Single-molecule spectroscopy of interfacial electron transfer

Article abstract of DOI:10.1021/ja0343104

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 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.

Full text of DOI:10.1021/ja0343104

Published on Web 09/23/2003
Single-Molecule Spectroscopy of Interfacial Electron Transfer
Michael W. Holman, Ruchuan Liu, and David M. Adams*
Contribution from the Department of Chemistry, Columbia UniVersity, 3000 Broadway,
New York, New York 10027
Received January 23, 2003; E-mail: dadams@chem.columbia.edu.
Abstract: 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 measure-
ment. 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.
A crucial aspect of the understanding and realization of any
future molecular electronics1-4 will be the connections between
the molecular-scale components, whatever their nature, and the
meso- and macroscopic metal or semiconductor components
which will transmit energy or information from these molecules
or molecular assemblies to more traditional electronic compo-
nents. Interfacial electron transfer (IET) between molecules and
bulk electrodes has received extensive theoretical5-8 and
experimental9-13 treatment, typically relying on ensemble-
averaged optical spectroscopic and electrochemical measure-
ments. There is a tremendous need for new methodology and
experiments designed to probe the discrete electronic and
molecular dynamic fluctuations of single molecules near elec-
trodes. Single-molecule spectroscopy (SMS)14-18 has emerged
as an important method for the study of the fluorescence
behavior of single molecules in ambient environments and has
been used to probe the discrete excited-state electronic19-22 and
molecular dynamic processes23,24 of molecules. SMS has been
used to estimate the average interfacial electron transfer excited-
state quenching rates of organic chromophores at semiconductor
electrodes25 and has been shown to be particularly well suited
for the study of low quantum yield excited-state deactivation
processes such as the photoinduced ionization of semiconductor
nanoparticles.26-28 Our laboratory has recently used SMS to
follow electron transfer processes in donor-bridge-acceptor
(14) Xie, X. S. Acc. Chem. Res. 1996, 29, 598-606.
(15) Xie, X. S.; Trautman, J. K. Annu. ReV. Phys. Chem. 1998, 49, 441-480.
(16) Moerner, W. E.; Orrit, M. Science 1999, 283, 1670-1676.
(17) van Hulst, N. F.; Veerman, J.-A.; Garc´ıa-Parajo´, M. F.; Kuipers, L. J. Chem.
Phys. 2000, 112, 7799-7810.
(1) Heath, J. R. Pure Appl. Chem. 2000, 72, 11-20.
(2) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541-8.
(3) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804.
(4) Cahen, D.; Hodes, G. AdV. Mater. 2002, 14, 789-98.
(5) Schmickler, W. J. Electroanal. Chem. 1986, 204, 31-43.
(6) Gao, Y. Q.; Georgievskii, Y.; Marcus, R. A. J. Chem. Phys. 2000, 112,
3358-3369.
(18) Moerner, W. E. J. Phys. Chem. B 2002, 106, 910-927.
(19) Xie, X. S.; Dunn, R. C. Science 1994, 265, 361-364.
(20) Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Science 1996,
272, 255-258.
(7) Gao, Y. Q.; Marcus, R. A. J. Chem. Phys. 2000, 113, 6351-6360.
(8) Gosavi, S.; Gao, Y. Q.; Marcus, R. A. J. Electroanal. Chem. 2001, 500,
71-77.
(21) Yip, W.-T.; Hu, D.; Yu, J.; Vanden Bout, D. A.; Barbara, P. F. J. Phys.
Chem. A 1998, 102, 7564-7575.
(22) Veerman, J. A.; Garcia-Parajo, M. F.; Kuipers, L.; van Hulst, N. F. Phys.
ReV. Lett. 1999, 83, 2155-2158.
(9) Chidsey, C. E. D. Science 1991, 251, 919-922.
(10) Lanzafame, J. M.; Miller, R. J. D.; Muenter, A. A.; Parkinson, B. A. J.
Am. Chem. Soc. 1992, 96, 2820-2826.
(23) Ruiter, A. G. T.; Veerman, J. A.; Garcia-Parajo, M. F.; van Hulst, N. F. J.
Phys. Chem. 1997, 101, 7318-7323.
(11) Miller, R. J. D.; McLendon, G. L.; Nozik, A. J.; Schmickler, W.; Willig,
F. Surface Electron Transfer Processes; VCH Publishers: New York, 1995.
(12) Fajardo, A. M.; Lewis, N. S. J. Phys. Chem. B 1997, 101, 11133-11151.
(13) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.;
Chidsey, C. E. D.; Feldberg, S. W. Science 2001, 291, 1519-1523.
(24) Stracke, F. B. C.; Becker, S.; Mullen, K.; Meixner, A. J. Chem. Phys. Lett.
2000, 325, 196-202.
(25) Lu, H. P.; Xie, X. S. J. Phys. Chem. B 1997, 101, 2753-2757.
(26) Nirmal, M.; Dabousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J.
K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802-804.
9
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J. AM. CHEM. SOC. 2003, 125, 12649-12654
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A R T I C L E S
Holman et al.
ITO electrode leads to creation of its lowest-energy singlet
excited state (P*) which can either relax radiatively to the
ground state (P) or undergo charge injection from the perylene
LUMO into the conduction band of the ITO to form the cation
radical (P+).10,25,37-38 At room temperature, the forward rate of
electron transfer (ket) is significantly slower than the rate of
fluorescent relaxation of the exited state (kfluor) due to weak
electron coupling between the chromophore and the electrode,
leading to slow IET through the long aliphatic bridge. While a
ket significantly slower than kfluor results in an extremely low
quantum yield of IET, such rare electron transfer events are
nevertheless easily detectible at the single-molecule level as
momentary losses in fluorescence. In this study, we show how
the fluorescence time trajectories can be analyzed to yield the
forward and back electron transfer rates for single molecules
in these layers.
Experimental Procedures
N-(1-Hexylheptyl)-N′-(12-carboxylicdodecyl)perylene-3,4,9,-
10-tetracarboxylbisimide 1 (Figure 1C), was synthesized by
standard methods39 (see Supporting Information for additional
details). Briefly, perylene-3,4,9,10-tetracarboxyldianhydride (Al-
drich) was reacted with 1-hexylheptylamine40 to give the
symmetric bisimide, which was then treated with KOH41 to
provide the monoimide monoanhydride. Reaction of this product
with 12-aminododecanoic acid (Aldrich) gave 1 in high yield.
Absorption and emission spectra (Figure 2) of 1 were mea-
sured in CHCl3 solution on a Perkin-Elmer Lambda 25 UV/vis
and a Cary Eclipse fluorescence spectrophotometer. The excita-
tion wavelength was 490 nm for emission spectra.
To form the monolayers, glass slides coated with ITO
(Metavac, ∼2 nm RMS roughness) were cleaned by sonicating
1 h in a 25:75 ethanolamine:water mixture, rinsing thoroughly
(>100 mL) with deionized water and then with methanol, and
drying in a vacuum oven (60 °C) 1 h. The slides were then
immersed for 3-4 h in a 5 ×10-3 M solution of undecanoic
acid in hexadecane,42 and then 24 h in an “exchange solution”9
of 5 ×10-3 M undecanoic acid and 5 ×10-9 M 1 in hexadecane
to form the self-assembled structure rendered in Figure 1C. After
removal from the final self-assembly solution the slides were
rinsed carefully with acetonitrile to remove excess hexadecane
and undecanoic acid. The contact angle of water with the slide
was found to increase from ∼20° to up to ∼100° upon
monolayer formation, indicating good coverage. Using oven-
dried glassware and keeping the self-assembly solution free of
water was found to improve the quality of the monolayers, which
were also observed to degrade in quality over the course of a
few hours when left out in air. Self-assembled films prepared
by this procedure show well-separated (>500 nm) isolated single
Figure 1. (A) Relative electronic energy levels and (B) kinetic scheme
for (C) the interfacial electron transfer system based on 1 self-assembled
in a mixed monolayer on an ITO electrode (R ) 1-hexylheptyl).
and molecular sensors systems.29,30 Yet, despite the intense
interest in the molecular electronic possibilities of organic self-
assembled systems at electrodes, similar studies have not been
attempted to follow the discrete electron transfer and charge
separation fluctuations in such systems at the single-molecule
level. Such single-molecule studies of ordered systems have the
potential to unlock information hidden in the ensemble average
of bulk experiments, and to contribute greatly to the theoretical
understanding of these systems.15,17,18,21
In this report we show how SMS can be used to measure
photoinduced IET rates in a prototypical chromophore-bridge-
electrode system. Our system is composed of an asymmetric
perylene bisimide chromophore 1 self-assembled in a mixed
monolayer on an ITO electrode (tin-doped indium oxide, a
p-type semiconductor) via a carboxylic acid terminated aliphatic
bridge (Figure 1C). Perylene bisimides show strong fluorescence
(ꢀ ≈ 80000; ΦF ≈ 1) and their synthetic flexibility and
accessible oxidation (1.61 eV vs SCE) and reduction (-0.59
eV) potentials31 make them good candidates for building blocks
of molecular electronic structures and studies of photoinduced
charge transfer.32-36 Optical excitation of a perylene near an
(34) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Mu¨llen, K.; Bard, A. J. J.
Am. Chem. Soc. 1999, 121, 3513-3520.
(27) Shimizu, K. T.; Neuhauser, R.; Leatherdale, C. A.; Empedocles, S. A.;
Woo, W. K.; Bawendi, M. G. Phys. ReV. B 2001, 63, 205316-1-205316-
5.
(28) Kuno, M.; Fromm, D. P.; Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. J.
Chem. Phys. 2000, 112, 3117-3120.
(29) Liu, R.; Holman, M. W.; Adams, D. M. J. Phys. Chem. A 2003, 107, 6522-
6526.
(35) Nollau, A.; Hoffmann, M.; Fritz, T.; Leo, K. Thin Solid Films 2000, 368,
130-137.
(36) Lukas, A. S.; Zhao, Y.; Miller, S. E.; Wasielewski, M. R. J. Phys. Chem.
B 2002, 106, 1299-1306.
(37) Wo¨hrle, D.; Kreienhoop, L.; Schnurpfeil, G.; Elbe, J.; Tennigkeit, B.; Hiller,
S.; Schlettwein, D. J. Mater. Chem. 1995, 5, 1819-1829.
(38) Willig, F.; Zimmermann, C.; Ramakrishna, S.; Storck, W. Electrochim.
Acta 2000, 45, 4565-4575.
(30) Zang, L.; Liu, R.; Holman, M. W.; Nguyen, K. T.; Adams, D. M. J. Am.
Chem. Soc. 2002, 124, 10640-10641.
(39) Langhals, H. Heterocycles 1995, 40, 477-500.
(40) Borch, R. F.; Bernstein, M. D.; Durst, H. D. J. Am. Chem. Soc. 1971, 93,
2897-2904.
(31) Kircher, T.; Lo¨hmannsro¨ben, H.-G. Phys. Chem. Chem. Phys. 1999, 1,
3987-3992.
(32) Adams, D. M.; Kerimo, J.; Olson, E. J. C.; Zaban, A.; Gregg, B. A.; Barbara,
P. F. J. Am. Chem. Soc. 1997, 119, 10608-10619.
(33) Gregg, B. A.; Cormier, R. A. J. Phys. Chem. B 1988, 102, 9952-9957.
(41) Kaiser, H.; Lindner, J.; Langhals, H. Chem. Ber. 1991, 124, 529-535.
(42) Yan, C.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Langmuir 2000, 16,
6208-6215.
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Single-Molecule Spectroscopy of Interfacial ET
A R T I C L E S
spectra closely match the solution spectra, as observed for other
single-molecule systems,43,44 and confirm that the fluorescence
seen is from the first singlet excited state of an isolated perylene
bisimide molecule.
Typical single-molecule traces show a single constant level
of fluorescence before photobleaching occurs and the fluores-
cence intensity returns to background in a single step (Figure
3A). When a 1 × 10-10 M solution of 1 in CHCl3 and then a
2 mg/mL solution of poly(vinylbutyral) (PVB) in THF are spin-
coated onto a quartz slide at 2000 rpm, 88% of 62 single
molecules studied by SCM show this typical “single-step”
behavior as shown in Figure 3A. Some of these molecules show
brief losses of fluorescence (down to background) before the
eventual irreversible photobleaching, and these “blinks” are
attributed to intersystem crossing (ISC) to a nonfluorescent
triplet state,21 or perhaps to photoinduced charge transfer to the
polymer or to impurity sites in the quartz for molecules lying
near the polymer-quartz interface,30 as 36% show blinks that
are longer than those generally associated with triplet states.
When similar films are prepared on an ITO electrode and studied
under identical conditions, this single-step behavior is seen for
only 38% of the 71 single molecules studied; presumably these
are from molecules that have been swept up into the polymer,
away from the interface, whose behavior should be unaffected
by the substrate. The remaining 62%, however, show very rapid
fluctuations in the fluorescence intensity similar to that shown
in Figure 3B. It has been shown that excited organic chro-
mophores on an ITO surface show rapid reversible electron
transfer to the ITO,25 and we attribute the fluorescence behavior
such as that in Figure 3B to molecules close to the polymer-
ITO interface showing rapid electron transfer, at or below the
time resolution (20 ms) of our experiment. In addition, among
the molecules that showed single-step behavior, 59% of these
displayed losses in fluorescence too long in duration to be
accounted for by intersystem crossing.
In contrast, however, 85% of the 168 single molecules studied
in SAMs described above show long-lived dark states, and many
show well-defined blinking behavior like that in Figure 3C and
3D. Incorporating the chromophore into a SAM both separates
the molecule from the ITO electrode to slow the electron transfer
down into a regime where it can readily be studied by our
methods and provides a well-defined geometry for understanding
and modeling the system. Single-molecule traces from the SAMs
often show traces like that shown in Figure 3C, having a
constant level of fluorescence but periodically showing “blinks”
in which the fluorescence intensity drops to background in a
single step as in photobleaching but returns to the original
intensity after a brief time. The blinks observed in these traces
are in general too long to be accounted for by triplet states,21,45
and translational diffusion is also not a plausible explanation
for these dark states, as repeated fluorescence images over the
same area over time periods from minutes to hours reveal no
change in the positions of the molecules. Similarly, conforma-
tional dynamics are also an unlikely source of the dark states,
given the general absence of intermediate intensity levels, except
in a few cases where the blinking is not fully resolved and the
Figure 2. (A) Solution absorption (solid-line blue curve) and emission
(solid-line red curve) spectra of 1 measured in CHCl3 solution and (B)
typical single-molecule fluorescence spectrum (dashed-line red curve) of 1
in a mixed self-assembled monolayer on ITO.
perylene chromophores in the monolayers in fluorescence
images. Similar films prepared with higher perylene concentra-
tion showed uniform fluorescence over large areas (>100 µm2)
with little evidence of defects or multilayer formation.
Single-molecule fluorescence images, traces, and spectra were
collected with a scanning confocal microscope (SCM), in which
a tightly focused laser spot (∼400 nm) was used to excite one
molecule at a time in these dilute films. In this microscope,
samples are raster scanned over the diffraction limited laser spot
as fluorescence images are collected to locate single molecules.
Fluorescence traces (fluorescence vs time) and fluorescence
spectra were collected over single molecules. These data were
collected using a Digital Instruments Aurora-3 near-field
scanning optical microscope (NSOM), with a 100×/1.25 NA
objective (Nikon), modified to function as an SCM. A dichroic
mirror (Chroma 505DCXRU) was used to bring in circularly
polarized 488 nm CW excitation light (Melles-Griot 543, 300
mW tunable argon-ion laser system) with an excitation intensity
at the sample of ∼1500 W/cm2. The collected fluorescence
passes through the dichroic mirror and a 488 nm holographic
notch filter (ThermoOriel SuperNotch) to remove backscattered
laser light, and can be diverted onto a liquid nitrogen cooled
CCD (Roper Instruments) via a diffraction grating (Acton
Research Corp. SpectraPro-300i) to collect spectra, or passed
through a 500-700 nm band-pass filter (Chroma HQ600/200m)
to further remove residual laser light and other background light,
and focused onto a single-photon counting avalanche photodiode
(APD) (EG&G SPCM-AQR-14) to monitor fluorescence in-
tensity for images and traces. The sample was kept under an
argon atmosphere to stave off eventual photobleaching and to
preserve the integrity of the monolayers.
Results and Discussion
(43) Vanden Bout, D. A.; Yip, W.-T.; Hu, D.; Fu, D.-K.; Swager, T. M.; Barbara,
P. F. Science 1997, 277, 1074-1077.
(44) Ying, L.; Xie, X. S. J. Phys. Chem. B 1998, 102, 10399-10409.
(45) Hernando, J.; van der Schaaf, M.; van Dijk, E. M. H. P.; Sauer, M.; Garc´ıa-
Parajo´, M. F.; van Hulst, N. F. J. Phys. Chem. A 2003, 107, 43-52.
Fluorescence spectra from single molecules in mixed mono-
layers on ITO were collected and compared to the solution
spectra (Figure 2). The background-subtracted single-molecule
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A R T I C L E S
Holman et al.
Figure 3. Fluorescence trajectories for 1 (A) on quartz, (B) on ITO, and (C,D) in a self-assembled monolayer on ITO.
molecule is frequently fluorescing for only a portion of the
integration time. Since the occurrence of the longer (.20 ms)
off times is clearly related to the proximity of an ITO electrode,
which is expected to readily accept an electron from the
photoexcited molecule,25,37 we believe this blinking results from
electron transfer to the ITO electrode as shown in Figure 1A:
the molecule stops fluorescing when it loses an electron into
the electrode, and remains “off” until the electron returns and
the molecule comes back “on” and resumes fluorescing. Each
blink off or blink on represents the observation of a single elec-
tron transfer event, and by analyzing the average duration of
the on and off times the electron transfer rates can be
determined.
To analyze the traces to obtain the average lifetimes of the
on and off states, τon and τoff, a method similar to that previously
used to analyze triplet blinking21 was used. Taking the fluo-
rescence trace shown in Figure 3C as an example, a histogram
of the fluorescence intensities (Figure 4A) shows clearly that
there are two states with two different levels of fluorescence:
a state with an average intensity of approximately 105 counts/
20 ms, which is equal to the background fluorescence (the “off”
state); and a state with an average intensity of roughly 170
counts/20 ms, which is equal to a typical level of fluorescence
above background for a single perylene chromophore under our
experimental conditions (the “on” state). Taking as a cutoff point
a value of 137 counts/20 ms the trace is “rounded off”: the
molecule is assigned to either the on or the off state at each
point in the trace (Figure 4B) as the fluorescence is greater or
less than this cutoff. The duration of the on and off times are
counted and histogrammed (occurrences versus duration), and
the histograms are fit with single-exponential curves to deter-
mine the average lifetimes of the on and off state (Figure 4C).
For this particular molecule, the average on time is τon ) 276
( 11 ms and the average off time, τoff ) 42.9 ( 0.7 ms.
Importantly, the observed single exponential behavior for this
and other molecules strongly suggests that the molecular system
is fixed and well behaved and that the blinking is due to a single
process, electron transfer to the electrode. Molecular reorienta-
Figure 4. (A) An intensity histogram of the trace shown in Figure 3D;
(B) a section of the trace and the same section rounded to on and off states;
(C) histogram and single-exponential fit of the on and off time durations.
tion and/or diffusion during data collection would likely lead
to dramatic changes in the electronic coupling between chro-
mophore and electrode, resulting in multiexponential behavior
in the observed kinetics.
Once τon and τoff have been determined the forward and back
electron transfer rates can be readily obtained. The back electron
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Single-Molecule Spectroscopy of Interfacial ET
A R T I C L E S
Figure 5. Representative fluorescence versus time trajectories observed for 1 in a self-assembled monolayer on ITO. Traces display the range of observed
blinking behavior.
Table 1. Measured ON and OFF Times and Calculated Electron
Transfer Rates for the Single-Molecule Traces Shown in Figure 5
transfer rate is simply the inverse of the average off time:
Figure 5
τ
on (ms)
τ
off (ms)
ket (s-1
)
kbet (s-1
)
ket/kbet
1
τoff
) kbet
(1)
A
B
C
D
E
F
15800 ( 4230 6780 ( 1740 16.2 ( 4.7 0.148 ( 0.041 109
276 ( 11
269 ( 20
55.0 ( 3.3
41.9 ( 1.9
22.6 ( 1.2
42.9 ( 0.7
37.9 ( 4.9
193 ( 17
21.6 ( 0.5
939 ( 39
956 ( 71
4670 ( 290
6140 ( 280
23.3 ( 0.4
26.4 ( 3.4
5.18 ( 0.47
46.3 ( 1.0
12.9 ( 0.4
40.3
36.2
902
133
884
2770
but the forward electron transfer rate also depends on the
excitation rate:21
77.5 ( 2.6 11400 ( 600
G
8.51 ( 0.68 91.8 ( 3.3 30200 ( 2400 10.9 ( 0.4
ket
1
τon
) kexc
(2)
ket + kfluor + kisc +kic
time-scale resolution of the experiment, as in Figure 5F or 5G.
The range of behaviors observed here is presumably related to
inhomogeneities in the SAMs and to variability on the surface
of the ITO (RMS roughness ≈ 2 nm) affecting the degree of
electronic coupling between the molecule and the electrodesa
narrow or monodisperse rate distribution is not expected for
single molecules in such a system. Molecules giving traces such
as that shown in Figure 5A are only weakly coupled to the
electrode, and are likely in well-packed regions of the monolayer
and thus a maximum distance from the ITO, while those such
as in Figure 5F and 5G represent molecules that are strongly
coupled, presumably because disorder in the monolayer or the
electrode has brought them closer to the surface of the ITO. It
should be noted, however, that even in a perfect monolayer on
an atomically flat surface, variability in coupling and thus in
rates might be expected due to differences in the local electronic
structure of the ITO related to the distribution of dopant sites
For this perylene chromophore ΦF ≈ 1 so kisc (rate of
intersystem crossing to the triplet state) and kic (rate of other
nonradiative decay) were neglected in these calculations. On
the basis of laser power and detection efficiency (1500 W/cm2,
∼100 counts/20 ms at 5% detection efficiency), we estimate
kexc ≈ 106 s-1, and kfluor is measured in solution to be 2.57 ×
108 s-1
.
Average on and off times and the corresponding forward and
back electron transfer rates for some representative single-
molecule traces (Figure 5) are shown in Table 1. While not all
molecules examined survived long enough before photobleach-
ing for single-molecule electron transfer rates to be successfully
calculated, for those that do, rates can be extracted from
molecules showing a wide range of possible behaviors, from
unusually long time-scale processes, as in Figure 5A, to cases
where the blinking takes place on a time scale at or below the
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A R T I C L E S
Holman et al.
and to different orientations of the chromophores relative to
the semiconductor lattice.
optoelectronic systems. Further studies using these techniques
should lead to a better understanding of the effect of the length
and nature (e.g., conjugated vs nonconjugated) of the spacer
and of binding functional groups on the electron transfer rates,
a question of enormous interest to both molecular electricians
and electron transfer experimentalists and theorists. In future
experiments a metallized probe can be positioned over the
sample to study the effect of an applied electric field on the
electron transfer rates. These methods can be readily applied to
a wide variety of systems and should provide a wealth of
information on these and other problems in electron transfer
and molecular electronics.
The forward and back electron transfer rates do not change
uniformly across this range, as shown by the dispersity in the
ratio ket/kbet (Table 1). The forward and back electron transfer
processes are inherently different, and it might be expected that
the forward transfer is more strongly dependent on distance and
orientation. The forward electron transfer process involves
charge injection from the perylene LUMO (lowest unoccupied
molecular orbital) into a large energetically accessible density
of states within the conduction band of the ITO while the back
electron transfer involves transfer of a thermally relaxed electron
from the bottom of the conduction band or from a local trap
(energetically discrete states) back to the SOMO (singly
occupied molecular orbital) of the perylene cation. The observed
slightly larger dispersion in the forward electron transfer rates
suggest differences in the structural and distance dependence
to electron transfer for the two processes. The methodology
employed here opens the possibility of studying this disparity
in more depth, as information about the back electron transfer
process is not accessible by previous methods.
Acknowledgment. We thank Dr. Stefan Ka¨mmer for his
assistance with developing the instrumental setup. This work
was supported primarily by Department of Energy Office of
Basic Energy Sciences under Award Agency Project DE-FG02-
02ER15375 and in part by the NSF MRSEC DMR-0213574,
NSF NSEC CHE-0117752. D.M.A thanks Research Corporation
for a Cottrell Scholar award RC#CS0937.
Supporting Information Available: Details of perylene
synthesis and characterization (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Conclusions
We have shown that single-molecule fluorescence techniques
can be used to measure interfacial electron transfer rates in the
millisecond time regime under ambient conditions in organic
JA0343104
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12654 J. AM. CHEM. SOC. VOL. 125, NO. 41, 2003