Triplet Spin Delocalization and Temperature Dependence for Adiabatic and Non-Adiabatic <i>Z</i>–<i>E</i> 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 <i>E</i>-form to a <i>Z</i>-form. The <i>Z</i>-form can thermally revert to the <i>E</i>-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 <i>Z</i>-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 <i>Z</i>-<i>E</i> 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.