H-shaped conjugated mesogens: synthesis and mesomorphic properties of 3,3′,5,5′-tetrakis(phenylethynyl)-2,2′-bithiophene derivatives

A new class of H-shaped conjugated mesogens, namely alkyl-and alkoxy-substituted derivatives of 3,3',5,5'-tetrakis( phenylethynyl)-2,2'-bithiophene (4PE2T), have been synthesised, and the relationships between molecular structures and mesomorphic properties investigated. The mesomorphic properties of the 4PE2T derivatives have been examined by differential scanning calorimetry and polarising optical microscopy. In the 4PE2Ts, (RC6H4CC)-C-1 [C8H2S2(CCC6H4R2)(2)]CCC6H4R1, where R-1 and R-2 = alkyl and alkoxy chains of various lengths, the tetraalkyl derivatives exhibit enantiotropic or monotropic nematic phases, whereas the tetraalkoxy derivatives all exhibit enantiotropic nematic phases. The derivatives with two alkyl and two alkoxy chains also show enantiotropic nematic behaviour. It is found that the formation of the mesophase and the crystal-nematic and nematic-isotropic transition temperatures are strongly affected by the length, number and position of the alkyl and alkoxy chains.