ja0343104_si_001.pdf (37.81 kB)
Single-Molecule Spectroscopy of Interfacial Electron Transfer
journal contribution
posted on 2003-09-23, 00:00 authored by Michael W. Holman, Ruchuan Liu, David M. AdamsIt 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 report we show how SMS can be used to measure photoinduced interfacial electron transfer
(IET) and back electron transfer rates in a prototypical chromophore−bridge−electrode nonadiabatic electron
transfer system. N-(1-hexylheptyl)-N‘-(12-carboxylicdodecyl)perylene-3,4,9,10-tetracarboxylbisimide was
synthesized and incorporated into mixed self-assembled monolayers (SAMs) on an ITO (tin-doped indium
oxide, a p-type semiconductor) electrode. Single-molecule fluorescence time trajectories from this system
reveals “blinks”, momentary losses in fluorescence (>20 ms to seconds in duration), which are attributed
to discrete electron transfer events: electron injection from the perylene chromophore into the conduction
band of the ITO leads to the loss of fluorescence, and charge recombination (back electron transfer) leads
to the return of fluorescence. Such blinks are not observed when an electrode is not present. The
fluorescence trajectories were analyzed to obtain the forward and back electron rates; the measured rates
are found to lie in the millisecond to second regime. Different rates are observed for different molecules,
but the lifetime distributions for the forward or back electron transfer for any given molecule are well fit by
single exponential kinetics. The methodology used is applicable to a wide variety of systems and can be
used to study the effects of distance, orientation, linker, environment, etc. on electron transfer rates. The
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.