Regulatory Mechanism and Kinetic Assessment of Energy Transfer Catalysis Mediated by Visible Light

The visible-light-mediated energy transfer catalysis plays a pivotal role in the photochemical synthesis. Although many significant advances in this field have been achieved within the past decade, the knowledge of the photochemically tunable metal-ligand interaction for photocatalysts, the manipulation principle of excited-state properties, and the available electronic excitation for the free and bound substrates, which makes it possible to design some photo- and auxiliary catalysts based on the proposed mechanism, is still sparse. In the present work, we investigated the paradigm example of intermolecular [2 + 2] photocycloaddition reactions for 2'-hydroxychalcones coordinated by the chiral Lewis acids, using tris(bipyridyl) ruthenium(II) as a photosensitizer. The electronic structure calculations at the CASPT2//CASSCF/PCM level of theory, as well as the kinetic assessment of energy transfer process using the Fermi's golden rule and the Dexter model, were performed to provide useful benchmarks for the elucidation of energy transfer photocatalysis. The excitation properties for the enone substrate are photochemically tunable in the presence of various metal ion based chiral Lewis acids, which rules out the background reaction of excited state intramolecular proton transfer (ESIPT). The preferable photosensitized pathway with dual catalysts can be also regulated cooperatively as a priority by the introduction of high valence d(0) ions that notably decreases the triplet energy for the photocatalysis reaction but without an efficient improvement on the intersystem crossing rate of metal-chelated substrates. Our kinetic evaluation method, which has been applied to different catalysis systems, reveals various factors that determine the energy transfer efficiency, including the rigidity of substrate-chiral Lewis acid complexes, the reasonable triplet energy gap between donor and acceptor, the molecular orientation of complexes, and the electronic characters of triplet excited states.