Quantitative Analysis of Thermal Equilibrium on Multistate Photo/Thermochromism in Binaphthyl‐Bridged Imidazole Dimers

Recently, multistate photochromism in a single compound has attracted considerable attention as a versatile photoswitch for controlling complex photochemical systems. However, elucidating the reaction kinetics and mechanisms of multistate photochromism has remained challenging due to time scale gaps among the reactions and the concerted spectral changes associated with multiple photoinduced processes. In this study, we investigated the photochemical properties of a newly synthesized fluorene‐functionalized binaphthyl‐bridged imidazole dimer. The compound exhibits unique multistate photochromism, in which the reddish isomer, the colorless isomer, and the yellowish isomer interconvert via a short‐lived blue‐colored biradical (BR). In our previous studies, quantitative analysis methods for these concerted photoisomerization processes were established; however, the kinetic and thermodynamic behavior of the yellowish isomer had not been experimentally evaluated. Here, we provide the first quantitative evaluation of its formation by employing a molecule that efficiently affords the yellowish isomer, thereby providing a comprehensive understanding of the multistate photochromism of the binaphthyl‐bridged imidazole dimer.