Singlet and Triplet Pathways Determine the Thermal Z/E Isomerization of an Arylazopyrazole-Based Photoswitch

Understanding the thermal isomerization mechanism of azobenzene derivatives is essential to designing photoswitches with tunable half-lives. Herein, we employ quantum chemical calculations, nonadiabatic transition state theory, and photosensitized experiments to unravel the thermal Z/E isomerization of a heteroaromatic azoswitch, the phenylazo-1,3,5-trimethylpyrazole. In contrast to the parent azobenzene, we predict two pathways to be operative at room temperature. One is a conventional ground-state reaction occurring via inversion of the aryl group, and the other is a nonadiabatic process involving intersystem crossing to the lowest-lying triplet state and back to the ground state, accompanied by a torsional motion around the azo bond. Our results illustrate that the fastest reaction rate is not controlled by the mechanism involving the lowest activation energy, but the size of the spin-orbit couplings at the crossing between the singlet and the triplet potential energy surfaces is also determinant. It is therefore mandatory to consider all of the multiple reaction pathways in azoswitches in order to predict experimental half-lives.