Extended conjugated mesogens: synthesis and mesomorphic properties of H-shaped mesogens based on 3,3′,5,5′-tetrasubstituted 2,2′-bithiophene with oligo(1,4-phenyleneethynylene) arms

A new class of extended conjugated mesogens, namely H-shaped mesogens based on 3,3',5,5'-tetrasubstituted 2,2'-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, [(RC6H4CCC6H2)-C-1(C2H5)(2)CC](2)[(RC6H4CCC6H2)-C-2(C2H5)(2)CC](2)C8H2S2 where R-1 and R-2 = 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 R-1 = R-2 = pentyl to heptyl exhibit enantiotropic mesophases, whereas the derivatives with octyl or nonyl chains exhibit monotropic mesophases. The tetraalkoxy derivatives in which R-1 = R-2 = 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 R-1 = alkyl; R-2 = alkoxy and in which R-1 = alkoxy; R-2 = alkyl (R-1 and R-2 = heptyl, nonyl, hexyloxy or nonyloxy) exhibit enantiotropic mesophases. The derivatives in which R-1 = alkoxy have a significantly lower crystalnematic transition temperature than the corresponding derivatives (R-2 = alkoxy), although the two types of derivatives have a similar nematicisotropic transition temperature. [GRAPHICS]