Electronic Structure and Interfacial Hole Transfer in a Di-Rhodium Photocatalyst on a p‑Type NiO Electrode.

Dye-sensitized photoelectrochemical cells are a promising route for solar-driven hydrogen production, using molecular dyes to generate electron-hole pairs that drive electrochemical reactions. A key challenge is achieving efficient charge transfer among the sensitizer, electrode, and catalyst. Paddlewheel dirhodium (DiRh) complexes have recently emerged as effective single-molecule photocatalysts, showing high activity when anchored to nickel oxide (NiO) electrodes for hydrogen evolution under red light. Here, we employ density functional theory (DFT) and projection-operator diabatization (POD) analysis to investigate the electronic structure of DiRh, its interaction with NiO, and the mechanisms of interfacial hole transfer. Our results show that DiRh binds strongly to NiO through stable mono- or bi- anchored configurations, with distinct ligand contributions to the charge-transfer pathway. While anchoring improves charge-separation efficiency, it has a minimal impact on the intrinsic properties of DiRh. Notably, the monoanchored s-DiRh/NiO interface exhibits stronger coupling to the NiO valence band and reduced charge recombination, making it the most favorable configuration for rapid hole injection. These findings provide atomistic insight into the structure-function relationships at dye-catalyst/electrode interfaces, offering design guidelines for next-generation photoelectrochemical systems for renewable hydrogen production. Beyond this case study, our work validates the use of a transferable hybrid-DFT/POD approach for realistic electrode-molecule systems, providing predictive atomistic insight into their interfacial electronic structure and charge-transfer characteristics.