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

Abstract

ABSTRACTA new class of extended conjugated mesogens, namely H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms, have been synthesised, and the relationships between molecular structures and mesomorphic properties investigated. Tetraalkyl, tetraalkoxy and dialkyldialkoxy derivatives, [R1C6H4CCC6H2(C2H5)2CC]2[R2C6H4CCC6H2(C2H5)2CC]2C8H2S2 where R1 and R2 = alkyl and alkoxy chains of different lengths, exhibit nematic phases. The length, number and position of the terminal chains strongly affect the mesomorphic properties. The tetraalkyl derivatives in which R1 = R2 = pentyl to heptyl exhibit enantiotropic mesophases, whereas the derivatives with octyl or nonyl chains exhibit monotropic mesophases. The tetraalkoxy derivatives in which R1 = R2 = pentyloxy to nonyloxy all exhibit enantiotropic nematic phases. The mesophase range increases with increasing alkoxy chain length, except that the octyloxy and nonyloxy derivatives have almost the same temperature range. The dialkyldialkoxy derivatives in which R1 = alkyl; R2 = alkoxy and in which R1 = alkoxy; R2 = alkyl (R1 and R2 = heptyl, nonyl, hexyloxy or nonyloxy) exhibit enantiotropic mesophases. The derivatives in which R1 = alkoxy have a significantly lower crystal–nematic transition temperature than the corresponding derivatives (R2 = alkoxy), although the two types of derivatives have a similar nematic–isotropic transition temperature.