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Multi-redox catalysis requires the transfer of more than one charge
carrier and is crucial for solar energy conversion into fuels and valuable
chemicals. In photo(electro)chemical systems, however, the necessary
accumulation of multiple, long-lived charges is challenged by recombination
with their counterparts. Herein, we investigate charge accumulation in two model
multi-redox molecular catalysts for proton and CO2 reduction
attached onto mesoporous TiO2 electrodes. Transient absorption
spectroscopy and spectroelectrochemical techniques have been employed to study
the kinetics of photoinduced electron transfer from the TiO2 to the
molecular catalysts in acetonitrile, with triethanolamine as the hole
scavenger. At high light intensities, we detect charge accumulation in the millisecond
timescale in the form of multi-reduced species. The redox potentials of the
catalysts and the capacity of TiO2 to accumulate electrons play an
essential role in the charge accumulation process at the molecular catalyst.
Recombination of reduced species with valence band holes in TiO2 is
observed to be faster than microseconds, while electron transfer from
multi-reduced species to the conduction band or the electrolyte occurs in the
millisecond timescale. Finally, under light irradiation, we show how charge
accumulation on the catalyst is regulated as a function of the applied bias and
the excitation light intensity.