Rate of interfacial electron transfer through the 1,2,3-triazole linkage

Article abstract of DOI:10.1021/jp057416p

The rate of electron transfer is measured to two ferrocene and one iron tetraphenylporphyrin redox species coupled through terminal acetylenes to azide-terminated thiol monolayers by the Cu(I)-catalyzed azidealkyne cycloaddition (a Sharpless click reaction) to form the 1,2,3-triazole linkage. The high yield, chemoselectivity, convenience, and broad applicability of this triazole formation reaction make such a modular assembly strategy very attractive. Electron-transfer rate constants from greater than 60,000 to 1 s^-1 are obtained by varying the length and conjugation of the electron-transfer bridge and by varying the surrounding diluent thiols in the monolayer. Triazole and the triazole carbonyl linkages provide similar electronic coupling for electron transfer as esters. The ability to vary the rate of electron transfer to many different redox species over many orders of magnitude by using modular coupling chemistry provides a convenient way to study and control the delivery of electrons to multielectron redox catalysts and similar interfacial systems that require controlled delivery of electrons.

Full text of DOI:10.1021/jp057416p

J. Phys. Chem. B 2006, 110, 15955-15962
15955
Rate of Interfacial Electron Transfer through the 1,2,3-Triazole Linkage
Neal K. Devaraj, Richard A. Decreau, Wataru Ebina, James P. Collman,* and
Christopher E. D. Chidsey*
Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080
ReceiVed: December 20, 2005; In Final Form: April 6, 2006
The rate of electron transfer is measured to two ferrocene and one iron tetraphenylporphyrin redox species
coupled through terminal acetylenes to azide-terminated thiol monolayers by the Cu(I)-catalyzed azide-
alkyne cycloaddition (a Sharpless “click” reaction) to form the 1,2,3-triazole linkage. The high yield,
chemoselectivity, convenience, and broad applicability of this triazole formation reaction make such a modular
assembly strategy very attractive. Electron-transfer rate constants from greater than 60,000 to 1 s-1 are obtained
by varying the length and conjugation of the electron-transfer bridge and by varying the surrounding diluent
thiols in the monolayer. Triazole and the triazole carbonyl linkages provide similar electronic coupling for
electron transfer as esters. The ability to vary the rate of electron transfer to many different redox species
over many orders of magnitude by using modular coupling chemistry provides a convenient way to study
and control the delivery of electrons to multielectron redox catalysts and similar interfacial systems that
require controlled delivery of electrons.
Introduction
and a terminal acetylene group where one of these groups is
incorporated at the surface of the monolayer and the other is
Self-assembled monolayers (SAMs) of organic thiols on gold1
are reproducible and controllable interfaces on which to study
redox processes dependent on the rate of electron transfer from
an electrode surface.2-19 One of the major limits to the routine
use of this method is the difficulty of forming the desired
assemblies of electrode, monolayer, and redox species. Typical
assembly strategies rely on appending a spacer or electron-
transfer “bridge” terminated with a thiol group to the redox
species of interest and then coadsorbing this molecule with a
more plentiful unmodified or “diluent” thiol. This coadsorption
strategy has proven successful for incorporating appropriate
amounts of relatively simple and unreactive redox species onto
the surfaces of thiol monolayers. Unfortunately, some of the
most interesting species to which one would like to control
electron transfer (porphyrins, enzymes, DNA, etc.) are too
structurally or functionally complex to be reliably immobilized
in this way.
An alternate strategy20-26 is to form a mixed monolayer from
a diluent thiol and a thiol terminated with a functional group or
ligand that can subsequently be coupled covalently to the redox
species of interest. While this post-adsorption coupling strategy
is intrinsically more flexible, no one coupling reaction has yet
been broadly adopted. This can be attributed to one or more of
the following limitations of most coupling reactions: (1)
incomplete coupling, (2) unwanted side reactions, or (3)
disruption of the monolayer by harsh coupling conditions.
Recently we and others have explored a particularly attractive
and broadly applicable reaction for coupling species of interest
to surfaces which we believe overcomes the limitations of
existing coupling reactions and may become widely adopted
for post-adsorption modification of monolayers. The procedure
relies on the recently discovered Cu(I)-catalyzed variant27,28 of
the Huisgen 1,3-dipolar cycloaddition29 between an azide group
appended to the species to be coupled to the monolayer.30-32
The resulting linkage is a five-member aromatic heterocycle
(specifically a 1,4-disubstituted 1,2,3-triazole), which is ther-
mally and hydrolytically very stable. Sharpless has dubbed this
Cu(I)-catalyzed reaction a “click” reaction27 by virtue of its
chemoselectivity, high yield, and overall convenience.33 We
have previously used the disappearance of the strong asymmetric
IR stretch of the azide on the monolayer after incubation with
a Cu(I) catalyst and an acetylene-terminated species in solution
to demonstrate that the surface reaction is quantitative:30
omission of the acetylene-terminated species but with otherwise
identical incubation conditions results in no loss of the azide
stretch, confirming the exquisite chemoselectivity of the cou-
pling. We also showed that, because of the quantitative yield
of the coupling, acetylene-terminated redox probes can be used
to quantify the number of azide groups initially present in the
mixed monolayers.34 Following the work of Sharpless, Fokin,
and co-workers,35 we have also shown that the oxygen-tolerant
complex of Cu(I) with tris(benzyltriazolylmethyl)amine (TBTA)
is an effective catalyst for the click coupling reaction at
monolayer surfaces.34 This catalyst has the advantage that it is
free of side reactions with O2 that form partially reduced oxygen
species and so can be safely used under aerobic conditions in
the presence of oxidatively sensitive compounds, such as the
coupling of single-stranded DNA to an azide-terminated mixed
monolayer.36 Most recently, we have used this chemistry to
selectively modify an array of independently addressable
microelectrodes by electrochemically localizing the copper(I)
catalyst.37
The work reported here demonstrates that monolayer as-
semblies coupled to a redox species by Cu(I)-catalyzed triazole
formation are well suited for studies where the electron-transfer
rate between a given surface redox species and the electrode
needs to be reproducibly and predictably controlled but where
assembly of the redox-active mixed monolayer in one step is
* To whom correspondence should be addressed. E-mail: jpc@stanford.edu
or chidsey@stanford.edu.
10.1021/jp057416p CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/21/2006

15956 J. Phys. Chem. B, Vol. 110, No. 32, 2006
Devaraj et al.
Figure 1. Scheme depicting the formation of mixed monolayers on gold electrodes coupled to redox species via Cu(I)-catalyzed azide-acetylene
cycloaddition. The modular nature of the approach facilitates studies by allowing the diluent thiol, the nature of the tunneling bridge, X, and the
redox species, R, to be independently varied.
prohibited by the reactivity of free thiols and/or the overall
complexity of the redox species. In this paper, we report the
electron-transfer rates for a number of assemblies of azide-
terminated thiol monolayers coupled to different redox species
(see Figure 1). By tailoring the length and degree of conjugation
of the azide-terminated thiol and the surrounding diluent, we
can control the electron-transfer rate to the redox species over
many orders of magnitude. In future work we hope to use this
methodology to determine how the rate of electron transfer
affects the mechanism of oxygen reduction by synthetic models
of the active site of cytochrome c oxidase.
and that monolayers treated in this fashion showed slower and
more reproducible rates of electron transfer. We have used this
post-assembly exchange method with the same effect. Immers-
ing our post-coupling monolayers in solutions of diluent thiols
slowed the electron-transfer rates and eventually gave repro-
ducibly lower limiting values. We infer that the remaining redox
species are in ordered domains of the monolayer where the redox
species is held a well-defined distance from the electrode by
the densely packed monolayer. Although this post-coupling
exchange does yield reproducibly slow rates of electron transfer,
it is not an ideal solution; for instance, metal-based redox species
with open coordination sites can react with the free diluent thiols
during the exchange step. Fortunately, we have found that
exchange of the azide-terminated thiols by diluent thiols prior
to the coupling reaction also results in reproducibly slow rates
of electron transfer. After an overnight incubation in an ethanol
solution containing 100 mM diluent thiol, the mixed monolayers
are coupled with the redox species and the electron-transfer rates
are reproducibly slowed to the slowest values obtained by post-
coupling exchange and can be predictably controlled by tailoring
the nature of the underlying monolayer. We infer that the
exchange process yields monolayers with coupling sites exclu-
sively immobilized in the ordered domains.
Results and Discussion
Our studies were carried out on gold electrodes modified with
a mixed monolayer using previously reported procedures.30,36
Mixed monolayers were formed from an azide-terminated thiol
(11-azidoundecanethiol, 16-azidohexadecanethiol, or azidophen-
yleneethynylenebenzylthiol (N3(PEB)SH)) and a diluent thiol
that was slightly shorter than the azide-terminated thiol to ensure
that the azide group was sterically unhindered for reaction. The
coverage of redox species coupled to the surface of the
monolayer is controlled by the ratio of azide-terminated to
diluent thiol in the monolayer formation step. As documented
in detail in our previous work,34 the yield of coupling is 100%
up to azide coverages at which steric limitations prevent
complete coupling.
Initially, we coupled the acetylene-bearing redox species to
the as-formed azide-terminated mixed monolayers. However,
we found the rates of electron transfer through the longer alkyl
bridging unit to the redox species to be faster than expected
and to be irreproducible. Chidsey and co-workers38 have
previously reported similar findings for mixed monolayers of
longer ferrocene-terminated alkane thiols and methyl-terminated
alkane thiols. They found that exposing the as-formed mixed
monolayer to an ethanolic solution of the diluent thiol replaced
some of the ferrocene thiols with the redox-inactive diluent thiols
Figure 2 shows typical cyclic voltammograms of mixed
monolayers of N3(CH2)16SH and CH3(CH2)15SH after coupling
with either ferrocene acetylene or an acetylene-terminated iron
tetraphenylporphyrin. The background charging currents are
typical of a hexadecanethiol monolayer indicating that neither
the presence of the azide nor the coupling chemistry disrupt
monolayer packing. For instance, at 0 V vs Ag/AgCl/3 M NaCl,
the capacitance determined from the 300 mV/s scan is 1.3 µF/
cm2, the same as that previously reported for a simple hexa-
decanethiol monolayer.39 As the scan rate for the cyclic
voltammogram is increased, the positive and negative current
peaks split further apart, indicating that the time to scan through
the peak is comparable to or less than the reciprocal of the

Controlling the Rate of Interfacial Electron Transfer
J. Phys. Chem. B, Vol. 110, No. 32, 2006 15957
the standard overpotential, E - E0.2 Equation 1 shows the
expected exponential decay of the redox current, iredox(t), for a
kinetically homogeneous population of one-electron redox sites
on the surface of an electrode:2
iredox(t) ) eA∆Γoxket(E) exp[-ket(E)t]
(1)
where e is the electronic charge, A is the area of the electrode,
∆Γox is the change in coverage (number per unit area) of the
oxidized form of the redox species during the course of the
measurement, and the first-order electron-transfer relaxation rate
constant, ket(E), is the sum of the first-order reduction rate
constant, kred(E), and the first-order oxidation rate constant,
kox(E):
ket(E) ≡ kred(E) + kox(E)
(2)
Equations 1 and 2 are derived from the first-order rate law for
reversible single-electron transfer, dΓox(t)/dt ) kox(E)Γred(t) -
kred(E)Γox(t), and conservation of the redox species at the
monolayer surface, Γred(t) + Γox(t) ) constant.
Figure 2. Cyclic voltammograms of N3(CH2)16SH + CH3(CH2)15SH
mixed monolayers on gold after click coupling with ferrocene acetylene
(left) or the acetylene-terminated iron tetraphenyl porphyrin (right). The
arrow indicates the start of the scan. Scan rates: 0.3 (solid), 0.5
(dashed), and 1.0 V/s (dotted). The electrolyte was 1 M HClO4. For
the case of the porphyrin, 1 M pyrazine was included to serve as an
axial ligand and the electrolyte was degassed thoroughly with nitrogen.
The monolayers were formed from an ethanol solution of 0.015 mM
N3(CH2)16SH and 0.09 mM CH3(CH2)15SH followed by exchange with
the CH3(CH2)15SH as described in the text.41
Figure 3a shows an example of the current transient for a
mixed monolayer formed from 0.015 mM N3(CH2)16SH and
0.09 mM CH3(CH2)15SH in ethanol with overnight exchange
in 100 mM CH3(CH2)15SH and then click coupled to ferrocene
acetylene. The potential was stepped from 10 mV negative of
the ferrocene standard potential to the standard potential of the
ferrocene redox couple. Thus, according to eq 1, we would
expect an exponential decay of the current with a decay rate
constant of ket(E0). A semilog plot (Figure 3b) reveals the decay
to be a single exponential over 2 orders of magnitude of the
current. The linearity of the semilog plot is an indication that
the redox sites are kinetically identical and independent of each
other.
electron-transfer rate.40 Note that for identical scan rates the
splitting for the porphyrin-modified monolayer is not as large
as that for the ferrocene-modified monolayer. This is an
indication that the electron-transfer rate for the porphyrin is
faster than that for the ferrocene. Note also that the area under
the voltammetric peaks and thus the redox charge to completely
oxidize the two different redox species are very similar. Indeed,
integration of the peaks and normalization by the scan rates30
shows that the redox charges are 3.9 ( 0.01 and 4.0 ( 0.1
µC/cm2 for the ferrocene and porphyrin modified monolayers,
respectively. This constancy of redox charge is as expected if
the coverage of the redox species on the surface after the
coupling reaction is equal to the coverage of reactive azide on
the surface before the coupling reaction. Because nominally
identical azide-terminated mixed monolayers were used for both
coupling reactions, we expected to obtain identical redox
coverages. This constancy of coverage demonstrates another
advantage of using a preformed monolayer as a platform for
click coupling: coverages can be controlled simply by control-
ling the amount of azide in the monolayer. Once the azide-
terminated mixed monolayer has been characterized, changing
the nature of the redox species does not require one to once
again empirically determine the conditions necessary for a
specific coverage. Thus, because of the chemoselective reactivity
and quantitative yield of the click reaction, an azide-terminated
mixed monolayer can be treated as a platform for obtaining
reliable coverages of a wide range of acetylene-terminated
species of interest.
The electron-transfer relaxation rate constants at E0 for all
the mixed monolayer redox assemblies examined in this paper
were determined in this way and are displayed in Table 1. Both
shorter azide-terminated thiols and shorter diluent thiols give
larger rates. Use of the conjugated bridge resulted in a rate that
was too fast to be accurately measured with conventional
chronoamperometry.42 The chronoamperometrically determined
lower limit to the rate is given in Table 1. These results are
expected based on previous work on mixed monolayers formed
by coadsorption of diluent thiols with thiols terminated by redox
species.3,4,6,9,12,15,18 One sees from Table 1 that the rate of
electron transfer to the click-coupled redox couples can readily
be scaled by over 4 orders of magnitude by modifying the nature
of the azide-terminated mixed monolayer.
Near the standard potential of a simple outer-sphere redox
couple, one expects the Butler-Volmer parametrization of the
potential dependence of the rate constants with a transfer
coefficient of 0.5 43 to be an appropriate approximation, in which
case eq 2 becomes:
ket(E) = k0{exp[-0.5e(E - E0)/(kBT)] +
exp[+0.5e(E - E0)/(kBT)]} (3)
One can obtain a value for the standard electron-transfer rate
constant, k0, between the gold electrode and the surface-
immobilized redox species from the voltammetric peak splitting
using the analysis method of Laviron.40 However, a preferable
method is measurement of the current transient following a
potential step to a final potential, E, extraction of the electron-
transfer relaxation rate constant, ket(E), and from that determi-
nation of the standard rate constant, k0. This method reveals
any multiphasic character of the electron-transfer kinetics and
allows measurement of the electron-transfer rate not only at the
standard potential, E0, of the couple but also as a function of
At the standard potential (E ) E0), the electron-transfer
relaxation rate constant should thus be twice the standard rate
constant:
ket(E0) ) 2k0
(4)
Figure 4 displays the measured dependence of ket(E) on
potential for a N3(CH2)16SH + CH3(CH2)15SH mixed monolayer

15958 J. Phys. Chem. B, Vol. 110, No. 32, 2006
Devaraj et al.
Figure 3. (A) Current transient for N3(CH2)16SH + CH3(CH2)15SH mixed monolayers on gold after click coupling with ferrocene acetylene. (B)
Semilog plot of the current transient in part A. The potential was stepped from E0 - 10 mV to E0. The electrolyte was 1 M HClO4.
TABLE 1: Electron-Transfer Relaxation Rate Constants
azide
diluent
redox species
ket(E0)/s-1
N3(PEB)SH
N3(CH2)11SH
N3(CH2)11SH
N3(CH2)16SH
N3(CH2)16SH
N3(CH2)16SH
N3(CH2)16SH
N3(CH2)16SH
N3(CH2)16SH
CH3(CH2)7SH
CH3(CH2)6SH
CH3(CH2)9SH
CH3(CH2)11SH
CH3(CH2)13SH
CH3(CH2)15SH
CH3(CH2)15SH
CH3(CH2)11SH
CH3(CH2)15SH
ferrocene acetylene
ferrocene acetylene
ferrocene acetylene
ferrocene acetylene
ferrocene acetylene
ferrocene acetylene
ferrocene carbonyl acetylene
Fe porphyrin acetylene
Fe porphyrin acetylene
>6 × 104
(1.07 ( 0.05) × 104
500 ( 2
20 ( 6
7.2 ( 0.2
2.2 ( 0.2
3.4 ( 1.0
(1.11 ( 0.05) × 103
34 ( 4
modified by either ferrocene or iron porphyrin. One should in
principle be able very accurately determine k0 by fitting such
plots to eq 3 (solid lines). This works well for the faster of the
two casessthe iron porphyrin. However, in the case of the
ferrocene, the experimental values of ket(E) do not change as
much with potential as expected by the Butler-Volmer param-
etrization. Such “shallow” potential dependences near E0 are
expected if there is a pathway for the redox process that is not
limited by electron transfer to and from the electrode. In the
case of this particularly slow electron transfer, it may be that
there is some residual electron transfer through defects followed
by self-exchange among the redox sites. Indeed, if the initial
mixed monolayer of azide-terminated and diluent thiol is not
incubated in the solution of pure diluent thiol prior to the
coupling, then the value of ket(E) is larger near E0 and the
potential dependence is even more shallow. If, for the defect
pathway, the self-exchange is rate-limiting rather than the
electron transfer between the electrode and redox species at the
defective site, then one would obtain a potential independent
contribution due to the self-exchange, here labeled kse, in parallel
with the electrochemical rate constant:
ket(E) = k0{exp[-0.5e(E - E0)/(kBT)] +
exp[+0.5e(E - E0)/(kBT)]} + kse (5)
The ferrocene data in Figure 4 are indeed well fit by this
equation. The fit parameters suggest that about 40% of the
electrons are transferred by the defective pathway. We have no
evidence for this pathway for the faster porphyrin redox species.
We conclude that, for mixed thiol monolayers on gold prepared
in this way, rate constants slower than about 1 s-1 may be prone
to defective electron transfer, but that rates faster than about 1
s-1 can be reliably controlled by the structure of the electron-
transfer bridge, the intrinsic packing of the monolayer, and the
nature of the redox species.
Figure 4. Dependence of ket(E) on potential for a N3(CH2)16SH +
CH3(CH2)15SH mixed monolayer modified by either ferrocene acetylene
(circles) or Fe monoaminotetraphenylporphyrin with appended acetylene
(squares). Solid lines are best fits of the data to the Butler-Volmer
form with a transfer coefficient of one-half (k0 ) 0.99 and 18 s-1, and
E0 ) +0.325 and +0.279V vs Ag/AgCl/3M NaCl(aq), respectively,
for the ferrocene and porphyrin species). The dashed line shows that
the ferrocene data are better fit with the modified Butler-Volmer form
An important question is how the 1,2,3-triazole linkage affects
the rate of electron transfer. Forster and co-workers have
previously shown that the standard rate constant of electron
transfer through a 3,5-bis(pyridin-4-yl)-1,2,4-triazole bridge is
of order 1 × 106 s-1 44
. If the 1,2,3-triazole linkage is comparable
to the 1,2,4-triazole then we would expect in our longer bridges
that the 1,2,3-triazole would have a small to negligible effect
on the rate of electron transfer. A mixed monolayer of
including self-exchange (k0 ) 0.65 s-1, E0 ) +0.330 V, and kse
)
0.40 s-1).

Controlling the Rate of Interfacial Electron Transfer
J. Phys. Chem. B, Vol. 110, No. 32, 2006 15959
N3(CH2)16SH + CH3(CH2)15SH modified with ferrocene acet-
ylene should, at least at a structural level, closely resemble the
mixed monolayer of FcCO2(CH2)16SH + CH3(CH2)15SH that
was studied by Chidsey.2 The reported ket(E0) for that monolayer
was 2.5 s-1. The difference between that assembly and ours is
that the ester link has been replaced by a triazole link. Our
reported ket(E0) of 2.2 s-1 is identical within error, indicating
that the triazole seems to behave similarly to an ester with regard
to electron tunneling. We modified the same mixed monolayer
of N3(CH2)16SH + CH3(CH2)15SH with ferrocene carbonyl
acetylene30 and found a ket(E0) of 3.4 s-1. This is only slightly
faster than the simple triazole case and seems to indicate that
the addition of a carbonyl to the linking scheme has a mild
accelerating affect on electron transfer. We can hypothesize that
this acceleration is due to the carbonyl allowing better π-con-
jugation of the substituted cyclopentadienyl ring with the triazole
by reduction of ortho C-H strain. Previous work with oli-
gophenylene ethynylene and oligophenyl vinylene bridges
between ferrocene and gold has shown that π-conjugated
linkages have much faster rates of electron transfer over similar
distances.6,9 Thus it is not too surprising that similar rates are
seen for the ester, triazole, and carbonyl triazole linkages, all
of which can be π-conjugated to the cyclopentadienyl ring to a
greater or lesser degree depending on the exact steric conforma-
tion adopted.
terminated thiol monolayers on gold. The electron-transfer rate
depends strongly on the length and degree of conjugation of
the azide-terminated thiol to which the redox species is coupled
as well as the length of the diluent thiol. These assemblies seem
nearly ideal for applications where one is interested in predict-
ably controlling and changing the electron-transfer rate to a
functionally complex redox species. Although we are currently
pursuing studies using these systems to control electron transfer
to synthetic bimetallic catalysts one could imagine adapting such
systems for electron-transfer studies involving both biomolecules
such as redox enzymes or oligonucleotides and also nanoscale
electronic materials such as metal or semiconductor nanopar-
ticles or nanowires.
Experimental Section
Substrate Preparation. The gold substrates were prepared
by condensation of an adhesion layer of electron-beam evapo-
rated titanium (99.99% purity) followed by gold (99.99% purity)
onto 4-in. silicon wafers. Silicon was pre-cleaned for 10 min
in hot piranha (1 volume 30% by mass aqueous H2O2:3 volumes
H2SO4), and rinsed in deionized water. (Warning: Piranha
solution reacts violently, even explosively, with organic materi-
als. It should not be stored or combined with significant
quantities of organic material.)
The nearly ideal voltammetry of the ferrocenes and porphyrins
attached to the mixed alkane thiol monolayers via the polar
1,2,3-triazole linkage (90-100 mV full width at half maximum
at slow scan rates) is also reminiscent of the nearly ideal
voltammetry observed with the polar ester linkage to ferrocene
in the earlier coabsorbed system.38 In that work, a direct,
nonpolar CH2 linkage to the ferrocene was also examined and
shown to result in much broader, nonideal voltammograms, even
at coverages as low as 10%. It was hypothesized that the more
polar linkage prevented the formation of aggregates of redox
species. It would thus appear that the 1,2,3-triazole linkage has
a desirable mixture of conjugation and polarity that could prove
useful in many contexts. Among many possible applications,
the ease with which functionality can be introduced with the
click coupling chemistry should be useful for future studies of
how chemical structure in monolayers affects electron tunneling.
The titanium and gold depositions were carried out in a
cryogenically pumped deposition chamber. Titanium thicknesses
(monitored with a quartz oscillator) were on the order of 10-
20 nm and gold thicknesses were on the order of 50-100 nm.
After deposition, the chamber was backfilled with purified
argon.
Formation of Mixed Thiol Monolayers on Gold. Freshly
evaporated gold substrates were immersed for 24-36 h in
deposition solutions made by dissolving the desired ratio of
azide-terminated thiol and diluent thiol in ethanol. The total thiol
concentration was always 0.1 mM. After deposition, the
monolayer-coated samples were rinsed with ethanol and water
and dried with N2.
Surface Coupling via Cu(I)-Catalyzed Azide-Alkyne
Cycloaddtion. Coupling solutions consisted of 3 volumes of
dimethyl sulfoxide:1 volume of water containing 400 µM
TBTACuBF4, 8 mM hydroquinone (to ensure that the catalyst
remained in the active Cu(I) oxidation state), and 30 µM of the
desired acetylene. For convenience, the copper complex was
formed as a 10 mM stock solution in dimethyl sulfoxide and
appropriate aliquots were used. Because this stock solution
slowly oxidizes with time, the mild reducing agent, hydro-
quinone, was added to ensure that the catalyst remained active.
Mixed azide- and methyl-terminated monolayers were exposed
to coupling solutions for roughly 10-15 min. After reaction,
the surfaces were rinsed with ethanol, methylene chloride,
ethanol, and water in that order.
Electrochemical Measurements. The electrochemical cell
area was defined by pressing a cylindrically bored Teflon cone
(4 or 8 mm inner diameter) against the sample. The bore was
filled with 1 M aqueous perchloric acid. A platinum counter
electrode and a glass frit-isolated Ag/AgCl/3 M NaCl(aq)
reference electrode were suspended in the cell.
One striking aspect of Table 1 is the large difference in
electron-transfer rate depending on whether the redox species
is a ferrocene or an iron porphyrin. In the two mixed monolayers
on which iron porphryins were studied (N3(CH2)16SH +
CH3(CH2)15SH or N3(CH2)16SH + CH3(CH2)11SH) the rates
were 10 to 50 times the value for the identical monolayer
modified with ferrocene acetylene. A marked difference in
reorganization energy between ferrocene and an iron porphyrin
with two axial ligands is a likely explanation for this dramatic
difference in rates. It is well-known from past work that the
reorganization energy of ferrocene on a monolayer is ap-
proximately 0.85 eV. In this work, pyrazine was used as an
additive to axially ligate the immobilized iron porphyrin.45 The
relatively small changes expected in the inner-sphere structure
and the steric bulk of the aromatic porphyrin and pyrazine
ligands should result in small inner- and outer-sphere contribu-
tions to the reorganization energy of this redox species. A future
study of the temperature dependence of the rate of electron
transfer for both the ferrocene and porphyrin-coupled mono-
layers is planned to verify this hypothesis.
Synthesized Compounds. Tris(benzyltriazolylmethyl)amine
(TBTA) was received as a gift from the Sharpless group.
The synthesis of 1-azidoundecan-11-thiol has been previously
described.30 1-Azidohexadecan-16-thiol was synthesized in an
identical manner with the exception that 1-bromohexadecanol
was used as the starting material and during the last step
We have demonstrated that electron-transfer rates can be
predictably controlled to acetylene bearing redox species that
have been coupled by click chemistry to dilute mixed azide-

15960 J. Phys. Chem. B, Vol. 110, No. 32, 2006
Devaraj et al.
1
SCHEME 1a
yielded a reddish brown oil (3.94 g, 94%). H NMR (CDCl3,
200 MHz) δ 7.59 (d, 2H), 7.14 (d, 2H), 3.73 (q, 4H), 1.24 (t,
6H).
Preparation of 1c. The synthesis was adapted from a
previously reported procedure.47 1b (3.76 g, 12.4 mmol),
Pd(PPh3)4 (90 mg), and CuI (130 mg) were placed in a round-
bottom flask. The flask was evacuated and backfilled with
nitrogen, after which triethylamine (18 mL) was added. After
three cycles of evacuation and backfilling with nitrogen,
trimethylsilylacetylene (1.83 g, 18.6 mmol) was added, and the
mixture was stirred at 40 °C for 2 days. The mixture was filtered,
and solvent was evaporated from the filtrate. The resulting
residue was chromatographed (silica, hexane/CH2Cl2, 1:1 v/v)
to yield a tan solid (3.35 g, 99%). 1H NMR (CDCl3, 200 MHz)
δ 7.41 (d, 2H), 7.32 (d, 2H), 3.74 (q, 4H), 1.24 (t, 6H), 0.23 (s,
9H).
a Reagents and conditions: (a) (1) HCl, NaNO2, -20 °C, (2) K2CO3,
Et2NH. (b) Trimethylsilylacetylene, Pd(PPh3)4, CuI, Et3N. (c) K2CO3,
MeOH.
Preparation of DEEPT. To a solution of 1c (2.5 g, 9.2 mmol)
in methanol (40 mL) was added potassium carbonate (1.9 g,
14 mmol). The mixture was stirred for 2 h at room temperature
after which it was extracted with CH2Cl2. The organic phase
was washed with water and then brine, and dried over MgSO4.
Rotary evaporation of the solvent yielded a brown oil (1.8 g,
SCHEME 2a
1
97%). H NMR (200 MHz, CDCl3) δ 7.41 (d, 2H), 7.31 (d,
2H), 3.72 (q, 4H), 3.01 (s, 1H), 1.22 (t, 6H).
Preparation of 2b. A suspension of 3a (38.8 g, 178 mmol),
azobis(isobutyronitrile) (3.2 g), and N-bromosuccinimide (36.5
g, 205 mmol) in benzene (70 mL) was placed in a round-bottom
flask fitted with a reflux condenser. The reaction chamber was
evacuated and backfilled with nitrogen three times. The reaction
mixture was refluxed overnight in the dark. The solvent was
evaporated and the residue was chromatographed (silica, hexane)
to yield a cream-colored solid (28.7 g, 55%). 1H NMR (CDCl3,
200 MHz) δ 7.66 (d, 2H), 7.12 (d, 2H), 4.40 (s, 2H).
Preparation of 4-Iodobenzylthioacetate. The synthesis was
adapted from a previously reported procedure.47 2b (1.45 g, 4.9
mmol) and potassium thioacetate (663 mg, 5.8 mmol) were
dissolved in dimethylacetamide (10 mL). The solution was
stirred for 2 days, then poured into water (200 mL) and extracted
with CHCl3. The organic phase was washed with water and
brine, and then dried with anhydrous MgSO4. The solvent was
evaporated, and the residue was chromatographed (silica,
a Reagents and conditions: (a) NBS, AIBN, benzene, reflux. (b)
Potassium thioacetate, DMA. (c) DEEPT, Pd(PPh3)4, CuI, Et3N.
SCHEME 3a
1
hexane:CHCl3) to yield a pale solid (597 mg, 42%). H NMR
(CDCl3, 200 MHz) δ 7.60 (d, 2H), 7.02 (d, 2H), 4.02 (s, 2H),
2.33 (s, 3H).
Preparation of Pre-APEB. The synthesis was adapted from
a previously reported procedure.47 4-Iodobenzylthioacetate (195
mg, 0.67 mmol), Pd(PPh3)4 (4.6 mg), and CuI (6.6 mg) were
placed in a round-bottom flask. The flask was evacuated and
backfilled with nitrogen, after which triethylamine (5 mL) and
THF (8 mL) were added. After three cycles of evacuation and
backfilling with nitrogen, DEEPT (134 mg, 0.67 mmol) was
added, and the mixture was stirred at 40 °C for 2 days. The
solvent was evaporated, and the residue was chromatographed
(silica, hexane/CH2Cl2, 1:1 v/v) to yield Pre-APEB (240 mg,
98%). H NMR (CDCl3, 200 MHz) δ 7.43 (d, 2H), 7.39 (d,
2H), 7.33 (d, 2H), 7.20 (d, 2H), 4.06 (s, 2H), 3.73 (q, 4H), 2.31
(s, 3H), 1.22 (t, 6H).
Preparation of Pre-APEB* (conVersion of triazene to azide).
The synthesis was adapted from a previously reported proce-
dure.48 To a solution of Pre-APEB (250 mg, 0.67 mmol) in
CHCl3 (4.5 mL) was added trifluoracetic acid (382 mg, 3.35
mmol). After the solution was stirred for 15 min, azidotrim-
ethylsilane (93 mg, 0.8 mmol) was added. After being stirred
for 1 h, the mixture was poured into water (100 mL) and
a Reagents and conditions: (a) Azidotrimethylsilane, trifluoroacetic
acid. (b) K2CO3, MeOH.
(thioacetate deprotection) ethanol was substituted for methanol
to maintain solubility.
Synthesis of Azidophenyleneethynylenebenzylthiol (N3(PEB)-
SH or APEB). Preparation of 1b. The synthesis was adapted
from a previously reported procedure.46 A suspension of
4-iodoaniline (3.02 g, 13.8 mmol) in water (30 mL) and
concentrated hydrochloric acid (30 mL) was cooled in a ice/
salt bath. To the suspension was slowly added a solution of
sodium nitrite (1.5 g, 20.7 mmol) in ice cold water (10 mL)
under vigorous stirring. After being stirred for 15 min, the
diazonium solution was slowly added to a solution of diethy-
lamine (2.52 g, 35 mmol) in water (30 mL), in an ice bath.
Potassium carbonate was added until CO2 evolution ceased. The
mixture was stirred for 1 h at room temperature, extracted with
CH2Cl2, and washed with brine. The organic phase was dried
with anhydrous MgSO4 and the solvent was evaporated. Column
chromatography (silica, hexane/CH2Cl2, 1:1 v/v) of the residue
1

Controlling the Rate of Interfacial Electron Transfer
J. Phys. Chem. B, Vol. 110, No. 32, 2006 15961
SCHEME 4: Synthesis of Acetylene-Containing
Porphyrin 3a
7.5 Hz), 7.78 (m, 9H), 7.54 (t, 1H, J ) 7.5 Hz), 7.14 (s, 1H),
1.97 (s, 1H), -2.73 (s, 2H); 13C NMR (75 MHz, CDCl3) δC
(ppm) 149.80, 142.13, 141.89, 137.96, 135.15, 134.80, 134.75,
134.62, 131.73, 129.88, 128.12, 127.01, 126.94, 123.84, 121.51,
121.36, 120.88, 112.21, 74.09; MS (ESI+) m/z 682.9 [M + H]+
(Calcd for C47H31N5O); UV/vis (CH2Cl2) λ (nm) 10-3 × ꢀ (M-1
cm-1) 374 (12.0), 400 (3.7, sh), 418 (158.2), 482 (2.3, sh), 514
(8.5), 548 (3.6), 588 (2.9), 648 (2.5), 678 (1.1); TLC (SiO2,
hexane/dichloromethane 1:1 v/v) Rf 0.6 (purple spot); mp 185
°C.
meso-5-Mono-o-propynamidophenyl-10,15,20-triphenylpor-
phyriniron(III) Chloride (3). To an O2-free solution of free-
base porphyrin 6 (90 mg, 0.13 mmol) in a CH3OH-THF
mixture (1:4 vol, 25 mL) was added FeBr2 (71 mg, 0.330 mmol)
and the resulting mixture was heated for 45 min. The solution
was cooled and air was bubbled through the solution for 1 h.
After addition of benzene (50 mL), the µ-oxo complex was
hydrolyzed by washing with 8% HCl solution (3 × 50 mL).
Solvents were distilled leaving the paramagnetic species 3 as a
dark blue residue (85.1 mg, 85%). MS (ESI+, m/z) 735.3 [M -
Cl]+ (Calcd for C47H29ClFeN5O); UV/vis (CH2Cl2) λ (nm) 10-3
× ꢀ (M-1 cm-1) 320 (18.6), 410 (62.6), 508 (4.0), 570 (4.8),
610 (1.9), 674 (0.6); TLC (Al2O3, hexane/ethyl acetate 2:1 v/v)
Rf 0.5 (green-brownish spot).
a Reagents and conditions: (a) AcOH, reflux, 1 h, 3%. (b) (i)
SnCl2‚2H2O, HCl, room temperature, 16 h; (ii) NH4OH, 75%. (c)
Propiolic acid, DCC, EtOAc, room temperature, 24 h, 70%. (d) (i)
FeBr2, CH3OH/THF, reflux, 45 min; (ii) O2, CH3OH/THF/C6H6, room
temperature, 1 h; (iii) 8% aq.HCl, room temperature, 85%.
extracted with CHCl3. The organic phase was washed with brine
and then dried with anhydrous MgSO4. Rotary evaporation
yielded Pre-APEB* as an orange solid (201 mg, 98%). 1H NMR
(CDCl3, 200 MHz) δ 7.48 (d, 2H), 7.42 (d, 2H), 7.24 (d, 2H),
6.98 (d, 2H), 4.10 (s, 2H), 2.34 (s, 3H); mp 61-63 °C.
Preparation of APEB (deacylation of thioacetate). Potassium
carbonate (90 mg, 0.65 mmol) was added to a solution of Pre-
APEB* (100 mg, 0.33 mmol) in methanol (5 mL). The mixture
was stirred for 4 h under nitrogen and then poured into water.
The mixture was extracted with CHCl3 and washed with brine.
The organic phase was dried with anhydrous MgSO4, and the
solvent was evaporated. The residue was chromatographed
Acknowledgment. N.K.D. acknowledges a Stanford Gradu-
ate Fellowship, R.A.D. acknowledges a Lavoisier Fellowship,
and W.E. acknowledges a Bing Undergraduate Fellowship. This
material is based upon work supported by the NIH under grants
GM-69568 and GM-17880.
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benzaldehyde, o-nitrobenzaldehyde, and pyrrole49 followed by
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new porphyrins 3 and 6 are provided below and follow standard
procedures in porphyrin chemistry.49,51
meso-5-Mono-o-propynamidophenyl-10,15,20-triphenylpor-
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7.30 mmol), and DCC (150 mg, 0.73 mmol) were mixed in
ethyl acetate (20 mL) and the resulting mixture was stirred at
room temperature for 24 h. After most of the starting material
had been consumed (monitored by TLC), the volume of the
solution was expanded (50 mL EtOAc) and the mixture was
washed with NaHCO3 solution (2 × 50 mL) and water (2 ×
100 mL) and dried (Na2SO4). After evaporation of the solvent,
the residue was subjected to chromatography [SiO2, 63 µm
(230-400 mesh), 20 × 3 cm, eluent: gradient hexane/
dichloromethane]. Evaporation of the solvent led to 6 as a dark
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