Polyviologen synthesis by self-assembly assisted grafting

Article abstract of DOI:10.1039/c5ra19487c

In this work a new method for electrochemical polymerization of a viologen derivative is presented. A precursor, having an isothiocyanate group as a "head"-group and a cyanopyridine group as a "tail"-group (SCNCP), suitable for self-assembly was designed

Full text of DOI:10.1039/c5ra19487c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Polyviologen synthesis by self-assembly assisted
grafting†
Cite this: RSC Adv., 2015, 5, 101232
ab
a
a
a
a
Nianxing Wang, Anniina Kahkonen, Timo A¨ aritalo, Pia Damlin, Jouko Kankare
¨
¨
¨
a
and Carita Kvarnstrom*
¨
In this work a new method for electrochemical polymerization of a viologen derivative is presented. A
precursor, having an isothiocyanate group as a “head”-group and a cyanopyridine group as a “tail”-group
(
SCNCP), suitable for self-assembly was designed and synthesized. An adsorbed layer of SCNCP was
achieved by assembling isothiocyanate “head”-groups onto gold electrodes or onto gold with an amino-
terminated monolayer of cysteamine (SNH ). The purpose of the cyanopyridine “tail”-group is to act as
hook and linker to cyanopyridine based monomers. In this work dimerization between two
2
a
cyanopyridines, the “tail”-group and a synthesized linear cyanopyridine monomer (LCP), were induced by
electrochemical reduction. The SAMs and films were characterized by electrochemical and
spectroscopic techniques (UV-vis and FTIR), and show characteristc FTIR spectra and electrochemical
redox responses of viologens. AFM and SEM images show wires extending out from the electrode
Received 21st September 2015
Accepted 11th November 2015
2
substrate when viologen films were formed on SNH –SCNCP modified gold surfaces. The new viologen
DOI: 10.1039/c5ra19487c
derivative not only broadens the field of applications of viologens in various electronics, but also enables
structural regularity and formation at different surfaces.
www.rsc.org/advances
cyanopyridines induced by electrochemical reduction, (reduc-
tion steps shown in Scheme S1†). The latter approach has
Introduction
15
0
0
Viologens, which are usually termed 1,1 -disubstituted-4,4 - initiated activity around the electrochemical synthesis and
1
5,10,16,17
bipyridilium salts, have attracted intensive attention due to characterization of viologen and polyviologen materials.
their unique redox properties. In recent years viologens have Also other methods have been developed to immobilize viol-
been applied as active material in various electronics or utilized ogens on electrodes including incorporation of viologen into
2–8
to design supermolecules. The key feature of viologens is their a polymer matrix, deposition of a chemically preformed poly-
redox property; the well-known two electron reduction process and mer and covalent linkage of viologen to a polymerizable
the subsequent three possible redox forms are shown in Scheme 1. monomer. In comparison to the immobilization method the
The dication form of viologen is the most stable and usually color- electrochemically made lms have some advantages, such as
less, however, the radial cation form is strongly colored and utilized less manufacturing procedures and direct deposition on small
in applications. The neutral form can function as a strong reductive and even complicated patterns.
9
agent. By changing the steric structure, e.g. the length of alkyl chain
of the substituted groups (R1 and R2) the redox potentials and the
10,11
colors of the viologens can be tuned.
Due to the different redox
forms, the viologen materials have been widely utilized in sensors
and fuel cells, and by designing viologen materials with different
12–14
colors, a series of electrochromic devices can be made.
Traditionally viologens are produced by chemical synthesis,
1
another approach is to utilize the dimerization between two
a
Turku University Center for Materials and Surfaces, c/o Laboratory of Materials
Chemistry and Chemical Analysis, University of Turku, Vatselankatu 2, FI-20014
Turku, Finland. E-mail: carita.kvarnstrom@utu.; Fax: +358 2 333 6700; Tel: +358
2
333 6729
b
University of Turku Graduate School (UTUGS), FI-20014, Turku, Finland
†
Electronic supplementary information (ESI) available: The ESI contains the
scheme of electrochemical formation of viologen, PM-IRRAS spectra and AFM
images. See DOI: 10.1039/c5ra19487c
Scheme 1 Three redox forms of viologens.
101232 | RSC Adv., 2015, 5, 101232–101240
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Self-assembly of monolayers, SAM, rst reported by Nuzzo in anchoring groups. Furthermore, since the isothiocyanate can
18
1
983, offers unique advantages as easy preparation on large or form a stable bond with the amine group, e.g. with cysteamine
19
patterned substrates.
Through proper selection of the (SNH ), the monolayer can be formed onto different organic
2
terminal functional group, utilizing specic surface/solution layers which contains amine. The deposition procedure used in
interactions, molecules can be immobilized at a surface. Thin this work is sketched in Scheme 2.

lms of a variety of materials can easily be fabricated by self-
The isothiocyanate group can form a stable monolayer at
+
26,27
assembly on top of metals, metal oxides, semiconductive an Au surface (Au–S–CN –)
or alternatively on an Au surface
19
28
materials and even nanoparticles and nanorods. It has been functionalized with cysteamine layer Au–S/NH–CS–NH
shown that SAMs can be used for modifying the work function according to Scheme 2. Characterization of the various steps
of metals. This might result in enhanced carrier injection involved in the assembly was provided by a surface-sensitive
20,21
leading to improved electronic device performance.
The self- IR technique (polarization modulation infrared-reection absorp-
assembly method has also been applied in the development of tion spectroscopy (PM-IRRAS)). The viologen lms could be
electrochemical detection systems and chemical sensors and successfully electrochemically synthesized from the cyanopyridine
for controlling the wetting behavior of surfaces. A series of groups on these monolayers by electroreduction. Electrode surfaces
studies have been published on viologen and its derivatives were characterized before and aer modication and electro-
12
utilizing SAM for designing biosensors with redox proteins
polymerization by atomic force microscopy, electrochemical and
where this redox active material played an important role as spectroscopic techniques. Contact angle measurements were used
electron relay in an electrochemical process. The electrochemical to access the wetting characteristics of bare gold, SAM-modied
properties of redox-active SAMs, such as the electron transfer and for electrodes functionalized with SAM and polyviologen. The
mechanism have been the subject of study but our focus lies on prepared lms show well dened redox responses and in the case
the chemical transformation of the SAMs by electrochemical when a SAM layer consisting of both cysteamine and isothiocyanate
methods for production of ordered polyviologen lms.
was used a brush shaped lm was obtained.
Our research group has reported on electroreductive
This synthesis procedure and the unique structure of the
synthesis and characterization on viologen/polyviologen lms viologen lms make them important materials where deposition
using in the group synthesized linear and branched cyanopyr- at different surfaces is needed. Additionally, as the isothiocyanate
22–25
idine based monomer materials.
In this work a new linker group forms a stable bond with an amine group a self-assembly
molecule with isothiocyanate and cyanopyridine groups has concept for lm formation can be used. This extends the use of the
been synthesized (SCNCP) and the electrochemical synthesis of electrochemical synthesis of different viologens derivatives at various
a polyviologen derivative using a linear cyanopyridine monomer substrates through the unique isothiocyanate-based monomers.
(LCP) to this (SCNCP) was studied. The isothiocyanate can form
a SAM layer on gold (Au) surface whereaer the viologen can be
Experimental
formed by electrochemical coupling to monomers containing
22,23
a cyanopyridine group.
Thus, these polyviologen lms are Materials
expected to have better structural regularity as well as dynamic
behavior compared to lms electropolymerized without
0
The linear cyanopyridine monomer 4,4 -bis(4-cyanopyridinium-
yl)biphenyl bistosylate (LCP) was synthesized in our group
1
Scheme 2 Electrochemical synthesis of viologen utilizing dimerization between cyanopyridines on a gold surface functionalized with cyste-
amine and an isothiocyanate layer.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 101232–101240 | 101233

View Article Online
RSC Advances
structure shown in Scheme 2) and has been described in
previous work. The cyanopyridine monomer linked with iso- a conventional three-electrode one-compartment cell connected
thiocyanate groups, 4-cyano-1-(4-isothiocyanatobenzyl)pyridin- to an Ivium Technologies potentiostat by using the cyclic vol-
Paper
(
Viologen lms were synthesized electrochemically in
22
1
-ium bromide (SCNCP), was synthesized as following. The p- tammetry (CV) technique. The plasma cleaned Au covered Si
tolueneisothiocyanate (9.68 g) and N-bromosuccinimide (12 g) wafers (6 mm Ø) primed with SCNCP or SNH /SCNCP and the
were added into CCl , a catalytic amount of dibenzoyl peroxide Au-minielectrode were used as working electrode. A platinum
2
4
was added and the mixture was reuxed for 6 h. Aer ltration wire (1 mm Ø) worked as counter electrode and an Ag/AgCl
and evaporation to dryness, the pure product (7.54 g) was ob- electrode (eDAQ) was used as reference electrode (calibrated
+
tained by recrystallization from methanol. The p-(bromo- vs. ferrocene (E1/2 (Fe/Fe ) ¼ 0.45 V)). All following potentials
methylene)isothiocyanatebenzene and p-cyanopyridine (5.2 g) mentioned are given vs. this Ag/AgCl electrode. For electro-
were added to acetonitrile and the solution was reuxed over- polymerization of viologen lms the potential was cycled between
night. Aer ltration and evaporation, 7.7 g (35.9%) of pure 0.3 to ꢀ1.2 V (20 consecutive cycles) in an aqueous solution of
1
ꢀ1
2 4
product was obtained. H NMR (DMSO): 9.52 (2H, d), 8.74 2 mM LCP in 0.1 M Na SO at a scan rate of 10 mV s . All CV
(
1
1
3
2H, d), 7.67 (2H, d), 7.54 (2H, d), 5.98 (2H, s). C NMR (DMSO): tests were carried out under oxygen free reaction conditions.
46, 134, 133, 131, 131, 130, 127, 126, 114, 63.
Na SO (Oy-FF-Chemicals Ab, $99%) was used as electrolyte
0.1 M). H (AnalaR Normapur, $30%) and H SO (Baker
2
4
Instrumentation
(
2
O
2
2
4
Analyzed, $95%) were used as received. Quartz distilled water The polyviologen modied SAMs were electrochemically char-
was used in all experiments. 2-Mercaptoethylamine hydrochlo- acterized applying the electrochemical setup described above in
ꢀ1
ride (Aldrich, 98%) (cysteamine) was used as 10 mM solution.
2 4
0.1 M Na SO using scan rates of 10, 20, 50 and 100 mV s .
Water contact angles were measured on a CAM 200 contact
angle measurement system (KSV Instruments).
Preparation of the substrates
The UV-vis spectra were recorded from gold evaporated quartz
Si(100) wafers (Okmetic, Finland) and quartz slides were slides (10 nm) for self-assembled structures and aer electro-
cleaned in fresh piranha solution (concentrated H SO /30% polymerization between 300 and 1100 nm using Hewlett Packard
(3 : 1)) for one hour, rinsed thoroughly with quartz 8453 and Agilent Technology Cary 60 spectrophotometers.
distilled water and dried. On the piranha cleaned wafers Fourier transform infrared (FTIR) spectra were measured of
2
4
2 2
H O
a 10 nm thick layer of chromium was rstly evaporated followed the viologen modied SAMs, deposited on Au evaporated Si
by a layer of approximately 100 nm of Au using the Edwards wafer electrodes, by using a dry-air-purged Nexus 870 FTIR
E306A coating system. Prior to use the Au evaporated wafers spectrometer (Nicolet) equipped with a Harrick Seagull™ vari-
2
were additionally cleaned with O plasma using the Harrick able angle reection accessory and a liquid nitrogen cooled
PDC-002 plasma cleaner. For some experiments an Au-electrode mercury–cadmium–telluride (MCT-A) detector. Spectra were
ꢁ
(
CHI101, 2 mm Ø, CH Instruments Inc.) was used. This electrode measured at 40, 45 and 50 angle of incidence relative to the
was mechanically polished with an EtOH wetted polishing cloth surface normal and for each spectrum 512 interferograms were
ꢀ
1
(
(
Struers, DP-Nap) using diamond paste of gradually ner grades recorded at spectral resolution of 4 cm . A surface sensitive IR
1/4 to 3 mm, Struers A/s) with rinsing in between by EtOH.
technique (PM-IRRAS) was used for the study of thin self-
assembled lms (Si/Au/SCNCP and Si/Au/SNH /SCNCP) using
2
a dry-air-purged Bruker VERTEX 70 FTIR spectrometer equip-
ped with an external PMA 50 accessory. The PMA 50 module has
a built-in photoelastic modulator (PEM) that operates with
Formation of self-assembled layers and electrochemical
synthesis
Self-assembled thin lms of the SCNCP monomer was formed a modulation frequency of 84 kHz. A liquid nitrogen cooled
on plasma cleaned Au covered Si and quartz wafers by MCT (narrow-band) detector was used and the angle of inci-
ꢁ
immersing into a 20 mM SCNCP solution for 24 hours (Si/Au/ dence, with respect to the surface normal, was adjusted to 83 .
2 p s
SCNCP). The SCNCP solution was purged by N for 30 min Due to PMA 50's dual-channel data acquisition the sum (R + R )
prior use and the solution was kept in an oxygen free environ- and difference signals (Rp ꢀ R ) could be collected simulta-
s
ꢀ
1
ment during the adsorption process. Aer rinsing, the neously. For each spectrum, 1024 scans at 4 cm spectral
substrates were used as working electrodes in the electro- resolution were collected.
chemical synthesis of viologen lms.
The surface morphology of the lms was imaged by a Leo
For some wafers priming with cysteamine (SNH
2
) prior to (Zeiss) 1530 Gemini FEG scanning electron microscope (SEM)
adsorption of the SCNCP monomer was made. Self-assembly of and atomic force microscopy (AFM) on a diCaliber AFM
the SNH monolayer on the cleaned Au covered Si and quartz microscope from VEECO Instruments Inc. Analysis was carried
wafers was obtained by immersion in a 10 mM aqueous solution out in tapping mode. A NSG-11 tip from NT-MDT with an 11.5 N
2
ꢀ
1
of cysteamine (pH ¼ 7) for 2 hours. Aer careful rinsing with
m
force constant was used. The raw AFM data obtained were
water the primed substrates were immersed into 20 mM SCNCP processed by attening and plane tting with the WSxM 5.0
solution (pH ¼ 9) for 24 hours (Si/Au/SNH /SCNCP), rinsed data visualization and analysis soware.
2
1
13
again and thereaer used as working electrodes in the electro-
chemical synthesis of viologen lms.
The H and C spectra of the synthesized linker were
recorded with a Bruker AVANCE 600 MHz spectrometer
101234 | RSC Adv., 2015, 5, 101232–101240
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
equipped with a direct X-detection probe (BBO-5mm-Zgrad) thickness of the lm was slowly increasing. The redox response
using dimethyl sulphoxide (DMSO-d ) as the solvent.
of the electrochemically formed black deposit in Fig. 1b consists
of two irreversible reduction peaks at potentials ꢀ0.55 V and
close to ꢀ0.9 V indicating a loosely adsorbed lm showing
increased solubility upon reduction involving simultaneous
charge neutralization of the polyviologen.
6
Results and discussion
Electrochemical polymerization of viologen lms
The electrochemical deposition of viologens depends on the
type of substituents, especially on the degree of hydrophobicity. In
this case the biphenyl unit is alternating with the viologen unit in
the chain and increases the hydrophobic character. In an earlier
work we studied the difference in redox properties of viologen
Fig. 1a shows the CVs obtained upon viologen lm synthesis
(polyLCP) on an Au-disc electrode using potentiodynamic
cycling between 0.3 to ꢀ1.2 V (20 consecutive cycles) in
a aqueous solution of 2 mM LCP in 0.1 M Na SO at a scan rate
2
4
ꢀ1
of 10 mV s . In the polymerization a sharp reduction peak
occurs in the rst cycle at ꢀ0.45 V together with a corresponding
oxidation peak at ꢀ0.1 V. As the polymerization proceed the
peak currents of both reduction and oxidation peaks dimin-
ishes and the oxidation peak shis to 0 V whereas the reduction
peak successively shis from ꢀ0.45 to ꢀ0.55 V. By the end of
polymerization three weak reduction peaks can be observed at
potentials ꢀ0.55, ꢀ0.8 and ꢀ1.05 V. The shi of the redox peaks
can be explained by the formation of the dimer or oligomer
leading to the deposition of the formed viologen lms at
the electrode surface, with increasing number of cycles the

lms when using a linear monomer structure to that obtained
22
upon copolymerization with a branched structure. From this
work it could also be concluded that electropolymerization using
the linear monomer structure alone resulted in poor lm forma-
tion, presumably due to high solubility of the intermediates. In
this work promotion of polyviologen deposition was studied using
the self-assembly concept by priming the Au electrode surface
with a linker molecule (SCNCP). This bifunctional molecular
linker was specially designed and synthesized having an iso-
thiocyanate group for assembly onto gold and a cyanopyridine
end group. Through electrochemical coupling of cyanopyridines,
between the linker and the linear monomer structure, viologen
lms with good surface adhesion and controlled structure could
be obtained. The isothiocyanate group was chosen as it has shown
to form rather vertical stances on gold surfaces when binding
27
through the sulfur atom.
Fig. 2a shows the CVs of the electrochemical synthesis of
viologen lms in 0.1 M Na SO containing 2 mM LCP on an Au
covered Si wafer with a SCNCP layer as working electrode. For
2
4
ꢀ
1
electrodeposition 20 cycles at 10 mV s in the range of 0 (0.1 V)
to ꢀ1.2 V (vs. Ag/AgCl) was used. Again a sharp reduction peak
appears in the rst scan at ꢀ0.46 V, due to the initially reduced
cyanopyridine units (at the tails of the SCNCP and at LCP) to
their radical cation form. The radicals undergo a reversible
coupling reaction aer which the cyanogroups are expelled
from the pyridine forming a viologen unit between the SCNCP
and the LCPs. Upon further reduction the viologen units are
reduced to their neutral form. In the reverse scan an oxidation
peak at ꢀ0.32 V, from the re-oxidation of the viologen units into
their dication form can be observed. In the consecutive cycles
viologen units are continuously formed due to available cya-
nopyridine groups on the linker and at the endings of the LCPs.
However, as the number of LCP units linked together is growing
and the active tail groups of the SCNCP monolayer decrease,
the peak currents decrease and the peak potentials are shied
to more negative (red ꢀ0.70 V) respectively less positive
(
ox ꢀ0.22 V) values. The thicker and more uniformly covering
layer is formed and the higher the lm resistivity, the larger the
peak potential difference (DE ¼ 0.10 V).
Aer polymerization the electrochemical response of the
viologen lm was characterized by CV in 0.1 M Na SO in the
2
4
potential range 0.1 to ꢀ1.2 V using different scan rates (Fig. 2b).
One redox couple can be found at approx. ꢀ0.70 and ꢀ0.26 V,
Fig. 1 (a) CVs of the electrodeposition of the monomer LCP (2 mM) on
ꢀ1
(
at scan rate 50 mV s ) which is from the reduction of viologen
top of a gold disc electrode in 0.1 M Na
2
SO
4
in the potential range 0.3
to ꢀ1.2 V with the scan rate 10 mV s (b) redox response of the
electrocoupled polyLCP film in 0.1 M Na SO
ꢀ
1
units from dication form to the radical cation form. The redox
response of the viologen lm is similar to what reported in our
2
4
.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 101232–101240 | 101235

View Article Online
RSC Advances
Paper
Fig. 2 The electrochemical polymerization of LCP on SCNCP and on SNH
2
–SCNCP primed electrodes and their redox responses in 0.1 M
ꢀ1
2 4
Na SO at 20, 50 and 100 mV s are shown in (a), (c), (b) and (d), respectively. Number of scan during polymerization is marked with colour in the
insets.
previous work: the peak currents increased with the increasing of alkanethiol SAMs, because they are easy to prepare and give
of scan rates, which indicated polymerization of LCP monomer rise to highly stable and reproducible close-packed structures. A
23
on top of the SCNCP layer. This can also be observed from the series of organic monolayers containing amine and amide
inserted plot in Fig. 2a with an enlargement from the potential functional groups on silicon and gold substrates were charac-
th
range 0.1 to ꢀ0.4 V (every 5 cycle is shown). Furthermore, in terized using X-ray photoelectron and infrared spectroscopies
Fig. 2b the peak currents are higher comparing to Fig. 2d, which showing that the amine-terminated monolayers had 70% of the
can be explained by that the bond formed by the SCNPCP group amine groups in the free amine (–NH
2
) form aer removal from
30
facilitates electron transfer more efficiently than the bond pure water. The SAM forming alkanethiol molecule, cyste-
2
9
which was formed by the thiol group. Another reason is the amine (SNH
2
), was in this work studied as linking adlayer. Here,
allow the sulfur
increased distance between the viologen and the Au surface the bi-functional molecular properties of SNH
2
when the SCNCP monolayer was formed on top of the alkane head-group to bind to the gold surface forming a uniform and
thiol monolayer, which increases the resistance and mitigates active amino-terminated monolayer that can be used for the
the electron transfer between the electrode and the solution. immobilization of the target linker molecule SCNCP. With this
The redox potential shis quite much with increasing scan rate, concept we can tune the layer formation leading to upright
which might be explained by rather high resistivity in the layer. geometries due to binding of the linker to gold via the sulfur
The second reduction, typical for viologens, showing the further atom. Scheme 2 depicts a proposed deposition procedure with
+
reduction of the radical cation to a neutral form cannot be ob- –NH
2
, leaving out the –NH
3
form, as it represents the amine
tained from these layers, at least not in the available potential terminal used in the coupling of SCNCP.
window (due to the use of H O electrolytes). Fig. 2c shows the CVs of electrochemical synthesis of viol-
The combination of organo-sulfur compounds and gold has ogen lms in 0.1 M Na SO containing 2 mM LCP on an Au
2
2
4
been well-established, especially those leading to the formation covered Si wafer with a SNH –SCNCP (Si/Au/SNH –SCNCP) layer
2
2
101236 | RSC Adv., 2015, 5, 101232–101240
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
ꢀ
1
31
as working electrode. Again 20 cycles at 10 mV s in the range absorbance of the viologen unit. The viologen lm synthesized
of 0 (0.1 V) to ꢀ1.2 V (vs. Ag/AgCl) was used for electrodeposi- using a SNH –SCNCP modied Au substrate (SNH –SCNCP–
2
2
tion. The inserted graph shows increase in current with the polyLCP) show a strong absorbance at around 400 nm, typical
amount of scans indicating lm formation. The CV response for polyviologens.
of the deposition process is almost identical to the one shown
in Fig. 2a. However, as the electrode surface was modied by spectroscopy has evolved into a very important method of
a layer of SNH before deposition of SCNCP, the distance for monitoring both chemical composition and molecular orientation
electron transfer is longer (the electrochemical resistance will of thin lms and monolayers deposited on metal substrates.
Vibrational spectral characterization. Reectance infrared
2
32,33
be higher) in comparison to deposition on the electrode surface The PM-IRRAS spectra for a cysteamine primed gold substrate
with just the SCNCP group. Therefore the reduction peak is in Fig. S1† show that the asymmetric and symmetric methylene
shied more negative, to ꢀ0.85 V, and the oxidation peak stretching vibrations belonging to the short ethylene chain in
ꢀ
1
shied less negative, to ꢀ0.24 V.
the monolayer appear at 2930 and 2857 cm , respectively. The
The redox response of the resulting viologen lm is shown in formation of aminothiol SAMs involves the cleavage of the S–H
Fig. 2d at different scan rates. The potential for the redox couple bond and formation of an Au–S. The absence of the S–H band
ꢀ1
can be found at approx. ꢀ0.79 and ꢀ0.26 V. As already mentioned (2575 cm ) in the spectra provides the evidence for a thiolate
ꢀ
1
above, the slightly larger DE is probably due to the higher resis- intermediate. The region between 1650 and 1570 cm
tance involved with the SAM layer deposited under the SCNCP. is assigned to N–H bending vibration of primary amines and the
ꢀ
1
However, the current response of the deposit is quite similar in peaks in the region 1050 to 1250 cm
are from the C–N
both cases as seen when comparing to the CV in Fig. 2b.
stretching vibration of aliphatic amines. The absence of the
+
ꢀ1
characteristic NH3 peaks in the region 3100–2900 cm indi-
cates that the terminal group is mainly in its free state (NH ),
2
Optical properties
not in the protonated form. Chemisorption and structural
characterization, employing surface-enhanced Raman scat-
tering (SERS), has been used for the study of isothiocyanate
UV-vis spectra of SAMs and viologen lms. Fig. 3 shows the
UV-vis absorption spectra of electrochemically deposited viol-
ogen lms on top of Au covered quartz glass modied by SAMs
27
SAMs on gold nanoparticle surfaces. From the studies by
Joo et al. it was concluded, that as for organic thiol SAMs, the
isothiocyanate SAMs were formed via the chemical bond of
S–Au between the NCS group sulfur atoms and the gold surface.
Both equatorial and axial chair conformers were formed at low
bulk concentrations, whereas at high concentrations the former
conformation appeared to be dominant. Isothiocyanates are
also commonly applied as chemical group in biomolecular
probes for labeling and crosslinking to primary amines.
Fig. S2† show the PM-IRRAS spectra obtained upon linking
of isothiocyanate to the cysteamine primed Au substrate. The
most profound changes observed are the aromatic C–H stretch,
of SCNCP or SNH
2
–SCNCP and from the SNH
2
–SCNCP linker.
–SCNCP
The absorbance from the SAM using the linker SNH
is very weak showing a broad band at 510 nm, which is due to
the absorbance of pyridine groups in the layer. The viologen
2

lm, electrosynthesized on Au with a SAM of SCNCP (SCNCP–
polyLCP), shows a strong absorbance centered at 350 nm with
a shoulder at 370 nm. The viologen precursor LCP shows
22
a strong and broad absorbance band at 350 nm, which indi-
cates that there is still plenty of uncoupled LCP embedded in
the lm. The absorbance at 370 nm can be explained by
the dimerization phenomenon between two viologen units
resulting in a radical cation dimer giving a blue shi in the
ꢀ
1
above 3000 cm (from the benzene and pyridine ring) and that
ꢀ
1
from the C^N at around 2100 cm . Some small changes can
also be observed for the N–H bending vibrations. These SAMs
were then applied as electrode substrates for the electro-
polymerization of viologen lms and the structures of the
synthesized lms characterized by infrared reectance spectros-
copy using a variable angle reection accessory. The obtained
absorbance spectra are split into two for clarication and shown
ꢀ
1
ꢀ1
from 2000 to 650 cm (Fig. 4a) and from 3150 to 2000 cm
(
ꢀ
1
Fig. 4b). Additionally, the region from 2300 to 2100 cm was
enlarged and is shown as insert in Fig. 4b.
In Fig. 4a it can be observed that the LCP monomer and the
viologen lms show bands in the wavenumber region 1400 and
ꢀ1
1
600 cm assigned to the C–C ring stretching (both from the
benzene and pyridine ring). The bands in the wavenumber
ꢀ
1
region 1000–1400 cm are mainly due to the C–H waging and
ꢀ1
bending vibrations. A sharp peak at 1642 cm is observed in
the monomer as well as in the polymer spectrum which is
assigned to ring C]N stretching vibrations. Notably, a peak
Fig. 3 UV-vis spectra of viologen films prepared on Au covered quartz
ꢀ
1
observed at 2246 cm due to C–N stretching is detectable in the
2
glass modified by SCNCP and by SNH –SCNCP and from a SAM of the
linker SNH –SCNCP.
2
monomer spectrum but is absent in the polymer spectrum
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 101232–101240 | 101237

View Article Online
RSC Advances
Paper
2 2
Fig. 5 AFM images of (a) SNH –SCNCP layer and (b) SNH –SCNCP–
polyLCP on Au.
layer forming several sites for further potential electrochemical
coupling.
This indicate that the link between the amine and iso-
2
thiocyanate group (SNH –SCNCP) have proceeded successfully.
The SAM formation by cysteamine on Au was veried by AFM
and shown in Fig. S3† together with the AFM picture from
a plasma cleaned Au covered Si plate. In Fig. S3† are also shown
AFM images of a SAM of SCNCP and of SCNCP–polyLCP. The
resulting deposited lm is thicker than in the case where
cysteamine is used as SAM Fig. 5b. The material however is less
ordered and the morphology resembles more the structure ob-
tained upon electropolymerization directly on Au.
Fig. 5b shows the AFM image of the polyviologen lm, which
was electrochemically polymerized of LCP on top of the SNH –
2
SCNCP layer. In comparison to Fig. 5a, it can be seen that the
thickness of the lm was enhanced, which is due to coupling of
LCP. In addition, the lm surface was turned to an inhomoge-
neous and rougher structure more clearly seen from the SEM
images in Fig. 6, indicating that the viologen lm was formed as
a brush shaped deposit with a maximum height of around 70 nm
together with areas of shorter brushes imaged by AFM (Fig. 5).
The contact angle measurements are sensitive to a change in
surface properties. The plasma cleaned Au substrate produced
Fig. 4 FTIR spectra of polyviologen films based on SNH
and SCNCP film from (a) 2000 to 650 cm and (b) 3150 to 2000 cm
2
–SCNCP film
ꢀ1
ꢀ1
.
(shown as enlargement in the insert in Fig. 4b). This indicates
ꢁ
an almost perfect wetting situation with a contact angle of 37 .
that the cyanopyridinium moieties are utilized for polyviologen
formation. As described above the viologen lms showed typical
redox responses and also the similarity of the obtained IR spectra
indicates that they have similar structures. However, the UV-vis
spectra showed differences between the lms indicating an
inuence of the SAM layer on the formation of viologen lms.
ꢁ
The low water contact angle of 32 on the amine surface
conrmed the presence of terminal amino group on the SAM
ꢁ
surface. Water contact angles of 44–46 have been reported for
34
surfaces modied with cysteamine. Aer additional linking of
SCN and furthermore electropolymerization (Si/Au/cysteamine/
ꢁ
ꢁ
SCN/polyLCP) contact angles of 40 and 51 were obtained,
respectively.
Changes in morphology of the viologens lms formed by
different deposition procedures were further studied by SEM.
Structural properties
In building supramolecular structures on electrodes, it is Fig. 6a and b show SEM images of polyviologen lms directly
important to provide evidence and full characterization of the electrodeposition from LCP onto the Au surface. The substrate
various steps involved in the assembly. Atomic force microscopy is well covered but an inhomogeneous cauliower like
(
AFM) is routinely employed in many research studies because morphology is obtained. The electrochemical characterization
of its capability to visualize surfaces of materials with the in Fig. 1b also indicated poor adhesion of the lm to the elec-
highest spatial resolution. The assembled structures and the trode substrate. Fig. 6c and d show the deposit of electro-
deposits formed aer electrochemical synthesis were therefore chemically synthesized viologen from LCP on top of an Au
imaged using the AFM and SEM technique in order to under- electrode linked with SCNCP. A slightly rope shaped material is
stand the chemistry of the attachments. Fig. 5a shows the AFM formed but hardly no growth perpendicular to the electrode
image of the isothiocyanate anchor linked to the cysteamine surface can be observed. This might be due to the way the
monolayer (SNH
2
–SCNCP). The surface is well covered by the SCNCP is anchoring to the Au structure. The R–NCS molecule
101238 | RSC Adv., 2015, 5, 101232–101240
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 6 SEM images (a) and (b) shows the morphology of electrochemically synthesized viologen films directly on an Au electrode. In (c) and (d)
the viologen film was electrochemically synthesized from LCP on an Au electrode with SAM of SCNCP and in (e) and (f) when using a SAM of
SNH –SCNCP.
2
has been shown to bind to metal surfaces through the sulfur was achieved using SCNCP, a synthesized molecule having an
26,27
but in several different conformations.
Two of the suggested isothiocyanate group as a “head”-group and a cyanopyridine
conformations that are dominating are the equatorial conformer group as a “tail”-group suitable for self-assembly. The electro-
pointing almost straight out from the substrate and the axial chemical polymerization of a linear cyanopyridine based
conformer, which is tilted and lies almost parallel to the surface. monomer (LCP) was successfully carried out on Au electrodes
2
From Raman measurements in ref. 26 and 27 it could be with SAMs of SCNCP or SNH –SCNCP showing typical redox
conrmed that several conformer types coexist. This can explain responses for viologen materials. Furthermore, the UV-vis and
the tilt rotation seen in the SEM image of the isothiocyanate FTIR spectra also proved that the viologen lms were success-
precursor formed self-assembly monolayer, leading to that the fully synthesized showing absorbance value unique for viol-
electrochemical coupling to the linear monomers, LCP, will ogens. With the analysis by AFM and SEM, it is clear that the
result in strains lying parallel to the surface.
SEM images for viologen lms electrochemically synthesized Au electrode with a SAM of cysteamine linked with SCNCP
from LCP when using Au electrodes that have been primed with (SNH –SCNCP) has a well-ordered structure. Additionally, due
viologen lm which was electrochemically formed on top of an
2
a SAM of cysteamine before anchoring of isothiocyanate are to the unique structure of the new synthesized linker monomer
shown in Fig. 6e and f. Compared to the images without SCNCP, the viologen lm formation can be extended from
cysteamine, the viologen deposit is now formed in shape of Au to other surfaces, which gives this unique viologen lm
small brushes. Cysteamine forms a well-ordered monolayer on huge potential when thinking about applications for various
top of gold, enabling the isothiocyanate anchor structure to electronics.
connect to the amine group in the monolayer in an ordered way.
Further electrochemical coupling to the linear monomer, LCP,
leads therefore to the formation of polyviologens strains
Acknowledgements
sticking out from the substrate.
The lengths of the brushes are however not uniform all over
the surface of the substrates. Consequently two different
regions can be observed for longer and shorter brush bundles of
which the region with the shorter ones can be seen from the
This work was nancially supported by CHEMSEM national
graduate school and by Academy of Finland. Arisara Muan-Oum
is gratefully acknowledged for the PM-IRRAS measurements.
Notes and references
AFM images.
1
P. M. S. Monk, The viologens: physicochemical properties,
synthesis, and applications of the salts of 4,4 -bipyridine, John
0
Conclusions
Wiley & sons Ltd, West Sussex, England, 1998.
In this work, a new well-ordered polyviologen lm has been
2 S. Cosnier, S. Da Silva, D. Shan and K. Gorgy,
electrochemical synthesized and characterized. The ordering
Bioelectrochemistry, 2008, 74, 47–51.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 101232–101240 | 101239

View Article Online
RSC Advances
Paper
3
B. B. Narakathu, M. S. Devadas, A. S. G. Reddy, A. Eshkeiti, 18 R. G. Nuzzo and D. L. Allara, J. Am. Chem. Soc., 1983, 105,
A. Moorthi, I. R. Fernando, B. P. Miller, G. Ramakrishna, 4481–4483.
E. Sinn, M. Joyce, M. Rebros, E. Rebrosova, G. Mezei and 19 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and
M. Z. Atashbar, Sens. Actuators, B, 2013, 176, 768–774. G. M. Whitesides, Chem. Rev., 2005, 105, 1103–1169.
A. S. Perera, N. K. Subbaiyan, M. Kalita, S. O. Wendel, 20 R. Nie, A. Li and X. Deng, J. Mater. Chem. A, 2014, 2, 6734–
4
5
T. N. Samarakoon, F. D'Souza and S. H. Bossmann, J. Am.
Chem. Soc., 2013, 135, 6842–6845.
6739.
21 S. F. J. Appleyard, S. R. Day, R. D. Pickford and M. R. Willis, J.
Mater. Chem., 2000, 10, 169–173.
N. Sano, W. Tomita, S. Hara, C.-M. Min, J.-S. Lee, K. Oyaizu
¨
¨
and H. Nishide, ACS Appl. Mater. Interfaces, 2013, 5, 1355– 22 N. Wang, P. Damlin, B. M. Esteban, T. Aaritalo, J. Kankare
361.
and C. Kvarnstr ¨o m, Electrochim. Acta, 2013, 90, 171–178.
X. Liu, M. Hao, M. Feng, L. Zhang, Y. Zhao, X. Du and 23 N. Wang, A. K ¨a hk ¨o nen, P. Damlin, T. Aaritalo, J. Kankare
G. Wang, Appl. Energy, 2013, 106, 176–183.
and C. Kvarnstr ¨o m, Electrochim. Acta, 2015, 154, 361–369.
L. Gao, G. Zheng, Y. Zhou, J. Jiang, G. Ding and Y. Wang, So 24 B. Gadgil, P. Damlin, T. Aaritalo, J. Kankare and
Mater., 2014, 12, 149–158.
C. Kvarnstr ¨o m, Electrochim. Acta, 2013, 97, 378–385.
E. Hwang, S. Seo, S. Bak, H. Lee, M. Min and H. Lee, Adv. 25 B. Gadgil, P. Damlin, T. Aaritalo and C. Kvarnstrom,
Mater., 2014, 26, 5129–5136. Electrochim. Acta, 2014, 133, 268–274.
W. W. Porter, T. P. Vaid and A. L. Rheingold, J. Am. Chem. 26 S.-W. Joo, Surf. Interface Anal., 2006, 38, 173–177.
1
¨
¨
6
7
8
9
¨
¨
¨
¨
¨
Soc., 2005, 127, 16559–16566.
0 C. L. Bird and A. T. Kuhn, Chem. Soc. Rev., 1981, 10, 49–82.
1 P. Monk, R. Mortimer and D. Rosseinsky, Electrochromism
27 H. Kang, J. Noh, E.-O. Ganbold, D. Uuriintuya, M.-S. Gong,
J. J. Oh and S.-W. Joo, J. Colloid Interface Sci., 2009, 336,
648–653.
1
1
and Electrochromic Devices, Cambridge University Press, 28 G. E. Slaughter, E. Bieberich, G. E. Wnek, K. J. Wynne and
007. A. Guiseppi-Elie, Langmuir, 2004, 20, 7189–7200.
2 A. K. M. Ka, D.-Y. Lee, S.-H. Park and Y.-S. Kwon, 29 M.-D. Fu, I.-W. P. Chen, H.-C. Lu, C.-T. Kuo, W.-H. Tseng
Microchem. J., 2007, 85, 308–313. and C.-h. Chen, J. Phys. Chem. C, 2007, 111, 11450–11455.
3 H. C. Ko, S. Kim, H. Lee and B. Moon, Adv. Funct. Mater., 30 A. E. Hooper, D. Werho, T. Hopson and O. Palmer, Surf.
005, 15, 905–909. Interface Anal., 2001, 31, 809–814.
4 C.-W. Hu, K.-M. Lee, K.-C. Chen, L.-C. Chang, K.-Y. Shen, 31 P. M. S. Monk, R. D. Fairweather, M. D. Ingram and
2
1
1
1
2
S.-C. Lai, T.-H. Kuo, C.-Y. Hsu, L.-M. Huang, R. Vittal and
K.-C. Ho, Sol. Energy Mater. Sol. Cells, 2012, 99, 135–140.
5 E. M. Kosower and J. L. Cotter, J. Am. Chem. Soc., 1964, 86,
J. A. Duffy, J. Chem. Soc., Perkin Trans. 1, 1992, 2, 2039–2041.
32 C. L. Hoffmann and J. F. Rabolt, Macromolecules, 1996, 29,
2531–2547.
33 J. T. Young, F. J. Boerio, Z. Zhang and T. L. Beck, Langmuir,
1996, 12, 1219–1226.
1
1
1
5524–5527.
6 K. Kamata, T. Suzuki, T. Kawai and T. Iyoda, J. Electroanal.
Chem., 1999, 473, 145–155.
7 K. Kamata, T. Kawai and T. Iyoda, Langmuir, 2001, 17, 155–
34 K. Chen, B. W. Caldwell and C. A. Mirkin, J. Am. Chem. Soc.,
1993, 115, 1193–1194.
163.
101240 | RSC Adv., 2015, 5, 101232–101240
This journal is © The Royal Society of Chemistry 2015