E → Z Isomerization Catalyzed by Organic Photosensitizers Explored by Nonorthogonal Multireference Wave Function Methods

Triplet-triplet energy transfer (TEnT) processes, where energy is transferred from the triplet state of a donor (or photosensitizer, PS) to the singlet ground state of an acceptor (substrate), were applied to investigate E → Z photocatalyzed isomerization of alkenes. Using Fermi's golden rule and the four-state model, it was demonstrated that the rate of the catalytic cycle involving E-alkene is faster, causing its depletion and conversely the accumulation of Z-alkene. Three different systems have been investigated, namely styrenyl-silane and styrenyl-boronate, using benzophenone as the PS, and the fluorenone/stilbene complex. The present study validates and generalizes a recently proposed theoretical model for the mechanistic description of the preferential accumulation of the Z-isomer and depletion of the E-isomer. The quantitative similarities as well as the differences between the two currently investigated systems and the previously reported system were highlighted. Finally, the nonorthogonal multireference wave function approach was compared with methods based on CAS-type wave functions using the benzophenone/styrenyl-silane as the prototype system.