X-ray crystallographic study on photochromic crystals has been introduced. Although a number of photochromic materials have been reported, compounds which show photochromic reactivity in the crystalline phase are very rare and detailed X-ray crystallographic study on the crystalline photochromism is limited, because of the difficulty to increase the conversion rates in the crystals. Recently, in situ X-ray crystallographic analysis has been successfully carried out for N-salicylideneaniline, hexaarylbiimidazole, and diarylethene, by employing new phototechniques, such as two-photon excitation or linear-polarized light illumination. This paper reports on such direct X-ray structural analysis of diarylethenes as well as N-salicylideneanilines.
