Direct Covalent Grafting of Conjugated Molecules onto Si, GaAs, and Pd Surfaces from Aryldiazonium Salts

Article abstract of DOI:10.1021/ja0383120

Using aryldiazonium salts that are air-stable and easily synthesized, we describe here a one-step, room-temperature route to direct covalent bonds between π-conjugated organic molecules on three material surfaces: Si, GaAs, and Pd. The Si can be in the fo

Full text of DOI:10.1021/ja0383120

Published on Web 12/16/2003
Direct Covalent Grafting of Conjugated Molecules onto Si,
GaAs, and Pd Surfaces from Aryldiazonium Salts
Michael P. Stewart,† Francisco Maya,† Dmitry V. Kosynkin,† Shawn M. Dirk,†
Joshua J. Stapleton,‡ Christine L. McGuiness,‡ David L. Allara,*,‡ and
James M. Tour*,†
Contribution from the Departments of Chemistry and Mechanical Engineering and Materials
Science, and Center for Nanoscale Science and Technology, MS-222, Rice UniVersity,
Houston, Texas 77005, and Department of Chemistry and Department of Materials Science,
PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
Received September 3, 2003; E-mail: tour@rice.edu
Abstract: Using aryldiazonium salts that are air-stable and easily synthesized, we describe here a one-
step, room-temperature route to direct covalent bonds between π-conjugated organic molecules on three
material surfaces: Si, GaAs, and Pd. The Si can be in the form of single crystal Si including heavily doped
p-type Si, intrinsic Si, heavily doped n-type Si, on Si(111) and Si(100), and on n-type polycrystalline Si.
The formation of the aryl-metal or aryl-semiconductor bond attachments was confirmed by corroborating
evidence from ellipsometry, reflectance FTIR, XPS, cyclic voltammetry, and AFM analyses of the surface-
grafted monolayers. A data-encompassing explanation for the mechanism suggests a diazonium activation
by reduction at the open circuit potential, with aryl radical secondary products bonding to the surface. The
synthetic details are included for preparing the surface-grafted monolayers and the precursor diazonium
salts. This spontaneous diazonium activation reaction offers an attractive route to highly passivating, robust
monolayers and multilayers on many surfaces that allow for strong bonds between carbon and surface
atoms with molecular species that are near perpendicular to the surface.
Introduction
these molecular devices, a less polar, more electronically
continuous chemical interface must be identified.7 A direct
Recent advances in “wet” surface chemistry offer an increas-
ingly sophisticated range of techniques for self-orienting mo-
lecular chemisorption on a wide variety of materials.1-3 These
new techniques expand the broad, general applicability of
synthetic chemistry to the heterogeneous phase and improve
the prospects of future “bottom-up” fabrication strategies in
nanotechnology. Specifically, work related to the construction
of post-CMOS hybrid electronic devices using chemical tech-
niques and molecular components4-6 to augment traditional
fabrication schemes will require more control at the molecule/
contact interfaces. In many experimental molecular electronic
systems, molecules assembled between bulk metallic electrodes
have chemical contacts that are highly polar, such as the sulfur-
metal bond. This allows for undesired interfacial capacitance,
possible electrochemical activity at the bond interface, and
generally causes the molecule in the electrode gap to behave
as a tunneling barrier.4 If we are to take further advantage of
the electronic properties of various chemical substituents on
covalent bond, allowing stronger electronic coupling between
the energy bands of a bulk contact and the frontier orbitals of
a conjugated organic molecule, may allow for a greater deal of
synthetic variation in device properties and make contact effects
less dominant.8 Using aryldiazonium salts that are air-stable and
easily synthesized, we describe here a one-step, room-temper-
ature route to direct covalent bonds between fully conjugated
organic molecules and three technologically important material
surfaces: Si, GaAs, and Pd.
The covalent surface chemistry of organic diazonium salts
has been known for some time on electrode surfaces. In the
best understood reactions, a diazonium salt in solution with an
electrolyte such as tetrabutylammonium cation is reduced with
an externally applied potential, typically at around -1 V versus
saturated calomel electrode (SCE), for reaction times of no more
than a few minutes. The aryl radical intermediates that are
locally formed rapidly react with the surface of the cathode.
Electrochemical grafting of aryl groups onto carbon,9 silicon,10
† Rice University.
‡ Pennsylvania State University.
(7) Cahen, D.; Hodes, G. AdV. Mater. 2002, 14, 789.
(8) Nitzan, A.; Ratner, M. A. Science 2003, 960, 153.
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1992, 114, 5883. (b) Kuo, T. C.; McCreery, R. L.; Swain, G. M.
Electrochem. Solid-State Lett. 1999, 2, 288.
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2505.
(4) Tour, J. M. Molecular Electronics; World Scientific: New Jersey, 2003.
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Phys. Lett. 2001, 78, 3735.
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370
J. AM. CHEM. SOC. 2004, 126, 370-378
10.1021/ja0383120 CCC: $27.50 © 2004 American Chemical Society

Direct Covalent Grafting of Conjugated Molecules
A R T I C L E S
Scheme 1. Example of Spontaneous Diazonium Activation by a Hydrogen-Passivated Si(111) Surfacea
a An electron transfer from the surface at the OCP generates a diazenyl radical, and then an aryl radical upon loss of N2. The complementary oxidative
process generates a proton which eliminates as HBF4. The radical generation process results in side products such as reduced aromatics (by aryl radical
attack of the Si-H surface) and the formation of covalently bound multilayers. FTIR evidence shows the loss of an average of 65% of the surface hydride
residues in the process of aryl radical generation. The multilayer formation can be retarded by the addition of BHT in situ, which may intercept the excess
aryl radicals before they can attack the nascent monolayer.
and iron11 electrode surfaces from diazonium salt precursors
has been demonstrated previously by this kind of heterogeneous
phase reduction.12 The grafting of similar groups onto carbon,13-15
carbon nanotube,16,17 and metallic surfaces18,19 has also been
carried out without electrochemical induction, using ancillary
reagents. We now report the activation of aryldiazonium salts20
alone, in the absence of an externally applied potential, or
exogenous reductant species, to assemble covalently bound
conjugated monolayers on hydride-passivated Si, GaAs, and Pd
surfaces. This chemistry has the ability to be applied to samples
where electrochemical means of surface activation are either
unwieldy or impossible, such as isolated, highly nonplanar, or
low-conductive substrates. Similar reactions might be possible
on many other surfaces and materials that have not yet been
studied, including many which have no known covalent attach-
ment chemistry with organic molecules.
experimental evidence. While specific details with regard to this
mechanism may differ from surface to surface, or remain
obscure, centrally we can propose a scheme in which the
diazonium salts are activated at the surface in the absence of
an externally applied potential, quickly forming a reactive
intermediate, followed by surface attachment. This is most likely
carried out by spontaneous electron transfer at the open circuit
potential (OCP) of the substrate material in solution, which leads
to the local generation of aryl radicals by loss of N2,21 some
diffusion of these radicals within the solution, and ultimately
the formation of irreversible surface-molecule bonds (Scheme
1).
Silicon hydride-passivated surfaces are known to activate
metal salts22 and dioxygen23 in solution, undergoing OCP
reduction at the Si(111):H electrode surface. This activation is
also considered to be a corrosion potential, as there is a
concomitant oxidation of the silicon surface in concert with the
reduction. Yet, this technique diminishes incorporation of
oxygen into the surface structure (in the form of SiO2 for Si, or
Ga2O3 and As2O3 for GaAs) and nonisolated contaminants such
as HBF4. This lack of contamination of the interface is
corroborated by X-ray photoelectron spectroscopy (XPS) and
by atomic force microscopy (AFM) studies which show that
surface roughening24 is minimal (vide infra). The required
complementary reverse electron transfer for the diazonium
activation at OCP has not been identified, but possibilities
include backside surface decomposition and the presence of trace
redox species in solution.
Results and Discussion
A tentative mechanism for the spontaneous heterogeneous
activation of diazonium salts is tested and reconciled with our
(11) (a) Adenier, A.; Bernard, M.-C.; Chehimi, M. C.; Cabet-Deliry, E.; Desbat,
B.; Fagebaume, O.; Pinson, J.; Podovorica, F. J. Am. Chem. Soc. 2001,
123, 4541. (b) Chausse, A.; Chehimi, M. M.; Karsi, N.; Pinson, J.;
Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2002, 14, 392. (c) Boukerma,
K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. Langmuir 2003, 19, 6333.
(12) Allongue, P.; Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson,
J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201.
(13) Belmont, J. A. 1996 U.S. Patent 5,554,739.
(14) Ranganathan, S.; Steidel, I.; Anariba, F.; McCreery, R. L. Nano Lett. 2001,
1, 491.
(15) Combellas, C.; Kanoufi, F.; Mazouzi, D.; Thie´bault, A.; Bertrand, P.;
Me´dard, N. Polymer 2003, 44, 19.
(16) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823.
(17) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156.
(18) Fan, F. F.; Yang, J.; Dirk, S. M.; Price, D. W.; Kosynkin, D. V.; Tour, J.
M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 2454.
(21) Zollinger, H. Diazo Chemistry I: Aromatic and Heteroaromatic Com-
pounds; VCH: Weinheim, 1994; p 188.
(22) Gorostiza, P.; Kulandainathan, M. A.; Diaz, R.; Sanz, F.; Allongue, P.;
Morante, J. R. J. Electrochem. Soc. 2000, 147, 1026.
(23) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1679.
(24) Magagnin, L.; Maboudian, R.; Carraro, C. J. Phys. Chem. B 2002, 106,
401.
(19) Kosynkin, D. V.; Yang, J.; Tour, J. M. Mater. Res. Symp. Proc. 2001,
660, JJ3.5.1.
(20) Kosynkin, D. V.; Tour, J. M. Org. Lett. 2001, 3, 993.
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J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004 371

A R T I C L E S
Stewart et al.
Table 1. Calculated and Observed Layer Thicknesses (in nm) for
Diazonium-Reacted Surfaces for Three Different Chemisorbates I,
III, and IIIa
I
II
III
calculated length without
surface bond
1.1
1.8
1.1
n
n
n
2+-Si(111):H
1 h
1.3 ( 0.3
2.5 ( 0.5
1.8 ( 0.1
1.0 ( 0.2
2.8 ( 0.4
1.8 ( 0.2
1.4 ( 0.4
2.1 ( 0.4
1.5 ( 0.1
2.1 ( 0.5
3.1 ( 0.8
2.2 ( 0.2
1.7 ( 0.3
3.0 ( 0.9
2.9 ( 0.2
2.2 ( 0.3
3.3 ( 0.7
2.2 ( 0.1
1.4 ( 0.2
2.1 ( 0.4
1.9 ( 0.2
0.9 ( 0.2
2.5 ( 0.7
1.9 ( 0.2
1.4 ( 0.3
2.3 ( 0.7
1.8 ( 0.2
2+-Si(111):H
6 h
2+-Si(111):H
6 h with 0.01 M BHT
GaAs(100)
1 h
GaAs(100)
6 h
GaAs(100)
6 h with 0.01 M BHT
Pd
Pd
Pd
Figure 1. Reflectance FTIR spectra of ultrathin layers on 2′′ wafers: (a)
compound IV (KBr pellet), (b) polycrystalline Pd wafer reacted with IV,
(c) n-type GaAs(100) wafer reacted with IV, (d) n2+-type Si(111):H wafer
reacted with IV. The spectra show a selective enhancement of vibrational
modes based on molecular orientation. In Figure 1c, molecules attached to
GaAs(100) show less enhancement of the νs(NO2) mode, most likely because
of a slight tilt with respect to the surface. All surface samples were checked
by ellipsometry for monolayer thickness (2 ( 0.2 nm on Si(111):H and
Pd, around 1.7 nm on GaAs(100)).
1 h
6 h
6 h with 0.01 M BHT
a Calculated length values are from Alchemy III software and do not
include the organometallic bond length. All experimental values given are
the average of three identically prepared samples for reactions of 0.5 mM
solutions of diazonium salts in CH3CN, reacted in the dark inside an inert
atmosphere glovebox for the indicated time period. A monolayer of the
chemisorbates tends to form within 1 h, followed by multilayer formation
over extended time periods. Generally, the most defect-free layers were
formed after a reaction time of 1 h, based on CV measurements. The
presence of 0.01 M BHT in situ improves the layer quality by retarding the
formation of multilayers over extended periods but does not prevent the
reaction from occurring entirely.
that is free of As or Ga oxides. The reaction with diazonium
salts has been successfully carried out on a wide range of oxide-
free hydride-passivated28 silicon surfaces: single crystal Si
including doped p-type Si, intrinsic Si, heavily doped n-type
Si, Si(111) and Si(100), and on n-type polycrystalline Si.
Because the presence of an inert passivating layer, such as
the native oxide of Si or GaAs, prevents any stable layers from
forming, the native oxides must be removed by fluoride or
ammonium hydroxide to see any reaction. This implies that a
chemically reactive surface plays a part in the activation or bond-
forming mechanism. Moreover, the reaction shows sensitivity
to the presence of a radical scavenger like butylated hydroxy-
toluene (BHT), added in small amounts (ca. 0.1 mM), which
retards the formation of organic multilayers but does not prevent
the reaction from occurring (Table 1). A high-quality monolayer
of the chemisorbates tends to be complete within 2 h under
nitrogen atmosphere, according to ellipsometry and cyclic
voltammetry (CV). The observed ellipsometric thickness for the
monolayer corresponds to a near-vertical single molecular
length. In light of these results regarding the BHT retarding-
effect, it is implied that the multilayer formation on Si, Pd, and
GaAs surfaces is radical-mediated.
For the assembly procedure, a cleaned surface is exposed to
a solution of the diazonium salt (I-VI) in anhydrous acetonitrile
(CH3CN), in the dark, over a period of 2 h under an inert
atmosphere. The presence of atmospheric oxygen seems to
promote the formation of disordered multilayers, which have
been observed in the electrochemical reactions as well.25 Pd
samples are freshly sputtered and used without further treatment,
and GaAs samples are chemically cleaned26,27 with concentrated
NH4OH, which results in a metastable, stoichiometric surface
To analyze the samples by Fourier transform infrared
spectroscopy (FTIR), a grazing angle experiment was used with
heavily doped Si and GaAs wafers and sputtered polycrystalline
Pd on Si wafers. The FTIR spectra in Figure 1b-d show
structural features related to the organic chemisorbate IV and
provide qualitative information about its alignment with respect
to the surface. IR reflectance spectra with p-polarized light at
angles of incidence greater than Brewster’s angle on dielectric
surfaces, such as Si and GaAs (Figure 1c and d), yield negative
absorbance peaks and vibrational modes with components
perpendicular to the substrate surface.29 As compared to the
structurally isotropic KBr pellet spectrum of the diazonium salt
IV (Figure 1a), the monolayer spectra (Figure 1b-d) are
dominated by vibrational modes with components perpendicular
to the sample surface.
Qualitatively, this indicates the molecules adopt a more
upright orientation rather than lying down with the molecular
(25) (a) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947. (b) Anariba,
F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837.
(26) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Ma¨rtensson, J.; Allara, D. L.
Jpn. J. Appl. Phys. 1991, 30, 3759.
(27) Sheen, C. W.; Shi, J. X.; Ma¨rtensson, J.; Allara, D. L. J. Am. Chem. Soc.
1992, 114, 1514.
(28) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys.
Lett. 1990, 56, 656.
(29) Brunner, H.; Mayer, U.; Hoffman, H. Appl. Spectrosc. 1997, 51, 209.
9
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Direct Covalent Grafting of Conjugated Molecules
A R T I C L E S
-NO2 group at 406.0 eV. The Pd 3d region and Si 2p regions
(Figure 2b) of the XPS spectra indicate no oxide present in Pd
or Si samples, respectively. For GaAs samples, the As 3d region
sometimes shows a small amount of AsIII oxide. If the samples
are prepared in O2-containing atmospheres, the degree of
oxidation in Si and GaAs samples increases slightly. Samples
prepared with nonfluorinated diazonium salt III show only one
signal in the C 1s region and no signal in the F 1s region (see
Supporting Information).
Cyclic voltammetry (CV) analysis gives evidence of consid-
erable physical passivation against ion currents on the Si and
Pd surfaces reacted with II (Figure 3a and b). Our n-type GaAs
samples showed passivation for only a few sweeps. The currents
for the Pd and Si samples are both relatively potential-insensitive
3-/4-
against the Fe(CN)6
couple, implying an overall low
concentration of defects and pinholes in the films. For the GaAs
samples, the putative Ga-C or As-C bond may not be
chemically stable enough to allow good passivation against ion
currents.
backbone parallel to the substrate surface. The most prominent
features are the in-plane ring breathing ν(CdC) modes at around
1525 and 1503 cm-1, the antisymmetric NO2 stretch νas(NO2)
mode at 1518 cm-1 (partially obscured by the ring breathing
mode at 1525 cm-1), the symmetric NO2 stretch νs(NO2) mode
at 1351 cm-1, and an in-plane aromatic C-H deformation γip-
(C-H) mode at 995 cm-1, which are seen in FTIR spectra of
all three substrates; the strong breathing mode near 1600 cm-1
and an out-of-plane aromatic C-H deformation γop(C-H) mode
at 825 cm-1 are present only in very weak intensity. The notable
absence of vibrational features related to the diazonium tet-
rafluoroborate moiety on any of the samples, such as the ν-
(NtN+) peak near 2280 cm-1 and the strong and broad ν(BF4)
mode expected at around 1050 cm-1 (Figure 1a), suggests that
it is no longer associated with the molecular layer. In Figure
1d, the spectrum for the assembly of IV on Si(111):H, loss of
surface hydride residues is evident in a upward sharp ν(Si-H)
stretching peak at 2080 cm-1. The peak is inverted as compared
to the molecular signatures because a freshly hydride-terminated
Si(111) wafer is used as the reference sample. No silicon oxide
signal is seen in the spectrum; if present, a large ν(SiOx) peak
around 1050 cm-1 and an oxide-back-bonded ν(OxSi-H)
stretching peak at around 2220 cm-1 would be visible. Com-
parison of the Si stretching regions of Si(111):H samples before
and after reaction shows an average 65% net consumption of
the surface hydride, indicating a maximal surface coverage on
the part of the phenyl groups.10c The hydride groups are possibly
lost during the grafting reaction by aryl radical abstraction
(leading to soluble side products) or by elimination as HBF4.
In either case, it is direct proof that the activation of the
diazonium salt and the subsequent formation of organic layers
on the silicon surface cause chemical changes at the Si-H level
of the Si(111):H surface without incurring direct oxidation.
Oxide-free organic functionalization of the Si surface is
corroborated by XPS. A Si(111) sample reacted with II shows
two peaks in the C 1s region, the smaller of the two being from
the fluorocarbon terminus appearing at higher binding energy
(Figure 2a), and a single peak in the F 1s region around 688.3
eV (see Supporting Information) corresponding to the pen-
Because of the relative ease of preparing atomically flat Si-
(111):H surfaces30 from wet treatment in 40% NH4F, Si samples
reacted with diazonium compounds were checked for surface
roughness effects by AFM. Our freshly made Si(111):H samples
typically showed RMS roughness of 0.2 nm or less over 1 µm2
areas, with atomic step edges being clearly visible. Samples
reacted with diazonium salts using the normal conditions (0.5
mM for 2 h) showed slight increases in surface roughness to
around 0.4 nm RMS, with complete loss of the atomic step edge
features. It was then found that short reactions using more dilute
solutions than normal (around 0.01 mM) left the surface sparsely
reacted, enabling small vertical islands to be visualized (Figure
4a and b). A series of diazonium salts of different lengths were
reacted with the flat surfaces to systematically observe these
molecular features. Surfaces made with dilute solutions of
diazonium salts V (observed height approximately 0.7 nm,
although at this short length confusion with step edge features
and defects is likely), VI (observed height approximately 1.5
nm), and II (observed height approximately 2.0 nm) showed
these features consistently, with vertical heights corresponding
to single perpendicular molecular lengths. The lateral dimension
of the islands is limited by the radius of curvature of the AFM
tip, which was typically around 30-40 nm in our images.
Although AFM does not give chemical information within the
images, given the XPS spectra that show the molecular
signatures and no detectable Si oxide, it is plausible to affirm
that the features are nucleation sites for film growth and not
merely etch pits or oxide.
The exceptional chemical stability of the layers is indirect
evidence of covalent bonding. Samples of the various substrates
were reacted with II, exposed to various cleaning or etching
treatments, and analyzed with ellipsometry and XPS. Pd samples
reacted with II showed no loss of the molecular layers upon
sonication in CH3CN (10 min). If the molecules were simply
physisorbed on the surface, this treatment would likely remove
most of the organic residue. Similarly reacted GaAs samples
were resistant to concentrated HCl etching solution (5 min),
30% aqueous citric acid etch solution, and concentrated NH4-
OH followed by sonication in CH3CN. Silicon samples reacted
-
tafluorophenyl terminus. Clearly, the absence of the BF4
counterion which composes part of the diazonium salt (typically
appearing at about 686.5 eV) is evidenced. There is no N 1s
signal for -N2+ (approximately 403.8 eV) in any of the spectra,
indicating that the diazonium moiety has been lost, although
assemblies of IV on all surfaces showed a clear signal for the
(30) Allongue, P.; de Villeneuve, C. H.; Morin, S.; Boukherroub, R.; Wayner,
D. D. M. Electrochim. Acta 2000, 45, 4591.
9
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A R T I C L E S
Stewart et al.
Figure 2. XPS spectra of reacted samples with monolayer thickness. (a) Spectrum of the C 1s region of the Pd sample reacted with II, taken at a 45° takeoff
angle. The small peak at 288 eV is due to the pentafluorinated phenyl ring. (b) The spectrum of the Si 2p region of n2+-type Si(111) reacted with II, taken
at a 30° takeoff angle, has a clear 2p1/2-2p3/2 doublet and is free of the oxidation signal normally seen at around 103 eV.
redox couple in 0.1 M KClO3, with a 1 cm2 area exposed. (a) Pd sample reacted with
3/4-
Figure 3. CV scans of reacted samples using a 0.01 M Fe(CN)6
II (solid line) as compared to clean Pd (dashed line). (b) n2+-type Si(111) wafer reacted with II (solid line) as compared to the Si(111):H surface (dashed
line).
Figure 4. Tapping mode AFM images of sparsely reacted Si(111) surfaces before and after reaction with diazonium salt II, showing initial nucleation of
perpendicularly oriented features of single molecular height. Scale bar is approximate. (a) Hydride-terminated Si(111) sample showing clear atomic step
edges and RMS roughness of 0.2 nm over 1 µm2. The vertical height of specklike features at step edge tips is less than 0.4 nm. (b) Si(111):H sample reacted
for 15 min with 0.5 mM diazonium compound II shows initial growth of perpendicular molecules on step edges. The vertical height of the island is approximately
2.0 nm (Section Analysis, based on dashed blue line) measured from the terrace side of the feature. The lateral resolution of the molecular features is limited
by the radius of curvature of the AFM tip, as depicted in the scheme.
with II were resistant to 7:1 buffered oxide etch solution (5
min), followed by pH 12 KOH solution at 25 °C (5 min),
followed by sonication (10 min) in CH3CN, CH2Cl2, or acetone.
Because of the chemical endurance of the surface-molecule
bonding, there was little or no loss of the chemisorbed molecular
species or substantial oxidation of the substrate from any of
these treatments.
Ellipsometric thickness measurements on monolayer films,
FTIR spectroscopy, and AFM microscopy all provide evidence
that implies that near-vertically oriented molecules are on the
Si(111) surface. This is consistent with a covalent attachment
between the aryl groups and the surface that would not allow
for a large degree of deformation. XPS spectra show a low
degree of chemical contamination within the attached layers and
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Direct Covalent Grafting of Conjugated Molecules
A R T I C L E S
at the critical surface-molecule interface. When combined with
chemical tests that verify the robustness of the attachment at
that interface, an overall review of the data is consistent with
our model of covalently bound monolayers in high coverage
on the surfaces in question. Now, we must use this model to
justify our theory of how this reaction may activate the
diazonium salts and form the layers.
on the multilayer formation implies that aryl radical intermedi-
ates play a significant role in the surface reaction, which would
be a secondary product of spontaneous electron transfer (the
direct product being a short-lived diazenyl radical). This effect
was most dramatic for diazonium assemblies on Pd and Si, and
less so for the GaAs surface.
This spontaneous diazonium activation reaction offers an
attractive route to highly passivating, robust monolayers and
multilayers on many surfaces that allow for strong bonds
between carbon and surface atoms. A data-encompassing
explanation for the mechanism calls for an OCP-driven diazo-
nium activation with aryl radical secondary products attacking
the surface and forming the bonds. Covalently bound mono-
layers on Pd, Si, and GaAs offer possibilities for new applica-
tions in processing, electronic transport, and antistiction coatings
for polysilicon-based MEMS devices, as well as a platform to
study through-bond interactions between molecules chemisorbed
on surfaces and the bulk materials to which they are attached.
Current work includes improving the long-range order of the
surface assemblies, determining molecular effects on the
electronic structure of the interfaces, and adapting molecular
assemblies with reported negative differential resistance (NDR)
properties into silicon-based hybrid molecular electronic devices
to assess molecular versus interfacial-based device behavior.4,8
Determining a consistent mechanism of the activation of the
diazonium compounds is complicated by the broad chemical
and physical differences between the metal, elemental semi-
conductor, and binary semiconductor materials used in this
study. It is tempting, especially when considering the hydridic
Si(111):H surface, to propose a cation exchange or charge
injection mechanism for the aryl-surface bond-forming reaction.
This might be qualitatively similar to charge injection from the
ferrocenium cation as a surface activation method.31 In such a
scheme (Scheme 1), elimination of N2 from the aryl diazenyl
radical would give an aryl cation intermediate, which would
attack the Si(111):H surface, and then eliminate a proton and
the counterion as HBF4. However, reactions of diazonium salts
with silanes,32 that likely do proceed via the aryl cation, tend
to simply abstract hydride and form reduced aromatics and silyl
halides. Although the hydride abstraction mechanism is a
possibility, and would explain the very high degree of Si-H
loss seen in FTIR spectra, inorganic surface fluoride from the
counterion is something that we have not systematically
observed on any of our surfaces. It also would not explain the
formation of Si-C or other organometallic bonds. Finally, such
a mechanism would not show significant sensitivity to the
presence of BHT. As the major component of spontaneous
diazonium activation, hydride abstraction can be ruled out.
Combellas and coauthors15 have proposed an OCP-based
mechanism for the spontaneous reduction of diazonium salts
on a carbonaceous material made from an electrochemically
reduced PTFE electrode surface. Is such a mechanism plausible
for materials such as Si, GaAs, and Pd? The tendency of
metastable surfaces such as Si(111):H to oxidize spontaneously,
at OCP, provides a complementary means of reduction, as seen
in Wade and Chidsey’s experiments on the OCP-driven reduc-
tion of dioxygen to superoxide in fluoride solution.23 If the OCP
of the Si(111):H surface33 in contact with 0.5 mM diazonium
salt in CH3CN is more negative than the reduction potential of
the diazonium itself, about +0.3 V versus SCE,34 it is possible
to have the spontaneous reduction. Yet, unlike the dioxygen-
superoxide system or reactions with metal salts,24 we do not
generally see the expected surface oxidation or major surface
etching that might accompany the spontaneous reduction. The
reverse electron transfer could instead have effects that were
beyond our ability to analyze, such as a roughening of the
opposite etched side or cleaved edges of the silicon wafer or
reaction with a solution impurity species that we could not
detect.
Experimental Section
Reagents and Solvents for Surface Reactions. Acetonitrile (CH3-
CN, 99.5+%) for surface reactions was purchased from Aldrich packed
under nitrogen in a SureSeal container. CH3CN, dichloromethane (CH2-
Cl2), ethanol (EtOH), methanol (MeOH), and water used for rinsing
were purchased at HPLC grade and used without further purification.
Concentrated ammonium fluoride (NHF4) was purchased at VLSI grade
from J. T. Baker. Concentrated hydrochloric acid (HCl), concentrated
NHF4, concentrated ammonium hydroxide (NH4OH), concentrated
sulfuric acid (H2SO4), 49% hydrofluoric acid (HF), and 30% hydrogen
peroxide (H2O2) were purchased at reagent grade. Before use, all
diazonium salts were stored under nitrogen in tightly capped vials, in
the dark at -30 °C.
Ellipsometric Measurements. Measurements of surface optical
constants and molecular layer thicknesses were taken with a single
wavelength (632.8 nm laser) Gaertner Stokes ellipsometer. The n value
for the clean Pd surface was 1.9, and k was -4.2. The n value for the
Si(111):H surface was 3.87, and k was -0.04. The n value for the
clean GaAs surface was 3.85, and k was -0.2. The surface thickness
was modeled as a single absorbing layer atop an infinitely thick substrate
(fixed ns). The observed error in repeated measurements of the same
spot was typically 0.2 nm or less.
Cyclic Voltammetry (CV) Measurements. Electrochemical char-
acterization was carried out with a Bioanalytical Systems (BAS CV-
50W) analyzer. The reference was a saturated calomel electrode (SCE).
The counterelectrode was a clean Pt wire. The aqueous redox couple
3/4-
and electrolyte were 0.01 M Fe(CN)6
in 0.1 M KClO4. Ap-
proximately 1 cm2 of sample was exposed to solution during CV
measurements. The scan rate was 100 mV s-1 from -200 to 600 mV.
Atomic Force Microscopy (AFM) Measurements. A Digital
Instruments Nanoscope IIIa tapping mode instrument was used with
freshly CHCl3-cleaned TESP tips. Images were taken in air without
using a purge box. Samples were prepared as normal and loaded into
the AFM within a few minutes of their final rinsing. Samples prepared
for Figure 4 were reacted for e15 min using approximately 0.05 mM
diazonium solution in CH3CN in a glovebox.
Nevertheless, the diazonium salts react with the surfaces and
cleanly eliminate the N2 and BF4- byproducts from the surface,
leaving no analytical traces on the surface. The effect of BHT
(31) Haber, J. A.; Lauermann, I.; Michalak, D.; Vaid, T. P.; Lewis, N. S. J.
Phys. Chem. B 2000, 104, 9947.
(32) Nakayama, J.; Yoshida, M.; Simamura, O. Tetrahedron 1970, 26, 4409.
(33) Zhang, X. G. Electrochemistry of Silicon and its Oxide; Kluwer Aca-
demic: New York, 2001.
X-ray Photoelectron Spectroscopy (XPS) Measurements. A
Physical Electronics (PHI 5700) XPS/ESCA system at 5 × 10-9 Torr
was used to take photoelectron spectra. A monochromatic Al X-ray
(34) Elofson, R. M.; Gadallah, F. F. J. Org. Chem. 1969, 34, 854.
9
J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004 375

A R T I C L E S
Stewart et al.
source at 350 W was used with an analytical spot size of 1.2 mm2 and
a 45° or 30° takeoff angle, with a pass energy of 11.75 eV.
General Procedure for the Coupling of a Terminal Alkyne with
an Aryl Halide Utilizing a Palladium-Copper Cross-Coupling
(Castro-Stephens/Sonogashira Protocol).37,38 To an oven-dried screw
cap tube or a round-bottom flask and a magnetic stir bar were added
the aryl halide, bis(triphenylphosphine)palladium(II) dichloride (5 mol
% based on aryl halide), and copper(I) iodide (10 mol % based on aryl
halide). The vessel was then sealed with a rubber septum, evacuated,
and backfilled with nitrogen (3×). A cosolvent of THF was added
followed by Hu¨nig’s base. The terminal alkyne was added, and the
reaction was heated, if necessary, until complete. The reaction vessel
was cooled to room temperature and quenched with water. The organic
layer was diluted with CH2Cl2 and washed with a saturated solution of
NH4Cl (3×). The combined aqueous layers were extracted with CH2-
Cl2 (3×). The combined organic layers were dried over anhydrous
MgSO4, and the solvent was removed in vacuo. The crude product
was then purified by flash or column chromatography (silica gel).
General Procedure for Alkaline Deprotection of Trimethylsilyl-
Protected Alkynes. The TMS-protected alkyne was added to an open
round-bottom flask equipped with a stirring bar and a solution of
potassium carbonate in MeOH or tetrabutylammonium fluoride (TBAF)
buffered with a mixture of acetic acid (AcOH) and acetic anhydride
(Ac2O). THF and CH2Cl2 were added to dissolve the organic compound.
The reaction was monitored by TLC every 5 min until deprotection
was completed. The reaction was quenched with water and extracted
with organic solvents (3×). The combined organic layers were dried
over anhydrous MgSO4, and the solvent was removed in vacuo. The
crude product was then purified by flash chromatography.
FTIR Measurements. A customized analytical system, based on a
Digilab FTS-7000 bench, was used, whose basic details are described
in a previous publication.35 FTIR spectra were obtained under an
extended dry air purge using a liquid N2 cooled wide-band MCT
detector. External reflection p-polarized spectra used 1200 scans at 2
cm-1 resolution at an 86° angle of incidence. A multipoint baseline
correction and H2O and CO2 subtractions in GRAMS/32 are used for
qualitative and presentation purposes.
Surface Preparation and Optical Constants. Pd samples were
deposited by ion mill sputtering onto a 2-in. undoped oxidized Si wafer
at 0.1 Å s-1 until a final thickness of 2000 Å was reached. No surface
adhesion layer was used. The Pd samples were reacted within 10 min
after coming out of the vacuum chamber, without any surface cleaning.
Highly doped 2-in. n-type Si(111) wafers (prime grade, As doped,
0.001-0.005 Ω cm) were first cleaned for 20 min in 2:1 H2SO4/H2O2
(piranha solution) followed by rinsing copiously with water and drying
in a stream of N2. The wafer was then hydride-terminated by immersion
in N2-sparged concentrated (40%) NHF4 for 15 min, rinsed with water,
and dried in a stream of N2. Undoped GaAs(100) samples were cut
(about 4 cm2) from a 3-in. wafer, sonicated in EtOH for 15 min, and
UV/O3 cleaned for 15 min. The oxidized GaAs shards were then treated
with concentrated (40%) NH4OH for 1 min, followed by a brief rinse
in water, then EtOH, and finally a gentle stream of N2. XPS experiments
on GaAs used shards from a lightly Te-doped GaAs(100) wafer
prepared and characterized with the same protocols.
Reactions with Diazonium Salts. The cleaned, prepared samples
were brought inside a low-oxygen N2-atmosphere glovebox (with levels
of 1 ppm H2O and O2). Inside the glovebox, a solution of the diazonium
salt was made to 0.5 mM concentration in CH3CN, providing enough
volume to completely cover the entire sample inside a glass screw-cap
Nalgene jar. To adequately cover a 2-in. wafer, at least 10 mL must be
prepared. For the smaller shards of GaAs, 5 mL of solution is sufficient.
The surface samples are immersed in the diazonium solution, sealed
to prevent evaporation, and covered with foil to prevent light exposure.
The reaction time was 2 h, although shorter reaction times may be
possible. Reaction times longer than 6 h tended to create multilayers
up to 3.5-5 nm thick, depending on the molecule that was used (layer
thicknesses were determined by ellipsometry). At the end of the reaction,
the samples were brought out of the glovebox, rinsed with CH3CN
and soaked for 5 min (to remove residual diazonium salt), and then
rinsed with CH2Cl2 (to remove physisorbed hydrocarbons) and soaked
for 1 min. The samples were removed from the CH2Cl2 and then dried
thoroughly with N2.
General Procedure for Diazotization of Anilines.39 Into an oven-
dried round-bottom flask equipped with a magnetic stir bar was added
nitrosonium tetrafluoroborate (1.1 equiv per aniline) in the drybox, and
the vessel was sealed with a rubber septum. CH3CN was added to
dissolve the salt and cooled to -30 °C. Separately, into an oven-dried
round-bottom flask equipped with a magnetic stir bar was added the
aniline that was dissolved in a minimum amount of CH3CN. The organic
solution was transferred dropwise via cannula, and the temperature was
allowed to rise to 0 °C. Et2O was slowly added until a precipitate came
out from the slurry. The precipitate was collected by filtration, washed
with Et2O (3×), and dried.
4-Styrylbenzenediazonium Tetrafluoroborate (I). To a 25 mL
round-bottom flask in a drybox was added NOBF4 (0.1 g, 0.95 mmol).
Acetonitrile (2 mL) was added, and the flask was cooled to -30 °C. A
solution of 4-styrylaniline40 (0.17 g, 0.86 mmol) and BHT (0.19 g, 0.86
mmol) in acetonitrile (5 mL) was added dropwise. The reaction was
allowed to warm to -5 °C over 20 min. Et2O (10 mL) was added, and
the precipitate was filtered. The title compound was purified by
reprecipitating from CH3CN (3 mL) with ether (15 mL) to yield 0.82
g (34%) of the desired compound. IR (KBr): 3360, 3108, 2271, 1654,
1581, 1497, 1497, 1421, 1079 cm-1. 1H NMR (400 MHz, CD3CN): δ
8.50 (d, J ) 8.8 Hz, 2H), 8.39 (d, J ) 8.8 Hz, 2H), 8.13 (d, J ) 8.8
Hz, 2H), 7.77 (d, J ) 8.8 Hz, 2H), 7.65 (d, J ) 16.8 Hz, 1H), 7.53 (d,
J ) 16.8 Hz, 1H). 13C NMR (100 MHz, CD3CN): δ 159.8, 155.3,
150.1, 133.3, 133.7, 130.0, 112.9, 112.22.
Synthetic Materials and General Synthetic Procedures. Unless
stated otherwise, reactions were performed in dried glassware under
dry nitrogen atmosphere. Reagent grade diethyl ether (Et2O) and
tetrahydrofuran (THF) were distilled from sodium benzophenone ketyl.
N,N-Diisopropylethylamine (Hu¨nig’s base), triethylamine (TEA), and
N,N-dimethylformamide (DMF) were distilled from calcium hydride.
Reagent grade hexanes, CH2Cl2, MeOH, EtOH, and ethyl acetate
(EtOAc) were used without further distillation. Trimethylsilylacetylene
(TMSA) was donated by FAR Research. All other commercially
available reagents were used as received. Unless noted, reactions were
magnetically stirred and monitored by thin-layer chromatography (TLC)
using E. Merck silica gel 60 F254 precoated plates (0.25-mm). In general,
the chromatography guidelines reported by Still were followed.36 Flash
chromatography (silica gel) was performed with the indicated solvent
4-Pentafluorophenylethynylaniline. Following the Sonogashira
coupling protocol, pentafluorobromobenzene (9.6 mL, 77 mmol), PdCl2-
(PPh3)2 (2.7 g, 3.9 mmol, 5 mol %), and CuI (1.5 g, 7.7 mmol, 10 mol
%) were dissolved in THF (100 mL). Hu¨nig’s base (53 mL, 307 mmol)
was added, followed by 4-ethynylaniline41 (9 g, 77 mmol), and the
1
systems using silica gel grade 60 (230-400 mesh). H and 13C NMR
spectra were taken at 400 and 100 MHz, respectively, and chemical
shifts are reported relative to CD3CN (1H, 1.96, 13C, 118.26) except
when specified. All new compounds were named using AutoNom v.
2.2 under license of Beilstein Informationssysteme.
(37) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 50, 4467.
(38) Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313.
(39) Kosynkin, D. V.; Tour, J. M. Org. Lett. 2001, 3, 993.
(40) Ziegler, C. B.; Heck, R. F. J. Org. Chem. 1978, 43, 2941.
(41) Mongin, O.; Gossauer, A. Tetrahedron 1997, 53, 6835.
(35) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927.
(36) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923.
9
376 J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004

Direct Covalent Grafting of Conjugated Molecules
A R T I C L E S
mixture was left overnight at 75 °C. Purification by flash chromatog-
raphy (CH2Cl2 eluent) afforded the desired product (4.4 g, 21% yield)
as a dark yellow solid. IR (KBr): 3482, 3394, 2206, 1610, 1501, 1446,
7.49 (m, 3H), 7.67 (m, 2H). 13C NMR (100 MHz, CD3CN): δ 137.1,
134.6, 134.0, 133.1, 131.4, 129.8, 121.9, 114.3, 101.5, 88.2.
1
1290, 1176, 1133, 1033 cm-1. H NMR (400 MHz, CDCl3 δ 7.27):
7.37 (m, 2H), 6.65 (m, 2H), 3.93 (br s, 2H). 13C NMR (100 MHz,
CDCl3 δ 77.23): 148.0, 133.6, 114.8, 110.8, 103.0, 71.4. HRMS calc’d
for C14H6F5N, 283.0420; found, 283.0423.
2-Nitro-4-pentafluorophenylethynylaniline. Following the general
coupling procedure, 4-ethynyl-2-nitroaniline44 (6.2 g, 38.7 mmol), PdCl2-
(PPh3)2 (1 g, 1.5 mmol), CuI (0.6 g, 3 mmol), and TEA (27 mL, 155
mmol) were dissolved in THF (50 mL). Pentafluorobromobenzene (19
g, 77.3 mmol) was added, and the reaction was heated at 70 °C
overnight. Purification by flash chromatography (CH2Cl2) afforded the
desired product (3.7 g, 30%) as an orange solid. IR (KBr): 3468, 3364,
3099, 2469, 2428, 2221, 1933, 1801, 1728, 1631, 1583, 1550, 1413,
1338, 1284, 1243, 1163, 1117, 1077, 1033 cm-1. 1H NMR (400 MHz,
CDCl3 7.27): δ 8.32 (s, 1H), 7.62 (d, J ) 8.8, 1H), 7.53 (br s, 2H),
7.21 (d, J ) 8.8, 1H). 13C NMR (100 MHz, CDCl3 77.23): δ 147.2,
138.0, 131.3, 130.1, 120.3, 108.4, 100.8, 71.8, 68.9. HRMS for
C14H5F5N2O2, 328.0271; found, 328.0273.
4-Pentafluorophenylethynylbenzenediazonium Tetrafluoroborate
(VI). Following the general procedure for diazotization, 4-pentafluo-
rophenylethynylaniline (400 mg, 1.4 mmol) was dissolved in CH3CN
(19 mL) and submitted under reaction with NOBF4 (182 mg, 1.5 mmol).
The desired product (487 mg, 91%) was isolated as a light yellow solid.
IR (KBr): 3421, 2950, 2883, 2356, 2161, 1475, 1372, 1244, 1183 cm-1
.
1H NMR (400 MHz, (CD3)2SO δ 2.54): δ 8.75 (d, J ) 9.2 Hz, 2H),
8.21 (d, J ) 9.2 Hz, 2H). 13C NMR (100 MHz, (CD3)2SO δ 40.4):
135.1, 134.4, 133.0, 117.8, 99.0, 82.9.
Pentafluoro-6-(4-iodo-3-nitrophenylethynyl)benzene. A solution
of aniline 2-nitro-4-pentafluorophenylethynylaniline (2.9 g, 9 mmol)
in CH3CN (40 mL) was slowly added to NOBF4 (1.5 g, 13.5 mmol) in
CH3CN (10 mL) at -20 °C. The resulting yellow solution was warmed
to 0 °C, stirred for 30 min, and transferred via syringe into a flask
containing a solution of NaI (2.7 g, 18 mmol) and I2 (4.5 g, 18 mmol)
in acetonitrile (50 mL). When the vigorous gas evolution was complete,
the reaction mixture was diluted with water (500 mL), treated with
urea (1.8 g, 30 mmol) and Na2SO3 (1.3 g, 11 mmol), and extracted
with ether. The extracts were washed with water (50 mL) and brine
(50 mL) and were evaporated. The crude product was purified by
chromatography (CH2Cl2) to give lemon yellow needles (3.5 g, 90%
yield). IR (KBr): 3093, 2879, 2629, 2428, 2229, 1940, 1780, 1594,
1518, 1501, 1443, 1356, 1266, 1151, 1036, 1017 cm-1. 1H NMR (400
MHz, CDCl3): δ 8.07 (d, J ) 8.0 Hz, 1H), 8.02 (d, J ) 1.6 Hz, 1H),
7.42 (dd, J ) 8.4, 2.0 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 165.9,
142.4, 135.8, 128.3, 123.2, 117.7, 89.6, 87.8. HRMS for C14H3F5INO2,
438.9129; found, 438.9125.
4-(4-Pentafluorophenylethynylphenylethynyl)aniline. 4-(4-Io-
dophenylethynyl)aniline42 (0.31 g, 1.0 mmol), PdCl2(PPh3)2 (28 mg,
0.04 mmol), CuI (8 mg, 0.04 mmol), TEA (2 mL), THF (2 mL), and
pentafluorophenylacetylene (0.28 g, 1.5 mmol) were used following
the general procedure for couplings. The tube was capped and stirred
at room temperature for 14 h. Flash column chromatography (1:1
hexanes:CH2Cl2) afforded the desired product as light yellow needles
(0.16 g, 43% yield). IR (KBr): 3417, 2206, 1623, 1595, 1526, 1497,
1446, 1298, 1138 cm-1. 1H NMR (400 MHz, CDCl3 7.27): δ 7.47 (m,
4H), 7.34 (m, J ) 8.7, 2.5, 1.8, 0.6 Hz, 2H), 6.64 (m, J ) 8.7, 2.5, 1.8,
0.6 Hz, 2H), 3.85 (br s, 2H). 13C NMR (100 MHz, CDCl3 77.23): δ
147.2, 133.3, 131.9, 131.8, 131.5, 125.6, 120.5, 114.9, 112.2, 93.3,
87.1, 74.6.
4-(4-Pentafluorophenylethynylphenylethynyl)benzenediazo-
nium Tetrafluoroborate (II). Following the general diazotization
procedure, 4-(4-pentafluorophenylethynylphenylethynyl)aniline (0.15
g, 0.04 mmol) was treated with NOBF4 (52 mg, 0.52 mmol) in CH3-
CN (4 mL) and sulfolane (4 mL). After the mixture was dried in vacuo,
the desired product was isolated (0.12 g, 64% yield) as a yellow solid.
IR (KBr): 3106, 2275, 2214, 1576, 1526, 1502, 1078, 989, 854, 832
4-(2-Nitro-4-pentafluorophenylethynylphenylethynyl)aniline.
4-Ethynylaniline (0.28 g, 2.4 mmol), bis(triphenylphosphine)palladium-
(II) dichloride (70 mg, 0.1 mmol), copper(I) iodide (19 mg, 0.1 mmol),
Hu¨nig’s base (0.51 g, 4.0 mmol), THF (4 mL), and 1-iodo-2-nitro-4-
pentafluorophenylethynylbenzene (0.87 g, 2.0 mmol) were used fol-
lowing the general procedure for couplings. The bright red slurry formed
after 14 h of stirring at room temperature was evaporated in vacuo,
suspended in CH2Cl2 (10 mL), and filtered. The filter cake was washed
with the same solvent (15 mL), dried, and recrystallized from THF-
heptane to afford red silky needles (0.74 g, 87% yield). IR (KBr): 3505,
1
cm-1. H NMR (400 MHz, (CD3)2SO δ 2.54): δ 8.71 (m, 2H), 8.15
(m, 2H), 7.74 (m, 4H). 13C NMR (100 MHz, (CD3)2SO δ 40.4): δ
133.8, 133.6, 133.0, 132.6, 132.2, 122.1, 114.9, 100.1, 98.3, 89.8, 75.5.
4-Phenylethynylbenzenediazonium Tetrafluoroborate (III). Fol-
lowing the general diazotization procedure, 4-phenylethynylaniline43
(0.57 g, 3 mmol) was treated with NOBF4 (0.36 g, 3.15 mmol) in CH3-
CN (20 mL). Yellow needles of the desired product were precipitated
with ether (0.75 g, 86% yield). IR (KBr): 3100, 2293, 2216, 1577,
3401, 2200, 1624, 1597, 1538, 1524, 1496, 1444, 1351, 1136, 985 cm-1
.
1H NMR (400 MHz, (CD3)2SO, 2.54): δ 8.26 (d, J ) 1.4 Hz, 1H),
7.87 (dd, J ) 8.1, 1.4 Hz, 1H), 7.76 (d, J ) 8.4 Hz, 1H), 7.24 (d, J )
8.4 Hz, 1H), 6.59 (d, J ) 8.4 Hz), 5.88 (s, 2H). 13C NMR (100 MHz,
(CD3)2SO, 40.4): δ 151.0, 148.46, 135.6, 134.1, 133.5, 127.5, 119.7,
119.3, 113.6, 106.4, 102.9, 98.4, 83.1, 76.5.
1
1415, 1071, 1033 cm-1. H NMR (400 MHz, CD3CN): δ 8.86 (m, J
) 8.7, 2.4, 1.7, 0.5 Hz, 2H), 8.16 (m, J ) 8.7, 2.4, 1.7, 0.5 Hz, 2H),
(42) Endo, Y.; Songkram, C.; Yamasaki, R.; Tanatani, A.; Kagechika, H.;
Takaishi, K.; Yamaguchi, K. J. Organomet. Chem. 2002, 48, 657.
(43) Price, D. W., Jr.; Tour, J. M. Tetrahedron 2003, 59, 3131.
(44) Dirk, S. M.; Price, D. W.; Chanteau, S. H.; Kosynkin, D. V.; Tour, J. M.
Org. Lett. 2001, 3, 5109.
9
J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004 377

A R T I C L E S
Stewart et al.
(300 mg, 2 mmol) was dissolved in CH3CN (2 mL) and treated with
NOBF4 (262 mg, 2.2 mmol). The desired product (350 mg, 71%) was
isolated as a pale yellow solid. IR (KBr): 3360, 3112, 2285.30, 2222,
1582, 1496, 1524, 1444, 1069 cm-1. 1H NMR (400 MHz, CD3CN): δ
8.47 (m). 13C NMR (100 MHz, CD3CN): δ 121.5, 121.2.
4-(2-Nitro-4-pentafluorophenylethynylphenylethynyl)benzene-
diazonium Tetrafluoroborate (IV). Following the general diazotization
procedure, 4-(2-nitro-4-pentafluorophenylethynylphenylethynyl)aniline
(0.42 g, 1 mmol) was treated with NOBF4 (1.4 g, 1.2 mmol) in CH3-
CN (20 mL) and sulfolane (20 mL). The product was precipitated with
Et2O (200 mL) and recrystallized from CH3CN to give orange needles
(0.26 g, 51% yield). IR (KBr): 3106, 2284, 2214, 15178, 1517, 1444,
Acknowledgment. This work was funded by the Defense
Advanced Research Projects Agency, the Army Research Office,
and the Office of Naval Research. Paul A. W. van der Heide
(University of Houston, MRSEC) is thanked for assistance with
XPS measurements. Dr. I. Chester (FAR Research, Inc.) is
thanked for providing trimethylsilylacetylene used in the
syntheses of the diazonium compounds.
1
1349, 1291, 1074 cm-1. H NMR (400 MHz, CH3CN): 8.61 (d, J )
2.4 Hz, 1H), 8.56 (m, 2H), 8.36 (dd, J ) 8.8, 2.4 Hz, 1H), 8.06 (m,
2H), 7.94 (d, J ) 8.8 Hz, 1H) δ. 13C NMR (100 MHz, CH3CN): δ
149.0, 136.5, 135.4, 134.8, 133.7, 130.9, 128.8, 126.1, 125.8, 115.4,
96.8, 93.4.
Supporting Information Available: XPS spectra of samples
reacted with fluorinated and nonfluorinated diazonium salts, and
an FTIR spectrum of the Si(111):H surface (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
3,4,5-Trifluorobenzenediazonium Tetrafluoroborate (V). Fol-
lowing the general procedure for diazotization, 3,4,5-trifluoroaniline
JA0383120
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378 J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004