Stark Effects after Excited-State Interfacial Electron Transfer at Sensitized TiO2 Nanocrystallites

Photophysical studies were performed with [Ru(dtb)2(dcb)](PF6)2 and cis-Ru(dcb)(dnb)(NCS)2, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine, dcb is 4,4′-(COOH)2-2,2′-bipyridine, and dnb is 4,4′-(CH3(CH2)8)2-2,2′-bipyridine), anchored to anatase TiO2 particles (∼15 nm in diameter) interconnected in a mesoporous, 10 μm thick film immersed in Li+-containing CH3CN electrolytes with iodide or phenothiazine donors. Pulsed-laser excitation resulted in rapid excited-state injection and donor oxidation to yield TiO2(e−)s and oxidized donors, while the metal-to-ligand charge-transfer (MLCT) absorption spectrum of the Ru(II) coordination compounds differed from that which was initially excited. The spectral data were consistent with an underlying Stark effect and indicated that the surface electric field was not completely screened from the molecular sensitizer. The magnitude of the electric field was estimated to be ∼270 MV/m from Li+ titration experiments, corresponding to a ∼40 mV potential drop. With iodide donors, the amplitude of the Stark effect decreased over time periods where charge recombination was absent, behavior attributed to “screening” of the electric field by interfacial ionic reorganization. The screening kinetics were nonexponential but were well described by the Kohlrausch−Williams−Watts model, from which a characteristic rate constant, τo −1, of ∼1.5 × 105 s−1 was abstracted. At least seven other sensitizers and five different cations, as well as on SnO2 nanoparticle films, exhibited similar transient absorption behavior with iodide donor molecules indicating that the effect was quite general. In the presence of phenothiazine donors (or in the absence of an external donor), there was no clear evidence for screening, and the Stark effect disappeared concurrent with interfacial charge recombination. Complementary spectroelectrochemical studies of these same sensitized films displayed similar absorption spectra when the TiO2 thin film was partially reduced with a forward bias. Spectral modeling in the absence of donor molecules as well as studies of TiO2 thin films sensitized with two different Ru(II) compounds demonstrated that the electric field created by excited-state injection from one sensitizer influenced the absorption spectra of other sensitizers that had not undergone photoinduced electron injection.