Study on structural phase transition induced by light in the organic dye crystal.

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Studies of photochromic materials have been made for a long time, but the photochromism in the crystal phase have not enough analyzed yet. Study of the photochromism in the crystal phase has a large advantage. As molecules array regularly in the crystal, we can know the direction of the atom in the photochromism. In the present study, we treat salicylideneanilines (SA) in which a proton transfers. The SA molecule in the enol phase changes to the keto phase after the ultraviolet light irradiation. Namely, the proton transfers and the benzen ring rotates. The absorption strength of the keto phase in the b-axis is stronger than that in the c-axis. On the contrary, the keto phase is generated more easily by ultraviolet light polarized parallel to the c-axis than by the light polarized parallel to the b-axis. This means that the direction of the photochromism is different from that of the electric dipole transition in the keto phase.Next, we measured temperature dependence of transformations from the enol to keto and vice versa, and obtained the thermal activation energies as 24meV and 17meV,respectively. These values are between those of the SA molecule in PMMA polymer and those of the SA molecule embedded in the dibenzyl crystal. These facts suggest taht the molecule rotates more easily compared to the SA in the dibenzyl crystal but it is difficult to rotate compared to the SA in the PMMA polymer.