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 <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b>. 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 <b>Pt-RDM1</b> to <b>Pt-RDM3</b> 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 <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b> 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 PtN₂S₂ ³CT 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

Deactivating Unproductive Pathways in Multichromophoric Sensitizers

The effects of solvent and substituents on a multichromophoric complex containing a boron-dipyrromethene (Bodipy) chromophore and Pt­(bpy)­(bdt) (bpy = 2,2′-bipyridine, bdt =1,2-benzenedithiolate) were studied using steady-state absorption, emission, and ultrafast transient absorption spectroscopy. When the Bodipy molecule is connected to either the bpy or bdt in acetonitrile, excitation ultimately leads to the dyad undergoing triplet energy transfer (TEnT) from the redox-active Pt triplet mixed−metal-ligand−to−ligand′ charge transfer (³MMLL′CT) state to the Bodipy ³ππ* state in 8 and 160 ps, respectively. This is disadvantageous for solar energy applications. Here, we investigate two methods to lower the energy of the ³MMLL′CT state, thereby making TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the ³MMLL′CT state to over 1 ns, the time frame of our experiment. Additionally, electron-withdrawing groups, such as carboxylate and phosphonate esters, on the bpy lower the energy of the ³MMLL′CT state such that the photoexcited dyad remains in that state and avoids TEnT to the Bodipy ³ππ* state. It is also shown that a single methylene spacer between the bpy and phosphonate ester is sufficient to eliminate this effect, raising the energy of the ³MMLL′CT state and inducing relaxation to the ³ππ*