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Updated: Jan 17, 2026

Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
Published on: February 7, 2022
Triplet Spin Delocalization and Temperature Dependence for Adiabatic and Non-Adiabatic Z-E Isomerization Pathways in
Francisco A Martins1, Renan V Viesser2, João Vitor Schober1
1Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States.
Abstract:
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.
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