Intramolecular Charge Transfer State to Suppress Back Electron Transfer in Singlet Photoredox Catalysis

Abstract

Back electron transfer (BET) is a critical process that impairs the singlet photoredox catalysis. In this study, we present that the S1 excited state of the 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 analyse 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 the ICT character, the FET coupling and the BET coupling are separately distributed, which will effectively suppress the occurrence of BET process. This finding confirms that employing photosensitizers with ICT S1 state is a simple yet effective strategy to achieve singlet photoredox catalysis, which may be extended to other light-driven chemical transformations.