Photoinduced electron transfer in platinum(II) terpyridyl acetylide chromophores: Reductive and oxidative quenching and hydrogen production

A series of luminescent platinum(II) terpyridyl acetylide complexes, ([Pt(tpy)(CCPh)]ClO4 (1) and [Pt(ttpy)(CC-p-C6H4R)]ClO4, where tpy = terpyridine, ttpy = 4'-p-tolylterpyridine, R = H, Cl, Me) (2-4) were studied with regard to excited-state quenching by dialkylated bipyridinium cations as electron acceptors and triethanolamine (TEOA) as an electron donor and the photogeneration of hydrogen from systems containing the chromophore, the dialkylated bipyridinium cations, TEOA, and colloidal Pt as a catalyst. The dialkylated bipyridinium cations include methyl viologen (MV2+) and a series of diquats prepared from 2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine. The quenching rates for the diquats for one of the chromophores (2) are close to the diffusion-controlled limit. The most effective electron acceptor and relay for hydrogen evolution has been found to be 4,4'-dimethyl-1,1'-trimethylene-2,2'-bipyridinium (DQ4) which on photoreduction by the chromohore provides the strongest reducing agent of the diquats studied. The rate of hydrogen evolution depends in a complex way on the concentration of the bipyridinium electron relay, increasing with concentration at low concentrations and then decreasing at high concentrations. The rate of H-2 photogeneration also increases with TEOA concentration at low values and eventually reaches a plateau. The most effective system examined to date consists of the chromophore 2 (2.2 x 10(-5) M), DQ4 (3.1 x 10(-4) M), TEOA (2.7 x 10(-2) M), and Pt colloid (6.0 x 10(-5) M), and has produced 800 turnovers of H-2 (67% yield based on TEOA as sacrificial electron donor) after 20 h of photolysis with lambda > 410 nm.