Interfacial electron transfer in TiO(2) surfaces sensitized with Ru(II)-polypyridine complexes.

Studies of interfacial electron transfer (IET) in TiO(2) surfaces functionalized with (1) pyridine-4-phosphonic acid, (2) [Ru(tpy)(tpy(PO(3)H(2)))](2+), and (3) [Ru(tpy)(bpy)(H(2)O)-Ru(tpy)(tpy(PO(3)H(2)))](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 TiO(2) 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-TiO(2) can be up to 1 order of magnitude faster when 1 is attached to TiO(2) in a bidentate mode (tau approximately 60 fs) than when attached in a monodentate motif (tau approximately 460 fs). The IET 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 IET triggered by the visible light excitation of 2-TiO(2) 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-TiO(2) 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 TiO(2). These results are particularly valuable to elucidate the possible origin of IET efficiency drops during photoconversion in solar cells based on Ru(II)-polypyridine complexes covalently attached to TiO(2) thin films with phosphonate linkers.