Polyoxometalate-based donor–acceptor-type photocatalytic-active cluster: Exploring structure–activity relationships in oxidation of alkylbenzene C(sp³)–H bond via ligand design

The selective oxidation of inert C(sp³)–H bonds in alkylbenzene remains a critical challenge in synthetic chemistry, necessitating advanced catalytic systems for sustainable C–H functionalization. Herein, we present a quinolinium-functionalized Anderson-type polyoxometalate (POM) (TBA-6MQ-Al) that achieves a toluene conversion of 64.24% and a benzoic acid yield of 57.32% under mild visible light (40 W 420−430 nm blue light-emitting diode (LED)), outperforming conventional POM-based photocatalysts. Systematic investigations reveal that methyl substitution on the quinolinium ligand enhances intersystem crossing efficiency, promoting triplet state formation for efficient O₂ activation. Radical quenching and electronic paramagnetic resonance (EPR) spectroscopy confirm superoxide/hydroxyl radicals and photogenerated electrons as key reactive species, whereas density functional theory (DFT) calculations elucidate the electronic structure–activity relationship. This work establishes a molecular engineering paradigm for optimizing POM redox properties, advancing sustainable C–H oxidation strategies with potential applications in green catalysis.