Understanding the Mechanism of Triplet-Triplet Energy Transfer in the Photocatalytic [2 + 2] Cycloaddition: Insights From Quantum Chemical Modeling

We investigate the mechanism of [2 + 2] photocycloaddition reaction of 3-(but-3-enyl)oxyquinolone using a chiral xanthone-containing triplet sensitizer. Quantum chemical computer models were utilized to examine the substrate-catalyst encounter complex structures, which were classified into syn- and anti-adducts. The photoactivation steps of the substrate were analyzed based on the Marcus equation of electron transfer, including intersystem crossings (ISC) and outer sphere triplet-triplet energy transfer (TTEnT). Our results show that the calculated rates of ISC are comparable for the two adducts, while the rates of TTEnT differ due to the orbital overlap between the donor and acceptor sites. After the TTEnT, a stereospecific cyclization occurs, completing the catalytic cycle. We propose a strategy to improve stereoselectivity by exploiting the intrinsic difference in TTEnT rates between the two encounter complex isomers.