Coupling of Diazonium to Carbon Nanotubes
FULL PAPER
polypropylene filtration membrane (0.45 mm pores). The crude material
was recrystallized in acetonitrile (10 mL) at RT and was precipitated
again with diethyl ether. After filtration on a polypropylene membrane,
the obtained residue was the diazonium salt. IR analysis was performed
on a Magna-IR 860 spectrometer. Methyl 4-diazobenzoate tetrafluorobo-
dicated a coverage yield of 1/18 carbon. Alternatively, the
reaction can stop when the diazonium has been fully con-
sumed in the coupling with SWNTs or with itself (step E).
This last competing reaction hinders the coupling to
SWNTs, especially in the case of diazonium compounds
with electron-withdrawing groups.
We have thus shown that the coupling of diazonium onto
SWNTs is a radical reaction with stable nanotube radical in-
termediates. Noticeably, this is a rare form of stable carbor-
adical in water. The chemistry of this new kind of organic
radical is still to be explored. In addition, we have unveiled
the mechanistic features determining the m- versus sc-NT
selectivity of the reaction. We note that the reaction rate
would change to a large extent depending on the ratio of m-
and sc-NTs: diazonium compounds will couple very slowly
on pure sc-NTs, while their coupling is fast on both types in
the presence of m-NTs. Working on both the electrophilicity
and the stability of the aryl radical, the aim of our further
work is to increase this selectivity up to a high enough level
for practical nanotube-type separation.
rate: white solid (1.08 g, 65%); IR (KBr): n˜ =3126 (m), 2300 (s; nACHTUNGTRENNUNG
(N+=
N)), 1726 (s; ns(C=O ester)), 1589 (vs), 1440 (vs), 1415 (vs), 1313 (vs),
869 cmꢀ1 (vs). Benzenediazonium tetrafluoroborate: white solid, becomes
pink. The product was unstable, even when kept under a nitrogen atmos-
phere. 4-Methylbenzenediazonium tetrafluoroborate: white solid (1.26 g,
60%); IR (KBr): n˜ =2269 (s; n (N+=N)), 1581 (vs), 1457 (s), 1390 (s),
1311 (s), 817 cmꢀ1 (vs). 4-Isopropylbenzenediazonium tetrafluoroborate:
white solid (1.02 g, 62%); IR (KBr): n˜ =3108 (s), 2971 (s), 2265 (s;
n(N+=N)), 1579 (vs), 1457 (vs), 1322 (s), 848 cmꢀ1 (vs).
Kinetics followed by absorption spectroscopy: The reaction of SWNTs
with diazonium compounds was carried out in a 1 cm quartz cuvette and
followed by visible/near-infrared absorption spectroscopy using a Perkin–
Elmer Lambda 900 spectrometer equipped with a thermostat. All reac-
tions were performed at 278C. In a typical experiment, the diazonium
compound was dissolved in pure water. Then the diazonium solution
(80 mL, 1–100 mm) was added to the SWNTs (720 mL) in F127 2% aque-
ous solution (usually 24 mgLꢀ1, that is, 2 mm of C atoms) in the cuvette.
Spectra of the solution were recorded over 2 h (successive spectra shown
in Figure 1). The peak height was determined from the spectra after sub-
traction of the full reaction baseline. A tangential baseline was calculated
between adjacent minima on the corrected spectra, to correct for back-
ground changes during the course of the reaction. The peak height was
measured as the difference between the corrected absorbance and this
baseline at a constant wavelength (see Figure S1 in the Supporting Infor-
mation). The relative peak height at a given time was defined as the ratio
of the peak height at that time over the initial peak height.
Experimental Section
Materials: The SWNTs used in this study were purchased from Nano-
ledge Company. Poloxamer F127, nitrobenzenediazonium tetrafluorobo-
rate (NO2-BDT), bromobenzenediazonium tetrafluoroborate (Br-BDT),
methoxybenzenediazonium tetrafluoroborate (MeO-BDT), N,N-diethyl
4-diazoaniline tetrafluoroborate (Et2N-BDT), aniline, p-toluidine, 4-iso-
propylaniline, methyl 4-aminobenzoate, and other chemicals (naphthol,
acetonitrile, diethyl ether, nitrosyl tetrafluoroborate, TEMPO, 4-carboxy-
TEMPO, nitric acid, sodium hydroxide, calcium dihydride) were pur-
chased from Sigma–Aldrich. Sephacryl S400 was purchased from GE
Healthcare. All diazonium syntheses were performed under an inert at-
mosphere of argon in glassware that had been flame dried. The acetoni-
trile was distilled from calcium hydride prior to use.
Electron spin resonance: The spectra were recorded on a Bruker EMX
spectrometer with a high-sensitivity cavity cooled with a nitrogen-flux
cryostat at 200 K. Samples (150 mL) were analyzed in quartz tubes. For
the kinetic follow-up, the tube was brought back to RT by a warm nitro-
gen flux and the reaction time corresponded to the accumulated time
passed at RT.
Acknowledgements
Preparation of SWNT dispersions: SWNTs were purified by a nitric acid
treatment to remove metal catalysts and most of the carbon black, fol-
lowed by size-exclusion chromatography to remove any remaining
carbon particles. Typically, SWNTs (100 mg) were suspended in water
(10 mL) by ultrasonication (80 W, 45 kHz ultrasound bath, maximum
power, 30 min). The resulting suspension was diluted in a mixture of
nitric acid (65%, 70 mL) and water (30 mL) and refluxed for 4 h under
stirring. The suspension was then cooled on ice, diluted by the addition
of cold water (110 mL), and filtered under vacuum on a hydrophilic poly-
propylene filtration membrane (0.45 mm pores). The black residue was
suspended in a sodium hydroxide solution (100 mg NaOH in 200 mL
H2O) by ultrasonication for 2 min and filtered again (same conditions).
The residue was rinsed with NaOH solution as long as the filtrate was
gray. Then it was briefly rinsed with pure water and stored wet in a
closed vial. A quarter of the black residue was suspended in F127 polox-
amer solution (20 mL, 2 wt% in water), with three cycles of heating at
708C and ultrasonication (80 W bath, 10 min). This solution was purified
by gel filtration on a Sephacryl S400 column (4 cm diameter, 2.5 cm
height) and eluted with F127 2% solution at 508C. The dark fractions
were pooled and used as pure SWNT stock. The concentration and
purity were tested by absorption spectroscopy. The concentration was de-
The authors thank Pascal Lavie for his precious support in the experi-
mental setup, Arianna Filoramo for Raman analysis, Bruno Jousselme
for his help in organic synthesis, and Pascale Jꢁgou for XPS analysis. G.S.
thanks the French Commissariat ꢄ lꢀEnergie Atomique (CEA) for his
PhD funding.
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(940 nm)=19.1 Lgꢀ1 cmꢀ1
termined by using the absorption coefficient eCAHTUNGTERNUNGN .
Synthesis of diazonium compounds:[53] Nitrosyl tetrafluoroborate (1 eq)
was added at ꢀ408C to a solution of the required para-substituted aniline
(1 g or 1 mL, 1 eq) in anhydrous acetonitrile (25 mL). The mixture was
stirred for 90 min at ꢀ408C. Then the diazonium salt was precipitated by
adding diethyl ether (75 mL) and was filtered under vacuum through a
Chem. Eur. J. 2009, 15, 2101 – 2110
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