Structural control of the horizontal double fixation of oligothiophenes on gold

Article abstract of DOI:10.1002/chem.200800133

Quaterthiophenes bearing one (1) or two (2) alkanethiol chains attached at the internal β-position of the outermost thiophene ring through a sulfide linkage have been synthesized. Cyclic voltammetric analysis of their electrochemical behavior in solution suggests that electrooxidation of the doubly substituted oligomer 2 leads to electrodeposition of a poly(disulfide) on the anode surface. Monolayers of 1 or 2 on gold surfaces have been investigated and characterized by cyclic voltammetry, ellipsometry, contact angle measurement, and X-ray photoelectron spectroscopy. The results of these investigations indicate that introduction of two thiol groups in the structure leads to double fixation of the oligothiophene chain with the main axis of the conjugated system oriented parallel to the surface. The effects of single versus double fixation of the quaterthiophene chain on the electrochemical properties and stability of the corresponding monolayers are discussed.

Full text of DOI:10.1002/chem.200800133

FULL PAPER
DOI: 10.1002/chem.200800133
Structural Control of the Horizontal Double Fixation of
Oligothiophenes on Gold
TruongKhoa Tran, [a] MaïtØna OÅafrain,[a] Sandrine Karpe,[a] Philippe Blanchard,*[a]
Jean Roncali,*[a] StØphane Lenfant,[b] Sylvie Godey,[b] and Dominique Vuillaume[b]
Abstract: Quaterthiophenes bearing
one (1) or two (2) alkanethiol chains
attached at the internal b-position of
the outermost thiophene ring through a
sulfide linkage have been synthesized.
Cyclic voltammetric analysis of their
electrochemical behavior in solution
suggests that electrooxidation of the
doubly substituted oligomer 2 leads to
electrodeposition of a poly(disulfide)
on the anode surface. Monolayers of 1
or 2 on gold surfaces have been investi-
gated and characterized by cyclic vol-
tammetry, ellipsometry, contact angle
measurement, and X-ray photoelectron
spectroscopy. The results of these in-
vestigations indicate that introduction
of two thiol groups in the structure
leads to double fixation of the oligo-
thiophene chain with the main axis of
the conjugated system oriented parallel
to the surface. The effects of single
versus double fixation of the quater-
thiophene chain on the electrochemical
properties and stability of the corre-
sponding monolayers are discussed.
Keywords: conjugation · dithiols ·
gold · monolayers · self-assembly
Introduction
to the conjugated system by a flexible linear alkyl chain. In-
teractions between the p-conjugated chains reinforced by
lipophilic interactions between alkyl spacers contribute to
ensure the cohesion of the SAMs and the quasi-vertical ori-
entation of the conjugated chains on the surface of the
substrate.[7]
Self-assembled monolayers (SAMs) based on the adsorption
of electroactive p-conjugated systems on metal or semicon-
ductor surfaces are of growing interest owing to their poten-
tial applications in the fields of molecular electronics,[1,2]
photovoltaic conversion,[3,4,5] and sensors.[6]
Taking advantage of this quasi-vertical orientation, Tour
and co-workers have investigated the conductance of single
conjugated molecules by scanning tunneling microscopy
(STM).[1d,f] Metzger and co-workers have used a similar ar-
rangement to analyze the rectifying properties of Langmuir–
Blodgett-based monomolecular junctions,[8] whereas SAMs
have also been developed for this purpose.[9] In another ap-
proach, monolayers of vertically oriented, stacked, conjugat-
ed systems have recently been used to realize organic field-
effect transistors in which the thickness of the channel cor-
responds to the length of the active conjugated molecule.[10]
Monolayers of linear oligothiophenes with a vertical or
quasi-vertical orientation immobilized through a single at-
tachment on a surface have been described by several
groups[9c,d,11–18] and investigated as molecular junctions,[13] di-
odes,[9c,d,f] switches,[14] and field-effect transistors,[10] and for
the development of a matrix-assisted laser desorption/ioni-
zation technique.[15] Otsubo and co-workers have reported
SAMs of oligothiophenes vertically immobilized on gold by
means of a tripodal anchoring group and their use as charge
injection layers in organic light-emitting diodes.[16] Hirayama
These SAMs are generally formed by chemical adsorption
of a conjugated molecule bearing one fixation group such as
a thiol, sulfide, disulfide, thiocyanate, or silane, often linked
[a] Dr. T. K. Tran, Dr. M. OÅafrain, Dr. S. Karpe, Dr. P. Blanchard,
Dr. J. Roncali
University of Angers, CNRS, CIMA
Linear Conjugated Systems Group
2Bd. Lavoisier, 49045 Angers (France)
Fax : (+33)241-735-405
E-mail: Philippe.Blanchard@univ-angers.fr
Jean.Roncali@univ-angers.fr
[b] Dr. S. Lenfant, Dr. S. Godey, Dr. D. Vuillaume
Molecular Nanostructures & Devices Group
Institute for Electronics Microelectronics & Nanotechnology, CNRS
University of Sciences and Technologies of Lille
BP60069, Avenue PoincarØ, 59652Villeneuve d ’Ascq (France)
Supporting information (1H NMR spectra of compounds 1, 2, and 6–
8, an illustration of the electrothermic properties of the polysulfide
film and its proposed structure, amd cyclic voltammograms of 8) for
this article is available on the WWW under http://www.chemeurj.org/
or from the author.
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6237

et al. have prepared monolayers consisting of C60-substituted
quater- and octithiophene linked to a gold surface with a di-
sulfide anchoring group, and have analyzed their photo-elec-
trochemical behavior.[3a] Improved photocurrent generation
was subsequently obtained when surface fixation was ach-
ieved by a tripodal anchoring group that ensured a denser
surface coverage than the single-point attachment.[3c]
More recently, Jen and co-workers have synthesized
SAMs based on a dithienylpyrrole disubstituted by C60
groups with an anchoring aryl thiol attached at the nitrogen
atom of the median pyrrole unit.[3d] The greater photocur-
rent generation than in previous systems was attributed to a
better absorption of the incident light thanks to the horizon-
tal orientation of the conjugated system.
The literature contains very few other examples of mono-
layers in which extended linear p-conjugated systems are
fixed horizontally on a surface.[11,19,20] Berlin, Zotti, and co-
workers have investigated the influence of molecular geom-
etry on the electrochemical coupling of bi- and terthiophene
adsorbed on indium tin oxide (ITO). They have shown that
SAMs of cyclopentabithiophenes, in which the conjugated
system is fixed parallel to the surface by a single attachment,
can be trimerized electrochemically.[11b]
The horizontal immobilization of linear p-conjugated sys-
tems on a substrate[3d,20] can contribute to the toolbox for
the fabrication of molecular electronic devices or electro-
chemical sensors. In this context, a possible approach to im-
posing a horizontal orientation on the conjugated system
can be to introduce two anchoring groups at two sufficiently
remote points of an extended p-conjugated system. Double
fixation of a dithiol molecule on gold was previously report-
ed by Gokel, Kaifer, and co-workers for a catenane-like
structure.[21] More recently, Kern and Sauvage have de-
scribed the double fixation on a gold surface of a disulfide
incorporating copper(I) catenane,[22] and Raymo has report-
ed that of a copper(I) complex based on 4,4’-bipyridinium
arms terminated by a thiol group.[23] Echegoyen and co-
workers have reported the formation of a pseudo crown-
ether loop on a surface by double fixation of an acyclic bis-
podant thiol, owing to the template effect of K+ ions during
monolayer formation.[24] Although a,w-alkanedithiols have
been shown to adopt an upright alignment of their hydrocar-
bon chains on the gold surface,[25] formation of a looped
configuration was also reported in several examples[26] and it
was found recently that the orientation of alkanedithiol mol-
ecules in monolayers has a strong impact on the electrical
transport in molecular junctions.[26c]
ACHTREUNG
Results and Discussion
Synthesis: Our approach to the synthesis of the target com-
pounds 1 and 2 (Scheme 1) is based on the chemistry of thi-
olate derivatives protected by a 2-cyanoethyl group.[27–29] Se-
lective deprotection of one thiolate group of 3,3’’’-bis(2-cya-
noethylsulfanyl)-2,2’:5’,2’’:5’’,2’’’-quaterthiophene 3[29a] with
cesium hydroxide (1 equiv) and subsequent reaction with an
excess of 1-iodopentane in DMF gave 4 in 77% yield. Simi-
larly, further treatment of 4 with cesium hydroxide (1 equiv)
and S-alkylation with an excess of S-4-bromobutyl ethane-
thioate 5 led to 6 in 87% yield. Thiol ester 5 was prepared
in 63% yield by reaction of potassium thioacetate with 1,4-
dibromobutane (1.5 equiv). Reduction of the thioester
group of 6 by diisobutylaluminum hydride (DIBAL-H) in
anhydrous CH2Cl2 followed by addition of hydrochloric acid
led to the target 3-(4-mercaptobutylsulfanyl)-3’’’-pentylsul-
fanyl-2,2’:5’,2’’:5’’,2’’’-quaterthiophene 1 in 90% yield. Simi-
larly, dithiol 2 was obtained by reduction of the two thioest-
er groups of 7 with DIBAL-H. Dithioester 7 was prepared
by double deprotection of the two thiolate groups of 3 with
In this context, we report here on the synthesis of two
quaterthiophenes 1 and 2 bearing respectively one and two
alkanethiol chains attached at the internal b-position of the
outermost thiophene ring by a sulfide linkage. The electro-
chemical properties of these compounds in solution have
been analyzed and their immobilization as monolayers on
gold electrodes has been investigated. The influence of the
presence of one versus two thiol group(s) on the formation,
structure, and properties of the monolayers is discussed on
the basis of their characterization by cyclic voltammetry,
Scheme 1. Synthesis of monothiol 1 and dithiol 2. i) CsOH·H2O (1.1 eq),
ii) IC5H11 (4.4 eq); iii) CsOH·H2O (2.2 eq), iv) Br
v) CsOH·H2O (1.2eq), vi) Br (CH2)4SCOCH3 (5); vii) DIBAL-H, 08C,
viii) aq. HCL; ix) DIBAL-H, 08C, x) aq. HCL.
A
AHCTREUNG
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Fixation of Oligothiophenes on Gold
FULL PAPER
a slight excess of cesium hy-
droxide followed by addition of
an excess of 5.
Cyclic voltammetry (CV) in
A
properties of quaterthiophene
(4T) mono- and dithiols 1 and 2
as well as their precursors 3, 6,
and 7 have been analyzed by
CV in the presence of
Bu4NPF6: 3, 6, and 7 exhibit
two one-electron reversible oxi-
dation waves corresponding to
Figure 1. CV (left) and deconvoluted CV (right) of dithioester 7 (1mm) in 0.10m Bu4NPF6/CH2Cl2–CH3CN
(1:1, v/v), scan rate 0.1 Vsꢀ1, Pt working electrode, SCE reference electrode.
the successive formation of the radical cation and dication
of the 4T backbone (Figure 1 and Table 1). As already ob-
served for parent 4T systems,[29b,c,e] the stability of these 4T
radical cations can be explained by the localization of the
sulfide group at the inner b-position of the terminal thio-
phene rings; in contrast, when introduced at the outer b-po-
sitions such electron donor groups enhance the reactivity of
Table 1. Absorption maxima (in CH2Cl2) and oxidation peak potentials
of quaterthiophenes (1mm) in 0.10m Bu4NPF6/CH3CN–CH2Cl2 (1:1, v/v),
scan rate 100 mVsꢀ1, Pt working electrode).
Compd
lmax [nm]
Epa1/SCE [V]
Epa2/SCE [V]
1
2
3
6
7
8
407
408
403
406
405
407
0.80
0.80
0.94
0.85
0.81
0.79
1.05
1.00
1.10
1.05
0.98
1.11
the radical cation, thus favoring polymerization.[30]
1
All compounds show a first oxidation potential (Epa
)
around 0.80 V except for 3, in which the electron-withdraw-
ing effect of the cyano groups induces a 140 mV positive
1
shift of Epa
.
toward more positive potentials (EpaSH =0.96 V) (Figure 3).
The irreversibility of this electrochemical process may be re-
lated to the rapid formation of a disulfide in the vicinity of
the electrode.
This hypothesis was confirmed by the analysis of the CV
of the disulfide 8 obtained by chemical oxidation of the
thiol group of 1 using FeCl3 (see the Supporting Informa-
tion, Figure S8).
The CV of the mono- and dithiols 1 and 2 also exhibits
2
two successive oxidation peaks at Epa1 =0.80 V and Epa
=
1.00–1.05 V/SCE (Figure 2). However, comparison with the
CV of compounds 6 (see the Supporting Information, Fig-
ure S7) and 7 (Figure 1), respectively, reveals a broadening
of the first wave for 1 and an intensification of the same
wave for 2. Integration and comparison of the oxidation
waves of the deconvoluted CVs
show that the first oxidation
peak of 1 corresponds to a two-
electron process, whereas that
of 2 involves a three-electron
process. These results indicate
that the first oxidation peak
corresponds to the superimposi-
tion of two processes, namely
the reversible oxidation of the
4T chain into its radical-cation
and the irreversible one-elec-
tron oxidation of the thiol
groups into disulfide or polydi-
sulfides.
The oxidation of the thiol
group of 1 is clearly evident
when the platinum working
electrode is replaced by gold.
1
2
Whereas Epa and Epa values
are nearly identical to those ob-
served on Pt, the anodic peak
associated with the oxidation of
Figure 2. CVs (top) and deconvoluted CVs (bottom) of monothiol 1 (left) and dithiol 2 (right) (all 1mm) in
the thiol group is shifted 0.10m Bu4NPF6/CH2Cl2–CH3CN (1:1, v/v), scan rate 0.1 Vsꢀ1, Pt working electrode, SCE reference electrode.
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P. Blanchard, J. Roncali et al.
0.80 V suggests that the effec-
tive conjugation length of the
deposited material is probably
limited to a 4T chain. This hy-
pothesis is further supported by
the UV/Vis spectrum of a poly-
mer film on ITO which shows a
lmax =430 nm. These results are
consistent with a polydisulfide
material containing 4T chains;
the red shift compared with the
solution spectrum of compound
2 (l=408 nm) can be attributed
to p interactions between the
conjugated chains in the solid
state.
Figure 3. CV (left) and deconvoluted CV (right) of monothiol 1 (2mm) in 0.10m Bu4NPF6/CH2Cl2–CH3CN
(1:1, v/v), scan rate 0.1 Vsꢀ1, Au working electrode, SCE reference electrode.
Recurrent potential scans be-
tween 0.20 and 1.20 V/SCE
were then performed on solu-
tions of 1 and of 2. A stable CV
was observed in the case of 1,
whereas the same procedure for
compound 2 leads to the pro-
gressive development of a broad
redox system indicative of the
electrodeposition of an electro-
active material (Figure 4). In ad-
dition to the two redox systems
associated with the 4T radical
Preparation and cyclic voltammetry of monolayers of 1 and
2: Monolayers of 1 and 2 were prepared under a controlled
argon atmosphere by immersion of cleaned gold bead elec-
trodes in a millimolar solution of 1 or 2 in CH2Cl2. The re-
sulting electrodes were then rinsed with pure CH2Cl2 and
CH3CN before immersion in CH2Cl2 for 4 h to eliminate
physisorbed molecules. Compounds 1 and 2 were stored
under a nitrogen atmosphere at +48C. However, reproduci-
ble monolayers of dithiol 2 were obtained only if 2 was
freshly generated from dithioester 7 and purified by chroma-
tography before monolayer preparation. Otherwise, as al-
ready reported, multilayers of disulfide compounds were ob-
tained.[23,31]
cation and dication,
a
first
system of weak intensity ap-
pears around 0.60 V. This indicates the coupling of the 4T
radical cation into more extended conjugated chains during
the initial steps of the overall electrooxidation process, be-
sides the electrochemical generation of a polydisulfide. This
result, which contrasts with the stability the radical cation in
solution (Figure 1), suggests that the coupling of the 4T radi-
cal cation of 2 could be favored in the confined environment
of the polydisulfide film deposited at the electrode. Further
work is needed to clarify this point.
Formation of a monolayer of 2 is relatively fast since a
CV characteristic of an immobilized molecule is obtained
after 3 h of immersion. In contrast, at least 24 h of immer-
sion in a solution of 1 are required to obtain a CV typical of
a monolayer. Another striking difference is that the repro-
ducibility of monolayer formation is only 30% for 1, but it
is close to 100% for 2.
The cyclovoltammetry of monolayers of each of 1 and 2
(Figure 5) exhibits two reversible oxidation peaks character-
istic of the 4T radical cation and dication respectively, at
Epa1 =0.83 V and Epa2 =1.15 V for monolayers of 1 and at
Epa1 =0.84 V and Epa2 =1.08 V for monolayers of 2, values
close to those recorded in solu-
The CV of the polymer recorded in a monomer-free
medium shows an intense oxidation peak at approximately
0.90 V/SCE. The absence of an oxidation signal below
tion. The linear variation of the
peak current versus scan rate
confirms that 1 and 2 are immo-
bilized on the electrode surface.
The surface coverage G has
been determined by integration
of the voltammetric peaks after
correction for double layer
charge.[32] A value of G=4”
10ꢀ10 molcmꢀ2
(G=2.4”1014
2
moleculescmꢀ2 or 41.7 Š per
molecule) was obtained for
monolayers of 1; this value is
somewhat smaller than is typi-
Figure 4. Potentiodynamic electro-oxidation of a solution of dithiol 2 (1mm) in 0.10m Bu4NPF6/CH2Cl2 (left)
and CV of the resulting material in 0.10m Bu4NPF6/CH3CN (right), scan rate 0.1 Vsꢀ1, Au working electrode,
SCE reference electrode.
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Fixation of Oligothiophenes on Gold
FULL PAPER
on silicon wafers in millimolar
solutions of freshly purified
compounds
1
and
2
for a
period of 72h and 3 h, respec-
tively.
The thickness of the mono-
layers of 2 determined on two
different samples by ellipsome-
try gave values ranging from
11.8 to 14.0 Š, which are in
good agreement with formation
of a monolayer and with the
calculated 13–14 Š height of
molecule 2 in the expected con-
formation for double fixation
(Figure 6). These latter values
have been estimated with the
MOPAC 3D software and as-
ꢀ
suming an S Au distance of
Figure 5. Left: CV of monolayers of 1 (top, scan rate=6 Vsꢀ1) t0 +2 4 h (a), t0 (c); and 2 (bottom, scan
rate=0.1 Vsꢀ1) in 0.10m Bu4NPF6/CH3CN, SCE reference electrode. Right: variation of the intensity of the
second oxidation peak versus scan rate.
about 2Š. [37] In this case, al-
though the main axis of the 4T
cal for close-packed monolayers of alkanethiol (G=4.7”1014
moleculescmꢀ2)[33a,34,35] or w-(3-thienyl)alkanethiol (G=6.5–
7.0”1014 moleculescmꢀ2) on gold.[33c] However, the stability
of the monolayer of 1 is limited since the intensity of the
CV waves decreases significantly after 24 h of storage in at-
mospheric conditions or after sonication for 1 min
(Figure 5).
Figure 6. Molecular structure of dithiol 2 drawn from ellipsometry data in
a conformation favorable to double fixation on gold (from MOPAC-
ChemDraw 3D optimization).
For monolayers of 2 the estimated surface coverage of
2
G=2”10ꢀ10 molcmꢀ2 (1.2”1014 moleculescmꢀ2 or 83.3 Š
per molecule), half that obtained for 1, is consistent with the
larger area occupied by the 4T molecule when doubly fixed
in a horizontal orientation.
segment is almost parallel to the surface, molecules of 2
stand up on the surface thus allowing p-stacking interactions
and the formation of densely packed domains, in agreement
with the relatively high value of G determined by CV. As-
suming that molecule 2 adopts the conformation shown in
Figure 6, the molecular area estimated with MOPAC 3D
Monolayers of 2 are considerably more stable than those
of 1. Thus, the CV remained unaltered after application of
400 recurrent potential scans between ꢀ0.40 and 1.20 V/
SCE at a scan rate of 0.5 Vsꢀ1 or after sonication for 1 min
in acetonitrile. No change in the CV was observed after stor-
age for one week in ambient conditions. This markedly im-
proved stability of monolayers of 2 can be related to the
double fixation of the 4T molecules to the gold surface.
To confirm the absence of free thiol groups in monolayers
of 2, a monolayer was immersed in a solution of cesium hy-
droxide (0.10m) in MeOH/DMF for 15 min (to generate thi-
olate from hypothetical remaining thiol groups) and then
immersed in a solution of 4-bromobutylferrocene (10mm)[36]
in DMF for 1 h to fix ferrocene groups on these hypothetical
thiolates. The absence of the typical signature of ferrocene
in the CV of the resulting modified electrodes provides indi-
rect support for the absence of free thiol groups in monolay-
ers of 2 as expected for a double fixation of 2.
2
would correspond to a 4.4 Š”15.6 Š rectangle of 68.6 Š
per molecule. Comparison with the 83.3 Š2 per molecule ob-
tained by CV shows that the experimental result represents
approximately 80% of the maximum theoretical coverage.
The water contact angle of monolayers of 2 (qH O =83.5ꢁ
2
18) is characteristic of a surface constituted by aromatic sys-
tems such as thiophene.[9d,33]
The thickness of monolayers of 1 varies from 9.3 to
11.7 Š. These values are much lower than expected for a
vertical orientation of the molecule relative to the surface.
The water contact angle (qH O =89ꢁ18) is higher than that
2
of monolayers of 2, suggesting a more lipophilic surface, in
agreement with the presence of the pentyl chain of 1. How-
ever, this value is much lower than the 1058 expected for a
molecular conformation in which a pentyl chain points out
of the surface.[38] These results may be due to the fact that
molecules 1 do not stand upright in a stretched conforma-
Structural characterization of the monolayers: Monolayers
of 1 and 2 for surface characterization were prepared by im-
mersion of substrates consisting of an evaporated gold layer
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6241

P. Blanchard, J. Roncali et al.
tion but adopt a rather disordered arrangement on the gold
surface.
the other sulfur atoms of compound 1 and 2, respectively.
Comparison of the areas of these two doublet signals leads
ꢀ ꢀ
Monolayers derived from 1 and 2 have been analyzed by
XPS. A first examination of the spectrum did not show any
contamination from residual CH2Cl2 solvent and CH2Cl2/
CH3CN rinsing solution. The high-resolution XPS spectra of
the S2p region (Figure 7) are decomposed in individual con-
to an estimated S C/S Au ratio of g=6.0ꢁ0.5 which is in
ꢀ
ꢀ
full agreement with the expected value (S C/S Au =6) for
a single fixation of molecule 1 to the gold surface via forma-
tion of an S Au bond. In the case of molecule 2, the experi-
ꢀ
ꢀ
ꢀ
mentally determined S C/S Au ratio of 3.7ꢁ0.5 is relative-
ly close to the theoretical value
expected for a double fixation
ꢀ
ꢀ
(S C/S Au =3). That the ex-
perimental ratio is higher than
the theoretical one may be a
result of the attenuation of the
signal of the sulfur at the Au/
monolayer interface by the
overlying organic layer.
A single fixation of molecule
2 to the surface would leave
ꢀ
one free S H group. In fact,
the S2p3/2,1/2 signals of a sulfur
atom exclusively linked to
ꢀ
carbon atoms (S C) and that of
ꢀ
a free alkanethiol (S H) are
very close in energy (for exam-
ple, the energy of the S2p3/2,1/2
ꢀ
signal of S H was reported to
be 163.5/164.8 eV).[39,40] Thus, in
this energy range, the integra-
tion of the S2p3/2,1/2 signal corre-
sponds to the different contri-
butions of the sulfur atom ex-
clusively linked to carbon
Figure 7. Curve-fitted high-resolution XPS spectra for the S2p region of molecules 1 (top) and 2 (bottom) ad-
sorbed on gold.
ꢀ
atoms (S C) and that of a free
ꢀ
alkanethiol (S H). A theoreti-
tributions whose binding energies (BE) are reported in
Table 2.
Table 2. XPS Binding energies (BE), full-width half-maximum (FWHM)
and assignment of emission peaks measured from high-resolution XPS of
monolayers of 1 and 2 adsorbed on gold.
Comparison of the peak area of the S2s or S2p signals
with that of the C1s signal allows us to estimate the experi-
mental S/C ratio in the monolayer. For a monolayer of 1,
the S2p/C and S2s/C ratio of 0.29 for both is very close to
XPS
signal
BE
[eV]
FWHM
[eV]
Corrected
Assignment
area[a] [A.U.]
the theoretical value of S/C=7= =0.28. For monolayer of 2,
Monolayer of 1
25
C1s
S2s
S2p
284.5
1.38
2.39
20437
5927
5914
3281
C1s-C; C1s-S
the S2p/C and S2s/C ratios of 0.27 and 0.24 respectively de-
viate slightly from the theoretical value (S/C=8= =0.33) re-
227.7
163.5
24
lated to the chemical structure of 2. The curve-fitted high-
resolution XPS spectra for the S2p region of a monolayer of
each of 1 and 2 show two doublets (Figure 7). Each doublet
results from spin–orbit splitting of the S2p level and consists
of a high-intensity S2p3/2 peak at lower energy and a low-in-
tensity S2p1/2 peak at higher energy separated by 1.2eV with
an intensity ratio of 2:1.[39] The value of the S2p3/2 peak at
161.9 eV for monolayers of 1 and 2 is in excellent agreement
with the binding energy for bound alkanethiolate on gold
(161.9–162.0 eV).[25,34] Thus, peaks at 161.9 and 163.1 eV of
the lower-energy doublet S2p3/2,1/2 of monolayers of 1 and 2
may be assigned to the thiol chemisorbed on the Au sur-
band 1
band 2164.7
band 3
band 4
Monolayer of 2
C1s
S2s
S2p
band 1
band 2164.7
band 3
band 4
1.15
S2p3/2-C
1/2-C
S2p3/2-Au
S2p1/2-Au
1.15
1.15
1640
1033
p
S2
161.9
163.1
1.15
1.15
547
274
284.6
1.30
2.64
17687
4785
4279
2068
C1s-C; C1s-S
227.6
163.6
1.15
S2p3/2-C
1/2-C
S2p3/2-Au
S2p1/2-Au
p
S2
161.9
163.1
1.15
1.15
563
281
[a] The corrected areas were calculated empirically using the sensitivity
factor (C1s 0.3; S2s 0.4; S2p 0.57). Each measured peak area was divided
by this factor to obtain the corrected area. The total corrected areas of
each peak are in italics. A.U.: arbitrary units.
face.[25,39,40] The other doublet, S2p3/2,1/2
164.7 eV for 1 and 163.6 and 164.7 eV for 2, corresponds to
, at 163.5 and
6242
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 6237 – 6246

Fixation of Oligothiophenes on Gold
FULL PAPER
ꢀ
ꢀ
ꢀ
was stirred for 1 h at 208C before addition of a solution of 1-iodopentane
(0.79 g, 4 mmol) in degassed DMF (5 mL). After 4 h of additional stirring
at 208C and evaporation of the solvents, the residue was dissolved in
CH2Cl2 and the organic phase was washed with water, dried over MgSO4,
and concentrated. Purification by chromatography on silica gel (CH2Cl2
as eluent) gave compound 4 as orange crystals (0.40 g, 77% yield).
1H NMR (500 MHz, CDCl3): d=7.32(d, 1H, 3J=3.9 Hz, Hthio), 7.30 (d,
1H, 3J=3.9 Hz, Hthio), 7.23 (d, 1H, 3J=5.3 Hz, Hthio), 7.19 (d, 1H, 3J=
5.3 Hz, Hthio), 7.16 (d, 1H, 3J=3.9 Hz, Hthio), 7.15 (d, 1H, 3J=3.9 Hz,
cal (S C+S H)/S Au ratio of 7:1 is expected if all mole-
cules 2 are singly fixed. By using a simple linear combina-
tion of the two extreme states where all molecules 2 are
either singly or doubly fixed on the surface, the experimen-
ꢀ
ꢀ
ꢀ
tal ratio g=(S C+S H)/S Au can be expressed as Equa-
tion (1), where a represents the fraction of doubly fixed
molecules 2.
H
thio), 7.08 (d, 1H, 3J=5.2Hz, H thio), 7.04 (d, 1H, 3J=5.2Hz, H thio), 3.04
3
3
ꢀ
ꢀ
g ¼ 3a þ 7ð1ꢀaÞ
ð1Þ
(t, 2H, J=7.3 Hz, CH2 S), 2.87 (t, 2H, J=7.4 Hz, CH2 S), 2.56 (t, 2H,
3
J=7.3 Hz, CH2 CN), 1.62(m, 2H, 3J=7.4 Hz), 1.40–1.34 (m, 2H), 1.32–
ꢀ
1.27 (m, 2H), 0.87 ppm (t, 3H, 3J=7.3 Hz, CH3); 13C NMR (125 MHz,
CDCl3): d=139.6, 138.4, 137.0, 135.6, 135.1, 133.5, 132.2, 128.3, 127.7,
126.9, 123.8 (2C), 123.5, 123.3, 117.8, 36.2, 31.7, 30.9, 29.3, 22.2, 18.5,
13.9 ppm; IR (KBr): n˜ =2246 cmꢀ1 (CN); UV/Vis (CH2Cl2): lmax (loge=
403 nm (4.30); MALDI MS: 517 [M+I] (M=517.02for C 24H23NS6); ele-
From our XPS result for a monolayer of 2 (g=3.7ꢁ0.5),
we can deduce that around (83ꢁ12)% of molecules 2 are
ꢀ
chemisorbed on the surface through two S Au fixation sites.
Although it remains difficult to fit accurately the experimen-
tal curves of the XPS spectra of the S2p region with decon-
voluted curves, and considering that the presence of differ-
ent types of sulfur atoms in the monolayer makes the XPS
analysis more complicated, our XPS study shows that most
of the molecules 2 are doubly fixed on the surface, although
it is likely that only a few singly fixed molecules of 2 subsist
inside the monolayer.
mental analysis calcd (%) for C24H23NS6:
C 55.62, H 4.65.
C 55.67, H 4.48; found:
S-4-Bromobutyl ethanethioate (5): Under a N2 atmosphere, a solution of
potassium thioacetate (5.28 g, 46.3 mmol) in EtOH was added dropwise
to a solution of 1,4-dibromobutane (15 g, 70 mmol) in CH2Cl2 at 208C. A
white precipitate was formed and the reaction mixture was stirred for
15 h at 2 08C. After evaporation of the solvents, the residue was dissolved
in CH2Cl2 and the organic phase was washed with water, dried over
MgSO4, and concentrated. Purification by chromatography on silica gel
(CH2Cl2/petroleum ether, 4:6, v/v) gave compound 5 as a slightly yellow
oil (6.13 g, 63% yield). 1H NMR (500 MHz, CDCl3): d=3.40 (t, 2H, 3J=
3
ꢀ
ꢀ
ꢀ
6.5 Hz, CH2 Br), 2.89 (t, 2H, J=7.2Hz, CH 2 S), 2.32 (s, 3H, CH3 CO),
Conclusion
1.92(m, H2 ,
(C=O).
C
H2 CH2Br), 1.73 (m, 2H); IR (film): n˜ =1680 cmꢀ1
ꢀ
AHCTREUNG
We have described a selective and straightforward synthesis
of oligothiophenes functionalized by one and two thiol
groups using 2-cyanoethyl as a thiolate protecting group.
Analysis of the electrochemical properties of these com-
pounds by cyclic voltammetry has shown that electrooxida-
tion of the quaterthiophene bearing two thiol groups leads
to the electrodeposition of an electroactive poly(disulfide)
on the electrode surface. Monolayers of 1 and 2 have been
prepared by chemisorption on a gold surface. Cyclic voltam-
metric results have shown that the doubly functionalized
quaterthiophene leads to faster and more reproducible for-
mation of a monolayer than the singly substituted system.
Characterization by CV, ellipsometry, and XPS of the mono-
layers obtained provided coherent results indicating that in
the monolayers derived from the doubly functionalized oli-
gomer, most of the conjugated molecules are doubly at-
tached on the surface with a horizontal orientation of the
conjugated system. This result, combined with the enhanced
stability of the doubly fixed monolayers, opens interesting
perspectives for the development of functional electroactive
monolayers for application in sensors and molecular elec-
tronics. Work in this direction is now under way and will be
reported in future publications.
3-(6-Oxo-5-thiaheptylsulfanyl)-3’’’-pentylsulfanyl-2,2’:5’,2’’:5’’,2’’’-quater-
thiophene (6): Under a N2 atmosphere, a solution of CsOH·H2O (0.14 g,
0.84 mmol) in N2-degassed MeOH (5 mL) was added dropwise to a solu-
tion of 4 (0.36 g, 0.70 mmol) in degassed DMF (25 mL). The reaction
mixture was stirred for 1 h at 208C before addition of a solution of 5
(0.61 g, 2.8 mmol) in degassed DMF (5 mL) and 4 h of additional stirring
at 208C. After evaporation of the solvents, the residue was dissolved in
CH2Cl2 and the resulting organic phase was washed with water, dried
over MgSO4, and concentrated. Purification by chromatography on silica
gel (CH2Cl2/petroleum ether, 1:1, v/v, as eluent) gave compound 6 as a
yellow oil (0.36 g, 87% yield). 1H NMR (500 MHz, CDCl3): d=7.28 (2d,
2H, 3J=3.8 Hz, Hthio), 7.18 (m, 2H, Hthio), 7.15 (m, 2H, Hthio), 7.04 (d,
1H, 3J=5.3 Hz, Hthio), 7.03 (d, 1H, 3J=5.3 Hz, Hthio), 2.85 (m, 6H, CH2
ꢀ
ꢀ
S), 2.30 (s, 3H, CH3 CO), 1.66 (m, 4H), 1.63 (m, 2H), 1.40 (m, 2H), 1.38
(m, 2H), 0.87 ppm (t, 3H, 3J=7.3 Hz, CH3); 13C NMR (125 MHz,
CDCl3): d=195.8, 137.5, 137.3, 136.4, 135.7, 134.7, 134.4, 132.5, 132.2,
127.9, 127.2, 126.9, 126.8, 123.5, 123.4, 123.2, 123.1, 36.1, 35.6, 30.8, 30.6,
29.2, 28.5, 28.4, 22.2, 14.0 ppm; IR (film): n˜ =1690 cmꢀ1 (C=O); UV/Vis
(CH2Cl2): lmax =406 nm; MALDI MS: 594 [M+I] (M=594.03 for
C27H30OS7).
3-(4-Mercaptobutylsulfanyl)-3’’’-pentylsulfanyl-2,2’:5’,2’’:5’’,2’’’-quaterthio-
phene (1): Under a N2 atmosphere, a solution (1m) of DIBAL-H in
CH2Cl2 (2.1 mL, 2.1 mmol) was added dropwise to a solution of com-
pound 6 (0.31 g, 0.53 mmol) in anhydrous CH2Cl2 (15 mL) cooled to 08C.
The reaction mixture was stirred for 2h at 0 8C before addition of an
aqueous solution of HCl (3m, 2mL). After 0.5 h of additional stirring at
208C, the reaction mixture was washed with water, dried over MgSO4,
and concentrated to dryness. Purification by chromatography on silica gel
(CH2Cl2/petroleum ether, 1:1, v/v, as eluent) gave compound 1 as an
orange oil (0.26 g, 90% yield). 1H NMR (500 MHz, CDCl3): d=7.29 (d,
2H, 3J=3.9 Hz, Hthio), 7.19 (d, 1H, 3J=5.3 Hz, Hthio), 7.18 (d, 1H, 3J=
5.2Hz, H thio), 7.15 (d, 1H, 3J=3.8 Hz, Hthio), 7.15 (d, 1H, 3J=3.8 Hz,
Experimental Section
H
thio), 7.04 (d, 1H, 3J=5.2Hz, H thio), 7.03 (d, 1H, 3J=5.2Hz, H thio), 2.86
Syntheses
(t, 4H, 3J=7.2Hz, CH 2 S), 2.50 (m, 2H, CH2 SH), 1.70 (m, 4H), 1.63
(m, 2H), 1.38 (m, 2H), 1.28 (m, 3H, CH 2 +SH), 0.87 ppm (t, 3H, CH3);
ꢀ
ꢀ
3-(2-Cyanoethylsulfanyl)-3’’’-pentylsulfanyl-2,2’:5’,2’’:5’’,2’’’-quaterthio-
phene (4): Under a N2 atmosphere, a solution of CsOH·H2O (0.18 g,
1.1 mmol) in N2-degassed MeOH (10 mL) was added dropwise to a solu-
tion of 3 (0.5 g, 1 mmol) in degassed DMF (50 mL). The reaction mixture
IR (KBr): n˜ =2563 cmꢀ1 (S H); UV/Vis (CH2Cl2): lmax =407 nm (loge=
ꢀ
4.68); MALDI MS: 552[ M+I] (M=552.02 for C25H28S7).
Chem. Eur. J. 2008, 14, 6237 – 6246
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6243

P. Blanchard, J. Roncali et al.
3,3’’’-Bis(6-oxo-5-thiaheptylsulfanyl)-2,2’:5’,2’’:5’’,2’’’-quaterthiophene (7).
Under a N2 atmosphere, a solution of CsOH·H2O (0.74 g, 4.40 mmol) in
N2-degassed MeOH (5 mL) was added dropwise to a solution of 3 (1 g,
2mmol) in degassed DMF (25 mL). The reaction mixture was stirred for
1 h at 208C before addition of a solution of 5 (1.10 g, 5 mmol) in de-
gassed DMF (5 mL) and 4 h of additional stirring at 208C. After evapora-
tion of the solvents, the residue was dissolved in CH2Cl2 and the organic
phase was washed with water, dried over MgSO4 and concentrated to
dryness. Purification by chromatography on silica gel (CH2Cl2 as eluent)
were then rinsed successively with deionized water, acetonitrile, and di-
chloromethane before immersion in the freshly prepared thiol solution.
The geometric area of the working gold bead electrodes was estimated
from the slope of the linear plots of the CV cathodic peak current inten-
sity versus the square root of scan rate, for the diffusion-controlled reduc-
3ꢀ
tion of Fe(CN)6 (10ꢀ2 m in 0.50m NaCl at 258C). Comparative experi-
ments with gold disk electrodes of known surface area (2mm diameter
from Radiometer Analytical SA) led to typical surfaces ranging from
0.02to 0.04 cm 2 for gold bead electrodes.
gave compound
7
as an orange oil (0.90 g, 70% yield). 1H NMR
Ellipsometry, contact angle measurements, and XPS analyses were per-
formed on SAMs prepared from gold films 200 nm thick, evaporated
onto silicon wafers covered by a titanium or chromium sublayer (10 nm)
deposited under ultrahigh vacuum. After gold deposition, annealing,[43]
and cleaning in an 37% HCl/ 65% HNO3/ deionized water (3:1:16, v/v/v)
mixture for 5 min led to atomically flat gold terraces. Silicon and gold
(500 MHz, CDCl3): d=7.29 (d, 2H, 3J=3.9 Hz, Hthio), 7.19 (d, 2H, 3J=
5.3 Hz, Hthio), 7.15 (d, 2H, 3J=3.9 Hz, Hthio), 7.03 (d, 2H, 3J=5.3 Hz,
3
3
ꢀ
ꢀ
Hthio), 2.86 (t, 4H, J=6.9 Hz, CH2 S), 2.84 (t, 4H, J=6.9 Hz, CH2 S),
2.30 (s, 6H, CH3 CO), 1.69–1.66 ppm (m, 8H, CH2 CH2); 13C NMR
(125 MHz, CDCl3): d=195.8, 137.5, 136.5, 134.5, 132.5, 127.2, 127.0,
123.5, 123.3, 35.6, 30.6, 28.53, 28.52, 28.48 ppm; IR (film): n˜ =1690 cmꢀ1
(C=O); UV/Vis (CH2Cl2): lmax =405 nm; MALDI MS: 654 [M+I] (M=
654.00 for C28H30O2S8); elemental analysis calcd (%) for C28H30O2S8:
C 51.34, H 4.62; found: C 51.25, H 4.71.
ꢀ
ꢀ
were purchased from Siltronix and Goodfellow respectively.
N
Contact angle measurements: Contact angles were measured, in a clean
room (class about 1000) at a well controlled relative humidity (40%) and
temperature (208C), with
a remote-computer controlled goniometer
3,3’’’-Bis(4-mercaptobutylsulfanyl)-2,2’:5’,2’’:5’’,2’’’-quaterthiophene
(2):
system (Digidrop; GBX, France). A drop (in the range 1–10 mL) of de-
ionized water (18 mWcmꢀ1) was deposited on the surface and the project-
ed image was acquired and stored by the remote computer. Contact
angles were then extracted by contrast contour image analysis software.
These angles were determined around 2s after application of the drop.
The precision of these measurements was ꢁ28.
Under a N2 atmosphere, a 1m solution of DIBAL-H in CH2Cl2 (7 mL,
7 mmol) was added dropwise to a solution of compound 7 (0.57 g,
0.87 mmol) in anhydrous CH2Cl2 (15 mL) cooled to 08C. The reaction
mixture was stirred for 2h at 0 8C before addition of a aqueous solution
of HCl (3m, 3 mL). After 0.5 h of additional stirring at 208C, the reaction
mixture was washed with water, dried over MgSO4, and concentrated to
dryness. Purification by chromatography on silica gel (CH2Cl2/petroleum
ether, 1:1, v/v, as eluent) gave compound 2 as an orange oil (0.40 g, 80%
yield). 1H NMR (500 MHz, CDCl3): d=7.29 (d, 2H, 3J=3.9 Hz, Hthio),
Spectroscopic ellipsometry: Spectroscopic ellipsometry data in the visible
range were obtained by using a UVISEL Jobin Yvon Horiba Spectro-
scopic Ellipsometer equipped with DeltaPsi 2data analysis software. The
system acquired a spectrum ranging from 2to 4.5 eV (corresponding to
300–750 nm) with 0.05 eV (or 7.5 nm) intervals (angle of incidence 708;
the compensator was set at 45.08). Data were fitted by regression analysis
to a film-on-substrate model as described by their thickness and their
complex refractive indices. Two spectra, before and after monolayer dep-
osition, were obtained to determine the thickness. In the software, we
used a two-layer model: layer 1 is gold, layer 2the organic monolayer.
To determine the monolayer thickness, we used the spectrum measured
on the sample before the monolayer deposition for layer 1, and we fixed
the refractive index at 1.50 for layer 2. Usual values in the literature are
in the range 1.45–1.50;[44] a change from 1.50 to 1.55 would result in less
than 1 Š error for a thickness less than 30 Š. To determine the thickness,
the software compared the measured data with the simulated data. The
estimated accuracy of the SAM thickness measurements was ꢁ2Š.
7.19 (d, 2H, 3J=5.3 Hz, Hthio), 7.15 (d, 2H, 3J=3.9 Hz, Hthio), 7.04 (d, 2H,
3
3J=5.3 Hz, Hthio), 2.86 (t, 4H, J=6.6 Hz, CH2 S), 2.50 (m, 4H, CH2
ꢀ
ꢀ
SH), 1.75–1.70 (m, 8H, CH2 CH2), 1.30 ppm (t, 2H, 3J=7.9 Hz, SH);
ꢀ
13C NMR (125 MHz, CDCl3): d=137.5, 136.5, 134.5, 132.5, 127.3, 127.0,
123.5, 123.3, 35.6, 32.8, 28.1, 24.2 ppm; UV/Vis (CH2Cl2): lmax =408 nm;
MALDI MS: 570 [M+I] (M=569.98 for C24H26S8).
Disulfide of 3-(4-mercaptobutylsulfanyl)-3’’’-pentylsulfanyl-2,2’:5’,2’’:5’’,2’’’-
quaterthiophene (8): A suspension of ferric chloride (40 mg, 0.25 mmol)
in anhydrous CH2Cl2 (3 mL) was added slowly to a solution of monothiol
1 (70 mg, 0.13 mmol) in anhydrous CH2Cl2 (10 mL). The reaction mixture
was stirred overnight at 208C. After addition of water and extraction
with CH2Cl2, the organic phase was concentrated. The residue was puri-
fied by chromatography on silica gel (CH2Cl2/petroleum ether, 3:7, v/v, as
eluent) to give disulfide 8 as a green oil (23 mg, 33% yield); 1H NMR
(500 MHz, CDCl3): d=7.29 (d, 4H, 3J=4.0 Hz, Hthio), 7.17 (d, 2H, 3J=
5.4 Hz, Hthio), 7.16 (d, 2H, 3J=5.8 Hz, Hthio), 7.14 (d, 2H, 3J=4.0 Hz,
X-ray photoelectron spectroscopy: The chemical composition of the
SAMs was analyzed and any contaminant that had not been removed
was detected by XPS by using a Physical Electronics 5600 spectrometer
fitted in a UHV chamber with a residual pressure of 2”10ꢀ10 torr. High-
resolution spectra were obtained with a monochromatic AlKa X-ray
Hthio), 7.13 (d, 2H, 3J=4.0 Hz, Hthio), 7.03 (d, 2H, 3J=5.6 Hz, Hthio), 7.02
3
(d, 2H, 3J=5.6 Hz, Hthio), 2.86 (t, 4H, J=7.1 Hz, CH2 S), 2.84 (t, 4H,
ꢀ
3
3
ꢀ
ꢀ ꢀ
source (hn=1486.6 eV),
a detection angle of 458 referenced to the
J=7.1 Hz, CH2 S), 2.60 (t, 4H, J=7.1 Hz, CH2 S S), 1.76 (m, 4H),
sample surface, an analyzer entrance slit width of 400 mm and an analyzer
pass energy of 12eV. In these conditions, the overall resolution measured
from the FWHM of the Ag 3d5/2 line was 0.55 eV. Semiquantitative analy-
sis was completed after standard background subtraction according to
Shirleyꢁs method.[45] Peaks were decomposed by using Voigt functions
and a least-squares minimization procedure and by keeping constant the
Gaussian and Lorentzian broadenings for each component of a given
peak.
1.68 (m, 4H), 1.64 (m, 4H), 1.38 (m, 4H), 1.28 (m, 4H), 0.87 ppm (t, 6H,
3J=7.1 Hz, CH3); UV/Vis (CH2Cl2): lmax =407 nm (loge=4.78); MALDI
MS: 1102[ M+I]; 551 [(M/2)+I] (M=1102.03 for C50H54S14).
Cyclic voltammetry and preparation of gold electrodes: Electrochemical
experiments were carried out by using a PAR 273 potentiostat–galvano-
stat in a three-electrode single-compartment cell equipped with a plati-
num or gold disk, diameter 2mm, and modified gold beads as working
electrodes, a platinum wire counter electrode, and a silver wire as a
pseudo-reference electrode. The ferricinium/ferrocenium couple Fc+/Fc
was used as an internal reference (E8
(Fc+/Fc)=0.405 V/SCE in 0.1m
C
Bu4NPF6/CH3CN or CH2Cl2). Potentials were then expressed relative to
a saturated calomel reference electrode (SCE).
Acknowledgements
Gold monolayers of mono- and dithiols 1 and 2 have been obtained by
using different substrates: gold bead electrodes for cyclic voltammetry
experiments and evaporated gold on silicon for structural analysis. Sub-
strates were cleaned thoroughly by previously reported methods.[41, 42] The
gold beads were prepared according to the literature.[41] They were sub-
jected to repetitive potential scans between 0 and 1.6 V/SCE in an aque-
ous H2SO4 solution (0.5m) until a sharp reduction peak was formed asso-
ciated with the reduction of oxidized gold species to gold. The gold beads
The Minist›re de la Recherche is acknowledged for the PhD grant for
T.K.T. The authors thank the Service Central dꢁAnalyses Spectroscopi-
ques de lꢁUniversitØ dꢁAngers for the characterization of organic com-
pounds. This research project is also supported by the Agence Nationale
de la Recherche PNANO Program, project OPTOSAM, ANR-06-
PNANO-016, and the CNANO Nord-Ouest. S.K. is indebted to ANR for
financial support.
6244
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 6237 – 6246

Fixation of Oligothiophenes on Gold
FULL PAPER
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