Revisiting photocyclization of the donor-acceptor stenhouse adduct: missing pieces in the mechanistic jigsaw discovered

Donor-acceptor Stenhouse adducts (DASA) have recently emerged as a class of visible-light-induced photochromic molecular switches, but their photocyclization mechanism remains puzzling and incomplete. In this work, we carried out MS-CASPT2//SA-CASSCF calculations to reveal the complete mechanism of the dominant channels and possible side reactions. We found that a new thermal-then-photo isomerization channel, i.e., EEZ -EZZ -EZE, other than the commonly accepted EEZ -EEE -EZE channel, is dominant in the initial step. Besides, our calculations rationalized why the expected byproducts ZEZ and ZEE are unobserved and proposed a competitive stepwise channel for the final ring-closure step. The findings here redraw the mechanistic picture of the DASA reaction by better accounting for experimental observations, and more importantly, provide critical physical insight in understanding the interplay between thermal-and photo-induced processes widely present in photochemical synthesis and reactions.

Automated summary

In this study, the photocyclization mechanism of donor-acceptor Stenhouse adducts (DASA) was revealed through calculations. It was found that the thermal-then-photo isomerization channel EEZ -EZZ -EZE is dominant, instead of the commonly accepted EEZ -EEE -EZE channel, and a competitive stepwise channel was proposed for the final ring-closure step. These findings redraw the mechanistic picture of the DASA reaction and provide critical physical insight into the interplay between thermal-and photo-induced processes in photochemical synthesis and reactions.