π-Bridge Substitution in DASAs: the Subtle Equilibrium Between Photochemical Improvements and Thermal Control.

Donor-Acceptor Stenhouse Adducts (DASAs) are playing an outstanding role as innovative and versatile photoswitches. Until now, all the efforts have been spent on modifying donor and acceptor moieties, to modulate the absorption energy and improve the cyclization and reversion kinetics. However, there is a strong dependence on specific structural modifications and a lack of predictive behavior, mostly due to the complex photoswitching mechanism. Here, by means of a combined experimental and theoretical study, we systematically explore the effect of chemical modification of the π-bridge linking the donor and acceptor moieties, finding significant impact on the absorption, photocyclization and relative stability of the open form. Especially, a position along the π-bridge is found to be the most suited to red-shift the absorption while preserving cyclization. However, thermal back-reaction to the initial isomer is blocked. These effects are explained in terms of an increased acceptor capability offered by the π-bridge substituent that can be modulated. This strategy opens the path toward derivatives with infra-red absorption and a potential anchoring point for further functionalization.