Non-linear disulphide-centred S-shaped oligomers containing inner and outer spacers with lateral Schiff base: synthesis and mesomorphic properties

A new series of non-linear S-shaped oligomers [bis-6-(2-((4-methoxyphenyl-imino)methyl)-5-(4'-hexyloxy-4-biphenyloxycarbonyl)-phenoxy)alkyl]disulphide have been synthesised and characterised. The present oligomers consist of a central disulphide (S-S) linkage, Schiff base, outer alkyl spacer (n1 = six carbons) and inner alkyl spacer (n2 = six to eleven carbons). Upon cooling, all the oligomers show monotropic nematic phase. The odd–even effect was found at the transition temperature of Cr-I (TCr-I) and temperature range from isotropization to crystallisation (∆TI-Cr). However, the odd–even effect was not observed at the transition temperatures of I-N and N-Cr as the values decrease when n2 increases. The representative oligomer with n2 = 7 undergone photoisomerisation behaviour and completed the conversion from trans to cis within a duration of 13 mins and vice versa (up to 1200 min). Optimized molecular structures of oligomers with n2 = 6 and 7 were obtained from TMoleX, and their length-to-breadth ratio can be reasonably employed to explain the odd–even effect in TCr-I, ∆TI-Cr and the reduction of its values in entropy change (∆SI-N/R).