Binary mixtures were prepared from an azobenzene derivative and a liquid-crystal (LC) compound that exhibits smectic C (SmC) and bicontinuous cubic (Cub(bi)) LC phases. Reversible switching between the two phases in response to UV-light irradiation was observed. This light-driven SmC-to-Cub(bi) transition is the first example showing the increased dimensionality of molecular ordering with isomerization of azobenzenes (see figure).

The self-assembly of tris(phenylisoxazolyl)benzene 1b with photochemically addressable azobenzene moieties produced toroidal nanostructures, the formation and dissociation of which were reversibly regulated upon photoirradiation. 1b displayed a mesogenic behavior. In the solution, the stacked assemblies along with their C3 axes were formed. In the mesophase, two molecules of 1b most likely adopted the antiparallel arrangement to stabilize the columnar organization. This assembling behavior most likely triggered the development of the supramolecular toroidal nanostructures.

The light-induced smectic C (SmC) to bicontinuous cubic (Cub(bi)) 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'-n-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(bi) 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(bi) 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(bi) phase at the temperature. This leads to a conclusion that the trans-cis 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(bi) phase with a twisted molecular arrangement.

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