Quantum interference effects in self-assembled asymmetric disulfide monolayers: Comparisons between experiment and ab initio/Monte Carlo theories

Article abstract of DOI:10.1021/jp962731k

Electron transfer measurements between solution redox probes and a Au electrode coated with self-assembled monolayers of symmetric and asymmetric ??-hydroxyalkane disulfides are used to probe the quantum interference between hydrocarbon chains of different lengths. While ab initio theory predicts a measurable destructive interference effect between hydrocarbon chains differing in length by a single methylene unit, electron transfer rates for the ferricyanide and horse heart cytochrome c are more consistent with rates estimated in the absence of the interference effect. The discrepancy between theory and experiment is discussed in light of possible interchain electronic coupling and phase segregation within the monolayers and limitations of the theoretical models.

Full text of DOI:10.1021/jp962731k

1058
J. Phys. Chem. B 1997, 101, 1058-1062
Quantum Interference Effects in Self-Assembled Asymmetric Disulfide Monolayers:
Comparisons between Experiment and ab Initio/Monte Carlo Theories
Jun Cheng and Cary J. Miller*
Department of Chemistry and Biochemistry, UniVersity of Maryland at College Park,
College Park, Maryland 20742
ReceiVed: September 6, 1996; In Final Form: December 4, 1996X
Electron transfer measurements between solution redox probes and a Au electrode coated with self-assembled
monolayers of symmetric and asymmetric ω-hydroxyalkane disulfides are used to probe the quantum
interference between hydrocarbon chains of different lengths. While ab initio theory predicts a measurable
destructive interference effect between hydrocarbon chains differing in length by a single methylene unit,
electron transfer rates for the ferricyanide and horse heart cytochrome c are more consistent with rates estimated
in the absence of the interference effect. The discrepancy between theory and experiment is discussed in
light of possible interchain electronic coupling and phase segregation within the monolayers and limitations
of the theoretical models.
1. Introduction
noted that the sign of the electronic coupling estimates changes
between even and odd hydrocarbon chain lengths.4,5 If one were
to produce a mixed alkanethiolate monolayer having both even
and odd chain lengths, the electronic coupling proceeding
through nearby chains with different lengths would be predicted
to interfere destructively, giving rise to lower electron transfer
rates.
In this paper we probe the electronic coupling properties of
monolayers formed by symmetric and asymmetric ω-hydroxy-
alkane disulfides. The monolayers formed by disulfides have
been shown to be functionally identical to those formed with
thiols.6 An advantage of using disulfides for the production of
monolayers containing different thiolate chains is that self-
segregation of the different chains into separate domains can
be minimized.7 The experimentally determined electronic
coupling through these asymmetric disulfide films is compared
with simple ab initio estimates and more sophisticated Monte
Carlo predictions.
Electron transfer reactions occurring over large distances play
important roles in biological and technological systems. The
rates of these electron transfers depend on the electronic
coupling between the donor and acceptor states, which is a
function of the intervening molecular material. The electronic
coupling interactions between the donor and acceptor generally
proceed through a large number of pathways which are
describable as the interactions between particular orbitals on
the intervening atoms. A notable feature of these electronic
coupling pathways is that they are signed quantities which can
sum constructively or destructively to give rise to the overall
electronic coupling.1 In building donor/acceptor systems for
biomimetic, photovoltaic, or other technological uses, the
quantum interference between electronic coupling pathways
should be an important design consideration. The investigation
of the interference between tunneling pathways has received
relatively little attention, particularly in regard to its use in
designing spacer geometries. Recently, Shephard and Paddon-
Row have presented a parity rule for the design of rigid
hydrocarbons with enhanced or diminished electronic coupling
due to pathway interference.2 While this previous work has
focused on intramolecular interference effects, the principle
should also be operative intermolecularly within spacers com-
prised of assemblages of molecules.
Studies using self-assembly of alkanethiols and disulfides on
Au electrodes have shed considerable light on the structural
dependence of the electronic coupling. These self-assembled
monolayers function as electron-tunneling barriers forcing
electron transfer reactions at such monolayer-coated electrodes
to proceed Via an extremely nonadiabatic electron transfer
mechanism. The relative electronic coupling between different
monolayers is obtained from a simple comparison of their
heterogeneous electron transfer rates to pendant or freely
diffusing redox probes.3 A particular strength of these mono-
layer systems is that their internal structures can be controlled
at the atom level simply by control of the structure of the
amphiphile used to form the monolayer.4 The relatively simple
structure of these tunneling barriers facilitates comparisons with
ab initio calculations. In these ab initio studies, it has been
2. Experimental Section
Syntheses: 11-Hydroxyundecyl 12′-Hydroxydodecyl 1,1′-
Disulfide (C11-12) and 10-Hydroxydecyl 11′-Hydroxyun-
decyl 1,1′-Disulfide (C10-11). HO(CH2)nOH (n ) 10, 12)
was converted to I(CH2)nI (n ) 10, 12) Via reflux in 51%
aqueous HI for 2 h. After flash chromatographic purification
(silica gel, heptane), the diiodoalkane was treated with 1 equiv
of thiourea, refluxed for 1 h in 95% ethanol, treated with a slight
excess of NaOH under nitrogen for 20 min, and then neutralized
with dilute HCl. The ethanol was removed under reduced
pressure, and the product was extracted from the aqueous phase
using HCCl3. The crude I(CH2)nSH (n ) 10, 12) was purified
by column chromatography (silica, heptane). To an equal molar
solution of I(CH2)nSH and HO(CH2)11SH (prepared from
Br(CH2)11OH) as described previously8) was added I2 (10 mM
in 95% ethanol) slowly while the solution was heated to 40 °C.
Iodine was added until a persistent yellow-brown color was
observed.7b,9 The I(CH2)nSS(CH2)11OH (n ) 10, 12) was
purified Via column chromatography (silica, chloroform). This
iodide was hydrolyzed to the diol Via heating at 100 °C in a
solution of 15% H2O in HMPA.10 The C11-10 and C11-12
were purified by column chromatography (silica, ethyl acetate:
chloroform; 15:85, w/w). The purity of the asymmetric
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, January 15, 1997.
S1089-5647(96)02731-9 CCC: $14.00 © 1997 American Chemical Society

Self-Assembled Asymmetric Disulfide Monolayers
J. Phys. Chem. B, Vol. 101, No. 6, 1997 1059
disulfides was determined to be in excess of 95% as measured
from high-resolution mass spectra.11 (C10-11, C21H44O2S2:
MW found 392.278 93, calculated 392.278 26. C11-12,
C23H48O2S2: MW found 420.310 31, calculated 420.309 57.)
11-Hydroxyundecyl 11′-Hydroxyundecyl 1,1′-Disulfide
(C11-11) and 12-Hydroxydodecyl 12′-Hydroxydodecyl 1,1′-
Disulfide (C12-12). HO(CH2)nBr (n ) 11, 12) was purified
by column chromatography (silica, ethyl acetate:chloroform; 15:
85, w/w) and converted to the corresponding thiols as described
above. The ethanol solution of HO(CH2)nSH (n ) 11, 12) was
titrated with I2 in ethanol to form the symmetric disulfides. The
HO(CH2)nSS(CH2)nOH (n ) 11, 12) was purified by column
chromatography (silica, ethyl acetate:chloroform; 15:85, w/w).
(C11-11, C22H46O2S2: MW found 406.293 72, calculated
406.293 91. C12-12, C24H50O2S2: MW found 434.323 76,
calculated 434.325 23.)
Figure 1. Drawing showing the origin of the sign alternation of the
electronic coupling with the alkane spacer length. The interactions of
the C-C antibonding orbitals for hexane and heptane spacers cause a
change in the symmetry of the HOMO.
through the intervening hydrocarbon chain. The sign of the
electronic coupling arises from the symmetry of the lowest
energy occupied and unoccupied orbitals. For hydrocarbons
containing even numbers of carbons, the antisymmetric com-
bination of Li radical orbitals is lower in energy. This results,
by definition, in a negative splitting energy.1,16 For hydrocar-
bons containing odd numbers of carbons, the symmetric
combination of Li radical orbitals is lower, resulting in a positive
coupling. A qualitative explanation of the origin of this
difference in the coupling between even and odd chain length
hydrocarbons can be presented from the work of Curtiss et al.
which identified that the most important coupling pathways for
these hydrocarbons stem from the overlap of the C-C anti-
bonding orbitals.13b,15 As seen in Figure 1, the electronic
coupling along these antibonding orbitals stabilizes the sym-
metric combination of Li radical orbitals for odd chain length
hydrocarbons, while the antisymmetric combination of Li radical
orbitals is stabilized for even chain length hydrocarbons.
In the Fermi’s golden rule limit appropriate for weakly
coupled systems, the square of the splitting energy is propor-
tional to the probability of electron tunneling through the
hydrocarbon. Because the probability of electron tunneling for
these nonadiabatic electron transfer reactions is directly pro-
portional to the rate of electron transfer, the ab initio splitting
energies can be used to predict electron transfer rates between
donor/acceptor pairs as a function of the spacer geometry. The
accuracy of these ab initio estimates has been demonstrated for
hydrocarbon spacers Via comparison to experimentally deter-
mined electronic coupling measurements.5c,17 The Koopman’s
theorem splitting energies for nonbonded lithium radicals
separated by a series of alkane spacers of increasing length yield
an exponentially decreasing electron transfer rate with a decay
constant, â, of 1.09 per methylene group,18 in good agreement
with the experimentally determined values.3c,d,8a The effect of
subtle structural changes in ω-hydroxyalkanethiol monolayers
on the electronic coupling has also been in good agreement with
experiment.4
Horse heart cytochrome c was purchased from Sigma. All
other chemicals were purchased from Aldrich.
Electrode Fabrication. Au electrodes were fabricated by
radio frequency sputtering ca. 3000 Å from a 99.99% Au target
onto microscope slides through a special mask. A ca. 500 Å
chromium layer was sputtered first to promote adhesion of the
gold films. The Au electrodes were cleaned through successive
exposures to chromic acid and aqueous HF, rinsed with water,
and immediately placed into ethanolic disulfide solutions as
described previously.8 The Au electrodes were kept overnight
in the ca. 5 mM solution of the corresponding ω-hydroxyalkyl
disulfides prior to their use in the electrochemical studies. The
geometric area of the electrodes was 0.13 cm2.
Electrochemical Measurements. All electrochemical mea-
surements were made using a BAS-100A electrochemical
analyzer (Bioanalytical Systems) in solutions which had been
purged with N2 and were held at 0 °C in a jacketed electro-
chemical cell. All kinetic parameters were extracted from cyclic
voltammograms obtained at a scan rate 5.12 V/s in aqueous
solutions. The voltammograms for ferricyanide were obtained
in 0.25 M CF3COONa and 3 mM of the redox probe, while
those for cytochrome c were obtained in 1.0 M KCl, 2 mM pH
7.1 phosphate buffer, and ca. 1 mM of the cytochrome. All
potentials were measured and are reported versus a saturated
calomel electrode.
Ab Initio Calculations. Ab initio computations were per-
formed using GAMESS.12 Using previous theoretical ap-
proaches,5,13 we have calculated R,ω-diradical splitting energies
for the neutral triplet diradical using an unrestricted Hartree-
Fock SCF calculation. The geometries of all the trans alkanes
were optimized at the 3-21G basis set level prior to the
introduction of the Li atom radical reporter groups.14 The Li
atoms were positioned 4 Å away from the hydrocarbon along
the symmetry axis.
3. Results and Discussion
In order to probe quantum interference effects using this KT
ab initio method, multiple hydrocarbon chains must be placed
between the Li radical reporters. A requirement of the KT
method is that the two radical reporter groups be in symmetry
equivalent positions. Because the electronic-coupling-induced
energy level splitting is small, even subtle asymmetry in the
environment of the Li atoms can introduce energy splittings
which overwhelm the electronic coupling component.19 In order
to probe the quantum interference between even and odd
hydrocarbon chains lengths, one must consider donor/acceptor
structures with symmetry equivalent Li sites. By surrounding
a central C2h symmetry hydrocarbon (even number of carbons)
with two C2V symmetry hydrocarbons (odd number of carbons)
as shown in Figure 2, one can maintain the overall C2h symmetry
Ab Initio Predictions. The electronic coupling through all-
trans hydrocarbon chains has been the focus of several recent
theoretical studies.13,15 Estimates of the efficiency of electronic
coupling between donor/acceptor pairs separated by hydrocar-
bons of differing lengths can be obtained from ab initio
calculations using several methods. We have focused on the
Koopman’s theorem (KT) approximation in which the neutral
triplet diradical splitting energies for a pair of nonbonded Li
radicals positioned along the axis of the hydrocarbon chain is
calculated. In these unrestricted Hartree-Fock calculations, the
energy differences of the two highest occupied and the two
lowest unoccupied eigenstates are summed to give a relative
measure of the energy splitting due to electronic coupling

1060 J. Phys. Chem. B, Vol. 101, No. 6, 1997
Cheng and Miller
The additivity of the electronic coupling pathways through
each hydrocarbon chain allows a simple prediction of the size
of the quantum interference effect. In a mixed monolayer
containing randomly dispersed even and odd ω-hydroxyal-
kanethiolate chains, the electronic coupling is assumed to
proceed equally through the shorter and longer chains. Because
of the difference in the chain lengths, these two tunneling
pathways will differ in their effectiveness at promoting electron
transfer. The removal of a single methylene group within these
hydroxyalkanethiol monolayers is observed to increase the rate
of electron transfer by a factor of 2.9, which is equivalent to an
Figure 2. Schematic representation of the alkane spacer geometries
used in the ab initio calculations. Beneath each structure is a
representative numerical symbol.
x
increase in the electronic coupling splitting energy of 2.9 or
1.7.8a Relative to an electrode coated with only the longer
ω-hydroxyalkanethiol monolayer, the electron transfer rate at
the mixed monolayer-coated electrode would reflect each of the
two pathways. Mathematically, one would sum the relative
electronic coupling components for the shorter chains which
occupy 50% of the sites, 0.5 × 1.7, and the longer chains which
have the opposite sign, 0.5 × (-1). This gives 0.35 which is
squared to give 0.12 as the predicted electron transfer rate
relative to the monolayer containing only the longer hydrocarbon
chain length. The mixed monolayer would be 8 times more
blocking as one composed of only the longer thiolate chains. If
in contrast the electronic coupling added constructively, inde-
pendent of the hydrocarbon chain length, this ratio would be
(0.5 × (1.7 + 1))2 or 1.84.
TABLE 1: Ab Initio Dilithium Splitting Energies for Model
Hydrocarbon Spacers
spacer
spacer
designationa
∆Vb (×106)
designationa
∆Vb (×106)
6
66
66′
666
666′
-128.1
-4.9
77
77′
767
767′
4.9
14.2
-4.9
-131.6
-131.9
-121.3
-112.1
a The spacer designations are the same as in Figure 1. b Sum of
anionic and cationic neutral triplet splitting energies in hartrees.
of the central hydrocarbon chain. We have considered all
possible orientations of these hydrocarbons which display
symmetry equivalent Li sites. The lithium radical splitting
energies for these different spacers are collected in Table 1.
The separation distance between the hydrocarbon chains of 4.75
Å was set to yield the same density of hydrocarbon chains found
in the disulfide monolayers.20
Comparison with Experiment. We have measured the
electron transfer rate for the reduction of ferricyanide and horse
heart cytochrome c at electrodes coated with a range of
symmetric and asymmetric ω-hydroxyalkane disulfides. These
two redox probes were chosen because of the large difference
in their radii. Figure 3 shows typical reduction rate curves for
C10-11-, C11-11-, C11-12-, and C12-12-coated Au elec-
trodes. The reduction currents measured at these electrodes have
been corrected for diffusion and double-layer effects as described
previously.22 The reduction currents for the electrodes coated
with the symmetric disulfide monolayers are in close agreement
with previous measurements made using Au electrodes coated
with ω-hydroxyalkanethiols of the same length.18 From the
simple quantum interference prediction, the currents for the
C10-11- and C11-12-coated electrodes should be significantly
lower than those coated with the C11-11 and C12-12,
respectively. This is not observed. As seen in the experimental
results collected in Table 2, we find that the electron transfer
rates are quite consistent with the predictions which neglect the
quantum interference effect.
The orientations of the hydrocarbon chains used in these ab
initio calculations do not exactly mimic the structure of the
monolayer on the Au surface.6b,21 However, comparing the elec-
tronic coupling estimates for all these geometries leads to several
useful generalizations. To a good approximation, the diradical
splitting energies in the three hydrocarbon chain geometry are
simply the sum of the splitting energies due to the central chain
and the flanking chains. This additive property of the electronic
coupling pathways is a likely result of weak interchain interac-
tions. From the computational results in Table 1, a quantum
interference effect between neighboring hydrocarbon chains is
clearly observed. When the hexane chain is flanked by hexane
chains, the Li orbital energy splitting increases. When heptane
chains are used to flank the central hexane chain, the splitting
energy decreases. This interference effect is independent of the
orientations of the hydrocarbons in keeping with the qualitative
explanation of the origin of this effect shown in Figure 1. The
rotation of the hydrocarbon chains or the internal vibrational
motion should not affect the sign of the electronic coupling
contribution of each hydrocarbon chain.
The magnitude of this interference effect is small. The
electronic coupling of the two Li radicals is mediated largely
by the central hydrocarbon chain. The energy level splitting
mediated by the two flanking hydrocarbons is approximately 1
order of magnitude weaker due to the larger separation between
the terminal methyl groups and the Li radicals. The interference
between these two pathways therefore yields a relatively small
effect. The asymmetry in the tunneling pathways is essentially
an artifact of the requirement to maintain the Li radicals in
symmetry equivalent sites. From the additivity seen in the
electronic coupling mediated through the chains, a stronger
quantum interference effect would be anticipated when the Li
atoms are positioned midway between the hexane and heptane
chains.
There are several explanations for the absence of quantum
interference in these measurements. The quantum interference
observed in the ab initio calculations may be simply an artifact
of the computational method and not relevant to the description
of electronic coupling through these organic monolayers. For
example, lateral coupling between adjacent hydrocarbon chains
may invalidate the separation of the electronic coupling between
individual hydrocarbon chains, which is at the heart of this
quantum interference effect. An analogy to this false separation
of tunneling pathways has been investigated by Liang and
Newton for electron- and hole-tunneling pathways within
individual hydrocarbon spacers.5b In this theoretical work, Liang
and Newton found that for sufficiently long hydrocarbons the
notion of separate electron-tunneling pathways which couple
along unoccupied spacer orbitals and hole-tunneling pathways
which couple along occupied orbitals was inappropriate due to
the mixing between these pathways. Lateral electronic coupling
between adjacent hydrocarbon chains within these densely
packed monolayers may also mix the pathways involved in the

Self-Assembled Asymmetric Disulfide Monolayers
J. Phys. Chem. B, Vol. 101, No. 6, 1997 1061
TABLE 2: Comparison of Relative Electron Transfer Rates
between Experiment and Theoretical Predictions
d
d
kMCQI
kMCQI
a
b
c
monolayers
kex
knQI kQI (â′ ) 1.2) (â′ ) 2.0)
3-
Fe(CN)6
C10-11/C11-11 1.73 ( 0.23 (7) 1.84 0.12
C11-12/C12-12 2.02 ( 0.20 (6) 1.84 0.12
0.35
0.35
0.57
0.57
Cytochrome c
C10-11/C11-11 1.68 ( 0.14 (4) 1.84 0.12
C11-12/C12-12 1.75 ( 0.36 (4) 1.84 0.12
0.28
0.28
0.39
0.39
a kex is the experimentally determined electron transfer rate ratio for
the indicated monolayer-coated electrodes. The numbers in parentheses
correspond to the number of independent measurements used to
b
determine the reported ratio. knQI is the predicted electron transfer rate
c
ratio in the absence of the quantum interference effect. kQI is the
predicted electron transfer rate ratio in the presence of the quantum
d
interference effect. kMCQI is the predicted electron transfer rate ratio
in the presence of the quantum interference effect from the Monte Carlo
model. The standard deviations for these determinations were <0.04
as determined from 10 separate runs.
assumption of the simple model. The electronic coupling
through these asymmetric disulfide monolayers may not proceed
evenly through both chains. Because of the very local nature
of electronic coupling, each redox center will couple to the Au
electrode through a relatively small number of alkyl chains. Even
the small degree of phase segregation present within a random
disulfide array could dramatically decrease the quantum interfer-
ence effect.
Monte Carlo Predictions. In order to produce a more
realistic quantum interference estimate, we have developed a
Monte Carlo approach which probes the statistical nature of
these electron transfer measurements. First we set up a 54 ×
54 hexagonal grid onto which asymmetric disulfides are allowed
“to react” and fill the grid. In each reaction cycle, a single
grid site is selected at random along with one of the adjacent
sites. If both are empty, the disulfide fills both sites. Otherwise,
the adsorption fails and a new cycle is repeated. Typically
10 000 adsorption cycles are used. Following this adsorption
cycle, the few remaining open sites are filled at random with
either of the two thiolate chains. The electronic tunneling
estimates are obtained by randomly allowing the redox probe
to react with the grid. A point within the central 50 × 50 portion
of the grid is selected at random, and the distances between the
spherical redox probe and the 19 nearest neighbor ω-hydroxy-
alkanethiol chains are calculated. The tunneling estimate is
calculated as the square of the mean of the square root of the
terms for each thiolate chain (either -1 for the longer thiolate
or 2.9 for the shorter thiolate multiplied by an exponential decay
factor which takes into account each chain’s distance from the
redox probe.) The rates of a 1000 redox probes typically are
averaged to obtain the estimated tunneling rate. The ratios of
these Monte Carlo rates to those for the longer symmetric coated
Au electrodes are given in Table 2.
Figure 3. Heterogeneous electron transfer rate constants for Au
electrodes derivatized with symmetric and asymmetric ω-hydroxyalkane
disulfide monolayers as indicated in the legend. Figure 3A shows
electron transfer rates for Fe(CN)63-, while Figure 3B shows those
measured for horse heart cytochrome c. These data were obtained from
cyclic voltammograms and were corrected for diffusion and double-
layer effects. Because these kinetic data give consistent reorganization
values of 1.2 and 0.6 eV for the ferricyanide and cytochrome c redox
probes, respectively, the relative electron transfer rates can be used as
a direct measure of the electronic coupling differences between the
monolayer-coated electrodes.
long-range electronic coupling. Such lateral coupling within
alkanethiol monolayers has been recently reported by Slowinski
et al.23 Another complication is in the contact terms in the
electronic coupling pathways between the exterior of the
hydrocarbon chains and the redox molecule. The differing
lengths of the ω-hydroxyalkanethiolate chains should introduce
some surface corrugation within the asymmetric disulfide
monolayers. Simplistically, the presence of a different thickness
of solvent between the redox molecule and the thiolate chains
may eliminate this interference effect. However, the presence
of a different thickness of the solvent between the two thiolate
chains would also be expected to lower the electronic coupling
mediated through the shorter thiolate chain, giving rise to a more
blocking mixed monolayer. That the ratios of the electron
transfer rates for the asymmetric and the longer symmetric
disulfide monolayer coated Au electrodes are close to those
predicted for the absence of quantum interference argues against
the presence of an increased solvent layer at the shorter thiolate
chains.24
There are two important input parameters within this model
which affect the predicted quantum interference rate. The
locality of the electronic coupling depends on both the expo-
nential decay constant and the radius of the redox molecule.
The decay constant appropriate for donors/acceptors separated
in aqueous solutions has not been determined definitively.
Using the Koopman’s theorem ab initio approach for a pair of
Li radicals separated by a row of water molecules of variable
length, we have estimated this tunneling decay constant as 1.2
Å-1 18
The radius of the redox molecules was taken from
.
crystallographic determinations.25 For the smaller ferricyanide
redox probe, this statistical model predicts a reduced quantum
interference effect from that predicted if the electronic coupling
An alternate explanation for a lack of quantum interference
for these mixed length thiolate chains focuses on the key

1062 J. Phys. Chem. B, Vol. 101, No. 6, 1997
Cheng and Miller
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proceeds equally through the two chain lengths. By increasing
the radius of the redox probe to that estimated for the heme
redox center of cytochrome c, the quantum interference effect
becomes larger within this model.26
The 1.2 Å-1 estimate for the solution tunneling decay constant
is likely too small due to the high symmetry of the row of water
molecules used. In order to show the effect of this parameter
on the predicted magnitude of the quantum interference effect,
these calculations were repeated using an increased solution
electron-tunneling decay constant. For both redox centers, as
seen in Table 2, the quantum interference is observed to decrease
when the solvent decay constant is increased to 2.0 Å-1 27
.
Although the Monte Carlo model predicts a reduced quantum
interference effect for randomly assembled asymmetric disulfide
monolayers, the predicted ratios are still significantly lower than
the experimental values, particularly for the larger cytochrome
c redox probe. In order to eliminate the quantum interference
effect within this model, the distribution of thiolate chains must
be made distinctly nonrandom. However, the extent of phase
segregation within these mixed thiolate monolayers would be
expected to be quite small. In STM studies of asymmetric
disulfides with hydroxyl and methyl terminal groups, Takami
et al. were able to distinguish the different chains and found
no measurable segregation of the chains.7a For the more subtly
asymmetric disulfides used in the present work, the driving force
for segregation would be expected to be much smaller.
In order to strengthen the assertion that phase segregation of
the thiolate chains is not the reason for the absence of the
quantum interference effect, the discussions above suggest
several possible experimental improvements to probe for these
interference effects. The effect of lateral segregation could be
diminished by monitoring the electron transfer rate of a tethered
redox probe assembled in diluent alkanethiolates of different
lengths.3a,c,e,f At sufficient dilution, the covalently bound alkane
chain pathway should sum with neighboring diluent chains, all
with the same length. Alternately, one could tether the redox
probe to the electrode surface Via two covalent linkages of
different lengths.
(10) Hutchins, R. O.; Taffer, I. M. J. Org. Chem. 1983, 48, 1360.
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to symmetric disulfides from the MS spectrum. The asymmetric disulfides
were found to undergo thiolate chains exchange to a small extent during
the mass spectral analysis, making these purity figures a lower estimate to
the true purity.
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176, 387.
(16) Paddon-Row, M. N. Acc. Chem. Res. 1982, 15, 245.
(17) (a) Curtiss, L. A.; Naleway, C. A.; Miller, J. R. J. Phys. Chem.
1995, 99, 1182. (b) Beratan, D. N.; Hopfield, J.J. J. Am. Chem. Soc. 1984,
106, 1584.
(18) Cheng, J.; Miller, C. J. Unpublished data.
(19) Our attempts at isolating the electronic-coupling-induced splitting
for even subtly asymmetric Li radicals have been unsuccessful. The
asymmetry-induced splitting is typically several orders of magnitude larger
than that due to the electronic coupling. Calculating the asymmetry-induced
splitting by separately calculating the energy levels of a single Li atom on
each side of the hydrocarbon chains is not sufficiently accurate to allow
extraction of the electronic-coupling-induced splitting for these asymmetric
systems.
(20) The separation distance between the hydrocarbon chains was
calculated for hexagonal packing assuming that each chain occupies 19.5
Å2.
(21) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147.
(22) Terrettaz, S.; Becka, A. M.; Traub, M. J.; Fettinger, J. C.; Miller,
C. J. J. Phys. Chem. 1995, 99, 11216. The charge of horse heart cytochrome
c was chosen to be +2 for the double-layer correction.
(23) Slowinski, K.; Chamberlain, R. V., II; Bilewicz, R.; Majda, M. J.
Am. Chem. Soc. 1996, 118, 4709.
Acknowledgment. This work was supported by the National
Science Foundation (CHE 9417357).
References and Notes
(1) Newton, M. D. Chem. ReV. 1991, 91, 767.
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(3) (a) Chidsey, C. E. D. Science 1991, 251, 919. (b) Miller, C.;
Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (c) Finklea, H. O.;
Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (d) Xu, J.; Li, H.-L.;
Zhang, Y. J. Phys. Chem. 1993, 97, 11497. (e) Weber, K.; Creager, S. E.
Anal. Chem. 1994, 66, 3164. (f) Tender, L.; Carter, M. T.; Murray, R. W.
Anal. Chem. 1994, 66, 3173.
(4) Cheng, J.; Saghi-Szabo, G.; Tossell, A. J.; Miller, C. J. J. Am. Chem.
Soc. 1996, 118, 680.
(5) (a) Naleway, C. A.; Curtiss, L. A.; Miller, J. R. J. Phys. Chem.
1991, 95, 8434. (b) Liang, C.; Newton, M. D. J. Phys. Chem. 1992, 96,
2855. (c) Jordan, K. D.; Paddon-Row, M. N. J. Phys. Chem. 1992, 96,
1188.
(24) We have produced asymmetric ω-hydroxyalkanethiol-containing
chains differing in length by two methylene groups. The electron transfer
ratios for these monolayers are significantly lower than those predicted,
suggesting the presence of an extra layer of water between the shorter
thiolate chain and redox molecule.
(25) (a) Bushnell, G.; Louie, G. V.; Brayer, G. D. J. Mol. Biol. 1990,
214, 585. (b) Vannerberg, N.-G. Acta Chem. Scand. 1972, 26, 2863.
(26) The effective radius for the cytochrome, 8.1 Å, was taken to be
the distance from the iron to the solution edge of the heme proprionate.
(27) The 2.0 Å-1 was chosen as an upper limit for the solvent tunneling
decay constant. Its value was selected to be somewhat lower than that of
the vacuum tunneling decay constant. Koopman’s theorem ab initio
calculations yield a vacuum tunneling decay constant of 2.3 Å-1.18
.