Unraveling the Full Photochromic Reaction Pathway of Binaphthyl-Bridged Imidazole Dimers

Light-driven reversible molecular switching is a promising strategy for constructing stimuli-responsive dynamic materials. In this study, we present BN-DBOXPImD, a rationally designed negative photochromic molecule featuring dibenzoxepine units integrated into a binaphthyl-bridged imidazole dimer (BN-ImD) framework. This compound exhibits a markedly accelerated thermal reversion from the metastable colorless isomer to the stable colored isomer. Although BN-ImD derivatives are known to undergo multistate photoisomerization via a short-lived biradical intermediate, a quantitative assessment of each individual step has yet to be accomplished. Global analysis of the transient absorption spectra of the colored and colorless isomers enabled the determination of the rate constants and quantum yields for the individual photoisomerization processes. The photoconversion efficiency from the colored to the colorless isomer was determined to be 0.082, corresponding to an approximate 19% enhancement over the previously reported value of 0.069 for BN-ImD. Furthermore, a previously unknown isomer, exhibiting distinct photoresponsive behavior compared to that of the other isomers, was identified and successfully characterized. Carefully designed quantum chemical calculations successfully reproduced all photoisomerization diagrams, encompassing both thermal and photochemical isomerization pathways. These findings highlight the critical role of structural flexibility in governing the photochromic behavior of BN-ImD and offer a guiding framework for the rational design of next-generation molecular photoswitches exhibiting negative photochromism.