LAUSR

N-Salicylideneaminopyridine (SAP) is a derivative of the common organic photochromic compound, N-salicylideneaniline (SA), whose crystal changes colors from orange to red upon UV irradiation. Interestingly, the photochromic property of the crystal depends on the crystal structure. In this study, SAP derivatives and their co-crystals were synthesized, and their crystal structures were analyzed to determine the relationship between their structures and photochromic properties. The photochromic property was found to be closely related to the molecular conformation and the packing efficiency in the crystal. In general, crystals with large dihedral angles were photochromic, while those with small dihedral angles were non-photochromic. However, even for crystals with large dihedral angles, small free available spaces in the crystal structure led to non-photochromic properties. This study demonstrates the importance of a combination of structural factors to elucidate the photochromic property of SAP derivatives.