Dye sensitization of nanocrystalline titanium dioxide with square planar platinum(II) diimine dithiolate complexes

A series of platinum-based sensitizers of the general type Pt(NN)(SS), wher e NN is 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) or 4,7-dicarboxy-1,10-phenan throline (dephen) and SS is ethyl-2-cyano-3,3-dimercaptoacrylate (ecda), qu inoxaline-2,3-dithiolate (qdt), 1,2-benzenedithiolate (bdt), or 3,4-toluene dithiolate (tdt), that have various groundstate oxidation potentials has be en synthesized and anchored to nanocrystalline titanium dioxide electrodes for light-to-electricity conversion in regenerative photoelectrochemical ce lls with an I-/I-3(-) acetonitrile electrolyte. The intense mixed-Pt/dithio late-to-diimine charge-transfer absorption bands in this series could be tu ned from 440 to 580 nm by choosing appropriate dithiolate ligands, and the highest occupied molecular orbitals varied by more than 500 mV. Spectrophot ometric titration of the Pt(dcphen)(bdt) complex exhibits a ground-state pK (a) value of 3.2 +/- 0.1, which can be assigned to the protonation of the c arboxylate group of the dcphen ligand. Binding of Pt(dcbpy)(qdt) to porous nanostructured TiO2 films was analyzed using the Langmuir adsorption isothe rm model, yielding an adsorption equilibrium constant of 4 x 10(5) M-1. The amount bf dye adsorbed at the surface of TiO2 films was 9.5 x 10(-8) mol/c m(2), which is ca. 50% lower than the full monolayer coverage. The resultin g complexes efficiently sensitized TiO2 over a notably broad spectral range and showed an open-circuit potential of ca. 600 mV with an impressive fill factor of >0.70, making them attractive candidates for solar energy conver sion applications. The visible spectra of the 3,4-toluenedithiol-based sens itizers showed an enhanced red response, but the lower photocurrent efficie ncy observed for these sensitizers stems in part from a sluggish halide oxi dation rate and a fast recombination of injected electrons with the oxidize d dye.