Platinum(II) terpyridyl acetylide complexes on platinized TiO(2): toward the photogeneration of H(2) in aqueous media.

New platinum(II) terpyridyl acetylide complexes having the ability to bind to TiO(2) have been synthesized and assayed in their ability to sensitize platinized titanium dioxide for the photogeneration of H(2) using visible light (lambda > 410 nm). Specifically, the complexes [Pt(tpy-phen-COOH)(C[triple bond]C-C(6)H(5))]Cl (1), where tpy-phen-COOH = 4'-(4-carboxyphenyl)-[2,2';6',2'']terpyridine and C[triple bond]C-C(6)H(5) = phenylacetylide, and [Pt(tpy-COOH)(C[triple bond]C-C(6)H(5))]Cl (2), where tpy-COOH = 4'-carboxy-2,2';6',2''-terpyridine, were prepared to investigate the effectiveness of attachment and proximity to the TiO(2) surface on hydrogen yield. Both complexes 1 and 2 sensitize the photogeneration of hydrogen, but produce fewer turnovers than the unbound chromophore, [Pt(ttpy)(C[triple bond]C-C(6)H(5))]PF(6) (5). On the basis of these observations and electrochemical data, a major limitation to the effectiveness of these chromophores is their instability upon oxidation. To attempt to remedy this problem, two donor-chromophore (D-C) dyads, [Pt(tpy-phen-COOH)(C[triple bond]C-C(6)H(4)CH(2)-PTZ)]PF(6) (3), where C[triple bond]C-C(6)H(4)CH(2)-PTZ = N-(4-ethynylbenzyl)-phenothiazine and [Pt(tpy-COOH)(C[triple bond]C-C(6)H(4)CH(2)-PTZ)]Cl (4) were prepared to function as TiO(2)-attached sensitizers. Transient absorption measurements have shown that the PTZ moiety reductively quenches the Pt center in several picoseconds. While the resultant PTZ(+) radical cation is capable of oxidizing rapidly the triethanolamine sacrificial electron donor, dyads 3 and 4 attached to platinized TiO(2) do not function to generate hydrogen upon irradiation, in contrast with results seen for 1 and 2.