Vibrational State Dependence of Interfacial Electron Transfer: Hot Electron Injection from the S₁ State of Azulene into TiO₂ Nanoparticles

Strong dependence of the quantum yield of electron injection, Φ_injection, on the excess vibrational energy of the short-lived S₁ excited state of the donor was observed for a carboxyazulene chromophore, 6-MeAz-2-COOH, bound to the surface of colloidal TiO₂. The oxidation state of 6-MeAz-2-COOH was aligned with the CB of TiO₂ in such manner that the lowest vibrational levels of the S₁ state were below the band edge. Two distinct electron injection regimes were observed: a long wavelength one which was attributed to a low yield direct injection into trap sites and a short wavelength one corresponding to a much higher yield injection into the bulk CB of the TiO₂ nanoparticles. There is a 9-fold increase of Φ_injection as λ_excitation decreases from 690 to 525 nm, with the steepest, 4-fold rise occurring between 585 and 550 nm, that is, when the excess energy is approximately equivalent to 3 quanta of skeletal vibrations of the donor. The charge recombination kinetics in the 6-MeAz-2-COOH@TiO₂ system is also different for the low energy (λ > 585 nm) and high energy (λ < 585 nm) excitation of the donor. This behavior is consistent with excess energy dependent spatial range of injection and different trapping sites that are accessible to the “cold” and “hot” electrons, with the latter exhibiting a broader distribution of lifetimes and 24-times higher long-term yield at 30 ps after the excitation. Through demonstrating that it is possible to harvest electrons from unrelaxed, vibrationally hot donor states under ambient conditions in solution, these results open interesting directions for new developments in photovoltaics and photocatalysis