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), where NN is 4,4‘-dicarboxy-2,2‘-bipyridine (dcbpy) or 4,7-dicarboxy-1,10-phenanthroline (dcphen) and SS is ethyl-2-cyano-3,3-dimercaptoacrylate (ecda), quinoxaline-2,3-dithiolate (qdt), 1,2-benzenedithiolate (bdt), or 3,4-toluenedithiolate (tdt), that have various ground-state oxidation potentials has been synthesized and anchored to nanocrystalline titanium dioxide electrodes for light-to-electricity conversion in regenerative photoelectrochemical cells with an I -/I - 3 acetonitrile electrolyte. The intense mixed-Pt/dithiolate-to-diimine charge-transfer absorption bands in this series could be tuned from 440 to 580 nm by choosing appropriate dithiolate ligands, and the highest occupied molecular orbitals varied by more than 500 mV. Spectrophotometric 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 carboxylate group of the dcphen ligand. Binding of Pt(dcbpy)(qdt) to porous nanostructured TiO2 films was analyzed using the Langmuir adsorption isotherm model, yielding an adsorption equilibrium constant of 4 × 105 M-1. The amount of dye adsorbed at the surface of TiO2 films was 9.5 × 10-8 mol/cm2, which is ca. 50% lower than the full monolayer coverage. The resulting 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 conversion applications. The visible spectra of the 3,4-toluenedithiol-based sensitizers showed an enhanced red response, but the lower photocurrent efficiency observed for these sensitizers stems in part from a sluggish halide oxidation rate and a fast recombination of injected electrons with the oxidized dye.