This article was downloaded by:[University of Central Florida] [University of Central Florida] On: 28 May 2007 Access Details: [subscription number 769428830] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Liquid Crystals Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713926090 Synthesis and mesomorphic properties of super high birefringence isothiocyanato bistolane liquid crystals To cite this Article: Liao, Yung-Ming, Chen, Hsin-Lan, Hsu, Chain-Shu, Gauza, Sebastian and Wu, Shin-Tson , 'Synthesis and mesomorphic properties of super high birefringence isothiocyanato bistolane liquid crystals', Liquid Crystals, 34:4, 507 - 517 To link to this article: DOI: 10.1080/02678290701223954 URL: http://dx.doi.org/10.1080/02678290701223954 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. 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The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. © Taylor and Francis 2007 Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 Liquid Crystals, Vol. 34, No. 4, April 2007, 507–517 Synthesis and mesomorphic properties of super high birefringence isothiocyanato bistolane liquid crystals YUNG-MING LIAO{, HSIN-LAN CHEN{, CHAIN-SHU HSU*{, SEBASTIAN GAUZA{ and SHIN-TSON WU{ {Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC {College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA (Received 12 June 2006; accepted 27 November 2006 ) Four series of high birefringence bistolane liquid crystals containing isothiocyanato termal groups were synthesized and characterized. As well as the phenyl group, both biphenyl and naphthyl moieties were introduced to enhance the birefringence. These bistolane compounds exhibit reasonably low melting points and high birefringence of 0.5–0.8. A eutectic mixture was formulated from these compounds exhibiting a wide nematic range, high figure-of-merit and low viscosity. 1. Introduction High birefringence (Dn) liquid crystals (LCs) are useful not only in conventional display devices such as STNLCDs, but also in scattering-type PDLCDs as a reflective LCD, and in spatial light modulators. They are also of interest as componens of LCDs; for example, compensation films for improving the viewing angle, reflectors and polarizers. A number of LCs have been studied for these applications [1–3]. It is well known that high Dn values can be achieved by increasing the molecular conjugation length [2]; a considerable number of p-conjugated compounds have been developed as high Dn LCs [4]. Molecules that contain highly polarizable groups with high electron densisity, such as benzene rings or acetylene linking groups, will therefore have large optical anisotropies. Tolane-based LCs exhibit reasonably high Dn, low viscosity and good chemical, photo and thermal stability [5]. Bistolane LCs show birefringences greater than 0.3 in the visible spectrum and as a result have attracted particular attention [6–16]. The same properties are also found for isothiocyanates (NCS) [17]. Therefore, coupling of the tolane and NCS groups could lead to high optical and large dielectric anisotropies while preserving a relatively low viscosity [18]. Several molecular structures with high Dn values, e.g. diphenyldiacetylene [19, 20], naphthalene tolanes [21] and thiophenyldiacetylene [22–24] have been widely studied. The Dn values of these compounds were *Corresponding author. Email: cshsu@mail.nctu.edu.tw reported in the range of 0.4–0.6; however, the diacetylene compounds have in adequate UV and thermal stabilities, and thus their application is limited [25]. In this paper, we report the synthesis procedures and physical properties of highly birefringent bistolane LCs having a terminal isothiocyanato group. Phenyl, biphenyl and naphthyl moieties linked by an ethynyl unit were applied as the core structure. Lateral methyl substitution in the middle phenyl ring significantly reduces the melting temperatures of bistolane LCs [14]. By introducing different laterally substituted short alkyl chains and fluorine at various positions, the synthesized compounds were characterized with low melting point, relatively low viscosity, and high optical anisotropy, ready for immediate practical applications. 2. 2.1. Experimental Characterization techniques 1 H NMR spectra were measured with a Varian 300 MHz spectrometer. Infrared spectra were obtained using a Perkin-Elmer Spectrum One spectrophotometer in the range of 400–4000 cm21. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer Pyris Diamond DSC instrument at a heating rate of 10uC min21. A Carl-Zeiss Axiophot polarizing microscope equipped with a Mettler FP 82 hot stage and a Mettler FP 80 central processor was used to observe the mesomorphic textures. The molecular mass and elemental analysis results were obtained from a T-200 GC/ MS spectrometer and Heraeus CHN-OS RAPID instrument, respectively. Liquid Crystals ISSN 0267-8292 print/ISSN 1366-5855 online # 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/02678290701223954 Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 508 Y.-M. Liao et al. For electro-optic measurements, homogenous cells with cell gap d,8 mm were used. An a.c. voltage with 1 kHz square waves was used to drive the LC cell, whose inner surfaces were coated with indium tin oxide (ITO) electrodes. On top of the ITO, the substrate was covered with a thin polyimide alignment film. The cell was placed on a heating/cooling stage with a temperature stability of 0.2uC. Commercial LCs ZLI-1565 and E-63 (from Merck) were used as the host. A conventional guest–host method was applied to extrapolate the Dn value at T,23.5uC. 2.2. Synthesis The compounds 1–5, 14–18, 29–30 and 39–40 were prepared by reported methods [14, 26]. As mentioned above, a large variety of high birefringence compound Scheme 1. structures has been investigated. However, in all cases the synthesis of the final materials is greatly facilitated by the use of a palladium-catalysed cross-coupling reaction [27– 33]; in the synthesis of some materials this synthesis procedure is virtually essential. Scheme 1 shows the synthesis of several isothiocyanato-substituted bistolanes containing alkyl groups; scheme 2 shows the synthesis of analogues containing alkoxy groups; scheme 3 shows the synthesis of similar compounds containing the biphenyl moiety; and scheme 4 shows the introduction of the naphthyl moiety. Despite the development of the palladium-catalysed cross-coupling reaction to high levels, their exceptional versatility and their extreme tolerance to a wide range of functional groups, attempted couplings involving isothiocyanato substituents resulted in failure [34]. The Synthesis of compounds 24–28. 509 Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 Super high birefringence LCs Scheme 2. Synthesis of compounds 36–38. use of thiophosgene and chloroform on an aromatic amine in the presence of aqueous calcium carbonate is a very useful and efficient method of introducing the isothiocyanato group [34, 35] and this method has been used in this research. 2.2.1. 1-(4-Pentylphenyl)-2-(4-amino-3-methylphenyl)acetylene, 9. Compound 4 (2.74 g, 10 mmol), Pd(PPh3)2Cl2 (0.14 g, 0.2 mmol), triphenylphosphine (0.2 g, 0.8 mmol), copper(I) iodide (0.08 g, 0.4 mmol) and dry triethylamine (150 ml) were mixed and stirred at room temperature for 30 min under nitrogen. A solution of compound 5 (1.44 g, 11 mmol) dissolved in 75 ml of triethylamime was added dropwise and the mixture stirred at 70uC for 24 h. After cooling to room temperature, the mixture was filtered and filtrate concentrated in vacuo to remove triethylamine. The crude product was dissolved in diethyl ether and extracted with aqueous ammonium chloride solution. The organic phase was then washed with saturated aqueous sodium chloride and dried over MgSO4. The crude product was isolated by evaporating the solvent, and purified by column chromatography using ethyl acetate/n-hexane51/4 as eluant to give a deep yellow solid; yield 2.0 g (72%). 1H NMR (d, CDCl3): 0.87–0.92 (t, 3H), 1.25–1.34 (m, 4H), 1.56–1.63 (m, 2H), 2.10 (s, 3H), 2.58–2.61 (t, 2H), 3.71 (s, 2H), 6.72–7.42 (m, 7H). IR (KBr) nmax (cm21): 3392, 3180, 2960, 2923, 2860, 2220, 1640, 1462, 1380, 1247, 1070, 823. MS m/z 402 (M+), 345. 2.2.2. 1-(4-Pentylphenyl)-2-(4-iodo-3-methylphenyl)acetylene, 13. Compound 9 (1.95 g, 7 mmol) was dissolved in THF (10 ml) and cooled to 0uC in an ice Y.-M. Liao et al. Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 510 Scheme 3. Synthesis of compounds 47–50. bath. Aqueous HCl (0.8 g, 21.0 mmol) was added and the mixture stirred for 30 min. Sodium nitrite (0.63 g, 9.1 mmol) dissolved in 5 ml of water was slowly added to the reaction mixture which was stirred for a further 30 min. After checking that the pH of the reaction mixture was acidic, potassium iodide (1.71 g, 10.5 mmol) dissolved in 8 ml of water was added, maintaining a temperature of 0–5uC. After stirring for 3 h at 0–5uC the reaction mixture was slowly heated to 45uC and held for 5 min. It was then cooled, treated with aqueous sodium thiosulphate solution and extracted with n-hexane. The organic phase was washed with saturated aqueous sodium chloride and dried over MgSO4. The crude product was isolated by evaporating the solvent and purified by column chromatography using ethyl acetate/nhexane51/10 as eluent to give a yellow liquid; yield 2.15 g (79%). 1H NMR (d, CDCl 3): 0.89–0.93 (t, 3H), 1.25–1.38 (m, 4H), 1.58–1.63 (m, 2H), 2.43 (s, 3H), 2.57–2.62 (t, 2H), 6.96–7.60 (m, 7H). IR (KBr) nmax (cm21): 2980, 2935, 2870, 2200, 1640, 1450, 1366, 1254, 1113, 843. MS m/z 277 (M+), 220. 2.2.3. 2-Methyl-{4-[2-(4-pentylphenyl)-1-ethynyl]-1-ethyny}-3-fluoroaminobistolane, 22. By following the procedure for compound 9 and using 13 and 15 as starting materials, compound 22 was obtained as a deep yellow solid; yield 1.18 g (60%). 1H NMR (d, CDCl3): 0.89–0.92 (t, 3H), 1.24–1.32 (m, 4H), 1.58–1.63 (m, 2H), 2.46 (s, 3H), 2.56–2.62 (t, 2H), 3.68 (s, 2H), 6.52–7.42 (m, 10H). IR (KBr) nmax (cm21): 3376, 3120, 2930, 2903, 2880, 2210, 1650, 1460, 1377, 1240, 1170, 833. 2.2.4. 2-Methyl-{4-[2-(4-pentylphenyl)-1-ethynyl]-1-ethyny}3-fluoroisothiocyanatobistolane, 27. Compound 22 (1.1 g, 2.5 mmol) was dissolved in 15 ml of chloroform 511 Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 Super high birefringence LCs Scheme 4. Synthesis of compounds 71–78. and added to a stirred, cooled (0uC) mixture of water (10 ml) calcium carbonate (0.39 g, 3.8 mmol), chloroform (8 ml) and thiophosgene (0.38 g, 3.3 mmol); stirring at 0uC was continued for 4 h. The stirred mixture was allowed to come to room temperature, heated to 45uC and held for 20 min, then poured into water. The aqueous layer was washed with dichloromethane; the combined organic extracts were washed with 1% aqueous HCl, then washed with water, brine and dried (MgSO4). The crude product was isolated by evaporating the solvent, then purified by column chromatography using ethyl acetate/nhexane51/10 as eluent to give a white solid; yield 0.81 g (80%). 1H NMR (d, CDCl3): 0.86–0.91 (t, 3H), 1.25–1.32 (m, 4H), 1.59–1.63 (m, 2H), 2.47 (s, 3H), 2.58–2.63 (t, 3H), 7.11–7.44 (m, 10H). 13C NMR (d, CDCl3): 14.0, 20.5, 22.5, 30.9, 31.4, 35.9, 88.5, 90.9, 91.6, 92.7, 119.0, 119.2, 120.1, 120.3, 121.8, 123.4, 123.5, 124.0, 126.3, 128.0, 128.5, 128.8, 131.5, 131.9, 132.5, 140.3, 143.7, 156.4, 159.8. IR (KBr) nmax (cm21): 2950, 2926, 2855, 2212, 2046, 1659, 1510, 1411, 1271, 1035, 835. MS m/z 437 (M+), 380. Elemental analysis: calc. for C29H24FNS, C 79.60, H 5.53, N 3.20; found, C 79.72, H 5.46, N 3.23%. 2.2.5. 1-(4-Pentylbiphenyl)-2-(4-amino-3-ethylphenyl)acetylene, 41. By following the procedure for compound 9 and using 39 and 40 as starting materials, compound 41 was obtained as a deep yellow solid; yield 2.6 g (70%). 1H NMR (d, CDCl3): 0.91–0.93 (m, 6H), 1.26– 1.37 (m, 4H), 1.63–1.65 (m, 2H), 2.62–2.68 (t, 2H), 2.84– 2.89 (q, 2H), 3.82 (s, 2H), 7.13–7.75 (m, 11H). IR (KBr) nmax (cm21): 3363, 3150, 2959, 2927, 2869, 2199, 1661, 1619, 1502, 1463, 1312, 1240, 1004, 891. 2.2.6. 1-(4-Pentylbiphenyl)-2-(4-iodo-3-ethylphenyl)acetylene, 42. By following the procedure for compound 13 and using 41 as starting material, compound 42 was obtained as a yellow solid; yield 1.57g (82%). 1H NMR (d, CDCl3): 0.87–0.91 (t, 3H), Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 512 Y.-M. Liao et al. 1.19–1.24 (t, 3H), 1.28–1.35 (m, 4H), 1.61–1.66 (m, 2H), 2.61–2.66 (t, 2H), 2.70–2.75 (q, 2H), 7.01–7.79 (m, 7H). IR (KBr) nmax (cm21): 2955, 2925, 2853, 2210, 1632, 1514, 1495, 1462, 1388, 1260, 1225, 1134, 887. 2.2.7. 2-Ethyl-{4-[2-(4-pentylbiphenyl)-1-ethynyl]-1-ethyny}-2methylaminobistolane, 46. By following the procedure for compound 9 and using 42 and 18 as starting materials, compound 46 was obtained as a yellow solid; yield 1.0 g (63%). 1H NMR (d, CDCl3): 0.88–0.93 (t, 3H), 1.25–1.37 (m, 7H), 1.63–1.68 (m, 2H), 2.17 (s, 3H), 2.62–2.67 (q, 2H), 2.84–2.92 (t, 2H), 3.79 (s, 2H), 7.01– 7.79 (m, 14H). IR (KBr) nmax (cm21): 3357, 2923, 2851, 2213, 1631, 1467, 1355, 1052, 892. MS m/z 481 (M+), 466. 2.2.8. 2-Ethyl-{4-[2-(4-pentylbiphenyl)-1-ethynyl]-1-ethyny}2-methylisothiocyanatobistolane, 50. By following the procedure for compound 27 and using 46 as starting material, compound 50 was obtained as a white solid; yield 0.71 g (81%). 1H NMR (d, CDCl3): 0.86–0.93 (t, 3H), 1.25–1.37 (m, 4H), 1.63–1.68 (m, 2H), 2.17 (s, 3H), 2.62–2.67 (t, 2H), 2.84–2.92 (q, 2H) 7.01–7.79 (m, 14H). 13 C NMR (d, CDCl3): 14.0, 14.6, 18.3, 22.5, 27.6, 31.1, 31.5, 35.6, 89.6, 89.9, 91.1, 93.6, 121.6, 121.9, 122.3, 123.6, 126.1, 126.8, 128.9, 130.0, 130.2, 131.1, 132.0, 132.1, 133.5, 135.2, 136.3, 137.5, 141.1, 142.7, 146.2. IR (KBr) nmax (cm21): 2964, 2926, 2196, 2070, 1659, 1463, 1408, 1261, 1094, 833. MS m/z 523 (M+), 508. Elemental analysis: calc. for C37H33NS, C 84.85, H 6.35, N 2.67; found, C 84.69, H 6.42, N 2.72%. 2.2.9. 2-Bromo-6-ethoxynaphthalene, 51. 6-Bromo-2naphthol (10.0 g, 44.82 mmol), bromoethane (6.35 g, 58.79 mmol), potassium carbonate (12.39 g, 89.64 mmol), potassium iodide (1.48 g, 8.91 mmol) and acetonitrile (80 ml) were mixed and heated under reflux with constant stirring for more than 16 h. The solvent was removed under vacuum and the reaction mixture poured into water. The product was extracted with ether (3640 ml) and treated with dilute hydrochloric acid, washed with water and dried. The crude product was purified by column chromatography using hexane as eluent to give a white solid 51; yield 11.2 g (99.5%). 1 H NMR (d, CDCl3): 1.20–1.70 (t, 3H), 3.80–4.30 (t, 2H), 7.05–7.88 (m, 6H). IR (KBr) nmax (cm21): 2963, 2938, 1265, 1064, 852. 2.2.10. 4-[2-(6-Ethoxy-2-naphthyl)-1-ethynyl]-2-ethylaniline, 55. By following the procedure for compound 9 and using 51 and 40 as starting materials, compound 55 was obtained as a brown solid; yield: 6.46 g (51.4%). 1H NMR d51.30–1.50 (t, 3H), 2.45–2.52 (q, 2H), 3.78 (s, 2H), 4.0–4.20 (q, 2H), 7.06–7.91 (m, 9H). IR (KBr) nmax (cm21): 3359, 3059, 2980, 2934, 2210, 1622, 1599, 1471, 1385, 1256, 1041, 858. 2.2.11. 2-[2-(3-Ethyl-4-iodophenyl)-1-ethynyl]-6-ethoxynaphthalene, 59. By following the procedure for compound 13 and using 55 as starting material, compound 59 was obtained as a yellow solid; yield 5.31 g (65.5%). 1H NMR d51.13–1.26 (t, 3H), 1.30–1.50 (t, 3H), 2.45–2.52(q, 2H), 4.00–4.20 (q, 2H), 7.07–7.91 (m, 9H). IR (KBr) nmax (cm21): 2920, 2197, 1626, 1480, 1390, 1258, 1017, 856. 2.2.12. 2-Ethyl-{4-[2-(6-ethoxy-2-naphthyl)-1-ethynyl]}49-amino-39-fluorobistolane, 67. By following the procedure for compound 9 and using 59 and 15 as starting materials, compound 67 was obtained as a brown solid; yield 3.04 g (59.8%). 1H NMR d51.23–1.32 (t, 3H), 1.44–1.50 (t, 3H), 2.81–2.89 (q, 2H), 3.89 (s, 2H), 4.10–4.17 (q, 2H), 7.09–7.95 (m, 12H). IR (KBr) nmax (cm21): 3337, 2962, 2205, 1630, 1471, 1384, 1254, 1041, 856. 2.2.13. 2-Ethyl-{4-[2-(6-ethoxy-2-naphthyl)-1-ethynyl]}39-fluoro-49-isothiocyanatobistolane, 75. By following the procedure for compound 27 and using 67 as starting material, compound 75 was obtained as a white solid; yield 2.51 g (76.3%). 1H NMR d51.23–1.32 (t, 3H), 1.44–1.50 (t, 3H), 2.81–2.89 (q, 2H), 4.10–4.17 (q, 2H), 7.05–7.95 (m, 12H). 13C NMR (d, CDCl3): 14.5, 14.8, 18.3, 27.6, 31.1, 63.5, 88.9, 90.7, 92.0, 92.3, 106.6, 118.9, 119.8, 120.5, 123.6, 124.2, 126.8, 128.0, 128.4, 128.8, 128.9, 129.3, 131.1, 131.4, 132.2, 134.3, 142.0, 146.4, 156.4, 158.0, 159.8. IR (KBr) nmax (cm21): 2923, 2195, 2093, 1622, 1475, 1390, 1256, 1040, 834. Elemental analysis: calc. for C37H33NS, C 78.29, H 4.84, N 2.95; found, C 78.27, H 4.66, N 2.95%. 3. 3.1. Results and discussion Thermal transitions and mesomorphic properties The chemical structure, phase transition temperatures, associated enthalpies, and optical anisotropy values for the reported novel compounds and some known materials for comparison are shown in tables 1–5. Table 2 summarizes the phase transitions of compounds 24–28. In all these bistolanes, a methyl lateral substituent was introduced at the cental phenyl ring to obtain lower melting point LCs. All these compounds exhibit an enantiotropic nematic phase. Compounds 24–27 contain the same terminal isothiocyanato group on the right-hand side and different alkyl chain length on the left. Their phase transitions are plotted against 513 Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 Super high birefringence LCs Table 1. Purity and elemental analysis data for some representative bistolanes. Elemental analysis, found (calc.)/% Compound 24 25 26 27 28 36 37 38 47 48 49 50 71 72 73 74 75 76 77 78 Purity/% C H N 99.0% 99.0% 98.6% 99.1% 98.7% 98.4% 98.8% 98.8% 99.0% 99.0% 98.8% 99.1% 99.0% 99.3% 99.3% 99.0% 99.4% 98.6% 99.0% 98.6% 78.96(79.17) 79.19(78.95) 79.40(79.77) 79.60(79.72) 83.02(82.71) 76.51(76.08) 73.50(73.37) 73.50(73.15) 84.83(85.04) 81.94(82.17) 84.87(84.47) 84.85(84.69) 81.37(80.93) 81.50(81.22) 81.62(81.90) 81.73(81.26) 78.29(78.27) 78.50(77.94) 78.70(78.29) 78.89(78.53) 4.59(4.40) 4.92(5.06) 5.24(4.96) 5.53(5.46) 6.01(6.22) 5.04(5.39) 4.63(5.02) 4.63(4.74) 6.13(6.01) 5.73(5.74) 6.56(6.50) 6.35(6.42) 5.07(5.06) 5.34(5.36) 5.60(5.43) 5.85(6.04) 4.84(4.66) 4.94(5.22) 5.20(5.07) 5.45(5.61) 3.54(3.52) 3.42(3.48) 3.31(3.18) 3.20(3.23) 3.34(3.25) 3.19(2.91) 3.06(2.96) 3.06(3.15) 2.75(2.81) 2.65(2.45) 2.60(2.76) 2.67(2.72) 3.06(2.99) 2.97(2.92) 2.88(2.81) 2.80(2.62) 2.95(2.95) 2.86(2.79) 2.78(2.77) 2.71(2.72) the carbon number of the left-hand alkyl chain in figure 1. Both melting and clearing temperatures decrease gradually with increasing carbon number. Compound 28 contains similar structures to those of compound 27, without an additional lateral fluoro group at the C-3 position of the right-hand phenyl ring. For comparison purposes, we synthesized compound 27 which contains a lateral fluoro group at the C-3 position of the phenyl ring. Its clearing point is lower than those of compound 28. It seems that the lateral group can hinder molecular packing and thus decrease transition temperatures. Table 2. Phase transition temperature (uC) and corresponding enthalpy changes (kcal mol21) in parentheses, for compounds 24–28. Cr5crystal, N5nematic, I5isotropic. Compound n X Y 24 2 H F 25 3 H F 26 4 H F 27 5 H F 28 5 H H Table 3 lists the phase transition temperatures of compounds 36–38. Their alkyl side chains were changed to the equivalent alkoxy groups. In all these bistolanes a fluoro lateral substituent was introduced at the central phenyl ring, and they all contain the same terminal iscthiocyanato group. Compound 36 exhibits enantiotropic nematic and smectic phases. The introduction of a lateral fluoro group at the C-3 position of the phenyl ring in compound 37 did not suppress the smectic phase by destroying the symmetry, in fact it enhanced the melting point to 150uC. The position of the fluorine atom was changed from C-3 to C-2 to provide a pure nematic phase in compound 38; as expected, a wide nematic range from 127 to 259uC was obtained. Table 3. Phase transition temperatures (uC) and corresponding enthalpy changes (kcal mol21) in parentheses, for compounds 36–38. Cr5crystal, Sm5smectic, N5nematic, I5isotropic. heating scan cooling scan Cr 162:8ð3:04Þ N 216:8ð0:03Þ I I 193:8ð0:02Þ N 103:8 ð2:26Þ Cr Cr 172:2ð3:67Þ N 217:9ð0:01Þ I I 196:2ð0:01Þ N 130:4 ð3:05Þ Cr Cr 130:1ð2:79Þ N 202:8ð0:05Þ I I 202:2ð0:04Þ N 71:1 ð2:05Þ Cr Cr 103:5ð6:30Þ N 188:3ð0:16Þ I I 186:7ð0:14Þ N 60:8 ð6:50Þ Cr Cr 103:1ð5:81Þ N 225:0ð0:22Þ I I 223:5ð0:24Þ N 79:7 ð4:59Þ Cr Compound X Y 36 H H 37 H F 38 F H heating scan cooling scan Cr1 96:0ð0:98Þ Cr2 126:2ð1:18Þ SmA 160:8ð0:26Þ N 231:6ð0:11Þ I I 229:2ð0:02Þ N 157:6ð0:14Þ SmA 123:1ð0:82Þ Cr2 60:8ð0:57Þ Cr1 Cr 150:9ð9:14Þ SmA 174:9ð0:20Þ N 231:6ð0:29Þ I I 245:9ð0:13Þ N 160:8ð0:18Þ SmA 117:5ð8:76Þ Cr Cr1 98:0ð2:05Þ Cr2 126:6ð2:01Þ N 259:2ð0:01Þ I I 252:8ð0:01Þ N 70:4ð1:24Þ Cr Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 514 Y.-M. Liao et al. Table 4. Phase transition temperatures (uC) and corresponding enthalpy changes (kcal mol 21) in parentheses, for Compounds 47–50. Cr5crystal, N5nematic, I5isotropic. heating scan cooling scan Compound X Y 47 H H 48 H F 49 H C2H5 50 CH3 H Cr 115:9ð10:08ÞN 241:3ð0:38Þ I I 234:8ð0:21Þ N 89:7ð15:44Þ Cr Cr 107:3ð10:82ÞN 241:4ð0:22Þ I I 229:5ð0:18Þ N 60:8ð9:01Þ Cr Cr1 44:0ð3:80ÞCr2 80:3ð8:90Þ N 225:6ð2:09Þ I I 224:2ð1:85Þ N 37:9ð4:57Þ Cr Cr 114:7ð42:90ÞN 240:9ð0:29Þ I I 229:4ð0:17Þ N 62:1ð35:03Þ Cr Table 4 lists the phase transition temperatures of isothiocyanato biphenyl-bistolanes 47–50. In this series a pentyl group was placed on the biphenyl ring, and different lateral groups were introduced on the righthand side containing an isothiocyanato phenyl ring. All the listed compounds exhibit a pure nematic phase within the whole mesomprphic region. Moreover, the smectic phases which normally appear in NCS-based biphenyl tolanes or tolanes are completely suppressed. Such an unusual behaviour is believed to be due to the laterally substituted ethyl chain(s) and/or the single fluorine atom. The lateral substitutions increase molecular separation and break the molecular symmetry so that formation of the smectic phase is inhibited. Table 5. Phase transition temperatures (uC) and corresponding enthalpy changes (kcal mol21) in parentheses, for compounds 71–78. Cr5crystal, N5nematic, I5isotropic. Compound n X Y 71 2 H H 72 3 H H 73 4 H H 74 5 H H 75 2 H F 76 3 H F 77 4 H F 78 5 H F a Overlapped transition. Figure 1. Dependence of transition temperatures on the number of alkyl chain carbon atoms for compounds 24–27. Compound 47 shows the highest transition temperature because of its right-hand side phenyl ring without lateral substitutents. In compound 48 a fluoro lateral substituent was introduced at the C-3 position of the righthand side phenyl ring, reducing the melting point from 115.9 to 108uC. In compound 49 an ethyl group was introduced at the C-3 position of the right-hand side phenyl ring, and both melting and clearing points were reduced. In compound 50 a methyl lateral substituent was introduced at the C-2 position of the right-hand side phenyl ring; the resulting transition temperature was similar to that of compound 47. Table 5 summarizes the phase transition temperatures of isothiocyanato naphthyl-bistolanes 71–78. In this series a pentyl group was placed on the biphenyl ring, and different lateral groups were introduced on the right-hand side containing an isothiocyanato phenyl ring. Compounds 71–74 contain the same terminal isothiocyanato group on the right-hand side phenyl ring and alkoxy chains of varying length on the naphthyl ring. Their phase transitions are plotted against the carbon number of the alkyl chain in heating scan cooling scan Cr 154:1ð2:85Þ N 278:1ð0:01Þ I I 274:9ð{Þ N 114:5ð0:71Þ Cr Cr 137:9ð4:25Þ N 279:5ð0:2Þ I I 274:9ð{Þ N 137:1ð{Þ Cr Cr 157:0ð1:96Þ N 266:1ð{Þ I I 247:7ð{Þ N 104:0ð{Þ Cr Cr 119:9ð2:0Þ N 187:2ð{Þ I I 160:8ð{Þ N 60:5ð{Þ Cr Cr 153:9ð8:66Þ N 283:6ð0:36Þ I I 282:8ð{Þ N 144:6ð5:94Þ Cr Cr 144:4ð2:78Þ N 277:7ð{Þ I I 268:4ð{Þ N 125:8ð2:73Þ Cr Cr 135:3ð2:32Þ N 259:0ð{Þ I I 255:3ð{Þ N 78:9ð0:26Þ Cr Cr 122:3ð3:32Þ N 237:3ð0:13Þ I I 235:0ð{Þ N 72:8ð2:01Þ Cr Figure 2. Dependence of transition temperatures on the number of alkyl chain carbon atoms for compounds 71–74. 515 Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 Super high birefringence LCs 75–78 contain similar structures to those of compounds 71–74, with an additional lateral fluoro group at the C-3 position of the right-hand side phenyl ring. Both melting and clearing points decrease gradually with increasing carbon number, see figure 3. To reduce the transition temperatures further, a lateral fluoro group was introduced at the C-3 position of the right-hand side phenyl ring, but the transition temperatures of most of the compounds were not reduced as expected. Only the melting point of compound 76 was decreased to that of compound 72, from 157.0 to 135.3uC. Figure 3. Dependence of transition temperatures on the number of alkyl chain carbon atoms for compounds 75–78. figure 2. It can be seen that melting points show an odd–even effect with increasing carbon number, while clearing temperatures decrease gradually. Compounds Table 6. 3.2. Optical anisotropy The Dn value, defined as the difference between the two principal refractive indices of a uniaxial material, was estimated by a guest–host method. The Dn value of a guest–host system can be approximated from the Dn value for single compound. Data calculated from guest-host systems. Compound Structure Dn 28 0.47a 37 0.49a 38 0.49a 47 0.73b 48 0.67b 49 0.55b 50 0.62b 74 0.61b a Host ZLI 1132. bHost mixture E-63. Downloaded By: [University of Central Florida] At: 14:54 28 May 2007 516 Y.-M. Liao et al. following equation: ðDnÞgh ~xðDnÞg zð1{xÞðDnÞh References ð1Þ where the subscripts g, h and gh denote guest, host, and guest–host cells, respectively; x is the concentration (in wt %) of the guest compound. By comparing the measured results for the guest–host mixtures with those of the host mixture, the Dn values of the guest compounds can be extrapolated. The Dn values of some synthesized compounds are listed in table 6. Commercial LCs ZLI-1132 or E-63 was used as host mixture. It can be seen that the Dn values of these compounds are in the range 0.5 to 0.8. For those compounds with biphenyl and naphthyl moieties, the Dn value is higher than that for the corresponding phenyl moiety. This is because biphenyl and naphthyl moieties have a more extended electron conjugation than the phenyl moiety. The lateral substitutions affect the Dn values significantly. The Dn values of compounds 28, 37 and 38, were similar, approaching to 0.5. Compound 49 has the lowest birefringence among the compounds 47–50 due to its two laterally substituted ethyl chains, which would lead to a higher Dn value due to the molecular packing density effect. The Dn values of compounds 48, 50 and 74 are 0.67, 0.62 and 0.61, respectively. Compound 47 has the highest Dn of 0.73 and is expected to be useful for PDLC, cholesteric display, and laser beam steering applications. 4. Conclusions Four series of novel super high Dn bistolane laterally substituted liquid crystals with terminal isothiocyanato groups were synthesized. Lateral alkyl or fluoro groups were introduced to modify the LC properties. The extrapolated Dn of some of these compounds is greater than 0.7. Some of these compounds exhibit an odd–even effect in their phase transition temperatures. A eutectic mixture consisting of these compounds and some NCS tolanes was developed. The Dn value was determined by the guest–host method using a commercial LC as host matrix. Compounds containing biphenyl and naphthyl moieties have a high Dn value; compound 47 has the highest Dn of 0.73, making it a good candidate for many display applications. Acknowledgement The authors would like to thank the National Science Council (NSC) of the Republic of China (NSC 94-2216E-009-001) for financial support of this research. [1] H.H.B. Meng, L.R. Dalton, S.T. Wu. Mol. Cryst. liq. Cryst., 259, 303 (1994). [2] S.T. Wu, J.D. Margerum, M.S. Ho, M. Fung, C.S. Hsu, S.M. Chen, K.T. Sai. Mol. Cryst. liq. Cryst., 261, 79 (1995). [3] C.S. Hsu, K.T. Tsay, A.C. Chang, S.R. Wang, S.T. Wu. Mol. Cryst. liq. Cryst., 19, 409 (1995). [4] H. Takatsu, K. Takeuchi, Y. Tanaka, M. Sasaki. Mol. Cryst. liq. Cryst., 141, 279 (1986). 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