The photocatalytic generation of hydrogen (H2) from protons by two cyclometalated ruthenium-platinum polypyridyl complexes, [Ru(bpy)2(2,5-bpp)PtIS](2+) (1) and [Ru(dceb)2(2,5-bpp)PtIS](2+) (2) [where bpy = 2,2'-bipyridine, 2,5-bpp = 2,2',5',2″-terpyridine, dceb = 4,4'-di(carboxyethyl)bipyridine, and S = solvent], is reported. Turnover numbers (TONs) for H2 generation were increased by nearly an order of magnitude by the introduction of carboxyethyl ester units, i.e., from 80 for 1P to 650 for 2P after 6 h of irradiation, with an early turnover frequency (TOF) increasing from 15 to 200 h(-1). The TON and TOF values for 2P are among the highest reported to date for supramolecular photocatalysts. The increase correlates with stabilization of the excited states localized on the peripheral ligands of the light-harvesting Ru(II) center.

A new approach to increasing the faradaic efficiency of dye-sensitised photocathodes for H2 evolution from water is described, using integrated photocatalysts based on a ruthenium 4,4′-diethoxycarboxy-2,2′-bipyridine chromophore linked via terpyridine or triazole to a Pd or Pt-based H+ reduction catalyst.

A series of supramolecular assemblies of the type [Ru(L-L)2(L'-L)MX2)](n+) are reported where L-L is 2,2'-bipyridine (bipy), 4,4'-di-tetra-butyl-bipyridine (tbbipy) or 4,4'-diethoxycarbonyl-2,2'-bipyridine (dceb), L-L' is tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz), 2,2':5',2''-terpyridine (2,5-bpp), 2,2':6',2''-terpyridine, (2,6-bpp), 2,5-di(pyridine-2-yl)pyrazine (2,5-dpp) or 2,3-di(pyridine-2-yl)pyrazine (2,3-dpp), and MX2 is PdCl2, PtCl2 or PtI2. The photocatalytic behaviour with respect to hydrogen generation of these compounds and their ultrafast photophysical properties are discussed as a function of the nature of the peripheral ligands, the bridging ligands and the catalytic centre. The results obtained show how differences in the chemical composition of the photocatalysts can affect intramolecular photoinduced electron transfer processes and the overall photocatalytic efficiency.

Recent studies on hydrogen-generating supramolecular bimetallic photocatalysts indicate a more important role of the peripheral ligands than expected, motivating us to design a Ru/Pt complex with 4,7-diphenyl-1,10-phenanthroline peripheral ligands. Photoinduced intra- and inter-ligand internal conversion processes have been investigated using transient absorption spectroscopy, spanning the femto- to nanosecond timescale. After photoexcitation and ultrafast intersystem crossing, triplet states localised on either the peripheral ligands or on the bridging ligand/catalytic unit are populated in a non-equilibrated way. Time-resolved photoluminescence demonstrates that the lifetime for the Ru/Pt dinuclear species (795±8 ns) is significantly less than that of the mononuclear analogue (1375±20 ns). The photocatalytic studies show modest hydrogen turnover numbers, which is possibly caused by the absence of an excited state equilibrium. Finally, we identify challenges that must be overcome to further develop this class of photocatalysts and propose directions for future research.