Light-induced charge separation and photocatalytic hydrogen evolution from water using (RuPtII)-Pt-II-based molecular devices: Effects of introducing additional donor and/or acceptor sites

In our hopes to improve the photocatalytic efficiency of photo-hydrogen-evolving molecular devices, several new dyads and triads possessing a photosensitizing Ru(bpy)(phen)(2)(2+) (or Ru(phen)(3)(2+)) chromophore (abbreviated as Ru-II) attached to both/either a phenothiazine moiety (abbreviated as Phz) and/or H-2-evolving PtCl2(bpy) units (abbreviated as Pt), such as Phz-Ru-II-Pt2 (triad), Ru-II-Pt2 (dyad), and Ru-II-Pt3 (dyad), were synthesized and their basic properties together with the photo-hydrogen-evolving characteristics were investigated in detail. The (MLCT)-M-3 phosphorescence from the RuII moiety in these systems is substantially quenched due to the highly efficient photoinduced electron transfer (PET). Based on the electrochemical studies, the driving forces for the PET were estimated as -0.07 eV for Phz-Ru-II-Pt2, -0.24 eV for Ru-II-Pt2, and -0.22 eV for Ru-II-Pt3, revealing the exergonic character of the PET in these systems. Luminescence lifetime studies revealed the existence of more than two decay components, indicative of a contribution of multiple PET processes arising from the presence of at least two different conformers in solution. The major luminescence decay components of the hybrid systems [tau(1) = 6.5 ns (Ru-II-Pt2) and tau(1) = 1.04 ns (Phz-Ru-II-Pt2) in acetonitrile] are much shorter than those of Phz-free/Pt-free Ru(bpy)(phen)(2)(2+) derivatives. An important finding is that the triad Phz-Ru-II-Pt2 affords a quite long-lived charge separated (CS) state (tau(CS) = 43 ns), denoted as Phz(+center dot)-Ru-Red-Pt2, as a result of reductive quenching of the triplet excited state of Ru(bpy)(phen)(2)(2+) by the tethering Phz moiety, where Ru-Red denotes Ru(bpy)(phen)(2)(+). Moreover, the lifetime of Phz(+center dot)-Ru-Red-Pt2 was observed to be much longer than that of Phz(+center dot)-Ru-Red. The photocatalytic H-2 evolution from water driven by these systems was examined in an aqueous acetate buffer solution (pH 5.0) containing 4-19% dimethylsulfoxide (solubilising reagent) in the presence of EDTA as a sacrificial electron donor. Dyads Ru-II-Pt2 and Ru-II-Pt3 were found to exhibit improved photo-hydrogen-evolving activity compared to the heterodinuclear Ru-Pt dyads developed so far in our group. On the other hand, almost no catalytic activity was observed for Phz-Ru-II-Pt2 in spite of the formation of a strongly reducing Ru-Red site (Phz(+center dot)-Ru-Red-Pt2), indicating that the electron transfer from the photogenerated Ru-Red unit to the PtCl2(bpy) unit is not favoured presumably due to the slow electron transfer rate in the Marcus inverted region.