Stability of the antiferroelectric phase in dimeric liquid crystals having two chiral centres with CF3or CH, groups; evaluation of conformational and electric interactions Yoshi-ichi Suzuki," Tadaaki Isozaki? Shigeharu Hashimoto," Tetsuo Kusumoto,b Tamejiro Hiyama,' Yoichi Takanishi,d Hideo Takezoe*d and Atsuo Fukudad aCentral Research and Development Laboratory, Showa Shell Sekiyu K.K., 123-1 Shimokawairi, Atsugi, Kanagawa 243-02, Japan bSagami Chemical Research Center, 4-4-1 Nishiohnuma, Sagamihara, Kanagawa 229, Japan 'Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226, Japan dDepartmentof Organic and Polymeric Materials, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152, Japan The synthesis of a novel series of dimeric liquid crystals with two chiral centres with CF, or CH, groups connected by an alkylene spacer is described.The two mesogenic units in the dimer tend to align in anticlinic zigzag ordering or in parallel ordering depending on whether the alkylene spacer has an odd or even number of -CH2 -groups, n, stabilizing the antiferroelectric SCA* phase or the ferroelectric Sc* phase, respectively. This odd-even rule in the conformational effect is no longer valid when the length of the alkylene spacer is increased to over n =10: the SCA*phase emerges instead of the Sc* phase for n =12, as already reported for the dimer with CF, groups. Furthermore, we found that the dimers with n =10 also show a SCA*phase, if the dimers with CH, or CF, groups are sandwiched between thick cells. In thin cells, on the other hand, the Sc* phase emerges instead of the SCA*phase in the dimers with CH, (n=10 and 12) and CF3 (n= lo), although the SCA*phase is stable even in thin cells in the dimer with CF, (n=12).Thus, the cell thickness dependence is pronounced in the dimer with CH, groups, indicating that the dimer with CF, groups shows more stable antiferroelectricity, the same as in the monomers.These facts suggest the importance of an electric interaction between mesogens for the emergence of the SCA*phase. A new chiral smectic liquid crystal phase exhibiting an anticlinic ordering of the molecular orientation in successive layers has been discovered. This phase, called the antiferro- electric liquid crystal (AFLC) phase, typically appears in 4-( 1-methylheptyloxycarbony1)phenyl4'-octyloxybiphenyl-4-carboxylate (MHPOBC)' and 44 l-trifluoromethyl-heptyloxycarbony1)phenyl 4'-octyloxybiphenyl-4-carboxylate (TFMHPOBC).2 AFLCs have been extensively studied, since they exhibit unique characteristics for device application: dc threshold and double hysteresis., On the other hand, extensive studies have also been carried out to clarify the origin of the antiferroelectric It was reported that the antiferroelec- tric SCA* phase is stabilized by a molecular pairing uiu a dipole-dipole interaction between two molecules.8 Recently, the importance of a dipole component parallel to the smectic layer was also pointed out.g With a view to the construction of new types of display devices,"." many AFLC compounds have been developed and the correlation between the molecular structure and the appear- ance of the antiferroelectric SCA*phase has been investigated. As a consequence, some empirical rules for the design of AFLC compounds have been rep~rted.".'~ According to these rules, a core structure composed of three phenyl rings more reliably induces the antiferroelectric phase than compounds with two phenyl rings.Most of the AFLC compounds have two ester groups, one in the mesogenic unit and the other in the linking group, which align in the same direction and in the same sense. This structure effectively induces polarization and conjugation along the molecular long axis in antiferroelectric molecule^.'^ In practice, however, it is not yet clear how to design new AFLC compounds which show the stable SCA* phase.We have already presented the synthesis of dimeric liquid crystal com- pounds 1 consisting of two chiral centres with two CF, groups in an alkylene spacer which connects the two mesogenic units, a dimer model of TFMHPOBC.159'6 These dimers exhibited a highly stabilized SCA*phase dependent on the odd-even number of the alkylene spacer. In this article, we report the synthesis of dimeric liquid crystals 1 and 2 having CF, or CH, groups at the chiral centres and discuss the importance of the electric interaction in addition to the conformational effect on the appearance of the SCA*phase.Experimental Synthesis Two series of dimeric liquid crystal compounds were synthe- sized as shown in Schemes 1 and 2. The chiral diol 3 was prepared by the treatment of the Grignard reagent obtained from 1,5-dibromopentane with (S)-(trifluoromethy1)oxirane (75% ee). Esterification of 3 with 4-benzyloxybenzoic acid followed by debenzylation of the resulting 4-benzyloxybenzoate 4 afforded bisphenol 5 as a diastereoisomeric mixture (S,S :S,R =3 :1). Resolution of the diastereoisomeric mixture of 5 by HPLC (Daicel, Chiralpak AD, hexane-propan-2-01, 7 : 1) afforded (S,S)-5 which was condensed with 4'-octyloxybi- phenyl-4-carboxylic acid to give rise to l(7).A similar trans- formation with 1,8-dibromooctane and (S)-propylene oxide (> 96% ee), followed by esterification and debenzylation, afforded bisphenol8.Bisphenol8 was also condensed with the corresponding acid to give rise to 2(,0,. In a similar manner 1(8)-1(12),2(*)and 2(11)-2(12) were obtained. The structures of the target products as well as intermediates were confirmed by IR, 'H NMR (J values are given in Hz) and mass spectral data, specific rotation ([@IDvalues are given in units of lo-' deg cm2 g-') and elemental analysis. J. Muter. Chem., 1996, 6(5), 753-760 753 iii / 3 Scheme 1 Reagents 1, Mg, 11, CuI, (S)-(trifluoromethyl)oxirane, 111, PhCH,OC,jH,COOH, PPE, IV, H2, 10% Pd-C, V, C8H,,0C6H,C6H4COOH, DCC, DMAP Scheme 2 Reagents I, Mg, 11, CuI, (S)-propylene oxide, 111, PhCHzOC,H,COOH, DCC, DMAP, IV, H2, 10% Pd-C, V, C8H170C6H4C6H4COOH,DCC, DMAP l,l,1,11,11,11-Hexafluoroundecane-2,10-dio1(3).In the pres- ence of a catalytic amount of copper(1) iodide (500 mg, 2 6 mmol), (S)-3,3,3-Trifluoro-1,2-epoxypropane(75% ee, 6 6 g, 59 mmol) was added slowly to the Grignard reagent prepared by the treatment of 1,5-dibromopentane [7 g, 30 mmol in tetrahydrofuran (THF) 90 ml] and magnesium (1 5 g, 62 mmol) The reaction mixture was stirred at room tempera- ture for 3 h, quenched with 2 M hydrochlonc acid, and then extracted with diethyl ether After removal of the solvent by evaporation, the crude product was purified by column chrom- atography (hexane-thy1 acetate, 5 1) followed by distillation (1 10 "C/O 1Torr) to give the title compound 3 (2 6 g, 29%) as a colourless oil, dB(CDC13) 1 30-1 48 (m, 8 H), 151-1 74 (m, 6 H), 2 06 (d, J 6 2, 2 H), 3 92 (m, 2 H), v(neat)/cm-l 3400, 2940, 2870, 1280, 1170, 1140, 700, m/z (re1 intensity) 297 (M' + 1, 30%), 139 (46), 90 (28), 73 (24), 70 (22), 69 (28), 68 (28), 67 (32), 57 (22), 55 (loo), 43 (41), 42 (23), 41 (80), 39 (28), 31 (25), 27 (32) l,l,l,ll,ll,ll-Hexafluoroundecane-2,lO-diylbis( 4-benzyloxy- benzoate) (4) Ethyl polyphosphate (solution win chloroform, 3 ml) was added to a mixture of 1,1,1,11,11,1l-hexafluoro-undecane-2,lO-diol [0 28 g, 0 94 mmol, (S,S) (S,R)=3 13 and 4-benzyloxybenzoic acid (048 g, 2 1mmol) suspended in dichloromethane (2 ml) The reaction mixture was stirred at room temperature for 17 h, quenched with sat aq sodium hydrogen carbonate and extracted with diethyl ether After removal of the solvent, the crude product was purified by column chromatography (hexane+thyl acetate, 9 1) to afford the title compound 4 [0 49 g, 72%, (S,S) (S,R)=3 11 as a colourless oil, dH(CDC1,) 124-1 46 (m, 10 H), 1 82 (br q, J 7 3, 4 H), 5 13 (s, 4 H), 5 50 (sextet, J 6 8, 2 H), 7 01 (d, J 9 0, 4 H), 7 3-7 42 (m, 10 H), 8 02 (d, J 9 0, 4 H), v(neat)/cm-l 2950, 2880, 1730, 1600, 1510, 1260, 1170, 1100, 1010, 850, 770, 740, 700, m/z (re1 intensity) 718 (M++2, trace), 716 (M', trace), 302 (34%), 211 (51), 121 (21), 91 (100) l,l,l,ll,ll,ll-Hexafluoroundecane-2,lO-diyl bis( 4-hydroxy- benzoate) (5).Debenzylation of 4 was carried out by stirring 6 [0 4 g, 0 56 mmol, (S,S) (S,R)=3 11 with 10% Pd-C (002 g) in ethyl acetate (6 ml) for 3 h Filtration of the catalyst, concentration, followed by column chromatography (hexane- ethyl acetate, 3 1) gave the title compound 5 [030 g, 99%, (S,S) (S,R)=3 11 Pure (S,S)-5 was obtained by HPLC separa- tion (Daicel, Chiralpak AD, hexane-propan-2-01, 7 1) as a COlOUrleSS Oil, [O!]D20 -58 8 (C =105, CHCl,), dH(CDC13) 120-1 40 (m, 10 H), 1 82 (br q, J 7 3, 4 H), 5 49 (sextet, J 6 7, 2H), 551 (s, 2H), 688 (d, J 88, 4H), 799 (d, J 88, 4H), v(neat)/cm-' 3400, 2940, 1700, 1610, 1510, 1440, 1260, 1170, 1100, 850, 770, 700, m/z (re1 intensity) 536 (M+, trace), 138 (15%), 121 (100) (S,S)-l,l,l,ll,ll,1l-Hexafluoroundecane-2,l0-diyl bis(4-[ 4-octyloxybiphenyl-4-yl)carbonyloxy] benzoate) [l,,,].Dicyclo-hexylcarbodiimide (1 15 mg, 0 56 mmol) was added to a solu- tion of 5 (0 1 g, 0 19 mmol), 4'-octyloxybiphenyl-4-carboxylic acid (125 mg, 0 38 mmol), dimethylaminopyridine (DMAP) (30 mg, 0 24 mmol) in dichloromethane (6 ml), and the result- ing solution was stirred at room temperature for 7 h before filtration of the precipitated material Concentration zn vucuo followed by column chromatography (hexane-dichlorometh- ane, 2 1 ) and recrystallization (hexane-dichloromethane, 10 I), gave the title compound (172 mg, 8O%), [O!]DZo -39 8 (c= 105, CHCI,), &(CDCl,) O 90 (t, J 7 0, 6 H), 124-1 54 (m, 30 H), 177-1 90 (m, 8 H), 401 (t, J 6 6, 4 H), 5 55 (sextet, J 6 6, 2 H), 7 00 (d, J 8 8,4 H), 7 36 (d, J 8 8, 4 H), 7 59 (d, J 8 8, 4 H), 7 70 (d, J 8 6, 4 H), 8 17 (d, J 8 8, 4 H), 8 23 (d, J 8 6, 4H), v(KBr)/cm-' 2930, 1740, 1600, 1260, 1180, 1160,830,770 (Found c, 69 89, H, 6 49 Calc for C67H74F6010 C, 69 78, H, 6 47%) Other homologues 1,81-1(121were prepared in a similar manner (S,S)-l,l,l,12,12,12-Hexa~uorododecane-2,1l-dzylbzs{ 4-[( 4'- octyloxybzphenyl-4-yl)carbonyloxy]benzoate} [l,,,] [O!]DZ0 -390 (c=lOl, CHCl,), dH(CDC13) 090 (t, J 70, 6 H), 1 24-1 54 (m, 32 H), 1 77-1 92 (m, 8 H), 4 02 (t, J 6 6, 4 H), 5 55 (sextet, J 6 6, 2 H), 7 01 (d, J 8 8, 4 H), 7 36 (d, J 8 8, 4 H), 7 59 (d, J 8 8, 4H), 7 70 (d, J 8 6, 4H), 8 17 (d, J 8 8,4H), 823 (d, J 86, 4H), v(KBr)/crn-l 2930, 1735, 1600, 1260, 1180, 1160, 1070, 830, 770 (Found C, 70 08, H, 6 52 Calc for C68H76F6010 C, 69 97, H, 6 56%) (S,S)-1,1,1,13,13,13-Hexa~uorotrzdecane-2,12-dzylbzs(4-[( 4'- octyloxybzphenyl-4-yl)carbonyloxy]benzoate) [l,,,] [a]D20 -33 8 (c=I 01, CHCI3), d~(cDCl3) 090 (t, J 70, 6 H), 1 23-1 53 (m, 34 H), 177-1 92 (m, 8 H), 401 (t, J 6 6, 4 H), 5 55 (sextet, J 6 6, 2 H), 7 01 (d, J 8 8,4 H), 7 36 (d, J 8 8, 4 H), 759 (d, J 88,4H), 770(d, J 86,4H), 8 17(d, J 88,4H), 823 (d, J 8 6, 4H), v(KBr)/cm-' 2920, 2850, 1730, 1600, 1260, 754 J Muter Chem, 1996, 6(5),753-760 1175, 1160, 1110, 1060, 830, 765 (Found: C, 70.15; H, 6.80.Calc. for C,j9H78F,Olo: C, 70.15; H, 6.66%). (S,S)-1,1,1,14,14,14-Hexa~uorotetradecane-2,13-diyl bis(4-[( 4'-octyloxybiphenyl-4-yl)carbonyloxy]benzoate} Cl(lOI* [a]~~'-40.5 (c= 1.01, CHC1,); d~(cDC1,) 0.90 (t, J 7.0, 6 H), 1.21-1.54 (m, 36 H), 1.77-1.92 (m, 8 H), 4.02 (t, J 6.6, 4 H), 5.55 (sextet, J 6.7, 2 H), 7.01 (d, J 8.8,4 H), 7.36 (d, J 8.8,4 H), 7.59 (d, J 8.8,4 H), 7.70 (d, J 8.5, 4 H), 8.17 (d, J 8.8,4 H), 8.23 (d, J 8.5, 4H); v(KBr)/cm-' 2930, 2850, 1735, 1600, 1260, 1180, 1160, 1100, 1070, 1010, 830, 765 (Found: C, 70.20; H, 6.81. Cak.for C~OH~OF~O~O: c, 70.34; H, 6.75%). (S,S)-1,1,1,15,15,15-Hexu~uoropentudecane-2,14-d~yl bis(4-[( 4'-oxtyloxybiphenyl-4-yl)carbonyloxylbenzoate} [l,,l,]. [a]D2' -38.3 (C= 1.01, CHC1,); &(CDCl,) 0.90 (t, J 7.0, 6 H), 1.20-1.53 (m, 38 H), 1.77-1.92 (m, 8 H), 4.01 (t, J 6.6, 4 H), 5.55 (sextet, J 6.6, 2 H), 7.01 (d, J 8.8, 4 H), 7.36 (d, J 8.8, 4 H), 7.60 (d, J 8.8, 4 H), 7.70 (d, J 8.6, 4 H), 8.17 (d, J 8.8, 4 H), 8.23 (d, J 8.6, 4H); v(KBr)/cm-' 2930, 2850, 1735, 1600, 1260, 1160, 11 15, 1165, 830, 765 (Found: C, 70.46; H; 6.92.Calc. for C,,Hg2F6Olo: C, 70.52; H, 6.83%). (S,S)-1,1,1,16,16,16-~exu~uorohexadecane-2,15-diyl bis(4-[( 4'-octyloxybiphenyl-4-yl)carbonyloxylbenzoate} [1(12J. [a]~~'-39.5 (c= 1.05, CHCl,); GH(CDCI3) 0.90 (t, J 7.0, 6 H), 1.20-1.53 (m, 40 H), 1.78-1.93 (m, 8 H), 4.02 (t, J 6.6, 4 H), 5.56 (sextet, J 6.6, 2 H), 7.01 (d, J 8.8, 4 H), 7.36 (d, J 8.8, 4 H), 7.60 (d, J 8.8, 4H), 7.71 (d, J 8.6, 4 H), 8.17 (d, J 8.8, 4 H), 8.23 (d, J 8.6, 4 H); v(KBr)/cm-' 2930, 2860, 1735, 1600, 1260, 1180, 1160, 1070, 830, 770 (Found: C, 7.082; H, 7.08.Calc. for C~~H,,F~O~O:c, 70.69; H, 6.92%). (S,S)-Tetradecane-2,13-diol (6). To the Grignard reagent prepared by the treatment of 1,8-dibromooctane (1.7 g, 6.2 mmol) in THF (8 ml) and magnesium (0.43 g, 18 mmol), (S)-propylene oxide (>96% ee, 0.85 g, 15 mmol) was added slowly in the presence of a catalytic amount of copper(1) iodide (0.130 g, 0.68 mmol). The reaction mixture was stirred at 0°C for 10 h and then quenched with 2 M hydrochloric acid. The mixture was extracted with diethyl ether. The solvent was removed by evaporation and the crude product was recrys- tallized from hexane-ethyl acetate (7: 1) to give the title compound 6 (0.59 g, 41%); mp 61 "c; [a],''+ 10.0 (c= 1.04, CHCI,); bH(CDC13) 1.19 (d, J 6.2, 6 H), 1.25-1.50 (m, 22 H), 3.75-3.84 (m, 2 H); v(KBr)/cm-' 3350, 2920, 2850, 1470, 1370, 1130, 1040, 1015, 945, 720cm-'; m/z (rel. intensity) 231 (Mf + 1, trace), 83 (20%), 69 (23), 55 (29), 45 (loo), 43 (22), 41 (22).(S,S)-Tetradecane-2,13-diyl bis(4-benzylox ybenzoate) ( 7). DCC (0.80g, 3.9mmol) was added to a solution of (S,S)-tetradecane-2,13-diol (0.40 g, 1.5 mmol), 4-benzyloxybenzoic acid (0.8 g, 3.5 mmol) and DMAP (80mg, 0.65 mmol) in dichloromethane (20 ml), and the resulting solution was stirred at room temperature for 24 h. The precipitated material was filtered, and the solvent was removed under reduced pressure. The residue was purified by column chromatography (hexane-ethyl acetate, 9 : 1) to give the title compound 7 (380 mg, 39%) as a colourless oil; [alD2' +32.9 (c =1.03, CHCI,); dH(C~cl,) 1.20-1.45 (m, 16H), 1.31 (d, J 6.3, 6H), 1.52-1.62 (m, 2 H), 1.64-1.74 (m, 2 H), 5.07-5.15 (m, 2 H), 5.12 (s, 4 H), 6.99 (d, J 9.0,4 H), 7.30-7.45 (m, 10 H), 7.99 (d, J 9.0,4 H); v(neat)/cm-' 2930, 2860, 1710, 1600, 1510, 1450, 1380, 1280, 1250, 1165, 1100, 1020, 1000, 845, 770, 700; m/z (rel.intensity) 651 (M' + 1, trace), 149 (27%), 91 (loo), 83 (20), 55 (36), 43 (24), 41 (38), 28 (21). (S,S)-Tetradecane-2,13-diyl bis( Qhydroxybenzoate) (8). A mixture of 7 (0.38 g, 0.58 mmol) and 10% Pd-C (0.02g) in ethyl acetate (6 ml) was stirred under an atmospheric pressure of hydrogen at room temperature for 5 h. Filtration of the catalyst, concentration in uucuo, followed by purification by column chromatography (hexane-thy1 acetate, 3 :1), afforded the title compound 8 (0.23 g, 84%) as a colourless oil; [C!lD20+22.2 (c=0.99, CHCl,); dH(CDC13) 1.16-1.41 (m, 16 H), 1.32 (d, J 6.3, 6 H), 1.53-1.63 (m, 2 H), 1.65-1.75 (m, 2 H), 5.09-5.19 (m, 2 H), 6.1-7.1 (br, 2 H), 6.88 (d, J 8.9, 4 H), 7.94 (d, J 8.9,4 H); v(neat)/cm-' 3350,2920,2850, 1680, 1605, 1590, 1510, 1440, 1355, 1310, 1280, 1230, 1160, 1110, 850, 770, 700, 620; m/z (rel.intensity) 470 (M', 2%), 139 (loo), 121 (67). (S,S)-Te tradecane-2,13-diyl bis{4-[( 4-oct ylox ybiphen yl-4-yl )-carbonyloxy] benzoate} [2(1J.DCC (245 mg, 1.2 mmol) was added to a solution of 8 (230mg, 0.49 mmol), 4'-octyloxybiphenyl-4-carboxylicacid (0.35 g, 1.1 mmol) and DMAP (10 mg, 0.08 mmol) in dichloromethane (10 ml), and the resulting solution was stirred at room temperature for 24 h.The precipitated material was filtered, and the filtrate was concentrated. The residue was purified by column chroma- tography (dichloromethane). The product was further purified by recrystallization from hexane-dichloromethane (5 : 1) to give analytically pure title compound 2(,,, (0.45 g, 85%); [a]D2'+22.9 (c= 1.02, CHCl,); GH(CDC1,) 0.90 (t, J 6.9, 6 H), 1.24-1.52 (m, 36 H), 1.34 (d, J 6.3, 6 H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4 H), 4.02 (t, J 6.6, 4H), 5.16 (br sextet, J 6.3, 2H), 7.01 (d, J 8.7, 4H), 7.31 (d, J 8.7, 4 H), 7.59 (d, J 8.7, 4 H), 7.70 (d, J 8.5, 4 H), 8.13 (d, J 8.7, 4 H), 8.23 (d, J 8.5, 4 H); v(KBr)/cm-' 2920, 2850, 1730, 1720, 1600, 1500, 1270, 1200, 1160, 1115, 1070, 1010, 830, 770, 690 (Found: c, 77.10; H, 8.14.Calc. for C70H86010: c, 77.32; H, 7.97%). Compounds 2(8), 2(,,,and 2(12Jwere prepared in a similar manner. (S,S)-Dodecane-2,ll-diyl bis(4-[( 4'-octyloxybiphenyl-4-yl)-carbonyloxylbenzoate) ~2(~,].[C!]DZo +23.2 (c=1.02, CHC1,); dH(CDC1,) 0.90 (t, J 7.0, 6 H), 1.24-1.52 (m, 32 H), 1.34 (d, J 6.2, 6H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4 H), 4.02 (t, J 6.6, 4 H), 5.16 (br sextet, J 6.3, 2H), 7.00 (d, J 8.8, 4H), 7.31 (d, J 8.8, 4H), 7.59 (d, J 8.8, 4 H), 7.70 (d, J 8.7, 4H), 8.13 (d, J 8.8, 4 H), 8.23 (d, J 8.7, 4H); v(KBr)/cm-l 2930, 2850, 1730, 1720, 1600, 1265, 1190, 1160, 1110, 1070, 1010, 830, 765 (Found: C, 76.86; H, 8.01.Calc. for C68H82010: C, 77.10; H, 7.80%). (S,S)-Pentadecane-2,14-diylbis(4-[( 4-octyloxybiphenyl-4-yl)-carbonyloxy]benzoate} [2(,,,]. [aID2'+21.8 (c =1.05, CHC1,); dB(CDC13) 0.90 (t, J 7.0, 6 H), 1.24-1.52 (m, 38 H), 1.34 (d, J 6.2, 6H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4 H), 4.02 (t, J 6.6, 4 H), 5.16 (br sextet, J 6.2, 2H), 7.01 (d, J 8.8, 4H), 7.31 (d, J 8.8, 4H), 7.59 (d, J 8.8, 4H), 7.70 (d, J 8.6, 4H), 8.13 (d, J 8.8, 4H), 8.23 (d, J 8.6, 4H); v(KBr)/cm-l 2900, 2850, 1730, 1710, 1600, 1260, 1190, 1160, 1110, 1070, 1010, 830, 760 (Found: C, 77.28; H, 8.20. CdC. for C71H88010: c, 77.42; H, 8.05Yo). (S,S)-Hexadecune-2,15-diylbis(4-[( 4-octyloxybiphenyl-4-yl)-carbonyloxy]benzoate) [2(12J.[a]D2' +22.9 (c = 1.01, CHCI,); GH(CDC1,) 0.90 (t, J 7.0, 6 H), 1.24-1.52 (m, 40 H), 1.34 (d, J 6.2, 6 H), 1.56-1.66 (m, 2 H), 1.68-1.76 (m, 2 H), 1.82 (br quintet, J 6.6, 4H), 4.02 (t, J 6.6, 4H), 5.16 (br sextet, J 6.2, 2H), 7.01 (d, J 8.8, 4H), 7.31 (d, J 8.7, 4H), 7.59 (d, J 8.8, 4H), 7.70 (d, J 8.5, 4H), 8.13 (d, J 8.7, 4H), 8.23 (d, J 8.5, 4H); v(KBr)/cm-' 2900, 2850, 1730, 1715, 1600, 1500, 1465, 1265, 1200, 1160, 1110, 1070, 1010, 830, 760, 690 (Found: C, 77.42; H, 8.30. Calc. for C7,H9oOlo: C, 77.53; H, 8.13%). Measurements The phase and transition temperatures of the compounds were determined by polarizing optical microscopy using an Olympus J. Muter. Chem., 1996, 6(5),753-760 755 BH2 instrument equipped with a temperature controller (Mettler FP82) and by differential scanning calorimetry (DSC) using a Rigaku DSC-8240D instrument with a TAS-200 data analysis system.Homogeneously aligned cells of ca. 2 pm thickness were prepared by rubbing polyimide thin films coated on the substrate glass plates with indium tin oxide (ITO). The spontaneous polarization was measured by the triangu- lar wave voltage method (5 Hz). The electrooptic properties were measured by applying a triangular wave voltage of 0.01 Hz frequency. The light transmittance through the sample cell was detected by a photomultiplier tube. The apparent tilt angle was determined by the extinction direction between crossed Nicols as a function of electric field.Results Phase sequences and transition temperatures The phase sequences and the transition temperatures of l(,,) and Z(,)were determined by DSC measurement, thermal polar- izing optical microscopy and their electrooptic behaviour. As shown in Fig. 1, the dimers l(,,)enantiotropically exhibit the SCA*phase or the Sc* phase depending on whether n Fig. 1 Phase sequences and transition temperatures of dimeric liquid crystals l(")and 2,"). The appearance of Sc* in parentheses indicates that it appears after field application and/or in thin cells (see text). (nd9) is odd or even, respectively. For an explanation of use of Sc* in parentheses, see below (microscopic observation section). This odd-even behaviour may be attributed to a conformational effect of the alkylene spacer -(CH,), -, as we In our previous paper,16 the chiral smectic phase in l(lo)was assigned to S,*.In the present study, we noticed that the SCA*phase emerges instead of the S,* phase, when the cell thickness is thick enough, as will be shown later. For 2,,,, the phase sequences are the same as those of l(,)for n d 11, and are also shown in Fig. 1. In Fig. 2, the molecular alignment based on the confor- mational effect of the alkylene spacer is schematically illus- trated. For odd values of n [e.g. l,,,], the two mesogenic groups are expected to align in a zigzag manner (a),whereas for even values of n [e.g. l,,,], they are arranged in a parallel manner (b).This odd-even rule is no longer valid when n is large as in l(lO),1(12), Z(10)and 2(12).We should note that l(,,)and Z(,,)with odd n values always show the Iso-SCA* phase transition. This is an unusual phase sequence in normal monomeric compound systems. The direct transition from Is0 to SCA*is interpreted by the suppression of the SA phase due to the bent structure shown in Fig.2(u). In fact the same phase sequence was observed for main chain polymers, in which mesogens are connected by a polymethylene chain with odd numbers of CH2 gro~ps.'~-'~ Microscopic observation In order to identify the phases in l(,)and 2,,,, texture obser- vation was made. The micrographs of the homogeneously aligned cells of 2(,) are shown in Plate 1; (a)a 2 pm cell of Z18, at 160"C, (b)a 3.5 pm cell of 2(10,at 155 "C and (c) a 9 pm cell of 2(,,, at 145 "C.In Plate l(a), two domains with different colours are observed and are attributed to twisted states in the Sc* phase. Hence we can conclude that 2(8)shows Sc* phase at 160°C. In Plate l(b) and (c), a typical herringbone texture was observed and the extinction direction under the crossed polarizers is parallel to the layer normal, suggesting the presence of the antiferroelectric SCA*phase at 155°C in 2(,,, and at 145 "C in Z(',). However, the situation is not so simple. The SCA*phase irreversibly changes to the Sc* phase on application of a field. After the application of a triangular voltage wave (0.1 Hz, Fig. 2 (a)Molecular orientational structures of the antiferroelectric SCA*phase of 1(7)and (b)the ferroelectric Sc* phase of l,,, 756 J.Muter. Chem., 1996, 6(5),753-760 Plate 1 Optical micrographs of (a)a 2 pm cell of 2(*, at 160"C, (b)a 3.5 pm cell of 2(,,, at 155 "Cand (c) a 9 pm cell of 2(,,, at 145 "C +5 V) to the 3.5 pm cell of 2(,,,, the texture changes from Plate 2(u) to (b).In Plate 2(b), ferroelectric domains appear, although antiferroelectric domains still remain. Thus, the SCA*phase is so unstable that the application of an electric field easily converts the phase to Sc*. The phase also depends on the cell thickness. Plate 3 shows the textures of 2(,,, and 2(12,.In thin cells they show the S,* phase, while they show the SCA*phase in thick cells. We confirmed that 1(,,, also shows the Sc* phase in a thin cell as already reportedi6 and that 4,,,shows the SCA*phase even in thin cells such as 2.5 pm, as shown in Plate 4.In this respect, l,,, has relatively strong antiferroelectricity compared with 2(n). In Fig. 1, shown above, Sc* in parentheses means that it appears after field application and/or in thin cells. Elec troop tic response Dimers with an odd value of n (n=7, 9 and 11) did not respond electrooptically, indicating the presence of the stable SCA*phase. In contrast, an electrooptic response was obtained in dimers with an even value of n (a= 8, 10 and 12). As already shown in Fig. 2(u) in our previous paper,16 a 2.5 pm thick cell of 1(12)shows antiferroelectric switching, while a 2.5 pm thick cell of l,,,, shows ferroelectric switching.In a thick cell, however, l(,,)shows antiferroelectric behaviour as shown in Fig. 3. In this figure, which displays the antiferroelec- Plate 2 Optical micrographs of the 3.5 pm cell of 2(,,, at 155"C (a) before and (b) after the application of a triangular voltage wave (0.1 Hz, +5 V) Plate 3 Optical micrographs of the thin and thick cells of (a)2,,,! at 140 "C(thin) and at 155 "C (thick) and (b)2(12)at 145 "C(thin and thick) tric transmittance change on application of a triangular voltage wave, one of the crossed polarizers was set parallel to the smectic layer. Fig. 4 shows the dependence on thickness of the electrooptic response in 2(,,, for (a) a 2 pm thick cell at 120"C, (6) a 3.5 pm thick cell at 155 "C and (c) a 7 pm thick cell at 150 "C.In the measurement showing ferroelectric switching [Fig. 4(u)],one of the polarizers was adjusted so as to obtain a dark state for one of the ferroelectric states. The thin [Fig. 4(a)] and thick J. Muter. Chem., 1996, 6(5),753-760 757 Plate 4 Optical micrographs of the 2.5 pm cells of (a) l(lo)at 130°C and (b)1(,,, at 120°C I 1I I I -20 -10 0 10 20 applied voltageN Fig. 3 Electrooptic responses in l(lo) [Fig. 4(c)] cells exhibit ferroelectric and antiferroelectric behav- iour, respectively. In the medium thick (3.5 pm) cell, ferroelec- tric behaviour overlaps with antiferroelectric behaviour because of the coexistence of the S,* and SCA*domains, as shown in Plate 2(6). The phase of 2(12) is also dependent on the cell thickness, while 1(12)always shows the SCA*phase even in thin cells.16 Fig.5 shows ferroelectric [Fig. 5(u)] and antiferroelectric [Fig. 5(b)] switching behaviour in thin (2.5 pm) and thick (9 pm) cells, respectively. Spontaneous polarization and tilt angle Fig. 6 shows the temperature dependence of the spontaneous polarization P, in five dimers. The temperature dependence and the absolute values are almost the same. The absolute values are much smaller than those in the monomers (MHPOBC and TFMHPOBC).2 This may be because the 758 J. Muter. Chem., 1996, 6(5),753-760 I 1 1 I -20 -10 0 10 20 I.'.'I....I ....-C E' $-a'b I ....1 ,,..I ..., -10 -5 0 5 10 -10 -5 0 5 10 applied voltageN Fig. 4 Thickness dependence of the electrooptic responses in 2(10,; (a) 2 pm, 120"C, (b)3.5 pm, 155 "C and (c) 7 pm, 150°C intramolecular directional relationship between the dipole moments of the carbonyl and CH, or CF, groups is fixed by the molecular conformation.The tilt angles of these dimers were also measured. The temperature dependences of the tilt angle in five dimers are shown in Fig. 7. The tilt angle of l,,,is larger than that in 2(") by about 5". Discussion Switching behaviour It is possible to consider a switching model of the mesogenic unit of a dimer and a monomer in the antiferroelectric SCA* phase, as illustrated in Fig. 8(u) and (b), respectively. In the dimer, the mesogenic units meet strong friction, since the two mesogenic units are interlinked and can only rotate around the end of the alkylene chain along a large cone.Thus, the antiferroelectric phase is highly stabilized and a higher energy is required for the switching of one of the mesogenic units, in other words for the conformational change of the dimer molecules, than for the switching in the monomer assembly, which can switch without changing the centre of mass of the molecules. This schematic model is consistent with the exper- imental results in that electrooptic switching cannot be I 1 I I I I D r r le I B-31::Y 8 -4 -2 0 2 4 c .... .... ....,.." .g I(b) -10 -5 0 5 10 applied voltageN Fig. 5 Thickness dependence of the electrooptic responses in 2(12,; (a) 2.5 pm, 150 "C and (b) 9 pm, 130 "C 60 50 .- OR .y 4052 30 .. 4" 20 lo t2 oli . I I I I 0 10 20 30 40 T-T, /"C 301 1I I I Lt 8200, 0 0 10 20 30 40 T,-TI"C Fig. 7 Temperature dependence of tilt angle in five dimers; (0)l(lo), (0)1(12)>(A)2@,>(m) 2(10,,(0)412, Fig. 8 Switching models in dimer (a)and monomer (b) systems Fig. 9 Formation of the anticlinic structure in l(lo,,1(,,,, 2(,,, and 2(12, Origin of stabilizationof AFLC For the inherent stabilization of the antiferroelectric ordering, the pairing of transverse dipole moments in the neighbouring layers is believed to be the major interaction in the formation of the antiferroelectric SCA* phase.* As mentioned above, antiferroelectric SCA*or ferroelectric Sc* appear depending on whether there is an odd or even number of the alkylene spacer -(CH,),-, respectively.Hence, at least in the dimer systems, a conformational effect is responsible for the formation of the antiferroelectric SCA* phase. In l(,,),1(12),2(,,, and 2(,,,, however, the dimeric mesogenic units tend to form the anticlinic structure, as shown in Fig. 9(b), resulting in the emergence of the SCA*phase instead of the Sc* phase. We can interpret this phenomenon as follows. Some electric interaction such as a dipole-dipole interaction between mesogenic units overcomes the conformational effect due to the increased flexibility in the longer alkylene chain. Thus, the antiferroelectric SCA*phase appears, even if the value of n of -(CH,), -is even.Conclusion A series of novel dimeric liquid crystals l(,)and 2(,) were synthesized. These dimers were found to show the antiferroelec- tric SCA* or the ferroelectric Sc* phases depending on whether they have an odd or even number of n in the alkylene spacer -(CH,), -chain, respectively. This odd-even rule becomes invalid when n increases: 1(,,), 1(,,), 2(,,, and 2(,,, exhibit an SCA*phase, though n is even. This fact indicates that an electric interaction is important for the stabilization of the SCA*phase. Thus, there exist two major forces responsible for the stabiliz- ation of the antiferroelectric SCA* phase, conformational and electric interactions. The emergence of the SCA* phase is governed by the balance between these two interactions.A complicated phase behaviour which depends on field appli- cation and cell thickness is also reported. The details will be reported in a separate paper. This work was partly supported by a Grant-in-Aid for Scientific Research (Specially Promoted Research No. 06102005) from the Ministry of Education, Science, Sports and Culture. J. Muter. Chem., 1996, 6(5), 753-760 759 References 1 K Furukawa, K Terashima, M Ichihashi, S Saitoh, K Miyazawa and T Inukai, Ferroelectrzcs, 1988,85, 63 2 Y Suzuki, T Hagwara, I Kawamura, N Okamura, T Kitazume, M Kakimoto, Y Imai, Y Ouchi, H Takezoe and A Fukuda, Liq Cryst, 1989,6,167 3 A D L Chandani, T Hagiwara, Y Suzuki, Y Ouchi, H Takezoe and A Fukuda, Jpn J Appl Phys ,1989,28, L1261 4 A D L Chandani, E Gorecka, Y Ouchi, H Takezoe and A Fukuda Jpn J Appl Phys, 1989,28, L1265 5 H Takezoe, K Hiraoka, T Isozaki, K Miyachi, H Aoki and A Fukuda, in Modern Topics in Liquid Crystals-From Neutron Scattering to Ferroelectriczty, ed A Buka, World Scientific Publishing, Singapore, 1994, p 290 6 T Isozaki, T Fujikawa, H Takezoe and A Fukuda, Phys Rev B, 1994,48,13439 7 A Fukuda, Y Takanishi, T Isozaki, K Ishikawa and H Takezoe, J Mater Chem ,1994,4,997 8 Y Takanishi, K Hiraoka, V K Agrawal, H Takezoe, A Fukuda and M Matsushita, Jpn J Appl Phys ,1991,30,2023 9 K Miyachi, J Matsushima, Y Takanishi, K Ishikawa, H Takezoe and A Fukuda, Phys Rev E, 1995,52, R2153 10 Y Yamada, N Yamamoto, K Mon, K Nakamura, T Hagwara, Y Suzuki, I Kawamura, H Onhara and Y Ishibashi, Jpn J Appl Phys, 1990,29,1757 11 Y Yamamoto, Y Yamada, N Koshobu, K Mori, K Nakamura, H Onhara, Y Ishibashi, Y Suzuki and I Kawamura, Jpn J Appl Phys, 1992,31,3186 12 Y Suzuki, 0 Nonaka, Y Koide, N Okabe, T Hagiwara, I Kawamura, N Yamamoto, Y Yamada and T Kitazume, Ferroelectrzcs, 1993, 147, 109 13 I Nishiyama and J W Goodby, J Mater Chem , 1992,2,1015 14 I Nishiyama, E Chin and J W Goodby, J Muter Chem, 1993, 3,161 15 Y Suzuki, T Isozaki, T Kusumoto and T Hiyama, Chem Lett, 1995,719 16 T Kusumoto, T Isozaki, Y Suzuki, Y Takanishi, H Takezoe, A Fukuda and T Hiyama, Jpn J Appl Phys ,1995,34, L830 17 J Watanabe, H Komura and T Niion, Lzq Cryst, 1993,13,455 18 Y Nakata and J Watanabe, J Mater Chem ,1994,4,1699 19 J Watanabe and M Hayashi, Macromolecules, 1988,21,278 Paper 5/07080E, Received 26th October 1995 760 J Mater Chem, 1996,6(5), 753-760