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

In recent years, significant progress has been made in the effort 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 π-extended indigoid photoswitches, i.e., peri-anthracenethioindigo (PAT), exhibiting all-red to near- infrared absorption. 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 (TD-)DFT and 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 E-to-Z photoisomerization on singlet potential energy surfaces (PESs) is highly unfavorable and instead shows that photoswitching proceeds on the T₁ PES. These insights enhance our understanding of thioindigoid photochemistry and demonstrate that extension of the π-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 from oxygen quenching. These results pave the way for advancing thioindigoid-based photoswitches to improved performance and functionality in a rational way.