The mechanism of charge transfer between quantum dots and redox molecules

Charge transfer between quantum dots (QDs) and redox molecules is not well described by the Marcus theory, the hall-mark theory for charge transfer in molecular systems. The Marcus inverted region, where the rate decreases with increasing the free energy difference, has never been observed in QDs. The previously reported hypothesis for the absence of the Marcus inverted region in QDs is an Auger-assisted charge transfer pathway. Here, we show that the Auger hypothesis does not hold to experimental tests. Instead, our experimental results suggest the presence of a distribution of molecular configurations on the QD surface that results in multiple energy surfaces. We measured the rate constants for two processes where Auger is either allowed or not: charge separation and charge recombination. We used ultrafast transient absorption spectroscopy to probe the rate of charge separation and recombination between PbS QDs and ferrocene derivatives ligands bound to their surface. We find that the rate constant for both charge separation and recombination increase by increasing the free energy difference, are temperature-independent, and increase with the number of molecular acceptors. All these results are against theoretical predictions for an Auger-assisted charge transfer and suggest the presence of multiple molecular configurations with charge transferring through the most favorable pathway.