Visible Light-Induced [2 + 2]-Cycloaddition and Photo-Ene Reactions: Spin-Dependent Catalysis and Quantum Tunneling...

Electron spin plays a vital role in photochemistry and its precise manipulation can enhance catalytic performance. However, the correlation between spin states and catalytic activity is still elusive, especially in the energy transfer (EnT)-catalyzed photocycloaddition and photo-ene reactions. Here, we rationalize the EnT process and clarify the chemoselectivity encompassing Marcus theory and ab initio methods. The initial photophysical process of photocatalyst isopropylthioxanthone (ITX) involves a nonadiabatic transition of ¹ππ*→³nπ*→³ππ* upon excitation, populating the long-lived ³ππ* state. In the presence of 1,5-dienes, the intrinsic deactivation pathway of photocatalyst ITX does not suppress the intermolecular spin-allowed EnT process with substrates, and this process is accompanied by spin multiplicity transfer within the Marcus normal region. From the triplet-state substrates, both the photocycloaddition and photo-ene reactions exhibit spin selectivity, whereas the chemoselectivity is governed in the open-shell singlet state. Furthermore, quantum mechanical tunneling effects play a decisive role in promoting the photo-ene reaction and reshaping the reaction pathways. Guided by Marcus theory, spin-dependent catalysis can be further regulated through amino-group substitution, which not only elevates the EnT efficiency but also enhances the photocycloaddition chemoselectivity.