A molecular photocatalyst consisting of a Ru(II) photocenter, a tetrapyridophenazine bridging ligand, and a PtX2 (X=Cl or I) moiety as the catalytic center functions as a stable system for light-driven hydrogen production. The catalytic activity of this photochemical molecular device (PMD) is significantly enhanced by exchanging the terminal chlorides at the Pt center for iodide ligands. Ultrafast transient absorption spectroscopy shows that the intramolecular photophysics are not affected by this change. Additionally, the general catalytic behavior, that is, instant hydrogen formation, a constant turnover frequency, and stability are maintained. Unlike as observed for the Pd analogue, the presence of excess halide does not affect the hydrogen generation capacity of the PMD. The highly improved catalytic efficiency is explained by an increased electron density at the Pt catalytic center, this is confirmed by DFT studies.

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.

The contrasting early-time photodynamics of two related Ru/Pt photocatalysts with very different photocatalytic H2 generation capabilities are reported. Ultrafast equilibration (535 ± 17 fs) creates an electron reservoir on the peripheral ligands of the ester substituted complex, allowing a dramatic increase in photocatalytic performance. This insight opens the way towards a novel design strategy for H2 generating molecular photocatalysts.

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.

In einem intramolekularen Photokatalysator,b estehend aus einem Ru II