Multistate Photochromism of 1‑Phenylnaphthalene-Bridged Imidazole Dimer That Has Three Colorless Isomers and Two Colored Isomers

A new type of the bridged imidazole dimer with a 1-phenylnaphthalene moiety that bridges two diphenylimidazole units at the 2- and 2′-positions was synthesized and the photochemical and thermochemical properties were investigated. This molecule shows unique multistate photochromism, in which the stable colorless 1,2′-isomers A and B photochemically isomerize to the colorless 2,2′-isomer through the short-lived biradical with a half-life of 180 ns at 25 °C. The 2,2′-isomer thermally returns to the 1,2′-isomers A and B through the colored isomer at elevated temperatures. The 1,2′-isomers A and B, the 2,2′-isomer, and the colored isomer were isolated, and their molecular structures were determined by X-ray crystallographic analysis. These isomers are stable at room temperature and can be almost fully converted to the 2,2′-isomer by light irradiation. This study serves the useful strategy for the molecular design of a new type of negative photochromic molecules applicable to switch molecular properties by visible light irradiation.