Photoinduced electron transfer from Ru am(m)ine compounds with low-lying ligand field excited states to nanocrystalline TiO2

The coordination compounds [Ru(NH3)5(eina)](PF6)2, [Ru(NH3)4(deeb)](PF6)2 and [Ru(en)2(deeb)](PF6)2, where eina is ethyl isonicotinate, deeb is 4,4′-(CO2CH2CH3)2-2,2′-bipyridine and en is ethylenediamine, were synthesized and characterized. [Ru(NH3)4(deeb)](PF6)2·2CH3COCH3 and [Ru(en)2(deeb)](PF6)2·CH3COCH3 single crystals were characterized by X-ray crystallography. Near-infrared photoluminescence was observed after photoexcitation of [Ru(en)2(deeb)](PF6)2 in butyronitrile at 77 K. The excited states of these compounds were found to be short-lived in fluid solution and when anchored to mesoporous nanocrystalline (anatase) TiO2 thin films immersed in CH3CN at room temperature, consistent with excited-state lifetimes <10 ns. Prolonged steady-state visible light excitation of the compounds in fluid solution and when anchored to ZrO2 films immersed in CH3CN resulted in a loss of the metal-to-ligand charge transfer (MLCT) absorption that was attributed to ligand-field photochemistry. Pulsed 532 nm light excitation of the compounds anchored to TiO2 thin films yielded an interfacial charge-separated state, i.e. RuIII/TiO2(e−), that formed within 10 ns and returned cleanly to the initial RuII/TiO2 state on a micro- to milli-second time scale. Quantum yields for formation of this state determined by comparative actinometry were excitation wavelength dependent suggesting the involvement of “hot” MLCT excited states. The quantum yields were also dependent on acid/base pre-treatments of the TiO2 surface. Photoelectrochemical performances of [Ru(NH3)4(deeb)](PF6)2 and [Ru(en)2(deeb)](PF6)2 on TiO2 in regenerative solar cells were consistent with excitation-wavelength-dependent electron injection.