Hybrid photocatalysts that integrate functional molecular units with semiconductor surfaces offer a promising route toward efficient artificial photosynthesis, yet controlling interfacial charge transfer dynamics remains a major challenge. Here we report a series of heteroleptic Ir(III) complex photosensitizers bearing 1-phenylisoquinoline ligands with phosphonic acid anchoring groups, designed to regulate the spatial localization of excited electrons. When combined with TiO₂-loaded polymeric carbon nitride and a supramolecular RuRe photocatalyst, these Ir(III) photosensitizers improve visible-light CO₂ reduction activity to selectively yield CO. Ir(III) complexes in which the excited electron is localized on the 2,2′-bipyridine ligand and is thus spatially separated from the semiconductor interface exhibited higher turnover numbers and apparent quantum yields than analogues with the excited state positioned closer to the semiconductor surface. Time-resolved photoluminescence and photoelectrochemical measurements confirmed that these molecular architectures suppress back electron transfer by facilitating long-lived one-electron-reduced species. This work demonstrates that precise control of excited-state electron localization in surface-immobilized photosensitizers provides an effective strategy to modulate interfacial charge recombination, thereby improving photocatalytic CO₂ reduction efficiency. The mechanistic insights gained here highlight a general molecular design principle for constructing integrated photocatalyst systems capable of efficient solar-to-chemical energy conversion.

Design Strategy for Heteroleptic Ir(III) Photosensitizers with Spatially Separated Excited Electrons toward Efficient CO₂ Reduction

Toshiya Tanaka, Masahito Oura, Rikuya Nagao, Joe Onodera, Yusuke Kuramochi, Ken Onda,* Osamu Ishitani,* Kazuhiko Maeda*

Artif. Photosynth. 2026, in press.