Liquid crystalline compounds in the thiophene series. X. Synthesis and characterization of liquid crystalline vinylenes with thiophene and benzene moieties

Abstract

Compounds with two mesogenic groups connected by a hexamethylene chain and terminal butyl groups were synthesized by the Wittig reaction from the newly described bis-Wittig salts 1,6-bis[4-(triphenylphosphoniomethyl)benzyl]hexane dichloride (1) or 1,6-bis[5-(triphenylphosphoniomethyl)-2-thienyl]hexane dibromide (2) and various aromatic aldehydes. All derivatives containing two or three vinylene units and three or four aromatic rings in each mesogenic group have liquid crystalline phases. These aromatic rings are 1,4-disubstituted benzene and/or 2,5-disubstituted thiophene rings. The transition temperatures of these compounds increase with increasing numbers of benzene rings in each core. All-E-1,6-bis{4-[4-(4-butylstyryl)styryl]phenyl}hexane (3) (which is one of the few examples of conventional rod-like liquid crystalline materials without any hetero-atoms) has the highest transition temperatures compared to all the other substances. The formation of smectic phases of a high order is dependent on the number of thiophene rings in each core. Therefore compounds with two thiophene rings and one benzene system in each core (8, 14, 15 and 16) normally have only nematic phases. The only exception is all-E-1,6-bis{4-[5-(5-butyl-2-thienylvinyl)-2-thienylvinyl]phenyl}hexane (7) which shows a smectic phase both on heating and cooling the sample. Most of the compounds with two benzene rings and one thiophene ring form smectic phases. It is obvious that the formation of a smectic phase is more probable if the mesogenic group is connected to the hexamethylene spacer by a benzene than by a thiophene system (see for example 4 and 7). For compounds 3 and 6, X-ray studies have been made. The compounds with four aromatic rings in each core show only nematic phases. They decompose either in the mesophase or the isotropic phase.