Interfacial electron transfer on TiO2 sensitized with an axially anchored trans etradentate 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 H-1 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 Ru-III/II couple, E degrees (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- [Ru-III (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, k(inj) > 10(8) s(-1). Comparative measurements with a Ru(bpy)(3)(2+)/PMMA thin film actinometer yielded an injection quantum yield (phi(inj)) of 0.8. Charge recombination required milliseconds for completion and followed a bi-second-order equal concentration kinetic model with k(1) = 1.0 x 10(8) s(-1), and k(2) = 3.0 x 10(5) s(-1). In regenerative solar cells with 0.5 M LiI and 0.005 M I-2 in acetonitrile, incident photon-to-current efficiencies were typically less than 10%. (c) 2007 Elsevier B. V. All rights reserved.