Rhodamine-Platinum Diimine Dithiolate Complex Dyads as Efficient and Robust Photosensitizers for Light-Driven Aqueous Proton Reduction to Hydrogen

Three new dyads consisting of a rhodamine (RDM) dye linked covalently to a Pt diimine dithiolate (PtN₂S₂) charge transfer complex were synthesized and used as photosensitizers for the generation of H₂ from aqueous protons. The three dyads differ only in the substituents on the rhodamine amino groups, and are denoted as Pt-RDM1, Pt-RDM2, and Pt-RDM3. In acetonitrile, the three dyads show a strong absorption in the visible region corresponding to the rhodamine π-π∗ absorption as well as a mixed metal-dithiolate-to-diimine charge transfer band characteristic of PtN₂S₂ complexes. The shift of the rhodamine π-π∗ absorption maxima in going from Pt-RDM1 to Pt-RDM3 correlates well with the HOMO-LUMO energy gap measured in electrochemical experiments. Under white light irradiation, the dyads display both high and robust activity for H₂ generation when attached to platinized TiO₂ nanoparticles (Pt-TiO₂). After 40 h of irradiation, systems containing Pt-RDM1, Pt-RDM2, and Pt-RDM3 exhibit turnover numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast transient absorption spectroscopy reveals that energy transfer from the rhodamine ¹π-π∗ state to the singlet charge transfer (¹CT) state of the PtN₂S₂ chromophore occurs within 1 ps for all three dyads. Another fast charge transfer process from the rhodamine ¹π-π∗ state to a charge separated (CS) RDM^(0•)-Pt^(+•) state is also observed. Differences in the relative activity of systems using the RDM-PtN₂S₂ dyads for H₂ generation correlate well with the relative energies of the CS state and the PtN₂S₂³CT state used for H₂ production. These findings show how one can finely tune the excited state energy levels to direct excited state population to the photochemically productive states, and highlight the importance of judicious design of a photosensitizer dyad for light absorption and photoinduced electron transfer for the photogeneration of H₂ from aqueous protons.