In situ photochemical conversion from cinnamoyl-functionalized liquid-crystalline monomers to liquid-crystalline dimers

Liquid-crystalline (LC) monomers, which were functionalized with a cinnamoyl group on their extremity, were synthesized and irradiated with UV light in their LC phases. In the presence of a triplet sensitizer, most LC monomers were converted into the corresponding dimers, which were produced by the cycloaddition reaction of the cinnamoyl group. The photodimerization reaction could proceed while the LC phases were maintained, because the dimers showed LC phases whose temperature ranges were wider than those of the corresponding monomers. A ¹H NMR study of the LC dimers indicated that the cyclobutane unit dominantly had an anti-head-to-head configuration, that is, δ-truxinate. As the LC monomers, which had a phenyl biphenyl-4-carboxylate moiety as a mesogen, showed smectic A phases and the corresponding dimers also exhibited smectic A phases, we estimated the smectic layer distances by X-ray diffraction analysis and found that the dimers adopted the structure in which the two mesogens aligned laterally and existed in the same smectic layer in the LC phases.