Excitonic processes of semiconductors open up the possibility for pursuing photocatalytic organic syntheses. However, the insufficient spin relaxation and robust nonradiative decays in semiconductors place restrictions on both quantum yield and selectivity of these reactions. Herein, by taking polymeric carbon nitride (PCN)/acetone as a prototypical system, we propose that extrinsic aliphatic ketones can serve as molecular co-catalysts for synergistically promoting spin-flip transition and suppressing non-radiative energy losses. Spectroscopic investigations indicate that hot excitons in PCN can be transferred to ketones, while triplet excitons in ketones can be transferred to PCN reversely. As such, PCN/ketone systems exhibit considerable triplet-exciton accumulation and extended visible-light response, leading to excellent performance in exciton-involved photocatalysis like singlet oxygen generation. This work provides a fundamental understanding of energy harvesting in semiconductor/molecule systems, and paves the way for optimizing exciton-involved photocatalysis via molecular co-catalyst design.