The kinetics and mechanism of fast electron transfer (ET) between tin oxide nanoparticles and electrostatically bound Os(III) and Ru(III) complexes have been examined via transient absorbance spectroscopy. Reaction-order studies establish that, at least in the short time regime, electrons are transferred directly from the tin oxide conduction band, rather than through localized redox trap states. The reactions occur in the high driving force regime (ΔG = -1.1 to -2.3 eV) and span the Marcus normal region, barrierless region, and inverted region. (Inverted reactivity, while commonplace in homogeneous solution-phase reactions, has only rarely been observed in interfacial reactions.) Depending on the reactant, normal or inverted kinetic behavior can also be observed via pH-induced manipulation of the conduction band-edge energy and, therefore, the overall reaction driving force. The observation of kinetically resolved ET over such a wide range of driving forces permits the reorganization energy to be evaluated directly from the maximum of a log(rate constant) versus driving force plot. The value obtained, 1.4 eV, is much larger than expected based on solvent contributions alone. Further analysis of driving force effects suggests that significant, but not dominant, nonclassical contributions (high-frequency vibrational contributions) to the reorganization energy exist. Rate measurements in the barrierless region yield an estimated initial-state/final-state electronic coupling energy, H(ab), of 15-30 cm-1, a value consistent with a moderately nonadiabatic ET pathway. Remarkably, even in the inverted region the reactions are thermally activated, with the activation effect evidently being amplified via an entropic driving force effect. Finally, the overall pattern of reactivity stands in remarkable contrast to the pH-independent, trapped-mediated kinetic behavior encountered for closely related metal complexes covalently bound to nanocrystalline TiO2 surfaces.
ASJC Scopus subject areas
- Colloid and Surface Chemistry