Interfacial Electron Transfer in TiO2 Surfaces Sensitized with Ru(II)-Polypyridine Complexes

Studies of interfacial electron transfer (IET) in TiO2 surfaces functionalized with (1) pyridine-4-phosphonic acid, (2) [Ru(tpy)(tpy(PO3H2))](2+), and (3) [Ru(tpy)(bpy)(H2O)-Ru(tpy)(tpy(PO3H2))](4+) (tpy = 2,2':6,2 ''-terpyridine; bpy = 2,2'-bipyridine) are reported. We characterize the electronic excitations, electron injection time scales, and interfacial electron transfer (IET) mechanisms through phosphonate anchoring groups. These are promising alternatives to the classic carboxylates of conventional dye-sensitized solar cells since they bind more strongly to TiO2 surfaces and form stable covalent bonds that are unaffected by humidity. Density functional theory calculations and quantum dynamics simulations of IET indicate that electron injection in 1-TiO2 can be up to 1 order of magnitude faster when 1 is attached to TiO2 in a bidentate mode (tau similar to 60 fs) than when attached in a monodentate motif (r similar to 460 fs). The JET time scale also depends strongly on the properties of the sensitizer as well as on the nature of the electronic excitation initially localized in the adsorbate molecule. We show that JET triggered by the visible light excitation of 2-TiO2 takes 1-10 ps when 2 is attached in a bidentate mode, a time comparable to the lifetime of the excited electronic state. IET due to visible-light photoexcitation of 3-TiO2 is slower, since the resulting electronic excitation remains localized in the tpy-tpy bridge that is weakly Coupled to the electronic states of the conduction band of TiO2. These results are particularly valuable to elucidate the possible origin of IET efficiency drops during photoconversion in solar cells based oil Ru(II)-polypyridine complexes covalently attached to TiO2 thin films with phosphonate linkers.