Aza‐Diarylethenes Undergoing Both Photochemically and Thermally Reversible Electrocyclic Reactions

Abstract

Exploring novel molecular photoswitches plays a crucial role in the field of photo‐functional materials chemistry. In this study, we synthesized aza‐diarylethenes with benzothiophene‐S,S‐dioxide as a part of hexatriene structure and investigated their photochromic properties. Unlike previously reported aza‐diarylethenes, which exhibit fast thermally reversible photochromism, the compounds synthesized here exhibited pseudo‐photochemically reversible photochromism. Due to their thermal stability, we successfully isolated the colored isomer. X‐ray crystallographic analysis revealed for the first time that the colored isomer adopts a closed‐ring structure with a bond between carbon and nitrogen atoms. Remarkably, these aza‐diarylethenes exhibited not only photochemical ring‐closing and ring‐opening reactions but also thermal ring‐closing and ring‐opening reactions, driven by a thermal equilibrium between the open‐ and closed‐ring isomers. This behavior, unprecedented for common diarylethenes, was elucidated through kinetic analysis, revealing an energy‐level diagram for the thermal equilibrium between these isomers. Furthermore, 1H NMR spectroscopy revealed that both photochemically and thermally generated closed‐ring isomers adopt the same structure, which was well explained based on the reaction mechanism of photochemical and thermal ring‐closing reactions. These findings not only advance the field of aza‐diarylethenes but also inspire future research in the development of new photoswitches.