Remarkable smectic phase behaviour in odd-membered liquid crystal dimers: the CT6O.m series

Abstract

The syntheses and characterisation of the first ten homologues of the (3⁴-[6-(4-{(E)-[(4-alkylphenyl)imino]methyl}phenoxy)-hexyl][1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitriles (CT6O.m) are reported. Below the conventional nematic phase, N, a rich smectic polymorphism is observed. CT6O.m homologues with m = 1–9 show a series of up to four smectic phases, namely: an intercalated biaxial smectic A_b phase, a modulated phase made of tilted biaxial SmC layer fragments, a non-tilted biaxial hexatic-type phase termed the SmX phase, and a soft crystalline lamellar phase, SmY. The longest homologue, CT6O.10, exhibits a different sequence of smectic phases below the N phase: an uniaxial SmA phase, a biaxial SmA_b phase, a tilted SmC phase, and the soft crystalline SmY phase. The assignment of these phases was confirmed using a combination of polarised optical microscopy, X-ray diffraction and optical birefringence studies. In CT6O.10 only the SmC has an intercalated structure of molecules; while the smectic phases either side show interdigitated molecular packing arrangement. The strong biaxiality of the SmC phase results in an unusual switching mode detected under the application of an electric field, in which by coupling the dielectric anisotropy with the electric field, molecules in cells with planar anchoring reorient to give an optical texture without tilt domains, and the switching is accompanied by a considerable increase in the measured birefringence.