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

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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.