Fast and Efficient Adiabatic Photoisomerization of Linearly Coupled Dithienylethenes.

Understanding and controlling the excited-state pathway dynamics of molecular photoswitches is critical for achieving rapid, efficient, and fatigue-resistant photoisomerization. Conventional diarylethenes (DAEs) undergo close-ring isomerization via nonadiabatic transitions involving conical intersections or intersystem crossing, which often introduce energetic losses and limited quantum yields. Herein, we report a new class of linearly coupled dithienylethene (DTE) derivatives that exhibit unprecedented photoisomerization behavior governed by an adiabatic excited-state mechanism. Spectroscopic analyses confirmed rapid switching and high conversion efficiency under ultraviolet irradiation, while quantum chemistry calculations revealed a smooth, barrierless evolution along the S surface without the involvement of surface crossings. This preoccupied adiabatic isomerization pathway enables efficient energy utilization and rapid photoreactivity of the system. This discovery introduces a rare example of adiabatic photoisomerization in DTEs and establishes a conceptual framework for designing high-performance photoswitches with tailored excited-state dynamics.