Stable Au(III) Benzohomoporphyrin: Synthesis, Structure, Near-Infrared Absorption, and Superoxide Radical Generation

Two Au(III) tetrapyrrole complexes were obtained from the reaction of a bent-shaped benzohomoporphyrin ligand and Au(III) acetate. Despite the small size of the tetrapyrrolic ligands, the Au(III) complexes exhibited a low-energy NIR absorption. Combined density functional theory and TDDFT calculations with natural transition orbital (NTO) (NTO) analysis show that the NIR absorption is dominated by intraligand π → π* excitations with only minor Au-centered acceptor mixing rather than by ligand-to-metal charge transfer. The NIR absorption is therefore attributed to a resonance- and structure-controlled narrowing of the frontier orbital gap within the non-aromatic benzohomoporphyrin framework. The generation of O₂^•- by gold complexes was investigated upon photoirradiation, and both Au(III) tetrapyrrole complexes showed high O₂^•- generation efficiency.