Interfacial electron transfer on TiO2 sensitized with an axially anchored trans tetradentate Ru(II) compound

The ruthenium complexes, trans-[Ru(phen-NH-phen)(eina)2](PF6)2 and trans-[Ru(phen-NH-phen)(ina)2](PF6)2 where phen-NH-phen = N,N-bis(1,10-phenanthroline-2-yl)amine, ina = isonicotinic acid and eina = ethyl isonicotinate, have been synthesized and characterized by 1H NMR, elemental analysis, and IR spectroscopy. The compounds were non-emissive at room temperature, but displayed intense photoluminescence in 4:1 ethanol/methanol glasses at 77 K with corrected emission maximum at 570–580 nm. A quasi-reversible wave observed in cyclic voltammetry experiments was assigned to the RuIII/II couple, E° (trans-[Ru(phen-NH-phen)(eina)2)3+/2+ = +1.22 V versus Ag/AgCl. The trans-[Ru(phen-NH-phen)(ina)2](PF6)2 compound was found to bind to nanocrystalline TiO2 thin films from acetonitrile solution. Pulsed 532 nm excitation of trans-[Ru(phen-NH-phen)(ina)2](PF6)2 anchored to mesoporous nanocrystalline TiO2 thin films resulted in an absorption difference spectra consistent with the formation of an interfacial charge separated state trans-[RuIII (phen-NH-phen)(ina)2]+/TiO2 (e−). The formation of this state could not be time resolved, consistent with rapid excited state injection into the TiO2, kinj > 108 s−1. Comparative measurements with a Ru(bpy) 3 2 + / PMMA thin film actinometer yielded an injection quantum yield (ϕinj) of 0.8. Charge recombination required milliseconds for completion and followed a bi-second-order equal concentration kinetic model with k1 = 1.0 × 108 s−1, and k2 = 3.0 × 105 s−1. In regenerative solar cells with 0.5 M LiI and 0.005 M I2 in acetonitrile, incident photon-to-current efficiencies were typically less than 10%.