Photochromic Phenoxyl–Imidazolyl Radical Complex via Homolytic C–O Bond Cleavage

We report a new photochromic phenoxyl imidazolyl radical complex (PIC) that exhibits reversible homolytic C–O bond cleavage upon UV light irradiation. Ultrafast transient absorption spectroscopy revealed pronounced spectral evolution on nanosecond time scales, reflecting rotational isomerization between two open form isomers. Time-resolved infrared absorption measurements confirmed their biradical character and supported the presence of both isomers in thermodynamic equilibrium. The thermal back-reaction of the photogenerated open form isomers occurs approximately 1000 times faster than that of a conventional PIC derivative, due to structural modifications of the phenoxyl unit. These results demonstrate that rational control over bond dissociation geometry enables the tuning of photochromic kinetics and provides a novel approach to designing fast-switching molecular systems.