Analysis of Organophotocatalyst-Substrate Interactions for Understanding Triplet-Triplet Back Energy Transfer

Abstract The excited energy levels of a photocatalyst (PC) and an organic substrate critically affect the feasibility of energy transfer (EnT) process. Despite having energy levels that are well-matched, the catalytic performance of the PC has not always been satisfactory, indicating there are other crucial factors that influence the catalytic efficiency and need to be considered. Finding additional critical factors can be challenging as negative factors often lead to unsuccessful reactions, making it difficult to conduct a thorough investigation. To address this challenge, we employed structurally similar indole-fused polycyclic complexes as organoPCs, which were developed by our laboratory for the study. Herein, we describe a binding interaction effect between a PC and a substrate for controlling the catalytic efficiency in an EnT-mediated DeMayo-type [2 + 2] cycloaddition process. A series of photophysical and computational investigations showed that the strong binding interaction of the PC with the substrate promotes the back triplet–triplet EnT process competently and thus deactivates the productive radical transformations. This finding provides an understanding of the inner-workings of EnT process, and will guide the rational design of new EnT processes. Furthermore, the excellent photocatalytic properties of the newly synthesized pentacyclic organoPC offer tremendous potential for applications in other photocatalytic reactions.