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 diphenylimi-dazole 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 degrees 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 irradiation.