Triplet Spin Delocalization and Temperature Dependence for Adiabatic and Non-Adiabatic Z–E Isomerization Pathways in Azoarenes

Photoswitching molecules like the azoarenes have myriad potential applications, ranging from energy storage to targeted drug delivery. Upon irradiation, azoarenes convert from an E-form to a Z-form. The Z-form can thermally revert to the E-form through a classical-adiabatic or a triplet-assisted rotational mechanism. We show that strategic placements of heteroatoms and substituents can modulate spin delocalization in the triplet state, thereby tuning the S0–T1–S0 crossing points. Increased spin delocalization lowers the crossing point and shortens the thermal half-life of the Z-form. Computed spin densities at the diazo N atoms, along with computed S0–T1–S0 crossing points, activation entropies, and rate constants for 15 azoarenes are compared with available experimental data. There is evidence that some of the studied Z–E isomerizations proceed simultaneously via a classical-adiabatic and a triplet-assisted mechanism. The ratio of the two reaction pathways is temperature-dependent. Triplet spin density at the diazo N atoms is recognized as a useful indicator for predicting the thermal half-lives of these species.