Enhanced Red-Light-Driven Hydrogen Evolution by a Diplatinum Photocatalyst by the Larger Wavefunction Leakage of Iodide Coordinated to the Platinum Center

The single-molecular photocatalyst Pt2(bpia)Cl3 (bpia = bis(2-pyridylimidoyl)amido), recently proven to promote red-light-driven hydrogen evolution reaction in water via singlet-to-triplet (S-T) transitions (Angew. Chem. Int. Ed., 2025, 64, e202418884), is shown to gain a significantly improved photocatalytic efficiency upon donating iodide instead of chloride to give Pt2(bpia)I3. Upon iodide donation, the absorption and emission energies both show a red shift basically due to destabilization of the HOMO by mixing of the iodide orbitals having essentially higher orbital energies compared to those of chloride. Consequently, the triplet lifetime and the luminescence quantum yield are both decreased by obeying the energy gap law. The unique S-T transition features of Pt2(bpia)Cl3 are preserved in Pt2(bpia)I3 with the excited-state redox properties remaining unchanged. In spite of apparently disadvantageous features induced by the iodide donation, Pt2(bpia)I3 is ascertained to promote the photocatalytic H2 evolution in a considerably higher rate in comparison with Pt2(bpia)Cl3, primarily attributed to the higher reductive quenching efficiency for 3*Pt2(bpia)I3. The observations are rationalized due to the substantially more excellent acceptor characteristics of iodide due to its larger wavefunction leakage required to have a larger electronic coupling factor in driving the outer-sphere electron transfer from the sacrificial electron donor.