Acid-Assisted Thermal Ring-Opening Reaction of Photochromic Diarylethene Closed-Ring Isomers

Abstract

Diarylperfluorocyclopentenes bearing thiophene rings as aryl groups are thermally stable and exhibit photochemically reversible photochromism (P-type photochromic behavior). However, we found that the addition of acid such as trifluoroacetic acid (TFA) converts it into T-type photochromism that exhibits a thermal ring-opening reaction at room temperature. Remarkably, the rate constant for the thermal ring-opening reaction increases by a factor of 10⁵ to 10⁷ compared to the reaction in the absence of acid, indicating an extraordinary acceleration effect. The rate constant rises exponentially with increasing amounts of TFA. Solvent effects were also pronounced; among n-hexane, toluene, and dichloromethane, the reaction proceeded fastest in dichloromethane. The absorption spectrum of the intermediate in dichloromethane appeared at a longer wavelength than that of the closed-ring isomer. Time-dependent density functional theory (TD-DFT) calculations suggested the formation of an intermediate cation, based on the predicted absorption spectrum. Herein, we propose a mechanism for the acid-assisted thermal ring-opening reaction.