Biphasic Electron Transfer Arising from Two Non-Interacting Donating States Unambiguously Revealed in C343-Sensitized Mesoporous SnO₂ by Combining Two Complementary Time-Resolved Spectroscopy Techniques

Dye-sensitized metal oxide (dye-MO) mesoporous films are promising building blocks in the field of solar energy conversion. At the dye-MO interface, the kinetic competition between donor-to-acceptor electron transfer (ET) and other recombination and relaxation channels largely determines the efficiency in photocatalytic and photovoltaic devices. Since the signals arising from an ET process and those linked with detrimental kinetic deactivation channels in conventional time-resolved spectroscopy techniques often have the same exponential fingerprint, it is still challenging to unambiguously characterize ET lifetimes on these architectures. Following previous works on sensitized oxide architectures, we show that by combining the information gathered in a sample when employing two complementary pump–probe methods, one sensitive to the photo excited electron depopulation in the dye donor and another sensitive to the photo-induced electron population in the MO acceptor, it is possible to remove any ambiguity in the determination of the ET rates in a dye-MO system. Specifically, we have employed fluorescence upconversion spectroscopy and time-resolved terahertz spectroscopy in a coumarin 343-tin oxide (C343–SnO2) mesoporous film to unambiguously resolve two independent noninteracting ET channels with characteristic ET rates and relative amplitudes almost quantitatively matching within error. Our approach stresses the potential of combining complementary spectroscopy techniques as a reliable procedure for the characterization of ET in sensitized systems and hence might contribute to the development of improved efficiencies in optoelectronic devices.