Regioselectivity control of photodimerization of liquid-crystalline cinnamoyl compounds by phase variation: dual functionality of p-terphenyl substituent as a mesogen and a triplet sensitizer

Cinnamoyl-functionalized liquid-crystalline (LC) compounds having a 4″-substituted [1,1′;4′,1″]terphenyl (p-terphenyl) as mesogens were synthesized to investigate the abilities of the mesogen to act as a triplet sensitizer. UV (365 nm) irradiation of the LC compound having 4″-cyano-p-terphenyl in the crystalline and LC phases regioselectively produced the photodimers with a head-to-tail and a head-to-head cyclobutane unit, respectively. The p-terphenyl thus played the role of triplet sensitizer because the cinnamoyl group does not absorb 365 nm light. Due to the dual functionality of the p-terphenyl as the mesogen and the triplet sensitizer, we successfully performed the photochemical and regioselective cycloaddition to obtain two different LC dimers from one LC monomer.