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An Electric-Field-Responsive Discotic Liquid-Crystalline Hexa-peri-Hexabenzocoronene/Oligothiophene Hybrid Nan Hu, Renfan Shao, Yongqiang Shen, Dong Chen, Noel A. Clark, and David M. Walba* Columnar liquid crystals (COL LCs) formed from discotic mesogens are promising materials for a number of applications in organic electronics[1] due to their advantageous properties, such as macroscopic self-assembly, self-healing, and ease of processing. Discotic materials possessing the hexa-peri-hexabenzocoronene (HBC) core show the highest charge carrier mobility observed to date in COL LCs.[2] Discotic mesogens, including HBC derivatives, are typically composed of more or less rigid planar aromatic cores with flexible chains laterally attached to the cores. Exploration for new COL LC materials enhances the molecular structural diversity achievable, and can allow tuning of physical properties of COL phases at the molecular and supramolecular levels. As for all COL LCs, realizing the full potential for applications in organic electronics requires achieving high-quality alignment (uniform macroscopic orientation of the LC).[1,3] For example, HBC-based columnar structures show high charge-transport anisotropy in well-aligned films.[4] If the director (column axis) is parallel to the substrates, the system is attractive for fabrication of field-effect transistors (FETs), while homeotropic alignment (director normal to the substrate surface) is well-suited for photovoltaic cells and light-emitting diodes.[1] Uniform parallel alignment of HBCs in thin films has been achieved by several methods, including self-assembly from solution on friction-deposited polytetrafluoroethylene (PTFE),[5] a multi-monolayer Langmuir-Blodgett technique,[6] zone casting,[7] and application of a magnetic field[4c] or electric field[8] to a drop-cast solution. None of these methods involves alignment of a sample in LC phase by applying an electric field; all require solution processing. In addition, the high clearing temperatures of most HBC mesophases (above 300 °C)[9] might be limiting with regard to thermal processing of the materials through the isotropic phase. Homeotropic alignment seems to be the most elusive organization for high-molecular-weight mesogens, such as those with the HBC core. Very little success in obtaining high-quality homeotropic alignment has been reported to date.[10] Furthermore, the examples of COL

N. Hu, Prof. D. M. Walba Department of Chemistry and Biochemistry and the Liquid Crystal Materials Research Center 215 UCB, University of Colorado Boulder, CO 80309–0215, USA E-mail: [email protected] Dr. R. Shao, Dr. Y. Shen, Dr. D. Chen, Prof. N. A. Clark Department of Physics and the Liquid Crystal Materials Research Center 390 UCB, University of Colorado Boulder, CO 80309–0390, USA

DOI: 10.1002/adma.201304371

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LCs responsive to electric fields are rare. With a few interesting exceptions,[11] most reported COL LC materials showing a response to an applied electric field in several micrometers thickness involve hydrogen-bonding motifs serving as a responsive handle.[12] The development of new HBC-based LC materials by tuning peripheral functionalities to enable high-quality alignment of mesophases with thickness of several microns is highly attractive. Furthermore, this is most useful if alignment can be achieved by well-known techniques typically used for nematics and smectics. Herein, we report a new HBC-based COL LC (1) with six pendant quadra-3-hexylthiophene units attached to the core through long alkyl chains (Figure 1). The oligothiophene-HBC hybrid is attractive, since thiophene-based materials can be easily functionalized,[13] have been extensively utilized for organic electronics,[14] and can self-assemble into LCs.[15] Even possessing bulky heterocyclic aromatic units at the ends of the tails, compound 1 shows a thermodynamically stable enantiotropic COL phase over a broad temperature range, supercools well below room temperature (the phase sequence and transition temperatures are given in Figure 1), and can be introduced into standard ITO/glass LC cells by capillary filling in the isotropic phase. Interestingly, this material can provide well-aligned cells with either homeotropic or parallel alignment, controllable by the applied electric fields in the LC phase. More importantly, these electric field induced alignments are maintained even after removal of the electric field. The synthesis of mesogen 1, outlined in Scheme 1, is remarkably efficient, taking advantage of the prototype click reaction, Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides[16] with terminal alkynes, to accomplish covalent tethering of six quadra–3–hexylthiophene units to a functionalized HBC core in one step. The synthesis of mesogen 1 starts from ester-terminated hexa-alkylphenylbenzene 2 (synthesis of compound 2, accomplished via the Vollhardt co-catalyzed aryl acetylene trimerization,[17] is described in the Supporting Information). The ester groups of compound 2 were reduced to alcohols with LiAlH4 in THF to provide hexa-alcohol 3, which was then converted to hexa-bromide 4 in high yield via the Appel reaction. Oxidative cyclization of the hexaphenylbenzene 4 under conditions reported by Müllen (FeCl3/MeNO2 in CH2Cl2) afforded the substituted HBC 5, possessing a mixture of bromo- and chloro-alkyl chains (approximately 2:1 Br/Cl ratio). Later, it was found that the alkyl chlorides could not be converted to azides by reacting with NaN3. Therefore, both the bromine and chlorine substituents were completely converted to the more reactive iodides by subjecting HBC 5 to excess NaI in dry acetone to afford the hexa-iodide 6, which was then converted to the desired hexa-azide 7 in quantitative yield. Finally, the target HBC-oligothiophene 1 was obtained by coupling hexa-azide 7 with ethynyl 3-hexyl-quadrathiophene (8) under

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consistent with a parallel-aligned hexagonal COL phase, with the inter-column spacing easily resolved (Figure 2b, c, and d). Here, we find that 1 self-assembles into long columns with diameter of ca. 5 nm in edge-on orientation. Fracture planes follow the interfaces between columns, and in some places across columns, which provides direct evidence for the 2D periodic stack of compound 1 in the COL LC phase. The hexagonal arrangement of the columns is substantiated by small-angle X-ray diffraction (XRD) experiments. The sequence of equatorial reflection peaks (Figure 2e) with d-spacings of 45.7 Å (100), 26.4 Å (110), 22.8 Å (200), 17.2 Å (210), and 15.2 Å √ (300) and √ a reciprocal d-spacing ratio of 1: 3 :2: 7 :3, characteristic of a hexagonal lattice of columns for an unoriented powder sample in the COL LC phase. The observed intercolumnar distance of the hexagonal lattice of 5.27 nm is shorter than the molecular length in a fully extended conformation (approximately 7 nm), inferring that the sidechains interpenetrate between neighboring columns, or that disks and/or sidechains randomly tilt relative to the column axis. Wideangle XRD (Figure 2f and g), shows a broad halo consisting of two reflections centered at about 3.53 Å, a reasonable value of the Figure 1. Molecular structure, phase sequence, and phase transition temperatures (°C) deterperiodic stacking of the HBC cores within −1 mined by DSC and XRD for mesogen 1. Transition enthalpies (kJ mol ) from DSC are given in parentheses. Crystalline, hexagonal columnar, and isotropic phases are denoted by Cr, Colh, columns, and 4.47 Å. We suggest that the latter is associated to the liquid-like correlaand Iso, respectively. tion of the pentacyclic triazole-oligo-3-hexylthiophene units, which is quite similar to stacking distances (ca. 4.5 Å) of other oligothiophenes in LC typical Cu(I)-catalyzed click conditions in 95% yield after puriphases[15b,19] and seems reasonable. Thus both the HBC core fication. Details of the synthesis are provided in the Supporting Information. and terminal pentacyclic aromatic units are mainly located in a plane normal to the column axis. Initial characterization of mesogen 1 was accomplished by A key advantage of LC materials in general is the strong differential scanning calorimetry (DSC) and polarized optical response to application of electric fields. The electro-optic microscopy (POM). The transition temperatures and enthalbehavior of the COL LC phase of mesogen 1 is particularly pies determined by DSC are given in Figure 1. The material interesting in this regard. Specifically, the columns in the nonforms a thermodynamically stable enantiotropic COL LC phase. uniform parallel-aligned samples of the COL LC phase between The transitions on heating from the crystal to COL LC phase ITO-coated glass plates, obtained directly by cooling from the and COL LC phase to isotropic phase, are first-order (transiisotropic phase, can be easily re-oriented to uniform homeotion enthalpies are 77.2 and 6.3 kJ·mol−1, respectively), while tropic alignment by application of an electric field along the cell on cooling, crystallization occurs well below room temperature normal. After switching off the field, this uniform alignment (though the enthalpy values suggest that some crystallization is maintained, and can only be changed by heating the sample occurs before the bulk of the sample crystallizes). The clearing above the clearing temperature and cooled again without an point Ti (106.2 °C) is 293 °C lower than that of the HBC meselectric field. ogen with six dodecyl pendent groups (399 °C).[18] Studies of This behavior is seen in clean ITO-glass cells, and in cells the phase for mesogen 1 by POM (Figure 2a) show a smooth with spin-coated alignment layers, including nylon (Du Pont focal conic texture on cooling from the isotropic to the COL LC Elvamide) and polyimide. For example, COL LC mesogen 1 was phase, with extinction brushes oriented parallel and perpenfilled by capillary action into a commercial ITO/glass LC cell dicular to the polarizers, indicating non-uniform parallel alignpossessing low pre-tilt rubbed polyimide alignment layers, with ment of an untilted COL phase. a cell gap of 4 μm. Upon cooling from the isotropic to the COL The topography of fracture surfaces obtained from LC cells of LC phase, POM under crossed polarizers exhibits the typical mesogen 1 quenched in the COL phase and imaged by freezebirefringent focal conic texture seen for COL phases, where the fracture transmission electron microscopy (FFTEM), are fully

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Scheme 1. Synthesis of HBC-hexa-3-hexyl-quadrathiophene 1. a) LiAlH4, THF, reflux, 97%; b) PPh3, CHr4, DCM, 95%; c) FeCl3, DCM/CH3NO2; d) NaI, THF/acetone, 80 °C, 60%(two steps); e) NaN3, THF/DMF, quant.; f) CuI, DIPEA, CHCI3, 95% after purification.

rubbed polyimide gives parallel alignment with no azimuthal orientation. Application of a 2.0 Hz square-wave AC electric field of about ±30 V·μm−1 at 80 °C provided an extremely high quality dark state (Figure 3a) within 1 min. This result is fully consistent with clean homeotropic alignment of a hexagonal COL LC. The switching voltage threshold for driving the reorientation of a parallel sample to homeotropic as a function of temperature is given in Figure S8. To confirm the conclusions of the POM studies, small-angle X-ray scattering (SAXS) experiments were accomplished using a LC cell prepared using thin glass substrates patterned with an ITO square surrounded by clean glass, where the native parallel alignment (off the electrode surface) could be compared directly to the vertical E-field driven homeotropic alignment (over the ITO electrodes). As shown in Figure 3c, the non-ITO area exhibits well-distinguishable circular reflections, indicating that the columns are randomly arranged with the column axes parallel to the surface (perpendicular to the k-vector of the beam). In contrast, the area between the ITO electrodes, after application of an AC

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field (±20 V·μm−1) 10 d prior to taking the XRD data, shows six sharp reflections arranged in a perfect hexagon, indicative of uniform homeotropic alignment of a hexagonal COL LC (Figure 3d). In addition, with a specially patterned ITO/glass electrode structure allowing application of in-plane and orthogonal electric fields, the LC columns can be reoriented to uniform parallel alignment. Thus, when the mesogen 1 is capillary-filled into a glass LC cell where there are interdigitated finger electrodes (electrode gap 10 μm), and a 2.0 Hz square-wave AC E field of ±15 V·μm−1 is applied while the sample is cooled from the isotropic into COL LC phase, POM imaging between crossed polarizer and analyzer demonstrates that the areas between the ITO electrodes show a maximum in transmission when the stripes are oriented at 45° to the polarizers (Figure 3b), and exhibit almost no light transmission when parallel or perpendicular to the polarizers (Figure S7). This demonstrates that clean parallel alignment of the COL LC phase can be achieved by application of in-plane E fields.

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COMMUNICATION Figure 2. a) Texture observed for the Colh phase of mesogen 1 at 80 °C (the red scale bar represents 100 μm). b) FFTEM image of a Pt-C replica showing the topography of mesogen 1 quenched from the Colh phase (at T = 80 °C) and fractured in the bulk; this is magnified in (c). d) Simplified model of the structure observed in (c). e) Synchrotron small-angle scattering observed for an unoriented powder sample of 1, and, f) an azimuthal scan of the wide angle scattering obtained from a 2D diffraction pattern for a powder sample of 1 at 84 °C. The original 2D wide-angle data are given in (g).

Based upon the observed switching from parallel to homeotropic alignment upon application of an electric field normal to the surfaces, and lack of any suggestion that the phase is ferroelectric (polarization reversal current cannot be detected (Figure S9)), we propose that the electro-optic switching is dielectric in nature, and the COL phase of discotic mesogen 1 exhibits positive low frequency dielectric anisotropy (Δε) (the switching speed was too slow to allow reliable measurement of the switching time vs. applied field strength). While positive Δε is well-known for COL phases, typically ester groups connect flexible chains to a rigid aromatic core in such mesogens, and re-orientation of the ester dipoles in the field is thought to be responsible for the large dielectric constant parallel to the columnar axis.[20] The COL phase formed from a novel discotic mesogen with a helicene core, and designed specifically to produce a large dipole normal to the disc, also shows positive dielectric anisotropy.[11b] For the mesogen 1, we propose that dipolar reorientation in the triazole-oligothiophene chains is responsible for the observed positive Δε. This strongly suggests that, on average, the dipoles of the thiophene units and triazole in each chain are predominantly oriented normal to the long axis of the pentacyclic aromatic chain since the oligomers are mainly oriented parallel to the HBC cores, as inferred from XRD.

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Understanding the degree of nanophase segregation of the HBC, aliphatic, and heterocyclic aromatic units within the columns is of special interest. XRD is consistent with some nanophase segregation of the pentacyclic aromatic units, since a broad but distinguishable wide-angle scattering peak for the oligothiophene units can be seen. However, the diffuse scattering at 4.47 Å does not prove the existence of well-defined sub-columns. In summary, we have prepared and provided basic characterization of a novel mesogen possessing an HBC core and six alkyl-triazole-quadra-3-hexylthiophene tails. The synthesis is efficient: the key step being the click coupling of the azideterminated HBC with 1-alkynyl-quadra-3-hexylthiophene. Even possessing the sterically demanding and complex 5-ring heterocyclic aromatic chains at the ends of the tails, HBC derivative 1 self-assembles into a hexagonal columnar liquid crystalline phase stable over a broad enantiotropic temperature range, and supercools below room temperature. The alignment of the mesogen can be easily controlled by application of electric fields, and is maintained after removal of the field. This switching is dielectric in nature, and the phase possesses positive dielectric anisotropy, making extremely clean homeotropic alignment in ITO-glass cells easy to obtain. To our knowledge, this is the first HBC-based liquid crystalline material, which

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements We thank the NSF SOLAR Collaborative (award no. CHE-1125935), and the Liquid Crystal Materials Research Center (NSF MRSEC award no. DMR-0820579), for financial support of this work. NH thanks Dr. ChengKang Mai for helpful discussions and editing. We also acknowledge Prof. Sean Shaheen for important contributions to this project in the planning stages. Received: August 30, 2013 Revised: November 1, 2013 Published online: December 19, 2013

Figure 3. a and b) POM images (the red scale bar represents 100 μm) obtained for LC cells of mesogen 1 at 80 °C between crossed polarizer and analyzer. Mesogen 1 was filled by capillary action into a sandwichtype glass LC cell with patterned ITO electrodes (the cell gap is 4 μm). a) An AC square wave electric field (2.0 Hz, ±28 V μm−1) was applied to the sample at 80 °C. The photomicrograph shows the edge of the electrode area. Domains of unoriented parallel alignment can be seen on the left, off the electrodes with no applied field, and on the right homeotropic alignment over the ITO-coated area. b) A 2.0 Hz AC square wave in plane field (±15 V μm−1) was applied to the sample to provide clean parallel alignment. One substrate of the cell was patterned with ITO electrodes with a 10 μm pitch, to provide the in-plane field in stripes between the electrodes (all adjacent electrodes are driven with opposite sign of E. The field was applied while cooling the sample from Iso to Colh. The insert shows a detail of the aligned sample, showing bright stripes of parallelaligned Colh phase. c,d) 2D-SAXS patterns obtained from an area off the ITO electrodes (c) and over the ITO electrodes (d) from an LC cell (cell gap 6.4 μm) fabricated using thin (80 μm) glass plates each coated with an ITO square. The sample was heated to 80 °C and driven by a vertical E field (±20 V μm−1) 10 d before the XRD data were collected. The homeotropically aligned sample (d) shows point-shaped reflections arranged in a hexagonal lattice. The lattice is indicated by lines added to the image after data collection to guide the eye. e and f) Schematic illustration of the different types of supramolecular arrangements in (c) and (d) with the incident X-ray beam (indicated by red arrow).

can be aligned over large areas by application of an electric field. Investigation of the new material for device applications is under way.

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A novel hexa-peri-hexabenzocoronene/oligothiophene hybrid is shown to self-assemble into a hexagonal columnar liquid crystalline phase, and respond to...
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