Phase Structure and Phase Transition Mechanism for Light-Induced <i>Ia</i>3<i>d</i> Cubic Phase in 4′‑<i>n</i>‑Docosyloxy-3′-nitrobiphenyl-4-carboxlic acid/Ethyl 4‑(4′‑<i>n</i>‑docosyloxyphenylazo)benzoate Binary Mixture

The light-induced smectic C (SmC) to bicontinuous cubic (Cub<sub>bi</sub>) phase transition was investigated using grazing-incidence X-ray diffraction (GI-XRD) and Fourier transform infrared (FT-IR) spectroscopy to elucidate the mechanism at the molecular level. The sample was a binary mixture of 4′-<i>n</i>-docosyloxy-3′-nitrobiphenyl-4-carboxylic acid with an azobenzene derivative having a similar structure. The GI-XRD analysis revealed that the lattice size of the light-induced Cub<sub>bi</sub> phase almost coincides with the extrapolated value of the thermally induced one to the irradiation temperature. The FT-IR analysis also showed that the UV irradiation shifts the peak positions toward their extrapolated wavenumbers that would be displayed by the thermally induced Cub<sub>bi</sub> phase at the temperature. These results indicate that both the molecular state and periodic structure realized by the irradiation may be regarded as the “postulated” state and periodic structure of thermally induced Cub<sub>bi</sub> phase at the temperature. This leads to a conclusion that the <i>trans</i>–<i>cis</i> photoisomerization of the azobenzene derivatives in the mixture gives rise to destabilization of the SmC phase with layered structure, alternatively favoring the formation of the Cub<sub>bi</sub> phase with a twisted molecular arrangement.