Tuning the Direction of Photoinduced Electron Transfer in Porphyrin-Protected Gold Clusters

The interfacial electron-transfer reaction in ligand-protected gold clusters (AuCs) has been extensively investigated, but there are limited reports on organic chromophore ligands for photoinduced electron-transfer reactions of chromophore-attached AuCs. Here, we focused on porphyrins as chromophore ligands because of their tunable redox properties through the insertion of metal ions. We synthesized 1.3 nm diameter AuCs face-coordinated by free-base porphyrin (H2P) or Au-III porphyrin (AuP+) as photofunctional ligands. The synthesized H2P- and AuP+-protected AuCs (H2P-AuCs and AuP+-AuCs) were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible-near-infrared absorption spectroscopy. Femtosecond transient absorption measurements revealed the photodynamics of H2P-AuCs and AuP+-AuCs. The AuCs in H2P-AuCs and AuP+-AuCs act as electron acceptors and electron donors, respectively, achieving control of the photoinduced electron-transfer direction by inserting the metal ion into the porphyrin ligand. This drastic change is caused by the high electrophilicity of AuP+, indicating that the precise design of the protecting ligand can expand the potential of AuCs as photofunctional materials.