From Light Absorption to Cyclization: Structure and Solvent Effects in Donor‐Acceptor Stenhouse Adducts

Abstract Donor‐acceptor Stenhouse adducts (DASAs) have emerged in the last years as novel reversible photoswitches characterized by the light‐induced interconversion between a linear open‐chain isomer and a compact closed‐ring isomer. Despite the considerable interest for potential applications (due to their changes in size, color and polarity), several steps of the photoswitching mechanism are still not completely understood, hence limiting the rational design of these compounds. Herein we propose a complete computational study of the switching mechanism by means of TD‐DFT and CASPT2//CASSCF calculations. Special attention is paid to the excited state pathway, leading to a previously unknown general scenario with common features for different DASAs: After light absorption, a rotational energy barrier needs to be overcome in order to reach a conical intersection region with the ground state, whose topology and relative energy is investigated based on different solvents and acceptor moieties. Then, the evolution of the compounds in the ground state is considered by calculating different minima until the formation of the final closed form. Finally, the eventual reverse thermal path was also considered. We offer herein a complete mechanistic description which allows for an overall comparison with available experimental results.