Investigation on thermal behaviour and optical properties of non-symmetric cholesterol-based twin liquid crystals with thioester linkages
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Herein, a series of novel asymmetric cholesterol-based liquid crystal compounds containing thioester linkage and an analogue containing carboxylate linking unit have been synthesised and characterised. Thermal analyses demonstrated that all desired target compounds possess broad mesophase temperature ranges (ΔT, 91–142.5°C) and high thermal stability (clearing points, 214.9–259.4°C). Due to the increasing chain length of the inner flexible spacer, clearing points and entropy changes (ΔS/R) of the COS-containing dimers showed pronounced parity effects. All dimers exhibited enantiotropic mesophase behaviour with an SmC* phase and a typical cholesteric phase (N*), which were validated by XRD and POM. The effect of structural changes of COO and COS unit in the linking group on the mesomorphic properties was analysed. Comparing C6SAR with C6OAR, it was found that the thioester-containing dimer exhibited more stable mesophase, higher phase transition temperatures (about 20°C higher) and enthalpy values (ΔH). All final products exhibited selective reflection to visible light, produced multiple colours. The higher temperatures led to the shorter pitch, which can further result in shorter reflection wavelengths, and the ‘blue shift’ occurred.