Intramolecular charge transfer state to suppress back electron transfer for singlet photoredox catalysis

Back electron transfer (BET) is a critical process that will impair singlet photoredox catalysis. In this study, we present that the S1 excited state of a photocatalyst with intramolecular charge transfer (ICT) character can effectively inhibit BET, due to the spatial separation of the electronic couplings of the forward and back electron transfer pathways. With phenoxazine-modified molecules as photocatalysts and chlorobenzene as the substrate, we analyze how the spatial distribution of electronic couplings with/without ICT states would affect the photocatalysis efficiency. Theoretical results indicate that when the initial state of the photocatalyst has ICT character, the FET coupling and the BET coupling are separately distributed, which will effectively suppress the occurrence of the BET process. This finding confirms that employing photosensitizers with an ICT S1 state is a simple yet effective strategy to achieve singlet photoredox catalysis, which may be extended to other light-driven chemical transformations.