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.