HETEROCYCLES

Short Paper | Regular issue | Vol. 85, No. 12, 2012, pp. 3021-3028
Received, 20th September, 2012, Accepted, 24th October, 2012, Published online, 25th October, 2012.
DOI: 10.3987/COM-12-12590

■ Synthesis and Mesomorphism of the Bipedal Liquid Crystals with a Tetrathiafulvalene/dithiole and Two Cholesterol Moieties

Shan Jiang, Han Wang, Ruibin Hou, Keli Zhong,* and Bingzhu Yin*

Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules, Ministry of Education, Yanbian University, Yanji 133002, China

Abstract

The bipedal 1,3-dithiole-2-thiones and the appropriate EDT-TTFs bearing two cholesteryl linked with two ω-thioalkanoyloxy spacers of varying length were synthesized. All of bipedals showed mesogenic phases in a wide temperature region, no crystallization but vitrifying to form glassy mesogens during cooling from the isotropic melt.

Liquid crystals (LCs),1 possessing nanometre-scale molecular architectures and unique chemical and physical properties, have attracted considerable attention of late on account of their applications in optical memory,2 display devices,3 holographic data storage devices,4 and so on. Tetrathiafulvalene (TTF) derivatives have played a pivotal role in the development of organic materials for optoelectronic application due to their excellent electron-donating properties.5 The transport properties of TTF-based organic materials are clearly dependent on the molecular architecture in the solid state and so a wide variety of substituents have been introduced at the periphery of the TTF core in order to achieve a suitable solid-state organization.6 Concerning tetrathiafulvalene derivatives, the most relevant results have been reached via two well-established techniques, namely, electrocrystallization and deposition of Langmuir-Blodgett films. In this respect, another effective approach is based on the preparation of mesogenic compounds. In particular, glassy liquid crystals (GLCs) hold a fast and good orientation, which can be readily processed into macroscopically ordered solid films.7 Although a considerable number of TTF derivatives have so far been synthesized, there are only a few reports describing TTF derivatives with mesomorphic properties8 and consequently, it is not yet possible to establish structure-property relationships for this family of compounds. Very recently, we reported that the polypedals derived from TTF and cholesterol exhibited smectic and /or hexagonal columnar phases, which depended on the length of spacers.9 In order to create new TTF-based mesomorphic states, a series of cholesteryl with various alkyl chain lengths was introduced onto one side of the EDT-TTF core, which has exhibited a good conductive property. Here, we report the synthesis and mesomorphic properties of the dithiole and/or EDT-TTF based unsymmetrical bipedals 1a-1f and 2a-2f bearing two cholesteryl linked with two ω-thioalkanoyloxy spacers, together with the electron donating property of TTF central core. The structures and synthetic route of the bipedals were shown in Scheme 1. The starting 1,3-dithiole-2-ones (1a-1f) bearing two cholesteryl linked with two ω-thioalkanoyloxy spacers of varying length were easily transformed to the appropriate EDT-TTF based bipedals (2a-2f) by cross-coupling reaction with 4,5-ethylenedithio-1,3-dithiole-2-thione (EDDT) in net triethyl phosphate at 120 ºC in reasonable yields (48–56%).

The phase sequences and phase structures of bipedals 1a-1f and 2a-2f were in vestigated by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and small-angle X-ray scatterings (SAXS). All of the synthesized bipedals exhibit mesophases in a wide temperature range including room temperature and no crystallization but vitrifying to form glassy smectic mesogen during cooling from the isotropic melt. In the case of dithiole-based bipetals, 1a-1c exhibited only one mesophase in a wide temperature region in heating as well as in cooling cycle (Table 1). For example, 1c (n=4) exhibited a liquid crystalline phase at a melting state (20.4 ºC), which was transformed to an isotropic phase at 93.3 ºC. On slow cooling of 1c from the isotropic liquid to liquid crystalline phase at 89.8 ºC, a fan-shaped texture was observed by POM experiment, which was transformed to glassy state at ca. 18.0 ºC during cooling (Figure 1a and Figure 2a). The SAXS of 1c measured at cooling to 60 ºC displayed three sharp reflections with d spacings of 4.90, 2.40, and 1.60 nm, which were in the ratio of 3:2:1 and agreed well with (100), (200), and (300) reflections of a lamellar packing structure (Figure 1b). Considering that the layer thickness (4.90 nm) obtained from the X-ray diffraction pattern is much larger than the estimated molecular length (2.93 nm by CPK model) and the S…S interaction between dithiole, bimolecule arrangement in lamellar structure is expected, in which dithiole segments interdigitated to fill the space (Figure 3a). In contrast, 1d-1f with the longer spacer (n=5-7) showed two phase transitions after

melting of the rod block, while they display a lack of layered smectic phase. For example, 1f with the longest spacer (n=7) displays two mesophases at 5.6 °C and 71.4 °C, followed by transformation to an isotropic phase at 88.9 °C (Figure 1a). Upon cooling from the isotropic liquid, a pseudo focal-conic texture was observed by POM experiment (Figure 2c). On further cooling to 69 °C, the pseudo

focal-conic texture was gradually transformed to a fan-like texture and the systems induce vitrification rather than crystallization during cooling (Table 1 and Figure 2c). The DSC curves together with optical textures preliminarily considered to be the oblique hexagonal columnar phase and disordered hexagonal columnar phase, respectively. To identify the detailed phase structure, SAXS studies were performed. The X-ray diffraction pattern of 1f measured at cooling to 50°C displays three sharp reflections corresponding to spacing of 4.74, 2.85, and 1.85 nm in the small-angle region which could be indexed as (100), (010), and (310) planes of a 2-D oblique hexagonal columnar structure (Colho) with lattice parameters a =5.57 nm, b = 3.35 nm, and γ = 58.3° (Figure 1b). Wide-angle X-ray scattering (WAXS) data of molecule 1f shows only a broad halo centered at approximately 0.50 nm. On the basis of the lattice constant, estimated molecular length (3.28 nm by CPK model) and the experimental values of the unit cell parameters (a, b, c, and γ) and the density (ρ),11 the average numbers of molecule per cross-sectional slice of the column is calculated to be about 4, and the schematic representation of the oblipue columnar structure of 1f can be illustrated as shown in Figure 3b. The X-ray diffractogram of 1f taken at 80 °C shows two similar 100 and 010 peaks, but the intensity of the peaks was obviously lower and the peaks were diffused. The weaker diffraction and wider full width at half maximum (FWHM) measured at 80 °C are indicative of poorer long-range ordering of molecular stacking than that measured at 50 °C. Moreover, a very diffused peak emerged in 120 plane at the cost of disappearance of the peak of 310 plane, supporting the presence of a disordered hexagonal columnar phase with lattice parameters a =5.24 nm, b = 3.26 nm, and γ = 58.3° (Figure 3c). Considering the similarity of the phase sequences and optical textures with those of 1f, 1d and 1e could also be expected to be the same phase structures. Unlike the TTF-based bipedals,9 the EDT-TTF- based bipetals 2a-2f exhibited only one mesophase (Table 1). In contrast the incorporation of the ethylenedithio unit (EDT) in TTF suppresses the columnar mesophase exhibited by its TTF-based analogue probably due to the increased rigid volume fraction.9 For example, compound 2f exhibits a birefringent liquid crystalline phase at a melting state (32 °C), followed by transformation to an isotropic phase at 72.6 °C (Figure 1a). On slow cooling of the isotropic liquid to 70.2 °C, a sanded texture was observed by POM experiment (Figure 2d), which also vitrifyied to from glassy mesogens with an above-ambient glass transition temperature (Tg) during cooling process upon further cooling. The SAXS of 2f measured at cooling to 60 °C displayed three sharp reflections corresponding to spacing of 6.13, 3.10, and 2.02 nm, which were in the ratio of 3:2:1, and agreed well with (100), (200), and (300) reflections of lamellar packing structure (Figure 1b). The obtained molecular length of 2f was 6.13 nm, which was close to the two molecular length (3.73 nm by CPK model), indicative of a bimolecule layer structure in which EDT-TTF segments interdigitated to fill the space (cf. Figure 3a). Considering the similarity of the optical textures with those of 2f, 2a-2e could also be indexed to lamellar arrangement.

To evaluate electron donating property of EDT-TTF based bipedal 2a-2f, the cyclic voltammetry (CV) measurements were performed in a dry CH2Cl2 solution with a scan rate of 100 mV s-1 at room temperature. The bipedal 2a-2f show two reversible oxidation peaks at ~0.49 and ~0.95 V, corresponding to the formation of radical cations and dications of TTF core, respectively, indicating two sequential reversible one-electron transfer steps (Figure 4a and Table 1). The oxidation potentials of 2a-2f are higher than those of DMTEDT-TTF10 probably due to the existence of electron withdrawing ester linkages in the distance. Therefore, as the methylene spacer lengthened (from n=2 to n=7), the halfwave potential was anodic shifted in a regular manner.

To investigate the potential of these structures to act as a conducting architecture, chemical oxidation of 2f in a CH2Cl2 solution was conducted with stepwise addition of I2. Upon stepwise addition of I2 (0.5→5.0 equiv) to a CH2Cl2 solution of 2f, two new CT absorption bands appeared at 488 and >800 nm (Figure 4b). The absorptions at 488 nm was assigned to an intramolecular electron transfer of radical cation, 2f+•, while the absorption band at >800 nm was due to a charge transfer arised from 2f to I2.12 All these are in agreement with the results obtained from the CV experiments.
In summary, two series of bipedal liquid crystals based on dithiole and/or EDT-TTF bearing two cholesteryl side chains linked with two ω-thioalkanoyloxy spacers was synthesized. The bipedal 1a-1c and 2a-2f exhibit smectic A phase while the dithiole-based 1d-1f with the longer spacer (n=5-7) exhibit the oblique columnar phase in a wide temperature range including room temperature and no crystallization but vitrifying to form glassy mesogen during cooling from the isotropic melt. These liquid crystals, in combination with the glassy mesogen and excellent electron-donating properties of TTF, may provide new opportunities in the development of soft materials.

EXPERIMENTAL
NMR spectra were recorded in CDCl3 with a Bruker AV-300 Spectrometer and chemical shifts were referenced relative to tetramethylsilane (δH/δC=0). Mass spectra was performed on a Shimadzu Axima CFRTM Plus using a 1,8,9-anthracenetriol (DITH) matrix. IR spectra were recorded on a Shimadzu FT-IR Prestige-21 instrument (KBr pressed disc method). UV-vis spectra were recorded on a Hitachi U-3010 spectrophotometer in CH2Cl2. Cyclic voltammetric studies were carried out on a Potentiostat/ Galvanostat 273A instrument in CH2Cl2 and 0.1 M Bu4PF6 as the supporting electrolyte. Counter and Working electrodes were made of Pt and Glass-Carbon, respectively, and the reference electrode was SCE. A Perkin-Elmer Pyris Diamond differential scanning calorimeter was used to determine the thermal transitions, the heating and cooling rates were controlled to 10 °C/min. An Olympus BX51-P optical polarized microscope (40×) was used to observe the thermal transitions and to analyze the anisotropic texture. X-Ray scattering measurements were performed in transmission mode with synchrotron radiation at the 3C2 X-ray beam line at Pohang Accelerator Laboratory (Korea) and Philips PW 1700 X-ray diffractometer. Starting compounds 1a-1f were synthesized according to the previous literature method.9
Typical synthetic procedure for 2
1 (0.5 mmol) and 4,5-ethylenedithio-1,3-dithiole-2-thione (1 mmol) was added to P(OEt)3 (5 mL) and the suspension was heated to 120 °C, causing dissolution within 1 min, leaving a yellow reaction mixture. The mixture was stirred for 5 h at 120 °C, cooled to room temperature. Addition of MeOH gave a yellow solid, which was filtered, washed with MeOH, and dried in vacuo. The products were purified by column chromatography (silica gel, CH2Cl2: Pet. ether =1:1; v/v) to give a yellow solid. The solid was recrystallized from THF/CH3COCH3 to give 2.
2a. Yellow solid (Yield: 60%). 1H NMR (300 MHz, CDCl3): δ 0.68-2.11 (m, 82H, aliphatic and cholesteric protons are overlapped), 2.33 (d, J = 7.4 Hz, 4H), 2.66 (t, J = 7.2 Hz, 4H), 3.05 (t, J = 7.4 Hz, 4H), 3.74 (s, 4H), 4.62-4.65 (m, 2H), 5.38 (brs, 2H); MS (EI): m/z (%) 1239 (M+, 100); FT-IR (KBr, cm-1): 2943 (C-H), 1730 (C=O), 1179 (C-O); Anal. Calcd for C68H102O4S8: C, 65.86; H, 8.29. Found: C, 66.01; H, 8.30.
2b. Yellow solid (Yield: 71%). 1H NMR (300 MHz, CDCl3): δ 0.68-2.11 (m, 86H, aliphatic and cholesteric protons are overlapped), 2.32 (d, J = 7.5 Hz, 4H), 2.45 (t, J = 7.2 Hz, 4H), 2.88 (t, J = 7.4 Hz, 4H), 3.29 (s, 4H), 4.61 (m, 2H), 5.38 (brs, 2H); MS (EI): m/z (%) 1267 (M+, 100); FT-IR (KBr, cm-1): 2945 (C-H), 1734 (C=O), 1175 (C-O); Anal. Calcd for C70H106O4S8: C, 66.30; H, 8.43. Found: C, 66.47; H, 8.31.
2c. Yellow solid (Yield: 42%). 1H NMR (300 MHz, CDCl3): δ 0.69-2.31 (m, 98H, aliphatic and cholesteric protons are overlapped), 2.83 (t, J = 7.0 Hz, 4H), 3.29 (m, 4H), 4.62 (m, 2H), 5.38 (brs, 2H); MS (EI): m/z (%) = 1295 (M+, 100); FT-IR (KBr, cm-1): 2942 (C-H), 1732 (C=O), 1171 (C-O). Anal. Calcd for C72H110O4S6: C, 66.72; H, 8.55. Found: C, 66.51; H, 8.59.
2d. Yellow solid (Yield: 42%). 1H NMR (300 MHz, CDCl3): δ 0.68-2.10 (m, 94H, aliphatic and cholesteric protons are overlapped), 2.25-2.38 (m, 8H), 2.89 (t, J = 7.2Hz, 4H), 3.30 (s, 4H), 4.61 (m, 2H), 5.38 (brs, 2H); MS (EI): m/z (%) = 1323 (M+, 100). FT-IR (KBr, cm-1): 2932 (C-H), 1734 (C=O), 1169 (C-O). Anal. Calcd for C74H114O4S8: C, 67.12; H, 8.68. Found: C, 66.99; H, 8.73.
2e. Yellow solid (Yield: 47%). 1H NMR (300 MHz, CDCl3): δ 0.67-2.32 (m, 106H, aliphatic and cholesteric protons are overlapped), 2.79 (t, J = 7.0 Hz, 4H), 3.29 (s, 4H), 4.59-4.62 (m, 2H), 5.37 (brs, 2H); MS (EI): m/z (%) = 1351 (M+, 100). FT-IR (KBr, cm-1): 2934 (C-H), 1732 (C=O), 1171 (C-O). Anal. Calcd for C76H118O4S6: C, 67.50; H, 8.80. Found: C, 67.71; H, 8.43.
2f. Yellow solid (Yield: 39%). 1H NMR (300 MHz, CDCl3): δ 0.68-2.12 (m, 102H, aliphatic and cholesteric protons are overlapped), 2.28-2.40 (m, 8H), 2.81 (t, J = 7.2 Hz, 4H), 3.29 (s, 4H), 4.62 (brs, 2H), 5.37 (brs, 2H); MS (EI): m/z (%) = 1380 (M++H, 100). FT-IR (KBr, cm-1): 2932 (C-H), 1732 (C=O), 1171 (C-O). Anal. Calcd for C78H122O4S6: C, 67.87; H, 8.91. Found: C, 67.81; H, 9.13.

ACKNOWLEDGEMENTS
This work was supported by the National Science Foundation of China (21062022, 21262039) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20102201110001).

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