Chiral calamitic organosiloxane liquid crystals with tunable properties

Abstract

A series of dissymmetric chiral calamitic liquid crystals were designed and synthesized, featuring a central phenyl–benzoate core linked via ester bridges to a chiral (S)-3,7-dimethyloctyloxy chain at one terminus and a biphenyl carboxylate with an undecenyloxy alkyl tail at the other. Multi-step synthetic procedures, including a key hydrosilylation step, enabled the construction of siloxane-bridged dimers, incorporating two identical chiral rod-like units tethered via a disiloxane spacer. These derivatives were fully characterized using ¹H, ¹³C, and ²⁹Si NMR, mass spectrometry, and elemental analysis. The liquid crystalline and thermal behaviors of all compounds were systematically studied using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and electro-optic (EO) techniques. All synthesized mesogens exhibited liquid crystalline phases. Notably, the vinyl-terminated siloxane derivative displayed both a stable chiral smectic C (SmC*) phase and an enantiotropic chiral nematic (N*) phase. The siloxane-bridged dimer showed an expanded smectic mesophase range, alongside the N* phase, underscoring the impact of siloxane connectivity on mesophase stability and organization. Furthermore, by inverting the orientation of one carboxylate linker within the molecular architecture, a broader and singular chiral SmC* phase was achieved, highlighting the subtle yet significant influence of structural asymmetry on mesomorphic behavior.