Tuning of Optical Properties and Thermal Cycloreversion Reactivity of Photochromic Diarylbenzene by Introducing Electron-Donating Substituents

Thermally reversible photochromic compounds having excellent physicochemical properties can be exploited for many practical applications, such as ophthalmic lenses, real-time holography, and super-resolution microscopy. In this study, we have designed and synthesized novel 1,2-diarylbenzene derivatives bearing various electron-donating substituents at the p-position of the phenyl ring to tune the optical and thermal properties for practical applications. The introduction of the electron-donating groups resulted in the red shift of the absorption spectra, an increase in the absorption coefficients of the open-ring isomers, and deceleration of the thermal cycloreversion of the closed-ring isomers. Theoretical analysis based on Hammett's substituent constant and density functional theory revealed that the rate of thermal cycloreversion became lower in proportion to the electron-donating ability of the substituent. The novel 1,2-diarylbenzene derivatives synthesized here could undergo photocyclization upon irradiation with UV-A light and fast thermal cycloreversion with a half-life of a few hundred milliseconds to seconds.