Iridium(III)-terpyridine photocatalysts with electron-rich and largely π-extended ligands for enhanced CO₂ reduction

The electronic structure of Ir(III) centers and their cyclometalated ligands dictates the performance of photocatalysts for CO₂ reduction, yet conventional Ir(III)-terpyridine systems are hindered by limited visible-light absorption capacity and slow charge transfer. Herein, we have designed two new Ir(III)-terpyridine complexes, namely Ir-iqbt and Ir-iqbf, by incorporating π-extended C^N ligands with benzo[b]thienyl and benzo[b]furan units, respectively. Ir-iqbf achieves a record CO turnover number (TON_CO = 144.5) and quantum yield (Φ_CO = 8.5 % at 450 nm), setting a new benchmark for Ir(III)-terpyridine photocatalysts. Experimental characterizations coupled with density functional theory (DFT)/time-dependent density functional theory (TDDFT) calculations reveal that Ir-iqbf, synthesized with an O-conjugated C^N ligand, exhibits significantly enhanced visible-light absorption and a high non-radiative transition rate(k_nr). This improvement is attributed to its optimized electronic structure, which facilitates efficient separation and transfer of photogenerated electron-hole pairs. Moreover, the enhanced electron delocalization ability of the ligand and nucleophilicity of the iridium center reduce the adsorption energy of CO₂, thereby accelerating the reduction process. These synergistic effects collectively contribute to the superior efficiency of CO₂-to-CO conversion observed in Ir-iqbf.