Theoretical study on thermal stability and absorption wavelengths of closed-ring isomers of diarylethene derivatives

Quantum chemical investigations on diarylethene derivatives with various substituents linked to different positions of thienyl rings have been carried out. The calculations on molecular structures show that the distance between two reactive carbons, which has close relationship with the photochromic reactivity and cyclization quantum yields, is prominently affected by different alkyl and alkoxy groups at 2- and 2′-positions. The ground-state energy gaps (ΔE) between closed-ring and open-ring isomers change in parallel with the aromatic stabilization energy (ΔE′) for the same series of substituents at reactive carbons. Once ΔE or ΔE′ becomes smaller, the thermal cycloreversion energy barrier becomes larger and closed-ring isomers would exhibit excellently thermal stability. The absorption wavelengths of closed-ring isomers have also been investigated using time-dependent density functional theory (TD-B3LYP/6-31G(d) or TD-PBE0/6-31G(d)). Theoretical calculations can well reproduce experimental absorption spectrum data. Furthermore, this work rationally predicts absorption wavelengths of some unsynthesized diarylethenes with various groups at 2- and 2′- or 4- and 4′-positions. The theoretical study would provide some basic insight for the novel photochromic molecular design.