Mechanistic Explanation of Enhanced E → Z Photocatalyzed Isomerization Kinetics

Synthetic access to the Z-isomer of substituted alkenes is a challenging task due to the thermodynamically disfavored energy profile. Rather than direct synthesis, irradiation in the presence of a photosensitizer can lead to accumulation with a high ratio of Z-isomer. While classical photochemical models can provide a qualitative explanation of this behavior, quantitative insight into the kinetics of the process remain limited. In this study, we investigate the mechanistic aspects of E → Z photocatalyzed isomerization using kinetic arguments based on triplet energy transfer. Using Fermi's golden rule and four-state model, we demonstrated that the rate of catalytic transformation involving the E-alkene is faster, causing its depletion and conversely the accumulation of the Z-alkene. To accomplish this, we utilize the nonorthogonal wave function method, enabling simultaneous study of all pathways, emphasizing the importance of charge transfer (CT) states. The findings of this work provide quantitative validation of photochemical concepts based on electronic structure theory, clarifying how two structurally similar isomers can exhibit such different kinetics, leading to the accumulation of the Z-isomer under light irradiation in the presence of a photosensitizer.