Rational Molecular Designs for Controlling Photochromism: Thermally Activated Valence Isomerization of Phenoxyl‐Imidazolyl Radical Complexes

Abstract

AbstractWe report a strategy to control the reversible photoinduced valence isomerization between a biradical and a quinoidal form based on a photochromic phenoxyl‐imidazolyl radical complex (PIC) framework. The thermal equilibrium of the valence isomerization is biased toward the biradical form by substituting the π‐conjugated substituents and the electron‐donating groups, while the equilibrium is biased toward the quinoidal form by twisting the substituting group at the 2‐position of the imidazole ring. The precise control of the reversible photoinduced valence isomerizations is especially important for fundamental spin chemistry. Moreover, because biradicals have several fascinating properties, for example, as initiators for polymerizations, broad absorption in the visible and near‐infrared regions, and large two‐photon absorption cross sections, light‐ and time‐gated open‐shell characteristics would stimulate the development of further photofunctional materials utilizing biradicals.