] structure, a lamellar shape, and a room temperature nematic phase has many advantages as an organic semiconductor. The lamellar molecular structure allows π-π interactions between neighboring molecules, which are required for efficient intermolecular charge. This represents an advantage compared to some polyfluorenes, where the helical structure and extended side chains do not allow close intermolecular spacing, although a planar structure can be obtained by appropriate processing.37 Nematics are easily proccessable and, as shown here, can be aligned to form a monodomain. This gives a very important advantage for semiconducting applications because, although charge transport in crystals may be fast, there is little connectivity between microcrystalline domains. There have been some structural studies on molecules having fluorene and thiophene groups with significantly different results to those obtained here. Oligofluorene-thiophene derivatives without side chains and with symmetric or asymmetric alkyl end caps are crystalline at room temperature.8,38 A molecule with three thiophene groups between two fluorene units also shows crystalline organization.33 Oligofluorenes with branched side chain used for polarized OLEDS and OFETs form nematic glassy phases, but have not been analyzed using X-ray diffraction.15,39 Similar oligofluorenes with slightly different side-chain structures show smectic B phases with intermolecular separations of 1.47 nm, due to layering of the side chains.40 An intermolecular spacing of

Dro¨ge et al. 0.4-0.5 nm, which would demonstrate π-π stacking is not observed. However, modeling the crystal structure suggests that the layers may consist of closely spaced pairs of molecules. AFM studies of thin deposits of alternating and statistical copolymers of fluorene and bithiophene aromatic rings show a fibrillar morphology.41 Molecular modeling showed that the most stable configurations are found when the bithiophene segments accommodate in front of fluorene units with an equilibrium interchain distance of 0.45-0.47 nm. In these configurations, the π-systems of the chains can efficiently overlap, giving rise to sizable π-π interactions. An important difference between the molecule discussed here and other fluorene-thiophenecontaining compounds is the nematic order. Nematic liquid crystals have no positional order and so avoid solid-state packing into low-energy crystalline conformations, which can inhibit intermolecular charge transport.42,43 Very recently, thin films of a uniaxially aligned nematic poly[(9,9-dioctylfluorenyl-2,7diyl)cobithiophene] was characterized by the X-ray diffraction pole figure technique.44 Similar intermolecular and longitudinal spacings were obtained as found in this work. The relatively high value of mobility, up to 3.4 × 10-3 cm2 V-1 s-1, confirms the suitability of these lamellar compounds as semiconductors. Indeed, significantly higher OFET mobilities would be expected for an aligned film since, as shown by Sirringhaus et al.,13 charge transport along the plane of the film involves fast intramolecular hopping as well as the slower intermolecular hopping. In conclusion, we show that the fluorene side chains of an extended molecule with an irregular alternating fluorene-thiophene structure do not inhibit close packing of neighboring molecules. Compound 2 is easily aligned by photoalignment techniques and lies in the plane of the substrate as a thin film with a lamellar molecular structure having two distinct intermolecular separations of 0.45 and 1.5 nm. The lamellae stack with only local order and the two short axes of the lamellae have no preferred orientation at the surface or bulk of the film. Neighboring molecules show a wide range of longitudinal displacements along the axis of the director, as expected for a nematic liquid crystal with no positional order. There is a dominant feature corresponding to a longitudinal offset of 0.51 nm. The lamellar structure and the short intermolecular spacing of 0.45 nm allow π-π interactions between neighboring molecules, which is important for efficient carrier transport through the film. A room temperature hole mobility of 3.4 × 10-3 cm2 V-1 s-1 is found using a time-of-flight technique. We expect that even higher mobility would be obtained in one direction, if long-range order of the lamellar stacking could be obtained. Future studies will involve the investigation of surface alignment techniques to obtain long-range biaxial order. Acknowledgment. We thank the EPSRC for funding both the project (GR/S08800) and the XMaS beamline, managed by C. Lucas and M. Cooper. We are grateful to D. Mannix, S. Brown, and P. Thompson at the beamline for technical help. We also thank Gordon Sowersby for technical support at Hull. We thank Sharp Laboratories of Europe for supporting a studentship (A.L.) and Dr. Paul Gass for helpful discussions. Supporting Information Available: The absorbance spectrum of 2 and a photocurrent transient from 3 are available free of charge via the Internet at http://pubs.acs.org References and Notes (1) Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539.

GIXRD of a Photoaligned Nematic Semiconductor (2) Mazzeo, M.; Pisignano, D.; Favaretto, L.; Barbarella, G.; Cingolani, R.; Gigli, G. Synth. Met. 2003, 139, 671. (3) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (4) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510. (5) Carrasco-Orozco, M.; Tsoi, W. C.; O’Neill, M.; Aldred, M. P.; Vlachos, P.; Kelly, S. M. AdV. Mater. 2006, 18, 1754. (6) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (7) Tang, M. L.; Roberts, M. E.; Locklin, J. J.; Ling, M. M.; Meng, H.; Bao, Z. N. Chem. Mater. 2006, 18, 6250. (8) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (9) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J. S.; Frechet, J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312. (10) Chang, J. F.; Sun, B. Q.; Breiby, D. W.; Nielsen, M. M.; Solling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H. Chem. Mater. 2004, 16, 4772. (11) Contoret, A. E. A.; Farrar, S. R.; Jackson, P. O.; Khan, S. M.; May, L.; O’Neill, M.; Nicholls, J. E.; Kelly, S. M.; Richards, G. J. AdV. Mater. 2000, 12, 971. (12) Chen, A. C. A.; Culligan, S. W.; Geng, Y.; Chen, S. H.; Klubek, K. P.; Vaeth, K. M.; Tang, C. W. AdV. Mater. 2004, 16, 783. (13) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406. (14) Tsoi, W. C.; O’Neill, M.; Aldred, M. P.; Kitney, S. P.; Vlachos, P.; Kelly, S. M. Chem. Mater. 2007, 19, 5475. (15) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y. H.; Culligan, S. W.; Chen, S. H. Chem. Mater. 2005, 17, 264. (16) O’Neill, M.; Kelly, S. M. J. Phys. D Appl. Phys. 2000, 33, R67. (17) Sainova, D. S.; Zen, A.; Nothofer, H. H.; Asawapirom, U.; Scherf, U.; Hagen, R.; Bieringer, T.; Kostromine, S.; Neher, D. AdV. Funct. Mater. 2002, 12, 49. (18) Aldred, M. P.; Vlachos, P.; Contoret, A. E. A.; Farrar, S. R.; Tsoi, W. C.; Hudson, R.; Woon, K. L.; Kelly, S. M.; O’Neill, M. J. Mater. Chem. 2005, 15, 3208. (19) Aldred, M. P.; Contoret, A. E. A.; Farrar, S. R.; Kelly, S. M.; Mathieson, D.; O’Neill, M.; Tsoi, W. C.; Vlachos, P. AdV. Mater. 2005, 17, 1368. (20) Neher, D. Macromol. Rapid Commum. 2001, 22, 1365. (21) Knaapila, M.; Stepanyan, R.; Lyons, B. P.; Torkkeli, M.; Monkman, A. P. AdV. Funct. Mater. 2006, 16, 599. (22) Woon, K. L.; Contoret, A. E. A.; Farrar, S. R.; Liedtke, A.; O’Neill, M.; Vlachos, P.; Aldred, M. P.; Kelly, S. M. J. SID 2006, 14, 557. (23) Woon, K. L.; O’Neill, M.; Richards, G. J.; Aldred, M. P.; Kelly, S. M. Liq. Cryst. 2005, 32, 1191. (24) Factor, B. J.; Russell, T. P.; Toney, M. F. Phys. ReV. Lett. 1991, 66, 1181.

J. Phys. Chem. B, Vol. 113, No. 1, 2009 53 (25) Factor, B. J.; Russell, T. P.; Toney, M. F. Macromolecules 1993, 26, 2847. (26) Kawana, S.; Durrell, M.; Lu, J.; Macdonald, J. E.; Grell, M.; Bradley, D. D. C.; Jukes, P. C.; Jones, R. A. L.; Bennett, S. L. Polymer 2002, 43, 1907. (27) Macdonald, J. E.; Durell, M.; Trolley, D.; Lei, C.; Das, A.; Jukes, P. C.; Geoghegan, M.; Higgins, A. M.; Jones, R. A. L. Radiat. Phys. Chem. 2004, 71, 811. (28) Jukes, P. C.; Das, A.; Durell, M.; Trolley, D.; Higgins, A. M.; Geoghegan, M.; Macdonald, J. E.; Jones, R. A. L.; Brown, S.; Thompson, P. Macromolecules 2005, 38, 2315. (29) 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. (30) Newsome, C. J.; O’Neill, M. J. Appl. Phys. 2000, 88, 7328. (31) Knaapila, M.; Stepanyan, R.; Lyons, B. P.; Torkkeli, M.; Hase, T. P. A.; Serimaa, R.; Guntner, R.; Seeck, O. H.; Scherf, U.; Monkman, A. P. Macromolecules 2005, 38, 2744. (32) Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S. Macromolecules 1999, 32, 5810. (33) Surin, M.; Sonar, P.; Grimsdale, A. C.; Mullen, K.; De Feyter, S.; Habuchi, S.; Sarzi, S.; Braeken, E.; Heyen, A. V.; Van der Auweraer, M.; De Schryver, F. C.; Cavallini, M.; Moulin, J. F.; Biscarini, F.; Femoni, C.; Roberto, L.; Leclere, P. J. Mater. Chem 2007, 17, 728. (34) Yang, L.; Feng, J. K.; Ren, A. I. J. Mol. Struct.: THEOCHEM 2006, 758, 29. (35) Woon, K. L.; Aldred, M. P.; Richards, G. J.; Vlachos, P.; Mehl, G. H.; Kelly, S. M.; O’Neill, M. Chem. Mater. 2006, 8, 2311. (36) Ungar, G. Physical Properties of Liquid crystals: Nematics; Dunmur, D. A., Fukuda, A., Luckhurst, G., Eds.; Institute of Electrical Engineers: Stevenage, UK, 2000. (37) Knaapila, M.; Garamus, V. M.; Dias, F. B.; Almasy, L.; Galbrecht, F.; Charas, A.; Morgado, J.; Burrows, H. D.; Scherf, U.; Monkman, A. P. Macromolecules 2006, 39, 6505. (38) Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B. C.; Van Patten, P. G.; Bao, Z. N. Chem. Mater. 2003, 15, 1778. (39) 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. (40) Chi, C.; Lieser, G.; Wegner, G. Macromol. Chem. Phys. 2005, 206, 1597. (41) Surin, M.; Sonar, P.; Grimsdale, A. C.; Mu¨llen, K.; Lazzaroni, R.; Leclerc, P. AdV. Funct. Mater. 2005, 1, 5–1426. (42) Donley, C. L.; Zaumseil, J.; Andreasen, J. W.; Nielsen, M. M.; Sirringhaus, H.; Friend, R. H.; Kim, J. S. J. Am. Chem. Soc. 2005, 127, 12890. (43) Cornil, J.; Gueli, I.; Dkhissi, A.; Sancho-Garcia, J. C.; Hennebicq, E.; Calbert, J. P.; Lemaur, V.; Beljonne, D.; Bredas, J. L. J. Chem. Phys. 2003, 118, 6615. (44) Werzer, O.; Matoy, K.; Smilgies, D.-M.; Rothmann, M. M.; Strohriegl, P.; Resel, R. J. Appl. Polym. Sci. 2008, 107, 1817.

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