Intrinsically Inhibiting Singlet Quenching of Organic Sensitizers for Enhanced CO2 Photoreduction.

High-performance, precious-metal-free photosensitizers (PSs) are crucial for driving photocatalytic CO2 reduction (PCO2R) in an efficient and sustainable manner. Organic PSs exhibiting thermally activated delayed fluorescence (TADF) are particularly promising owing to their ideal photophysics and well-understood electron-transfer mechanisms. However, rational improvement via structural design remains elusive. In this study, we present a comparative study on three TADF-type PSs for precious-metal-free PCO2R. Our results reveal that the kinetic competition between intersystem crossing (ISC) and singlet excited-state quenching determines the utilization efficiency of the long-lived triplet state, a key species to enable high cage-escape yield and eventually the PCO2R performance. Guided by this principle, we have developed a Br-substituted PS derived from 1,3-dicyano-2,4,5,6-tetrakis(diphenylamino)-benzene, in which the heavy-atom effect ensures rapid ISC while avoids reinforcing singlet quenching. Such strong steering toward triplet utilizations has led to record-high quantum yields near 50% for complete CO production with a cobalt catalyst, significantly surpassing the performances of precious-metal benchmarks (Ru and Ir-based) and most documented organic PS-driven systems.