Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

Abstract

Seven members of the homologous series of the liquid crystal dimers, the bis(ω-(cholesteryloxycarbonyl)alkyl) disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesised and their liquid crystal properties characterised. The dimers are referred to using the acronym Chol-n-SS-n-Chol in which n denotes the number of carbon atoms linking the cholesteryl-based groups and the sulfur atoms, and was varied between 3, 5, 6, 8, 10, 11 and 12. All seven homologues exhibit a chiral nematic phase and for the longest three members a smectic A phase was also observed. An odd−even effect is apparent in both the transition temperatures and the values of the entropy change associated with the chiral nematic−isotropic transition, S N * I /R, in which dimers with even values of n show the higher values. This is interpreted in terms of the average molecular shapes in which the C−S−S−C dihedral angle is around 90 • . The values of S N * I /R shown by these dimers are very small for liquid crystal dimers and this is attributed to the increased molecular biaxiality arising from the C−S−S−C dihedral angle. The smectic A phase exhibited by the homologues with n = 10, 11 and 12 is proposed to consist of an intercalated arrangement of the dimers which is driven by the mismatch in cross-sectional areas of the cholesteryl-based groups and alkyl chains.

Extended conjugated mesogens: synthesis and mesomorphic properties of H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms

“…The biaxial nature of the molecule reduces the orientational order in the nematic phase and hence the entropy change at the N-I transition. [30,35,36] The entropy changes of tetraalkyl (5a-c), tetraalkoxy (6a-e) and dialkyldialkoxy (7a, 7b, 8a and 8b) derivatives are slightly smaller than those of the corresponding 4PE2T derivatives (1a and 1c: 0.2; 2a-e: 0.3; 3a, 3b and 4a: 0.3; 4b: 0.35). [30] This can be attributed to the ethyl side groups attached to the 4(PE) 2 2T core.…”

Mesophase behaviour of laterally di-fluoro-substituted four-ring compounds

“…Similarly, reduced values of ΔS N-I /R have been observed for liquid crystal dimers either containing A branched terminal chain or for dimers in which the biaxiality of the mesogenic groups has been increased. [40][41][42][43] …”

1,2-Propanediol-linked chiral symmetric and non-symmetric liquid crystal dimers containing trifluoromethyl

“…Chiral liquid crystals (LCs), including chiral smectic phase, [1][2][3][4][5][6] chiral nematic phase, [2,[5][6][7][8][9] cholesteric phase, [4,[10][11][12] blue phase [12][13][14][15] and so on, have attracted much interest because of their unique optical and electrical properties, such as the selective reflection of light, [3,5,7,8,12] thermochromism [16,17] and ferroelectricity. [1,6,18,19] Fluorinated LCs are a class of important LCs for their fascinating properties such as the high polarity, dielectric anisotropy, optical anisotropy and viscoelastic properties.…”

“…LC dimers may be divided into two broad classes: symmetric [10,37,42] and nonsymmetric. [21,23,24,[29][30][31][32][33][34][35]42,43] In a symmetric LC dimer, the two mesogenic units are identical, whereas they are different in a non-symmetric dimer.…”