Photochemical Synthesis of a Main-Chain LC Oligomer from an α,ω-Dicinnamoyl-Functionalized LC Monomer: Photoirradiation in the Crystalline State

Abstract

An LC compound 1,4-bis[4-(6-cinnamoyloxyhexyloxy)benzoyloxy]benzene, 1, was synthesized, having cinnamoyl groups at the each end of the molecule, and was found to show polymorphism in the crystalline state. A rapid cooling from the isotropic melt of 1 to room temperature gave a K1 crystal, while annealing the mesophase of 1 at 95°C afforded a K2 crystal. UV irradiation (>300 nm) of both the K1 and K2 crystals at room temperature resulted in the formation of linear LC oligomers with cyclobutane rings in the main chain. ¹H NMR spectroscopy revealed that the cyclobutane units of the oligomer obtained from K1 assumed anti and syn head-to-head configurations, whereas those from K2 predominantly formed an anti head-to-tail structure.

Keywords:

• cinnamoyl
• liquid crystal
• liquid-crystalline polymer
• photocycloaddition
• topochemistry