Interfacial electron transfer on TiO 2 sensitized with an axially anchored trans tetradentate Ru(II) compound

Abstract

The ruthenium complexes, trans-[Ru(phen-NH-phen)(eina) 2](PF 6) 2 and trans-[Ru(phen-NH-phen)(ina) 2](PF 6) 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 Ru III/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](PF 6) 2 compound was found to bind to nanocrystalline TiO 2 thin films from acetonitrile solution. Pulsed 532 nm excitation of trans-[Ru(phen-NH-phen)(ina) 2](PF 6) 2 anchored to mesoporous nanocrystalline TiO 2 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] +/TiO 2 (e −). The formation of this state could not be time resolved, consistent with rapid excited state injection into the TiO 2, k inj > 10 8 s −1. Comparative measurements with a Ru(bpy) 3 2 + / PMMA thin film actinometer yielded an injection quantum yield ( ϕ 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 × 10 8 s −1, and k 2 = 3.0 × 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%.