Excited State Interfacial Electron Transfer from a Compound with a Single Pyridine Ligand

The coordination compound Ru(NH3)5(eina)(PF6)2, where eina is ethyl isonicotinate, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films, abbreviated Ru(NH3)5(eina)/TiO2. The metal-to-ligand-charge-transfer (MLCT) absorption was found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 486 nm in acetonitrile and 528 nm in dimethylformamide for Ru(NH3)5(eina)(PF6)2 and at 512 and 555 nm for Ru(NH3)5(eina)/TiO2, respectively. The compound was found to be nonemissive with an excited state lifetime <10 ns under all conditions studied. Light excitation in fluid solution and when attached to insulating ZrO2 films resulted in a loss of the MLCT absorption, consistent with ligand field photochemistry. Pulsed light excitation of Ru(NH3)5(eina)/TiO2 yields an absorption difference spectrum consistent with an interfacial charge separated state, RuIII(NH3)5(eina)/TiO2(e-). This state forms within 10 ns and returns cleanly to ground state product within milliseconds. The injection quantum yields were determined by comparative actinometry and were found to be excitation wavelength dependent: (phi)inj(417 nm) = 0.30 ± 0.05 and (phi)inj(532.5 nm) = 0.15 ± 0.03. Regenerative solar cells based on Ru (NH3)5(eina)/TiO2 with 0.5 M TBAI, where TBA is tetrabutylammonium, and 0.05 M I2 in acetonitrile were very inefficient. Sluggish iodide oxidation is expected, on the basis of the negative E° (RuIII/II) = +0.17 (V vs Ag/AgCl) reduction potential, and this presumably allows a greater fraction of the injected electrons to recombine with the oxidized compound thereby lowering the solar cell efficiency.