Electrochemical approach for constructing a monolayer of thiophenolates from grafted multilayers of diaryl disulfides

Article abstract of DOI:10.1021/ja0682430

An electrochemical approach is presented for constructing a monolayer or near monolayer of molecules on conducting surfaces by degradation of a multilayer film formed via reduction of aryldiazonium salts. Usually, grafting of surfaces employing aryldiazonium salts results in the formation of intertwined multilayers in a difficult controllable polymerization reaction. However, if the chemical system selected contains a cleavable group, a subsequent degradation of the polymeric network may allow the formation of a thin reactive molecular layer. As chemical systems, diazonium salts of diaryl disulfides (ArSSAr) were selected to produce surface-confined thiophenolates, but an extension of the approach to include other functionalities is straightforward. The thiophenolates (ArS^-) may be reversibly oxidized to disulfides to establish a covalently attached ArSSAr/ArS^- redox pair at the surface. Analysis of the surfaces was carried out by means of cyclic voltammetry, X-ray photoelectron spectroscopy, and atomic force microscopy. Copyright

Full text of DOI:10.1021/ja0682430

Published on Web 01/25/2007
Electrochemical Approach for Constructing a Monolayer of Thiophenolates
from Grafted Multilayers of Diaryl Disulfides
Lasse T. Nielsen,† Karina H. Vase,† Mingdong Dong,‡ Flemming Besenbacher,‡
Steen U. Pedersen,*,† and Kim Daasbjerg*,†
Department of Chemistry, UniVersity of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark, and
Interdisciplinary Nanoscience Center (iNANO), UniVersity of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark
Received November 17, 2006; E-mail: sup@chem.au.dk; kdaa@chem.au.dk
Scheme 1
One of the most widely used procedures for covalent modification
of conducting surfaces involves electroreduction of aryldiazonium
salts.1-13 Usually, the electrode derivatization is so effective that
intertwined multilayers are formed.4,7 Although the process to some
extent may be controlled through the applied potential and
electrolysis time,11-13 the derivatized surface will seldom be
sufficiently well-defined for developing, for example, sensors or
molecular devices. To accomplish such a goal, other approaches
would usually be required.
In this communication, we explore the possibility of preparing
thin layers on carbon surfaces through a degradation of multilayers
formed via reduction of diazonium salts. Rather than controlling
the grafting process, the key issue becomes selecting appropriate
molecular systems containing cleavable groups. The basic concept
involving disulfides is outlined in Scheme 1.
In the first step, a diazonium salt of a diaryl disulfide is reduced
electrochemically at a glassy carbon (GC) surface to produce a
covalently bonded multilayer. This polymerization process goes
the oxidation of the remaining thiophenolates on the reverse sweep
through the intermediate formation of reactive aryl radicals which
is a one-electron process, it may be concluded that the initially
attack either the surface or the aryl rings of already grafted
molecules.4,7 Because of the steric constraints, it is expected that
the outer ring B will be more accessible to attack than A.
Subsequent reductive cleavage14 of the disulfide bonds should,
therefore, in principle, lead to the generation of a monolayer of
thiophenolates. The formation of such a monolayer would be either
impossible or, at least, difficult to obtain through a direct elec-
troreduction of 4-mercaptobenzenediazonium.15
The grafting of the surfaces with the two diazonium salts, 4,4′-
disulfanediyldibenzenediazonium (1)16 or 4-[(4-chlorophenyl)dis-
ulfanyl]benzenediazonium (2), was carried out using standard
procedures (potentiostatic grafting for 300 s at a potential 200 mV
negative of the reduction peak; see Supporting Information). In
Figure 1, a cyclic voltammogram recording of a GC electrode
derivatized with compound 2 is shown.
formed coating consists of about two layers.
The second and subsequent cycles give rise to essentially the
same oxidative behavior, whereas the reduction wave decreases to
the same size as the oxidation wave while shifted in a positive
direction by 300 mV. Both the cathodic peak current, ip,c, and the
anodic peak current, ip,a, are found to be proportional to the sweep
rate ν (inset of Figure 1).17 These observations show that the
degradation of the outer layers on the first cycle is an efficient
process and that the molecules left on the surface constitute a
surface-confined redox pair, that is, ArSSAr/ArS-. The stability
of this pair is surprisingly high as evidenced by the fact that at
The most notable feature on the first cycle is the reduction wave
at -1.3 V versus SCE accompanied by the smaller oxidation wave
at -0.3 V versus SCE; the latter wave only appears if the first
sweep is extended past the reduction wave. We attribute these two
signals to the reductive cleavage of diaryl disulfide units, ArSSAr,
in the multilayer followed by oxidation of surface-confined ArS-
(Scheme 1). Integration of the waves reveals that about four times
as much charge is associated with the reduction as with the
oxidation process, indicating that thiophenolates generated in the
outer layer during the reductive sweep are able to escape not only
the surface but also detection on the oxidative sweep. On the
assumption that all disulfides are reduced on the forward sweep
and that the reduction proceeds by a two-electron process while
Figure 1. Cyclic voltammograms of a GC electrode derivatized with 2
recorded at a sweep rate ν of 0.1 V s-1 in 0.1 M Bu4NBF4/N,N-
dimethylformamide; sweep no. 1 (black) and 2 (red). Inset shows plots of
(a) anodic and (b) cathodic peak currents measured for the degraded film
as a function of ν.
† Department of Chemistry.
‡ Interdisciplinary Nanoscience Center.
9
1888
J. AM. CHEM. SOC. 2007, 129, 1888-1889
10.1021/ja0682430 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. XPS Surface Elemental Compositions of Modified Glassy
A. Future work will show to what extent the exact electrolysis
conditions and, in particular, the substituent X can be used to affect
the proportion of radicals reaching the inner part.
Carbon Platesa
atomic concentration (%)b
In summary, a versatile electrochemical approach employing
diazonium salts of diaryl disulfides has been developed for the
construction of a monolayer or near monolayer of ArS- on glassy
carbon surfaces. An extension of the approach to include other
cleavable functionalities as well as atomically smooth materials such
as Si, metals, or pyrolyzed photoresist films to better exploit the
properties of a monolayer seems straightforward. Interestingly, ArS-
may be reversibly oxidized to ArSSAr, thus allowing the establish-
ment of a relatively stable covalently attached ArSSAr/ArS- redox
pair. This feature gives simple access to adjusting and controlling
electrochemically the surface properties. For instance, preliminary
work has shown that the high reactivity of thiophenolates may be
exploited in further reactions to develop more advanced molecular
systems. In addition, the prospect in repeating the formation/
degradation procedure with the purpose of eliminating pinholes and/
or build up surfaces layer by layer is worth pursuing.
entry
electrode
C1s
S2p
Cl2p
1
2
3
4
5
GCc
94.5
85.5
89.4
84.8
88.7
GC/1d
5.1
0.9
5.3
0.8
GC/1/cleavede
GC/2f
1.0
0.0
GC/2/cleavedg
a Average of the results obtained for two GC plates. b In addition to the
reported elements, the surfaces contained oxygen. c Bare GC plate. d GC
plate modified with 1. e Entry 2 after reduction at -1.7 V vs SCE in 0.1 M
Bu4NBF4/N,N-dimethylformamide. f GC plate modified with 2. g Entry 4
after reduction at -1.4 V vs SCE in 0.1 M Bu4NBF4/N,N-dimethylforma-
mide.
least 100 cycles at ν ) 0.1 V s-1 are required to deactivate it
electrochemically. Thus, it appears that disulfide bonds can be
created between the covalently attached molecules at the surface
despite the steric constraints. In fact, the positive shift of the
reduction wave observed for the first two cycles could be partly
due to differences in the sulfur-sulfur bond strengths; that is, the
bonds in the degraded film are more strained and weaker than those
in the multilayer film. Also the increase of the heterogeneous charge
transfer rate brought about by the formation of a thinner film will
induce a shift in this potential direction.18
Acknowledgment. We are indebted to the Danish Natural
Science Research Council for continuing and generous financial
support. We also thank laboratory technician, Lene Hubert, Danish
Polymer Center, for recording XPS spectra.
Supporting Information Available: Synthetic details, grafting
procedures, and analyses of electrode surfaces by electrochemical
methods and AFM. This material is available free of charge via the
Internet at http://pubs.acs.org.
The surface coverage Γ of ArS- and ArSSAr in the degraded
film can be determined from an integration of the current signals
to be 4.0 and 2.2 × 10-10 mol cm-2, respectively (obtained at ν e
2 V s-1). While this shows that essentially all surface-confined ArS-
surprisingly are able to form a disulfide bridge with a neighbor
molecule upon oxidation, Γ is much lower than that calculated for
an ideal closed-packed monolayer at a plane surface (Γideal ) 12 ×
10-10 mol cm-2).19,20 In fact, a study using the smaller 4-nitroben-
zenediazonium salt as grafting agent has shown that at least five
layers were required to obtain a coverage comparable to Γideal on
carbon materials and that a single molecular layer would correspond
References
(1) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc.
1992, 114, 5883-5884.
(2) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson,
J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201-207.
(3) Downard, A. J. Electroanalysis 2000, 12, 1085-1096.
(4) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947-5951.
(5) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894-10902.
(6) Nowak, A. M.; McCreery, R. L. J. Am. Chem. Soc. 2004, 126, 16621-
16631.
(7) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429-439.
(8) Holm, A. H.; Møller, R.; Vase, K. H.; Dong, M.; Norrman, K.;
Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. New J. Chem. 2005, 29,
659-666.
(9) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21, 1672-1675.
(10) Baranton, S.; Be´langer, D. J. Phys. Chem. B 2005, 109, 24401-24410.
(11) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791-
8798.
to Γ ) 2.5 × 10-10 mol cm-2 21
.
To substantiate the proposed reaction scheme, GC plates de-
rivatized with 1 or 2 were analyzed (Table 1) before and after
degradation by means of X-ray photoelectron spectroscopy (XPS).
In entries 2 and 4, survey spectra of the two modified surfaces are
provided. Importantly, they contain nearly the same amount of S
(∼5.2%). For the surface derivatized with 2, the amount of Cl
(1.0%) is surprisingly small. We suspect that this may be attributed
to an ipso-attack of the aryl radical at the chlorine substituent in
the polymerization reaction followed by an elimination of chlorine.22
As expected, the sulfur content diminishes considerably after the
reductive cleavage (entries 3 and 5) to ∼0.9%. This is accompanied
by a complete disappearance of chlorine in the case of 2. These
observations are consistent with the proposition that the efficient
degradation of the modified surface results in the removal of the
outer part, thus leaving the inner ring A as the main moiety at the
surface.
(12) Anariba, F.; Viswanathan, U.; Bocian, D. F.; McCreery, R. L. Anal. Chem.
2006, 78, 3104-3112.
(13) Ricci, A.; Bonazzola, C.; Calvo, E. J. Phys. Chem. Chem. Phys. 2006, 8,
4297-4299.
(14) The bond cleavage can be accomplished using various chemical ap-
proaches, but an electrochemical procedure exploiting the high conductivity
of GC seems straightforward.
(15) Harnisch, J. A.; Pris, A. D.; Porter, M. D. J. Am. Chem. Soc. 2001, 123,
5829-5830.
(16) For the salt 1, the second N2+ group will be reduced off and most likely
replaced with a hydrogen atom, this being the reason for presenting the
substituent X in Scheme 1 as N2+/H.
(17) The slopes of plots of log ip,c and log ia,c versus log ν are 0.87 and 0.92,
respectively.
(18) In general, the voltammetric behavior is consistent with that of freely
diffusing diaryl disulfide, although the oxidation wave of surface-confined
ArS- appears at a less extreme potential than the solution counterpart
(Supporting Information). This might be attributed to an effect such as
charge repulsion between the surface-confined thiophenolate ions. The
possibility that the inner ring A has been attacked by aryl radicals during
the grafting process to produce the biphenyl bis(thiolate) system after
degradation cannot be excluded.
The film thickness for the GC plates in entries 4 and 5 in Table
1 was determined by atomic force microscopy (AFM) to be 3 ( 1
and 1.5 ( 0.5 nm (Supporting Information). Although the uncer-
tainty on these measurements does not allow a quantitative
description of the changes induced by the degradation procedure,
they substantiate the view that molecular units of the originally
derivatized surface (consisting of 2-3 layers of diaryl disulfides)
indeed have been cleaved off.23 The thickness of a monolayer of
thiophenolate is calculated to be only 0.6 nm,20 so we cannot
exclude that larger biphenyl bis(thiolate) units may have been
formed during the grafting because of aryl radicals attacking ring
(19) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254-11259.
(20) Calculations are based on an optimization of the structure using DFT and
assuming a perpendicular orientation of the molecules.
(21) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038-5045.
(22) Traynham, J. G. Chem. ReV. 1979, 79, 323-330.
(23) The thickness of the initially derivatized film is essentially equal to that
found under similar grafting conditions in other studies, in which typically
4-5 layers are formed using grafting agents of approximately half the
molecular size of our systems.4,21
JA0682430
9
J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 1889