Cycloaddition of benzyne to SWCNT: Towards CNT-based paddle wheels

Article abstract of DOI:10.1039/c0cc01907k

The cycloaddition of benzyne to SWCNT has been carried out for the first time. Raman spectroscopy, TGA, HR-TEM, UV-vis-NIR as well as XPS have been used for products characterization.

Full text of DOI:10.1039/c0cc01907k

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COMMUNICATION
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Cycloaddition of benzyne to SWCNT: towards CNT-based paddle
wheelswz
Alejandro Criado, Maria Jose Gomez-Escalonilla, Jose Luis G. Fierro, Antonio Urbina,
a
b
c
d
´
Diego Pena, Enrique Guitian* and Fernando Langa*
˜
´
´
a
a
b
Received 15th June 2010, Accepted 4th August 2010
DOI: 10.1039/c0cc01907k
The cycloaddition of benzyne to SWCNT has been carried out
for the first time. Raman spectroscopy, TGA, HR-TEM,
UV-vis-NIR as well as XPS have been used for products
characterization.
were generated by two different procedures (Scheme 1): (i)
thermal decomposition of benzenediazonium 2-carboxylate
(2)7 and (ii) fluoride induced elimination of TMS and triflate
from 2-(trimethylsilyl)aryltriflates 3.8 By procedure (i), after
heating at 80 1C a suspension of benzenediazonium 2-carboxylate
(2) and SWCNT in o-dichlorobenzene (ODCB) for 4 h followed
by filtration and centrifugation (see Experimental in ESIz)
covalent functionalization of SWCNT was achieved and
proved by Raman spectroscopy and TGA as indicated below
(see ESIz, Fig. S1 and S2, respectively). Following procedure
(ii) a solution of 2-(trimethylsilyl)aryl trifluoromethanesulfonate
3 in acetonitrile (CH3CN) was added to a suspension of
SWCNT in ODCB under Ar; then 18-crown-6 ether and dry
CsF were added and the mixture was refluxed overnight
affording f-SWCNT 1. It should be indicated that previous
studies by Dyke and Tour using 2-(trimethylsilyl)phenyl
trifluoromethanesulfonate and TBAF under solvent-free
conditions afforded unfunctionalized SWCNT only.9 f-SWCNT
1a and 1b were characterized by Raman spectroscopy, TGA,
HR-TEM, UV-vis-NIR, FTIR as well as XPS techniques.
Fig. S3 (see ESIz) shows the TGA profiles of pristine
SWCNT and f-SWCNT 1a and 1b prepared by method (ii).
A loss of weight of around 12% for pristine SWNCT, 21% for
1a and 27% for 1b was observed at 650 1C. The loss of weight
observed for pristine SWCNT between 100 and 650 1C may be
due to the destruction of the residual amorphous carbon still
Carbon Nanotubes (CNTs) have emerged as central building
blocks in nanoscience as a result of their unique mechanical,
geometric, thermal, and electrical properties opening doors to
their use in electronic, optical, magnetic and mechanical
applications. Chemical functionalization1 of CNTs allows
both to increase their solubility and to modulate their properties
and although some work has been done along the last years on
the chemistry of CNTs, the establishment of new protocols for
their functionalization is still needed. Functionalized CNTs,
due to their modulated properties, potentially offer possibilities
for unprecedented applications providing opportunities for the
fabrication of novel nanodevices; in this respect, it has been
theoretically suggested that it may be possible to make paddle
wheels by bonding rigid molecules (gear teeth) such as benzene
onto CNTs (Fig. 1).2
Following our research on SWCNT functionalization3,4 and
aryne cycloadditions5,6 we report herein the functionalization
of SWCNT by cycloaddition of benzynes. To this aim arynes
Fig. 1 Proposed molecular paddle wheels2 (reprinted with permission
from IOP Nanotechnology) (left). Cycloaddition product of benzyne
to SWCNT (right).
a
´
Departamento de Quımica Organica, Facultad de Quımica,
Universidad de Santiago de Compostela, 15782,
´
´
Santiago de Compostela, Spain. E-mail: enrique.guitian@usc.es
´
b
c
Instituto de Nanociencia, Nanotecnologıa y Materiales Moleculares
(INAMOL), Universidad de Castilla-La Mancha, 45071 Toledo,
Spain. E-mail: Fernando.Langa@uclm.es; Fax: +34 925268840
´
Instituto de Catalisis y Petroleoquımica, CSIC, Madrid, Spain.
E-mail: jlgfierro@icp.csic.es
´
d Universidad Polite´cnica de Cartagena, 30202-Cartagena(Murcia),
Spain. E-mail: antonio.urbina@upct.es
w This article is part of the ‘‘carbon nanostructures’’ web-theme issue
for ChemComm.
z Electronic supplementary information (ESI) available: Experimental
details for synthesis and characterization. See DOI: 10.1039/
c0cc01907k
Scheme 1 Cycloaddition of benzyne to SWCNT.
c
7028 Chem. Commun., 2010, 46, 7028–7030
This journal is The Royal Society of Chemistry 2010

Table 1 C1s and O1s core-level spectra of f-SWCNT 1a, f-SWCNT 1b and pristine SWCNT samples. In parentheses are peak percentages
BE/eV (C1s (%)) BE/eV (O1s (%))
sp2 C
sp3 C
C–O
CQO
COO
p–p*
O–C
OQC
Sample
SWCNT
f-SWCNT 1a
f-SWCNT 1b
284.8 (66)
284.8 (68)
284.8 (54)
—
—
286.2 (19)
286.2 (16)
286.2 (19)
287.7 (3.8)
287.7 (4.4)
287.5 (5.7)
289.2 (5.6)
289.2 (1.8)
289.4 (3.8)
291.4 (5.6)
291.4 (9.8)
291.4 (4.1)
533.5 (75)
533.8 (66)
533.5 (77)
532.2 (25)
532.2 (34)
531.9 (23)
285.4 (13.4)
present in the carbon nanotubes and to the decarboxylation of
the oxidized species. The corrected weight losses due to the
functional groups on nanotubes (weight loss differences of
1a ꢀ pristine SWCNT and 1b ꢀ pristine SWNCT) were then
estimated to be 9% and 15% for f-SWCNT 1a and 1b,
respectively. The number of aromatic functional groups in
f-SWCNT 1a was then estimated as 1 per 64 carbon atoms.
Finally, the number of functional groups for f-SWCNT 1b
corresponds to one dialkoxy benzene group per 115 carbon
atoms. The degree of functionalization for f-SWCNT 1a
obtained by method (i) was estimated as 16% (1 benzene
functional group per 35 carbon atoms).
this technique gives information about additional structural
defects disrupting the delocalized electron system in the
SWCNT sidewalls.14 An increase in the ID/IG ratio is observed
when the extent of SWCNT functionalization is increased. As
depicted in Fig. 2, f-SWCNTs 1a and 1b exhibit an increase of
the D-band (at around 1299 cmꢀ1) relative to the starting
SWCNT, indicating a significant conversion of SWCNT sp2
carbons to sp3 centers due to the functionalization in the walls
of the tube.
A higher ID/IG ratio is observed in f-SWCNT 1a (ID/IG
=
0.30) compared to f-SWCNT 1b (ID/IG = 0.21) suggesting a
superior degree of sidewalls functionalization. Finally, it is
important to remark that, as a SWCNT becomes functionalized,
the electronic properties of the material change and the
nanotube moves off resonance, and an apparent decrease in
the intensity of the normally resonance enhanced RBM is
observed.14
X-Ray Photoelectron Spectroscopy (XPS) was used to
identify the surface groups, the chemical state of the atoms,
and their relative abundance in the functionalized
SWCNTs.10,11 The high resolution C1s and O1s spectra of
f-SWCNT 1a and 1b, and SWCNT samples are displayed in
Fig. S4 (see ESIz). The binding energies and peak assignments
of C1s and O1s emissions are summarized in Table 1. Sample
1b was decomposed into five different components: (i) a major
one placed at 284.8 eV (FWHM = 1.4 eV) originated from sp2
C-atoms of the graphene sheets, together with its p - p*
shake-up transition at 291.4 eV,4,12 indicative of the presence
of nanotube structures; (ii) a component at 285.4 eV (FWHM =
1.7 eV) assigned to the sp3 C-atoms, namely from alkyl
groups; (iii) a peak at 286.2 eV (FWHM = 1.8 eV) usually
attributed to oxidized groups in C–O bonds;4,13 (iv) and (v)
peaks located at 287.5 eV (FWHM = 1.9 eV) and 289.4 eV
(FWHM = 2.2 eV) assigned, respectively, to CQO and COO
groups.4,13 Sample f-SWCNT 1a and SWCNT showed similar
C1s line shape although the whole FWHM was about 0.2 eV
smaller than for sample f-SWCNT 1b. Accordingly, C1s peaks
of 1a and reference SWCNT samples were satisfactorily fitted
using only four components (i, iii, iv and v) and the same
FWHM values (see Fig. S4, ESIz) indicating the absence of sp3
C-atoms in both samples. The O1s peak (Fig. S4z) showed a
component placed at ca. 532.0 eV assigned to CQO surface
groups whereas the second one at ca. 533.5 eV is associated to
C–O groups in phenol-like structures.4 The surface atomic
concentrations of the three samples are compiled as atomic
percentages in Table S1 (see ESIz). The pristine SWCNTs
sample shows a considerable oxygen amount (7.6% total). The
O-content drops drastically up to 1.8% for the benzyne-
modified f-SWCNT 1a sample, but increases again (6.5%
total), as expected for the –OC6H13 chains linked to the
benzyne structure during functionalization. In summary,
detailed C1s fitting analysis pointed out the presence of sp3
C-atom in the two benzyne-functionalized SWCNTs whereas
only sp2 C-atom is displayed by the pristine sample.
Cycloaddition of benzyne to SWCNT can form the [2+2] or
[4+2] adducts (Fig. 3). It is well established that benzyne adds
to naphthalene, anthracene and other PAHs forming the
[4+2] cycloadducts.15 However, the reaction of benzyne with
C60 leads to the [2+2] adduct forming a cyclobutene ring.16
For the addition of benzyne to CNTs calculations suggest that
Fig. 2 (top) Raman spectra (785 nm, 1.58 eV) of SWCNT (black
line), f-SWCNT 1a (red line) and f-SWCNT 1b (blue line), normalized
at the G-band; (bottom) RBM region of SWCNT (black line),
f-SWCNT 1a (red line) and f-SWCNT 1b (blue line).
Raman spectroscopy is one of the preferred tools for the
characterization of covalent functionalization of SWCNTs as
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7028–7030 7029

dialkoxy-benzyne group per 115 carbon atoms approximately.
The nanotubes are stable while focusing the microscope image,
but after some time under the electron beam current, some
degradation can be appreciated in the nanotubes. The ESIz
provides evidence of enough stability to provide good fixed
images (see ESIz, AVI-1 and AVI-2 movies).
In summary, we have demonstrated the viability of the
cycloaddition of o-benzyne onto the sidewall of SWCNTs
Fig. 3 [2+2] (left) and [4+2] (right) products of the benzyne–
SWCNT cycloaddition reaction.
offering
a new and powerful methodology to obtain
both cycloadditions are possible, the [4+2] product being
more stable than the [2+2] product, particularly in nanotubes
with larger diameter than C60 (0.683 nm).17 Consequently, it is
important to determine the diameter of the SWCNTs
employed in this work which can be done by analyzing the
RBM zone of the Raman spectra17 (Fig. 2 (bottom)) by the
application of relation (1):
functionalized carbon nanotubes. The construction of bigger
wheels by reacting CNTs with polycyclic benzynes is in progress.
Financial support from the Ministerio de Ciencia y Tecno-
logıa of Spain, FEDER funds (Project CTQ2007-63363/PPQ,
´
CTQ2007-63244 and Consolider Project HOPE), JJCC de
Castilla-La Mancha (Project PCI08-038) and Xunta de
Galicia (PGIDIT07PXIB209139PR and RCMM) is gratefully
acknowledged.
nRBM (cmꢀ1) = 248/diameter (nm).
(1)
Notes and references
The calculated diameters of our SWCNTs range from 0.88 to
1.49 nm (with laser of 1.58 eV, 785 nm) and from 0.79 to
1.77 nm (laser of 2.33 eV, 532 nm) (see ESIz, Fig. S5). So,
according to ref. 2 and considering these diameters of CNTs,
the [4+2] adduct is more stable than the [2+2] adduct and it
should be the preferred product as indicated in Scheme 1.
However, in a kinetic-controlled process this is only right if the
difference of stabilities is present in the transition states.
Finally, further confirmation of the addition of the
functional groups to the SWCNT is provided by Transmission
Electron Microscopy (TEM), performed on samples before
and after the functionalization procedure. In Fig. 4(A) we
show a reference image with pristine nanotubes before any
chemical procedure had been accomplished other than synthesis
and purification of the nanotubes. SWCNTs are easily found
in the image, where clean walls are visible. The average
diameter of the pristine nanotubes is 2 ꢁ 0.7 nm, where the
uncertainty comes from TEM resolution. Some dispersion in
diameters has also been observed. In Fig. 4(B) an image of a
functionalized nanotube (f-SWCNT 1b) is presented, where
the functionalization consists in the addition of dialkoxy-
benzene groups, the apparent diameter of this nanotube is
slightly larger (4 ꢁ 0.7 nm), which can be explained by the
uniform coverage of the original SWCNT wall. The dense
functionalization seen by TEM is compatible with the high
coverage predicted by the other characterization techniques
presented in this article, which yields an addition of one
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Fig. 4 Pristine (A) and functionalized (B) single wall carbon nano-
tube images obtained by TEM (200 kV, 250 000ꢂ). Uniform coverage
of dialkoxy-benzyne groups can be observed.
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7030 Chem. Commun., 2010, 46, 7028–7030
This journal is The Royal Society of Chemistry 2010