Single-molecule spectroscopy of intramolecular electron transfer in donor-bridge-acceptor systems

It is widely appreciated that single-molecule spectroscopy (SMS) can be used to measure properties of individual molecules which would normally be obscured in an ensemble-averaged measurement. In this article, we show how SMS can be used to investigate intramolecular electron transfer (IET) processes in model dimer systems composed of two perylene chromophores connected via an adjustable bridge. The fluorescence behaviors of a large number of molecules are cataloged and the results statistically analyzed to gauge information about the range of behaviors of the ensemble. Single-molecule fluorescence time trajectories reveal "blinks", momentary losses in fluorescence (>20 ms to seconds in duration), which are attributed to discrete IET excursions to the charge-separated (CS) state. We find that fluorescence blinking behavior is dependent on bridge length and chromophore geometry, which affect the electronic coupling and therefore the IET. The statistical trends observed in this analysis are used to corroborate the assignment of the blinking behavior to IET. These results and methodology have implications for molecular electronics, where understanding and controlling the range of possible behaviors inherent to molecular systems will likely be as important as understanding the individual behavior of any given molecule.