Sensitized photografting of diazonium salts by visible light.

Article abstract of DOI:10.1021/cm3032994

Visible irradiation of a gold surface dipped into a solution of diversely substituted aryldiazonium salts in the presence of a photosensitizer (Ru(bipy)₃²⁺ or eosinY) leads to a modification of the surface by attached aryl groups. A grafted nanometer thick polyphenylene film is obtained. The reaction is extended to a polyvinylchloride (PVC) surface. The mechanism proposed for the formation of this film involves the formation of an aryl radical that reacts with the surface and then with the first grafted aryl group. The film is characterized by infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), water contact angles, and ellipsometry.

Full text of DOI:10.1021/cm3032994

Article
pubs.acs.org/cm
Sensitized Photografting of Diazonium Salts by Visible Light.
Meriem Bouriga,† Mohamed M. Chehimi,‡ Catherine Combellas,† Philippe Decorse,‡ Frederic Kanoufi,†
́
́
Alain Deronzier,*,§ and Jean Pinson*,†,‡
†
́
Physico-Chimie des Electrolytes, des Colloïdes et Sciences Analytiques, Ecole Super
́
ieure de Physique et de Chimie Industrielles,
ESPCI, CNRS UMR 7195, 10 rue Vauquelin, 75231 Paris Cedex 05, France
‡ITODYS, CNRS UMR 7086, Universite
Paris Diderot, Sorbonne Paris Cite,
§Dep
́ ́
artement de Chimie Moleculaire, Laboratoire de Chimie Inorganique Redox, CNRS UMR 5250, ICMG FR-2607, Universite
́
́
15 rue J. A. de Baïf, 75013 Paris, France
́
Joseph Fourier, BP-53 38041 Grenoble Cedex 9, France
S
* Supporting Information
ABSTRACT: Visible irradiation of a gold surface dipped into a solution of
diversely substituted aryldiazonium salts in the presence of a photosensitizer
2+
(Ru(bipy)3 or eosinY) leads to a modification of the surface by attached aryl
groups. A grafted nanometer thick polyphenylene film is obtained. The reaction
is extended to a polyvinylchloride (PVC) surface. The mechanism proposed for
the formation of this film involves the formation of an aryl radical that reacts
with the surface and then with the first grafted aryl group. The film is characterized by infrared reflection absorption spectroscopy
(IRRAS), X-ray photoelectron spectroscopy (XPS), water contact angles, and ellipsometry.
KEYWORDS: surface chemistry, photochemistry, diazonium salts
spectroscopic methods15 as well as by density functional theory
(DFT) calculations.16 A large number of substituents of the aryl
group have been used to provide selective properties to the
films, from simple groups such as NO2, CH3, CF3, COOH,10a
CCH17 to macromolecules,18 dendrimers,19 proteins, and
enzymes.20 The thickness of the bonded films can be tuned
from monolayers21 to micrometric layers.22 Except for
electrochemistry, the methods described above10 apply to
non conductive surfaces such as polymers and inorganic oxides,
even if large concentrations of diazonium salts have to be used,
for example, in the grafting of polymers under ultra-
sonication.14j−l Patterning surfaces is possible by AFM,23 by
scanning electrochemical microscopy (SECM),24 with poly-
dimethylsiloxane PDMS stamps or nanoparticles shadowing the
surface.25
Surprisinly, although photochemical methods (especially
using visible light) would be interesting as they could be
achieved with a mask above the surface, this approach has rarely
been investigated to functionalize surfaces from diazoniums. In
this line, the photografting of diazonium salts, either by UV
irradiation of the salt or by visible irradiation of a
[aryldiazonium cation (0.1 M) − dimethoxybenzene (0.9
M)] charge transfer complex (CTC) has recently been
reported and films with up to 50 nm thickness have been
obtained.26 However, because of the poor absorption of visible
light of this CTC, this technique requires a large amount of the
donor (0.9 M) and the use of a high-power irradiation system
(300 W halogen lamp). Since grafting of surfaces by aryl groups
I. INTRODUCTION
Surface modification is important both in daily life and in the
academic field; for example, industrial processes permit to
protect the surface of materials and to improve their aesthetics1
while a large number of research papers and reviews are found
under these headings, covering for example bioanalysis,
microelectronics, energy related problems. These surface
treatments can be achieved by a variety of physical methods
(plasma, physical or chemical vapor deposition (PVD or CVD),
atomic layer deposition (ALD)), and also by chemical or
electrochemical methods and less commonly by photochemical
methods. Contrary to physical methods, chemical methods
permit the attachment of complex organic molecules to the
surface; they proceed, for example, by silanization,2 phospho-
natation,3 self-assembling of long alkyl chains (SAMs).4 In
addition to the industrial metal deposition (plating), electro-
chemical methods permit the attachment of organic molecules
to conductive surfaces, either by oxidation (amines,5 alcohols,6
carboxylates,7 Grignard reagents8) or by reduction (vinylics,9
diazonium10). UV photochemical grafting of surfaces is more
restricted; it has been applied to alkenes,11 arylazides,12 and
acetonitrile.13
Functionalization of surfaces (carbon, metals, semiconduc-
tors, polymers) by homolytic dediazonation of diazonium salts
10
+
−
(Ar−N2 BF4 ) is currently a well-established method that
has led to a variety of applications from analytical and
biochemical sensors to microelectronics and biomedical
industrial applications. It is easily performed under a variety
of conditions:14 electrochemical or homogeneous reduction in
ACN or aqueous media, or ultrasonication and heating. All
these methods provide strongly bonded polyaryl layers; the
surface-aryl bond has been characterized by different
Received: October 12, 2012
Revised: November 28, 2012
© XXXX American Chemical Society
A
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from diazonium salts has been previously assigned to the
formation of radicals that react with the surface,27 successful
photochemical grafting utilizing a more conventional photo-
redox sensitizer is expected. Indeed, the formation of aryl
radicals from diazonium by sensitized photochemistry (λ > 410
nm) has been reported in a series of papers.28,29
Scheme 3. Photosensitized C−H Arylation
For example, in the Pschorr cyclization, the stilbenediazo-
nium ion irradiated in ACN in the presence of tris(2,2′-
bipyridyl)ruthenium(II), Ru(bipy)32+, leads to the radical
through a reductive dediazonation reaction, and this radical
cyclizes along a Pschorr reaction with 100% yield and high Φ >
0.4 quantum yield (Scheme 1).
visible light, using a conventional LED that permits surface
patterning of insulating surfaces and should later on permit
obtaining optically addressable selective surface grafting. We
describe the photosensitized grafting of diazonium salts on gold
and polymers, using 4-nitro (DNO2), 4-bromo (DBr), 4-
carboxy (D-COOH), 4-carboxymethyl (DCH2COOH), 3,5-
bis-trifluoromethyl (D(CF3)2), and 3,5-dimethoxy (D(OMe)2)
benzenediazonium tetrafluoroborates. The aryl layers are
extensively characterized by infrared reflection absorption
spectroscopy (IRRAS), ellipsometry, contact angle measure-
ments, electrochemistry, and X-ray photoelectron spectroscopy
(XPS); particularly, XPS allows investigating the lateral
homogeneity of the tethered layers.
Scheme 1. Sensitized Photochemical Pschorr Cyclization
The proposed mechanism (Scheme 2)30−32 involves the
formation of an excited state, Ru(bipy)32+*, that is able to
II. MATERIALS AND METHODS
Scheme 2. Sensitized Photochemical Pschorr Reaction of
Stilbene Diazonium
Substrates. Gold coated (100 nm, Aldrich) Si wafer plates were
cleaned in concentrated (96%) sulfuric acid, rinsed in ultrapure water
and dried. Polyvinylchloride (PVC) was an industrial sample. They
were cut into ∼15 × 15 mm2 pieces, rinsed with ultrapure water and
alcohol.
Chemicals. Ru(bipy)3 (PF6)2 was synthesized along the procedure
previously described;36 eosinY, DNO2, and DBr were obtained from
Aldrich, and D(CF3)2, DCOOH, DCH2COOH, D(OMe)2 were
prepared from the corresponding anilines in aqueous HBF4 solution
by addition of 1.1 equiv of NaNO2.37 All other chemicals were
obtained from Aldrich and used without further purification.
Photografting. The small gold or polymer plates were placed in a
closed deoxygenated cell refrigerated (6 °C (general) or 0 °C
(D(CF3)2 photografting onto PVC and DNO2 photografting in
aqueous solution), see Supporting Information) to limit the
evaporation of the solvent and the spontaneous grafting of the
diazonium salt. Visible light irradiation was performed with a LED
bulb (4.8 W, 650 lm, cool white, Toshiba).
The irradiation solution was prepared from 2 mL of ACN (99.8%),
DMSO (99.9%), DMF (≥99.5%), or ultrapure water, the diazonium
salt (c = 0.155, 0.1, 0.05, or 0.01 M) and 0.0025 M Ru(bipy)3 (PF6)2
or eosinY. It was deposited on the gold plate, and two procedures were
used. In procedure A, the maximum possible volume of ACN solution
(50 μL/cm2) was deposited onto the gold surface, and irradiation was
started; after 30 min, the solution was completely evaporated, and
irradiation was stopped; with DMF and DMSO, evaporation was
negligible. In procedure B, after evaporation of the ACN solution,
more solution was added, and the process was repeated for 3 h. After
irradiation, the plate was rinsed under ultrasonication for 280 s
successively in ACN and ultrapure water and then dried.
Electrografting. XPS was used to compare photochemistry to
electrochemistry, using a brominated compound (DBr) that presents,
on a gold microelectrode, an irreversible voltammogram, with a
reduction peak at −0.25 V/(Ag/AgCl) and a prepeak at −0.22 V/(Ag/
AgCl). An electrolysis was performed on a gold plate (11 × 11 mm2)
in a 10 mM DBr deoxygenated ACN + 0.1 M NBu4BF4 solution at
−0.55 V/(Ag/AgCl) for 5 min; the plate was rinsed as above.
IR Spectra. IRRAS and ATR spectra were recorded using a JASCO
FT/IR-6100 Fourier Transform Infra Red Spectrometer equipped with
MCT detector. For each spectrum, 1000 scans were accumulated with
a spectral resolution of 4 cm−1. The background recorded before each
spectrum was that of a clean substrate.
reduce the diazonium salt by transfer of one electron to give an
aryl radical and Ru(bipy)33+. The latter species oxidizes the
intermediate cyclohexadienyl ring formed during the cyclization
2+
to restore the aromaticity and regenerate Ru(bipy)3
.
This same reaction33 was recently used for CH arylation
using either Ru(bipy)3 or eosinY as a sensitizer, as shown in
2+
Scheme 3.34
The mechanism in Scheme 2 is somewhat similar to that
described during the grafting of diazonium salts on surfaces.35
In the latter case, the generated radical reacts with (i) the
surface and (ii) the first grafted aryl group to give a
cyclohexadienyl moiety that is oxidized to restore the
aromaticity, leading to multilayers.
In this paper, we present a different and simple method
based on the sensitized photografting of diazonium salts under
B
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XPS Spectra. X-ray photoelectron spectra were recorded using a
Thermo VG Scientific ESCALAB 250 system fitted with a micro-
focused, monochromatic Al Kα X-ray source (1486.6 eV) and a
magnetic lens, which increases the electron acceptance angle and
hence the sensitivity. An X-ray beam of 650 μm size was used (15 kV
× 200 W) [500 μm (15 kV-150 W) for linescans]. The pass energy
was set at 150 and 40 eV for the survey and the narrow regions,
respectively. The Avantage Software, version 4.67, was used for digital
acquisition and data processing. The spectra were calibrated against
Au4f7/2 set at 84 eV. The surface composition was determined using
the integrated peak areas, and the corresponding Scofield sensitivity
factors were corrected for the analyzer transmission function.
The lateral uniformity of the layers along the main axis of the plate
sample was checked for photo- and electro-grafting of the diazonium
salt. The conditions are reported in Table 1.
Figure 1. IRRAS spectra of two gold plates modified by irradiating an
2+
ACN solution containing 0.1 M DNO2 and 0.0025 M Ru(bipy)3
−
Table 1. XPS Conditions for Perfoming the Linescan
Routine Experiment
(PF6 )2. Gold plate modified along (a) procedure A and (b)
procedure B.
X-ray spot
size (μm)
step size no. of total distance along
sample
(μm)
points
X axis (μm)
a
photografting
500
500
978
8
6847.5
would be inefficient as for that concentration the light is
completely absorbed in the thin layer of solution, while
reducing the concentration of the sensitizer from 0.0025 to
0.001 M decreases the absorbance of the −NO2 band at 1522
cm−1 by approximately nine times (Supporting Information,
Figure S9, red square).
b
electrografting
1193
15
16714.9
a
Five survey spectra were accumulated with pass energy = 100 eV;
dwell time = 100 ms; step size = 1.0 eV. Similar conditions for survey
spectra; in addition, high resolution spectra were recorded.
b
Other diazonium salts were photografted on gold in ACN
and their IR characteristics are gathered in Table 2 (Supporting
Information, Figures S2−S8, Table S1). Different solvents were
investigated, and photografting was also observed in DMSO
(Supporting Information, Figures S3, S4, S6 for DNO2, DBr,
and DCOOH, respectively), DMF (Supporting Information,
Figure S13), and in the presence of eosinY as a sensitizer
(Supporting Information, Figures S3 and S14). Interestingly,
aqueous solutions (0.1 M DNO2 and 0.0025 M eosinY) can
also be used (Supporting Information, Figure S14). The
intensity of the nitro bands does not vary significantly with the
solvent.
Since diazonium salts are prone to spontaneous homolytic
dediazonation and grafting even on materials of low reactivity
such as gold,40 we have evaluated by IRRAS the importance of
spontaneous grafting that always takes place simultaneously
with photografting. The corresponding ratio (spontaneous
grafting in the absence of irradiation/photochemical grafting
under irradiation, Supporting Information, Table S2) amounts
to 5% in ACN, 24% in DMSO, 56% in water at pH = 3, and up
to 70% in DMF (at 6 °C). This indicates that the photografting
reaction is more efficient in ACN but much less in other
solvents including water.
XPS Spectra on a Gold Substrate. The gold plates
modified by the sensitized [(Rubipy)32+] photografting of 4-
bromobenzenediazonium (DBr) along procedures A and B in
ACN were also examined by XPS. The spectra are presented in
Figures 2 and 3.
The survey spectrum of a gold plate modified along
procedure B (3 h irradiation, Figure 2a) evidences the presence
of Br3d (as a doublet at 69.6 and 70.7 eV in the ∼3:2 peak area
ratio, Figure 2b), Br3p (183−187 eV) and C1s (284.8 eV, close
to the value of 285.10 eV reported for bromobenzene41),
indicating that the bromophenyl group has been attached to the
surface. This is confirmed by the presence of a shakeup satellite
peak at 291 eV along procedure A in DMSO (Figure 2e). Note
the absence of the XPS signature of Ru, indicating that the aryl
radical does not attack the pyridine rings of the sensitizer, in
Ellipsometry. A Sentech SE 400 ellipsometer was used, with a
He−Ne laser λ = 6328 Å, angle = 70.00°. The values for ns and ks
were measured on a clean gold wafer and then used to determine the
thickness of the grafted layer: ambient: n = 1, organic layer: no = 1.46,
ko = 0.0; substrate: ns = 0.230, ks = 3.341. The thickness was evaluated
directly from these data by the ellipsometer software.
Water Contact Angles. They were measured with a Kruss DSA3
̈
instrument. Three microliters of Milli-Q water was automatically
deposited on the top of the test sample placed in a horizontal position
on the instrument stage. At least five measurements were made for
each sample. The values of the contact angles were calculated by the
tangent method using Drop Shape Analysis software.
Electrochemistry. Voltammograms were recorded with an EGG
Princeton Applied Research potentiostat 263A and an Echem 4.30
version software. The counter electrode was Pt, and the reference was
Ag/AgCl.
III. RESULTS
Two Different Grafting Procedures on a Gold
Substrate. In procedure A, a volume of an ACN solution of
DNO2 is deposited at once on the gold surface and subjected to
irradiation for 30 min. In procedure B, the irradiation is
maintained for 3 h; during this time, the solution deposited on
the surface has to be renewed about every 30 min. At the end of
these procedures, the gold plates are carefully rinsed and
examined by IRRAS.
IRRAS Spectra on a Gold Substrate. The spectra are
presented in Figure 1 when using Ru(bipy)32+ as the sensitizer.
As expected, the spectrum along procedure B is more intense
than along procedure A. On both spectra, the characteristic
NO2 vibrations are observed at 1522 and 1347 cm−1 as well as
the ring vibration at 1599 cm−1. The main difference between
these spectra is the presence, when procedure B is used, of a
band at 2233 cm−1 that can be assigned to cyano groups; their
formation from acetonitrile will be discussed below.
As expected, the optical density of the nitro band (1522
cm−1) varies linearly with the concentration of diazonium along
procedure A (Supporting Information, Figure S9). Increasing
2+
−
the concentration of Ru(bipy)3 (PF6 )2 above 0.0025 M
C
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Table 2. IRRAS Spectra of Gold Plates Photografted in ACN with Different Diazonium Salts
+
+
Ar−N2
IR-ATR of Ar−N2
wavenumber (cm−1
IR of Ar−H
wavenumber (cm−1
IRRAS of modified gold plates
wavenumber (cm−1
assignment
)
)
)
b
DNO2
2233
CN
2303
NN
ab
,
1612 1615 1574
1539, 1352
856, 738
1689, 1620, 1607
1599
ring vibration
NO2 as and s
CH out of plane
CN
ab
,
1521, 1347
855, 73938
1522, 1347
ab
,
857
b
DBr
2204
2285
NN
1554, 1464, 1414
1123, 1030
828,762
1579, 1476, 1443
1075−106539
812, 735
1568, 1484, 1445
ring vibration
1074, 1060
811
C−Br combination
CH out of plane
CN
D(CF3)2
DCO2H
2233
/
2154
1594
NN
1563
ring vibration
CF3
1353, 1277, 1208, 1153,1135, 1108
1345−1265, 1190−1130, 1165−110539
3073−2965
1377, 1277, 1246, 1198, 1143
3366
centered at 3117
2265
COOH
NN
1706
1689
1605,1585,146438
1745
CO
1589, 1509
3105
1585, 1526
3246
ring vibration
COOH
DCH2CO2H
D(OMe)2
2285
NN
1706
1708
CO
1589, 1419
3105
1599, 1498
ring vibration
aromatic C−H stretching
CH3
3093,307038
2950, 2846
2303
2959, 2942, 2836
NN
1633, 1547,1471
1606, 1593, 1548, 1494
1606
ring vibration
a
b
Procedure A. Procedure B.
−
agreement with the IR spectra that do not exhibit the sensitizer
signature (Supporting Information, Figure S1).
process and (ii) F1s indicating that no BF4 anion has been
trapped in the film and also that no Balz−Schiemann reaction
occurred that would have furnished fluorobenzene from
benzenediazonium.44
The presence of Au4f7/2 (84 eV) and Au4d5/2 (335 eV)
indicates that the organic layer is rather thin (<5−10 nm). It
does not completely shield the Au plate and does not retain the
inelastically scattered electrons, as attested by the intense
inelastic background beyond Au4f. The difference between
procedures A and B is evidenced in Table 3.
A linescan was performed to check the homogeneity of the
photo and electrografting. This experiment, presented in Figure
3, indicates that photografting provides homogeneous films
regarding the spot size of the X-ray beam (see also in
Supporting Information, Figure S17).
Ellipsometry on a Gold Substrate. In agreement with the
XPS and IR results, ellipsometry shows that, except for DBr
upon 90 min irradiation, the films are rather thin with
As the irradiation time increases from procedure A to
procedure B, the Au signal is more shielded by the organic layer
and the Br/Au and C/Au atomic ratios increase. In DMSO, the
ratios are higher than in ACN partly because of the
spontaneous reaction evidenced above. The N/C atomic ratio
is 0.03, both in DMSO and after electrografting in ACN; it is
due partly to contamination and partly to the -NN- groups
that are formed during the growth of multilayers by attack of a
diazonium group on a cyclohexadienyl radical.35 During
photografting by procedure A, the N/C atomic ratio in ACN
is 0.06; it increases to 0.10 during procedure B, which is
consistent with the appearance of CN bands on the IR spectra.
The location of the N1s peak at 399.6 eV is similar to that given
for polyacrylonitrile (399.6 and 399.2 eV)42 and valeronitrile on
Ni (399.7 eV).43 On tilting the specimen from 90° to 40°
(relative to the surface), the Br/N ratio (direct experimental
ratio) increases slightly from 1.6 to 1.8, suggesting that the
outermost layer of the coating is aryl-rich (Br is a unique
elemental marker for the diazonium salt) whereas the
electrode-organic layer interface is nitrogen- and thus
“acetonitrile”-rich. We also checked the absence of (i) S2p in
the layers prepared in DMSO, indicating that during procedure
A, the solvent has not been involved in the photografting
thicknesses always less than 5 nm (Table 4). For example,
the film obtained with DNO2 in the presence of [Ru(bipy)3
2+
]
along procedure A in ACN is 1.8 0.6 nm thick; such film
corresponds to a tetraphenyl group (∼ 1.7 nm), as shown in
Scheme 4. The films are hydrophobic and thinner than those
obtained by irradiation of charge transfer complexes.26
Water Contact Angles on a Gold Substrate. The values
for the contact angles can be compared with those reported in
the literature for grafted surfaces: Au-C6H4−C(O)C6H5: ∼
70°,45 C-C6H4−COOH: from ∼45 to ∼10° depending on
pH,46 C-C6H4−CF3: 82° and C-C6H4−SO3H: 63°,47 Au-
C6H4−C(O)C6H4−Polystyrene: 78°, Au-C6H4−C(O)-
C6H4-polyhydroxyethylmethacrylate: 44.5°, Au-C6H4−C(
O)C6H4-polymethyl methacrylate: 57°.48 The layers prepared
by photochemistry (Table 4) are quite hydrophobic. The value
obtained for the CF3 substituted film, Au-C6H3-(CF3)2: 87° is
in reasonable agreement with that in the literature for C-C6H4−
CF3: 82°.
D
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a
Table 3. Surface Chemical Composition of the Modified
Gold Plates
atomic
ratios
ACN
procedure A
DMSO
procedure A
ACN
ACN
procedure B electrochemistry
Br/Au
C/Au
N/C
0.068
1.05
0.06
0.114
2.17
0.03
0.455
4.55
0.10
3.33
16.7
0.03
a
In atomic ratios.
Table 4. Films Thicknesses Measured by Ellipsometry and
Water Contact Angles
solvent
thickness
(nm)
water contact
angles (degrees)
diazonium salt [sensitizer] (procedure)
none
bare gold
57
75
4
1
2+
DNO2 [Ru(bipy)3
]
ACN (A)
DMF (A)
DMSO (A)
ACN (B)
1.8 0.3
3.9 0.6
1.3 0.1
3.2 0.5
2.1 0.4
80
86
3
2
DNO2 [EosinY]
water,
pH = 3 (A)
2+
DBr [Ru(bipy)3
]
ACN (A)
DMSO (A)
ACN (B)
ACN (B)
ACN (A)
3.1 0.4
3.3 0.7
15.0 1.8
3.1 0.9
1.7 0.4
84
87
81
87
80
3
4
2
2
3
2+
D(CF3)2 [Ru(bipy)3
D(OMe)2 [Ru(bipy)3
]
2+
]
Scheme 4. Schematic Representation of the Film Formed
with DNO2 in ACN along Procedure A
Figure 2. XPS Spectra of a gold plate modified by photografting DBr
2+
in the presence of 0.0025 M Ru(bipy)3 (a,b,c) in ACN along
procedure B; (d) in ACN along procedure A; (e) in DMSO along
procedure A; (a) survey spectrum, (b) Br3d, (c, d) N1s, (e) C1s.
nonconducting substrates. We therefore investigated the
grafting of PVC as an example of polymer.
PVC Substrate. A DNO2 solution (c = 0.1 M) and eosinY
(c = 0.0025M) in ACN was deposited on PVC and subjected to
irradiation for 1 h at 0 °C. The bands of the nitrophenyl group
clearly appear at 1525 and 1347 cm−1, as shown in Figure 4,
and a blank experiment in the dark does not present any NO2
band.
Figure 3. XPS linescan along a gold plate modified by photografting of
2+
DBr (c = 0.1 M) in ACN in the presence of 0.0025 M Ru(bipy)3
−
(PF6 )2 along procedure B.
Irradiation allows to localize the grafting, as shown in Figure
5 that presents the image of ∼4 mm grafted spots obtained
after 1 h irradiation of droplets containing D(CF3)2 and
Ru(bipy)32+. An IRRAS spectrum recorded on a 100 × 100 μm2
surface inside the spot presents the same bands as those in
Figure 4, and the water drop angle inside the spots is 107 4°
while it is only 84 3° outside the spots on unmodified PVC.
In spite of the success of the process on gold, it cannot be
applied for the modification of oxidizable substrates. For
example, a 150 nm layer of copper on Si completely disappears
under the same experimental conditions; this is likely due to the
oxidation of copper by the oxidizing Ru(bipy)3 (E1/2 = 1.35
V/SCE). A distinctive characteristic of photografting, by
comparison with electrochemistry, is the possibility of using
3+
E
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Scheme 6. Possible Structures for CH2CN/aryl Films
Figure 4. IRRAS spectrum of PVC modified by photografting DNO2
(c = 0.1 M) and eosinY (c = 0.0025 M) for 1 h at 0 °C.
with the surface to give a surface bonded aryl group (R3).35 In
a second step, another aryl radical attacks the first attached aryl
group to give a cyclohexadienyl radical (R4); this radical can be
3+
reoxidized to a biaryl by Ru(bipy)3 (R5) or by the starting
Figure 5. Optical image of a PVC surface (15 × 20 mm2) patterned
with PVC−C6H4-(CF3)2 spots (∼ 4 mm).
diazonium itself. Since Ru(bipy)33+ is a much stronger oxidizing
species [E° (Ru3+/Ru2+) = 1.30 V/SCE] than diazonium salts
[E°(+N2C6H5/C6H5 ) = 0.05 V/SCE], the Ru species should
•
be much more efficient for the oxidation of the cyclohexadienyl
IV. DISCUSSION
radical than the diazonium salt (R5). We have not attempted to
IRRAS (for example, characteristic bands of the nitrophenyl
group), XPS (Br3d peaks and change in the survey spectra
background), electrochemistry (characteristic nitrophenyl re-
duction peak), and water contact angles (hydrophobic surface
obtained with D(CF3)2) indicate that the substituted phenyl
groups are attached to the surface. The film resistance to
ultrasonic rinsing, as for electrografted films for which the
existence of a bond between the surface and the aryl group has
been demonstrated,15 indicates the existence of a strong
bonding. The films are rather thin, generally less than 5 nm,
even for long irradiation times.
3+
characterize some possible remaining Ru(bipy)3 as the
amount of solution used was too small.
For longer irradiation times, cyano groups are observed by IR
and through the increase of %N in XPS, the location of the
peak being in agreement with published values for cyano
groups. This observation can be compared with previously
published results concerning the 2,6-dimethylbenzenediazo-
nium salt. When the latter is reduced electrochemically, the
ensuing sterically hindered radical cannot react with the surface,
but it is able to abstract a hydrogen atom from acetonitrile to
give the cyanomethyl radical (·CH2CN). The latter reacts with
the surface to give Surface−CH2CN, and it further attacks the
already grafted nitrile group to give Surface-CH2(NH2)-
[CH2(NH2)]n-CH2−CN.49
The mechanism we propose for the formation of these films
is given in Schemes 5 and 6. It is similar to that operating
during electrografting.
Upon irradiation with visible light, the excited state of
2+
Ru(bipy)3 is formed (R1, Scheme 5) that reduces the
The formation of cyanomethyl radicals likely also occurs in
the present experiments along reaction (R6, Scheme 6). The
issue is to know where they react; they do not form
-[CH2(NH2)n]- chains as previously observed,49 since in such
a case, the-CN bands would have disappeared from the IR
spectra and strong NH deformation bands would have
appeared at ∼1600 cm−1. Therefore, they either react with
the surface, in competition with the aryl radicals, or they react
on attached aryl groups along a SH homolytic aromatic
substitution. Such a substitution requires a further oxidation
step (similar to R5) that can be performed by Ru(bipy)33+. We
have shown above that when the sample is tilted from 90 to 40°
(relative to the surface), the N1s signal decreases slightly,
indicating that the nitrogen atoms are close to the gold surface,
and Br/N increases, indicating that the CH2CN groups are
closer to the gold surface than the bromoaryl groups. These
observations fit reaction (R9). Therefore, for long reaction
times, a mixed aryl/CH2CN surface seems the most likely
structure (R9, Scheme 6).
diazonium salt to give the aryl radical (R2); this radical reacts
Scheme 5. Mechanism for the Photografting Reaction
F
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Chemistry of Materials
V. CONCLUSION
Article
(h) Marquette, C. A.; Corgier, B. P.; Heyries, K. A.; Blum, L. J. Front.
Biosci. 2008, 13, 3827. (i) Podvorica, F.; Pinson, J. Chem. Soc. Rev.
2005, 34, 429.
Visible light sensitized photografting of aryldiazonium salts
provides homogeneous thin oligoaryl films that have been
characterized by IR, XPS, ellipsometry, and water contact
angles. Such strategy should be amenable to a large set of
substituted aryl groups. Unlike electrografting, photografting
allows the surface modification of non conducting substrates
such as polymers (PVC in this paper). Moreover, unlike
irradiation of charge transfer complexes,26 this process does not
require large amounts of the complexing species, and it
provides thinner films close to a monolayer. Besides, patterned
grafting can be obtained by localized deposition of the reacting
solution or through the use of masks. Therefore, this process
should be useful for the fabrication of sensors, the thin localized
aryl films serving as the anchoring place for the biosensing
molecule.
(11) Wang, X.; Landis, E. C.; Franking, R.; Hamers, R. J. Acc. Chem.
Res. 2010, 43, 1205.
(12) Gross, A. J.; Yu, S. S. C.; Downard, A. J. Langmuir 2010, 26,
7285.
(13) Berisha, A.; Combellas, C.; Hallais, G.; Kanoufi, F.; Pinson, J.;
Podvorica, F. Chem. Mater. 2011, 23, 3449.
(14) By electrochemistry: (a) Delamar, M.; Hitmi, R.; Pinson, J.;
Savea
Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.;
Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. By reducing reagents:
́
nt, J.- M. J. Am. Chem. Soc. 1992, 114, 5883. (b) Allongue, P.;
́
hypophosphorous acid: (c) Masheter, A. T.; Wildgoose, G. G.;
Crossley, A.; Jones, J. G. J.; Compton, R. G. J. Mater. Chem. 2007, 17,
3008. By iron powder: (d) Mevellec, V.; Roussel, S.; Tessier, L.;
́
Chancolon, J.; Mayne-L’Hermite, M.; Deniau, G.; Viel, P.; Palacin, S.
Chem. Mater. 2007, 19, 6323. By ascorbic acid: (e) Mesnage, A.;
Esnouf, S.; Jeg
6229. By reducing substrates: (f) Adenier, A.; Cabet-Deliry, E.;
Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem.
́
ou, P.; Deniau, G.; Palacin, S. Chem. Mater. 2010, 22,
ASSOCIATED CONTENT
* Supporting Information
IR spectra of the sensitizers and of the surfaces modified by
different diazonium salts; rinsing procedures; blank experi-
ments, electrochemical, and XPS experiments. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
́
S
Mater. 2005, 17, 491. (g) Le Floch, F.; Simonato, J.-P.; Bidan, G.
Electrochim. Acta 2009, 54, 3078. By spontaneous dediazonation:
(h) Podvorica, F. I.; Kanoufi, F.; Pinson, J.; Combellas, C. Electrochim.
Acta 2009, 54, 2164. (i) Schmidt, G.; Gallon, S.; Esnouf, S.; Bourgoin,
J.-P.; Chenevier, P. Chem.Eur. J. 2009, 15, 2101. By ultrasonication:
(j) Mangeney, C.; Qin, Z.; Dahoumane, S. A.; Adenier, A.; Herbst, F.;
Boudou, J.-P.; Pinson, J.; Chehimi, M. M. Diamond Relat. Mater. 2008,
17, 1881. (k) Dahoumane, S. A.; Nguyen, M. N.; Thorel, A.; Boudou,
J. P.; Chehimi, M. M.; Mangeney, C. Langmuir 2009, 25, 9633.
(l) Mirkhalaf, F.; Mason, T. J.; Morgan, D. J.; Saez, V. Langmuir 2011,
27, 1853. By heating: (m) Edward, J. J., U.S. Patent 6555175, 2003.
By mechanical grafting: (n) Shi, L.; Sun, T.; Yan, Y.; Zhao, J.; Dong, S.
J. Vac. Sci. Technol. B 2009, 27, 1399. By Microwave: (o) Liu, J.;
Rodriguez i Zubiri, M.; Vigolo, B.; Dossot, M.; Humbert, B.; Fort, Y.;
McRae, E. Nanosci. Nanotechnol. 2007, 7, 3519.
(15) (a) By XPS: Boukerma, K.; Chehimi, M. M.; Pinson, J.;
Blomfield, C. Langmuir 2003, 19, 6333. (b) By Raman: Itoh, T.;
McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894. (c) By Raman:
Laurentius, L.; Stoyanov, S. R.; Gusarov, S.; Kovalenko, A.; Du, R. B.;
Lopinski, G. P.; McDermott, M. T. ACS Nano 2011, 5, 4219. (d) By
ToF-Sims: Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I.
Langmuir 2005, 21, 280. (e) Hinrichs, K.; Roodenko, K.; Rappich, J.;
Chehimi, M. M.; Pinson, J. Analytical methods for the characterization
of aryl layers. In Aryl Diazonium Salts. New Coupling Agents in Polymer
and Surface Science; Chehimi, M. M., Ed.; Wiley-VCH: Weinheim,
Germany, 2012; Chapter 4, pp 71−101.
AUTHOR INFORMATION
Corresponding Author
*E-mail: alain.deronzier@ujf-grenoble.fr (A.D.), jean.pinson@
espci.fr (J.P.).
■
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) Wicks, Z. W., Jr.; Jones, F. N., Pappas, S. P., Wicks, D. A. Organic
Coatings: Science and Technology; Wiley-Interscience: Hoboken, NJ,
2007.
(2) Haensch, C.; Hoeppener, S.; Schubert, U. S. Chem. Soc. Rev.
2010, 39, 2323.
(3) Queffelec, C.; Petit, M.; Janvier, P.; Knight, D. A.; Bujoli, B.
Chem. Rev. 2012, 112, 3777.
(4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides,
G. M. Chem. Rev. 2005, 105, 1103.
(5) (a) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J.
Electrochem. Soc. 1990, 101, 1757. (b) Gallardo, I.; Pinson, J.; Vila,
Phys. Chem. B 2006, 110, 19521.
(6) Maeda, H.; Yamauchi, Y.; Ohmori, H. Curr. Top. Anal. Chem.
2001, 2, 121.
(7) Astudillo, P. D.; Galano, A.; Gonzalez, F. J. J. Electroanal. Chem.
2007, 610, 137.
̀
N. J.
(16) (a) Jiang, D.-e.; Sumpter, B. G.; Dai, S. J. Am. Chem. Soc. 2006,
128, 6030. (b) Jiang, D.-e.; Sumpter, B. G.; Dai, S. J. Phys. Chem. B
2006, 110, 23628. (c) de la Llave, E.; Ricci, A.; Calvo, E. J.; Scherlis, D.
A. J. Phys. Chem.C 2008, 112, 17611. (d) Zhao, J.-x.; Ding, Y.-h. J. Phys.
Chem. C 2008, 112, 13141.
(17) Mahouche-Chergui, S.; Ledebt, A.; Mammeri, F.; Herbst, F.;
Carbonnier, B.; Ben Romdhane, H.; Delamar, M.; Chehimi, M. M.
Langmuir 2010, 26, 16115.
(8) (a) Fellah, S.; Teyssot, A.; Ozanam, F.; Chazalviel, J.-N.;
Vigneron, J.; Etcheberry, A. Langmuir 2002, 18, 5851. (b) Fellah, S.;
Ozanam, F.; Chazalviel, J.-N.; Vigneron, J.; Etcheberry, A.;
Stchakovsky, M. J. Phys. Chem. B 2006, 110, 1665. (c) Fellah, S.;
Amiar, A.; Ozanam, F.; Chazalviel, J.-N.; Vigneron, J.; Etcheberry, A.;
Stchakovsky, M. J. Phys. Chem. B 2007, 111, 1310.
(9) Palacin, S.; Bureau, C.; Charlier, J.; Deniau, G.; Mouanda, B.;
Viel, P. ChemPhysChem. 2004, 5, 1468.
(10) (a) Aryl Diazonium Salts. New Coupling Agents in Polymer and
Surface Science; Chehimi, M. M., Ed.; Wiley-VCH: Weinheim,
(18) (a) Adenier, A.; Cabet-Deliry, E.; Lalot, T.; Pinson, J.;
Podvorica, F. Chem. Mater. 2002, 14, 4576. (b) Matrab, T.;
Chehimi, M. M.; Perruchot, C.; Adenier, A.; Guillez, A.; Save, M.;
Charleux, B.; Cabet-Deliry, E.; Pinson, J. Langmuir 2005, 21, 4686.
(19) (a) Ratheesh Kumar, V. K.; Gopidas, K. R. Chem.Asian J.
2010, 5, 887. (b) Ratheesh Kumar, V. K.; Gopidas, K. R. Tetrahedron
Lett. 2010, 52, 3102. (c) Ratheesh Kumar, V. K.; Krishnakumar, S.;
Gopidas, K. R. Eur. J. Org. Chem. 2012, 2012, 3447.
(20) Corgier, B. P.; Laurent, A.; Perriat, P.; Blum, L. J.; Marquette, C.
A. Angew. Chem., Int. Ed. 2007, 46, 4108.
(21) (a) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem.
2003, 75, 3837. (b) Nielsen, L. T.; Vase, K. H.; Dong, M.;
Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. J. Am. Chem. Soc.
2007, 129, 1888. (c) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica,
́
Germany, 2012. (b) Belanger, D.; Pinson, J. Chem. Soc. Rev. 2011,
40, 3995. (c) Gooding, J. J.; Ciampi, S. Chem. Soc. Rev . 2011, 40, 2704.
(d) Mahouche-Chergui, S.; Gam-Derouich, S.; Mangeney, C.;
Chehimi, M. M. Chem. Soc. Rev. 2011, 40, 4143. (e) Aswal, D. K.;
Koiry, S. P.; Jousselme, B.; Gupta, S. K.; Palacin, S.; Yakhmi, J. Phys. E
2009, 41, 325. (f) Barrier
2008, 12, 1231. (g) McCreery, R. L. Chem. Rev. 2008, 108, 2646.
̀
e, F.; Downard, A. J. J. Solid State Electrochem.
G
dx.doi.org/10.1021/cm3032994 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
F. I. J. Am. Chem. Soc. 2008, 130, 8576. (d) Leroux, Y. R.; Fei, H.;
(38) Spectral Data Base For Organic Compounds SDBS: http://
riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
(39) Socrates, G. Infrared and Raman Characteristic Group
Frequencies, 3rd ed; John Wiley: New York, 2001.
(40) (a) Podvorica, F. I.; Kanoufi, F.; Pinson, J.; Combellas, C.
Electrochim. Acta 2009, 54, 2164. (b) Lehr, J.; Williamson, B. E.; Flavel,
B. S.; Downard, A. J. Langmuir 2009, 25, 13503.
(41) Larsson, R.; Folkesson, B. Chem. Scr. 1976, 9, 148.
(42) (a) Beamson, G.; Briggs, D. High Resolution XPS of Organic
Polymers: the Scienta ESCA300 Database; Wiley: Chichester, U.K.,
1992. (b) Takahagi, T.; Shimada, I.; Fukuhara, M.; Morita, K.; Ishitani,
A. J. Polym. Sci. Part A 1986, 24, 3101.
Noel, J.-M.; Roux, C.; Hapiot, P. J. Am. Chem. Soc. 2010, 132, 14039.
̈
(22) (a) Ceccato, M.; Bousquet, A.; Hinge, M.; Pedersen, S. U.;
Daasbjerg, K. Chem. Mater. 2011, 23, 1551. (b) Jousselme, B.; Bidan,
G.; Billon, M.; Goyer, C.; Kervella, Y.; Guillerez, S.; Abou Hamad, E.;
Goze-Bac, C.; Mevellec, J.-Y.; Lefrant, S. J. Electroanal. Chem. 2008,
621, 277. (c) Mevellec, V.; Gonzales, J.; Suhr, D. U.S. Patent
Application 0003808 (to Alchimer), 2010. (d) Adenier, A.; Combellas,
C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Chem. Mater. 2006, 18,
2021.
(23) Of diazonium salts: (a) Brooksby, P. A.; Downard, A. J.
Langmuir 2005, 21, 1672. Of vinilics: (b) Duwez, A.-S.; Cuenot, S.;
(43) Nakayama, T.; Inamura, K.; Inoue, Y.; Ikeda, S.; Kishi, K. Surf.
Sci. 1987, 179, 47.
Jer
Nanotechnol. 2006, 1, 122.
(24) (a) Cougnon, C.; Gohier, F.; Bel
́ ̂ ́ ̂
ome, C.; Gabriel, S.; Jerome, R.; Rapino, S.; Zerbetto, F. Nat.
(44) (a) Dobele, M.; Vanderheiden, S.; Jung, N.; Brase, S. Angew.
̈
̈
́
anger, D.; Mauzeroll, J. Angew.
Chem., Int. Ed. 2010, 49, 5986. (b) Heredia-Moya, J.; Kirk, K. L. J.
Chem., Int. Ed. 2009, 48, 4006. (b) Matrab, T.; Combellas, C.; Kanoufi,
F. Electrochem. Commun. 2008, 10, 1230. (c) Hauquier, F.; Matrab, T.;
Kanoufi, F.; Combellas, C. Electrochim. Acta 2009, 54, 5127.
(d) Matrab, T.; Hauquier, F.; Combellas, C.; Kanoufi, F.
ChemPhysChem 2010, 11, 670. (e) Combellas, C.; Kanoufi, F.;
Nunige, S. Chem. Mater. 2007, 19, 3830. (f) Ghorbal, A.; Grisotto, A.;
Charlier, F.; Palacin, J.; Goyer, S.; Demaille, C. ChemPhysChem 2009,
10, 1053.
Fluorine Chem. 2007, 128, 674.
(45) Gam-Derouich, S.; Gosecka, M.; Lepinay, S.; Turmine, M.;
Carbonnier, B.; Basinska, T.; Slomkowski, S.; Millot, M.-C.; Othmane,
A.; Ben Hassen-Chehimi, D.; Chehimi, M. M. Langmuir 2011, 27,
9285.
(46) Packwood, D. M.; Brooksby, P. A.; Abell, A. D.; Downard, A. J.
Aust. J. Chem. 2011, 64, 122.
(47) Weissmann, M.; Baranton, S.; Clacens, J.-M.; Coutanceau, C.
Carbon 2010, 48, 2755.
(48) Gam-Derouich, S.; Carbonnier, B.; Turmine, M.; Lang, P.;
Jouini, M.; Ben Hassen-Chehimi, D.; Chehimi, M. M. Langmuir 2010,
26, 11830.
(25) (a) Garrett, D. J.; Lehr, J.; Miskelly, G. M.; Downard, A. J. J. Am.
Chem. Soc. 2007, 129, 15456. (b) Downard, A. J.; Garrett, D. J.; Tan,
E. S. Q. Langmuir 2006, 22, 10739. (c) Corgier, B. P.; Bel
́
anger, D.
Langmuir 2010, 26, 5991.
(26) Busson, M.; Berisha, A.; Combellas, C.; Kanoufi, F.; Pinson, J.
Chem. Commun. 2011, 47, 12631.
(27) With a few reported exceptions (a) during the spontaneous
(49) Berisha, A.; Combellas, C.; Kanoufi, F.; Pinson, J.; Ustaze, S.;
Podvorica, F. I. Chem. Mater. 2010, 22, 2962.
̀ ́
grafting of diazonium salts on gold: Mesnage, A.; Lefevre, X.; Jegou,
P.; Deniau, G.; Palacin, S. Langmuir 2012, 28, 11767. (b) during the
reduction of diazonium salts by H3PO2: Abiman, P.; Wildgoose, G.
G.; Compton, R. G. Int. J. Electrochem. Sci. 2008, 3, 104.
(28) (a) Cano-Yelo, H.; Deronzier, A. J. Chem. Soc., Perkin Trans. II
1984, 1003. (b) Cano-Yelo, H.; Deronzier, A. Tetrahedron. Lett. 1984,
25, 5517. (c) Cano-Yelo, H.; Deronzier, A. J. Chem. Soc., Faraday
Trans. I 1984, 80, 3011. (d) Cano-Yelo, H.; Deronzier, A. New J.
Chem. 1987, 11, 479. (e) Cano-Yelo, H.; Deronzier, A. J. Photochem.
1987, 37, 315.
(29) The formation of radicals has been reported during the
photosensitized (xanthone) cleavage of diazonium salts: Milanesi, S.;
Fagnoni, M.; Albini, A. J. Org. Chem. 2005, 70, 603.
(30) The intermediacy of the diazenyl radical during photolysis of
azoalkanes31a and (aryl)(arylthio)diazene31b is supported by its
observation by product analysis or rpe, respectively. This is different
from the electrochemical reduction of diazonium salts where the
electron transfer is concerted with the cleavage of nitrogen.32
(31) (a) Engel, P. S.; Gerth, D. B. J. Am. Chem. Soc. 1983, 105, 6849.
(b) Suehiro, T.; Masuda, S.; Nakausa, R.; Taguchi, M.; Mori, A.;
Koike, A.; Date, M. Bull. Chem. Soc. Jpn. 1987, 60, 3321.
(32) Andrieux, C. P.; Pinson, J. J. Am. Chem. Soc. 2003, 125, 14801.
(33) For recent reviews about visible-light photoredox catalysis in
organic synthesis see: (a) Narayanaman, J. M. R.; Stephenson, C. R. J.
Chem. Soc. Rev. 2011, 40, 102. (b) Teply, F. Collect. Czech. Chem
.Commun. 2011, 76, 859. (c) Tucker, J. W.; Stephenson, C. R. J. J. Org.
Chem. 2012, 77, 1617.
(34) (a) Kalianyi, D.; McMurtrey, K. B.; Neufeldt, S. R.; Sanford, M.
S. J. Am. Chem. Soc. 2011, 133, 18566. (b) Hari, D. P.; Schroll, P.;
Konig, B. J. Am. Chem. Soc. 2012, 134, 2958. (c) Schroll, P.; Hari, D.
̈
P.; Konig, B. Chemistry Open 2012, 1, 130. (d) Hering, T.; Hari, D. P.;
̈
Konig, B. J. Org. Chem. 2012, 77, 10347.
̈
(35) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S.
Chem. Mater. 2007, 19, 4570.
(36) Deronzier, A.; Meyer, T. J. Inorg. Chem. 1980, 19, 2912.
(37) Vogel, A. I.; Tatchell, A. R.; Furnis, B. S.; Hannaford, A. J.;
Smith, P. W. G. Vogel’s Textbook of Practical Organic Chemistry, 5th ed;
Longman: Harlow, U.K., 1989.
H
dx.doi.org/10.1021/cm3032994 | Chem. Mater. XXXX, XXX, XXX−XXX