From Triplet to Twist: The Photochemical E/Z-Isomerization Pathway of the Near-Infrared Photoswitch peri-Anthracenethioindigo.

In recent years, large progress has been made to shift the absorption of photoresponsive molecules into the long-wavelength region of the electromagnetic spectrum. A breakthrough in this field was the recent development of -extended indigoid photoswitches, i.e., peri-anthracenethioindigo (PAT), exhibiting all-red and near-infrared addressability. The excited-state isomerization mechanisms of this very new addition to the realm of photoswitching are currently not understood at all, prohibiting a rational further development. In this study, we present a combined theoretical and experimental approach, including time-dependent density functional theory (TD-DFT) and second-order algebraic diagrammatic construction (ADC(2)) calculations as well as steady-state and time-resolved femtosecond spectroscopy, to explore the isomerization pathways of this photoswitch. Our findings show that photoisomerization on singlet potential energy surfaces (PESs) is highly unfavorable and instead show that photoswitching proceeds on the PES. These insights enable a deep understanding of thioindigoid photochemistry and demonstrate that extension of the -system and peri-connectivity in the heterocycle unlock extremely favorable photoswitching properties along with the desirable red-shift in absorption. Reliable photoswitching from the triplet is achieved because of its favorable energy, which evades undesirable interference of oxygen quenching. These results pave the way for advancing thioindigoid-based photoswitches to improved performance and functionality in a rational way.