Microwave-assisted functionalization of carbon nanostructures in ionic liquids

Article abstract of DOI:10.1002/chem.200901408

The effect of microwave (MW) irradiation and ionic liquids (IL) on the cycloaddition of azomethine ylides to [60]fullerene has been investigated by screening the reaction protocol with regard to the IL medium composition, the applied MW power, and the sim

Full text of DOI:10.1002/chem.200901408

FULL PAPER
DOI: 10.1002/chem.200901408
Microwave-Assisted Functionalization of Carbon Nanostructures in Ionic
Liquids
Ivan Guryanov,[a] Francesca Maria Toma,[b, d] Alejandro Montellano Lꢀpez,[b]
Mauro Carraro,[a] Tatiana Da Ros,[b] Guido Angelini,[c] Eleonora DꢁAurizio,[c]
Antonella Fontana,*[c] Michele Maggini,*[a] Maurizio Prato,*[b] and Marcella Bonchio*[a]
Dedicated to the Centenary of the Italian Chemical Society
Abstract: The effect of microwave
(MW) irradiation and ionic liquids (IL)
on the cycloaddition of azomethine
ylides to [60]fullerene has been investi-
gated by screening the reaction proto-
col with regard to the IL medium com-
position, the applied MW power, and
the simultaneous cooling of the system.
[60]Fullerene conversion up to 98% is
achieved in 2–10 min, by using a 1:3
mixture of the IL 1-methyl-3-n-octyl
The mono- versus poly-addition selec-
tivity to [60]fullerene can be tuned as a
function of fullerene concentration.
The reaction scope includes aliphatic,
aromatic, and fluorous-tagged (FT) de-
rivatives. MW irradiation of IL-struc-
tured bucky gels is instrumental for the
group coverages of up to one function-
al group per 60 carbon atoms of the
SWNT network. An improved perfor-
mance is obtained in low viscosity
bucky gels, in the order [bmim]BF4 >
[omim]BF4 > [hvim]TF2N (bmim=1-
methyl-3-n-butyl imidazolium; hvim=
functionalization
of
single-walled
1-vinyl-3-n-hexadecyl
imidazolium).
carbon nanotubes (SWNTs), yielding
With this protocol, the introduction of
fluorous-tagged pyrrolidine moieties
onto the SWNT surface (1/108 func-
tional coverage) yields novel FT-CNS
(carbon nanostructures) with high af-
finity for fluorinated phases.
Keywords: cycloaddition
enes · ionic liquids · microwave
chemistry · nanotubes
· fuller-
imidazolium
tetrafluoroborate
AHCTUNGTRENNUNG
([omim]BF4) and o-dichlorobenzene,
and an applied power as low as 12 W.
Introduction
[a] Dr. I. Guryanov, Dr. M. Carraro, Prof. M. Maggini, Dr. M. Bonchio
ITM-CNR and Department of Chemical Sciences
University of Padova
Research on carbon nanostructures (CNS), such as ful-
lerenes, carbon nanotubes, and graphene, has recently been
the focus for scientists from many disciplines, owing to their
exclusive structures and properties. Shortly after their dis-
covery, the fullerenes were subjected to extensive studies of
chemical modification to understand their reactivity and to
tune their properties.[1] Exohedral addition of organic
groups helps to control solubility, morphology in the solid
state, and electronic properties of the fullerene core. The
azomethine ylide cycloaddition, yielding fulleropyrrolidines
(FPs),[2] has contributed significantly to the development of
new nanocarbon-based materials for strategic fields such as
solar energy conversion or molecular medicine.[3] FP synthe-
sis tolerates many functional groups and is significantly ac-
celerated by microwave (MW) irradiation.[4,5] Several FPs
have been prepared under MW irradiation by Langa and co-
workers, with yields up to 37% and interesting regioselectiv-
ities in the case of additions to C70.[4] In these reactions, tolu-
ene, benzene, or o-dichlorobenzene (o-DCB) have been
Via Marzolo, 1, 35131 Padova (Italy)
Fax : (+39)049-8275239
E-mail: michele.maggini@unipd.it
marcella.bonchio@unipd.it
[b] Dr. F. M. Toma, Dr. A. Montellano Lꢀpez, Dr. T. Da Ros,
Prof. M. Prato
Dipartimento di Scienze Farmaceutiche, INSTM unit of Trieste
Piazzale Europa 1, 34127 Trieste (Italy)
Fax : (+39)040-52572
E-mail: prato@units.it
[c] Dr. G. Angelini, Dr. E. DꢁAurizio, Prof. A. Fontana
Dipartimento di Scienze del Farmaco
Universitꢂ “G. d’Annunzio”
Via dei Vestini, 66013 Chieti (Italy)
Fax : (+39)0871-3554461
E-mail: fontana@unich.it
[d] Dr. F. M. Toma
SISSA
Via Beirut 2-4, 34151 Trieste (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901408.
Chem. Eur. J. 2009, 15, 12837 – 12845
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12837

used as solvents, although o-DCB should be preferred for
fullerene solubility, efficient microwave absorption, and rel-
atively high boiling point.
(FT-CNS) could be readily isolated. Preliminary investiga-
tions showed a remarkable solubility of FT-CNS in fluorous
phases, with potential applications in catalysis, material sci-
ence, and membrane-based technology.[16]
Building on the fullerene experience, a wide variety of re-
actions have been also described for the covalent functional-
ization of carbon nanotubes (NTs). At variance with the
fullerene analogues, NTs require harsh temperature and/or
pressure conditions, with greater use of organic solvents,
and long reaction times. Therefore, MW-assisted and sol-
vent-free protocols have been developed to accelerate reac-
tions and produce functionalized NTs efficiently. A recent
achievement was the double functionalization of single-
walled carbon nanotubes (SWNTs) with azomethine ylides
and arene radical additions, both promoted by MW irradia-
tion.[6] In this perspective, the optimization of functionaliza-
tion methods for CNS should also address the replacement
of hazardous volatile organic solvents with alternative reac-
tion media. To this end, room-temperature ionic liquids
(ILs) have been used successfully for a variety of organic
transformations.[7] Recognized advantages of ILs are their
tunable composition, polar character, non-volatility, thermal
resistance, complementarity with water or other green
media, and liquid electrolyte behavior. In addition, IL
phases are instrumental for fast and selective MW heating
by the ionic conduction mechanism, with negligible vapor
pressure and safer setup conditions.[8] The combination of
ILs and CNS has hardly been explored, with just a few re-
ports related to the synthesis of methanofullerenes[9] and hy-
drogenation reactions.[10] Aggregation phenomena, driven by
the low solubility of CNS in polar environments, are expect-
ed to affect the reactivity behavior in IL media. Notably, it
has recently been discovered that SWNTs, upon grinding
with IL, form highly viscous gels termed “bucky gels”, the
thermal,[11] electrochemical,[12] and rheological properties[13]
of which have been investigated by several authors. The ge-
lation refers to mixtures in which an exfoliation process of
nanotube bundles is induced by the tendency of the IL to
adsorb onto the SWNT surface. The use of bucky gels as re-
action media has been investigated for the addition of diazo-
nium ions to the SWNT surface.[14] Exfoliation of the SWNT
ropes by the IL environment accelerates the reaction, which
occurs in minutes at room temperature.
Results and Discussion
Synthesis of [60]fullerropyrrolidines in IL media under MW
irradiation: The reactivity of [60]fullerene was considered
first, as an evaluation benchmark to test and optimize the
conditions for an efficient functionalization of CNS by MW-
induced flash heating in IL phases. In particular, we focused
on the use of MW-activated IL phases to accelerate the ther-
mal azomethine ylide cycloaddition, while providing an ad-
ditional tool for tuning the selectivity outcome of the poly-
adduct distribution (Scheme 1). A key observation was that
Scheme 1. Azomethine ylide cycloadditions to [60]fullerene yielding
mono- and poly-adducts under MW irradiation, in IL/o-DCB mixtures.
cycloreversion of FPs could be readily achieved in IL under
MW irradiation within minutes, by unlocking the pyrrolidine
ring to back-release pristine [60]fullerene.[17] No other addi-
tive is required, as the IL solvates the incipient 1,3-dipole by
electrostatic interactions, while the MW-induced dielectric
heating speeds up the thermal cycloreversion with unprece-
dented rates.[17,18] As a matter of fact, in pure IL media, cy-
cloreversion turned out to be the dominating process, be-
cause [60]fullerene, which is hardly soluble in such a polar
environment, precipitates, thus hampering the thermody-
namic equilibration of the reagent/product distribution. In
pure IL, functionalization of [60]fullerene failed to yield any
cycloadduct, regardless of the IL type, applied MW power,
and irradiation time (entry 1, Table 1). However, when o-
DCB is used as a co-solvent, the rate of retro-cycloaddition
is remarkably slowed down.[17]
In this work, we investigate the effect of MW irradiation
and ILs on the cycloaddition of azomethine ylides to
[60]fullerene and CNTs by screening the reaction protocol
in relation to the IL medium composition, the concentration
factor, the applied MW power, and the simultaneous cooling
of the system. This latter turned out to be crucial for directly
dosing high levels of MW power to the reaction without
overheating.[15] Our results include: 1) the evaluation of the
cycloaddition kinetics and selectivity in both o-DCB- and
IL-containing media; 2) optimization of the synthetic proto-
col towards multisubstitution on the fullerene core and the
CNT surface; 3) examination of the reaction scope, with
both aliphatic and aromatic aldehydes; 4) extension of the
optimized protocol to fluorous-tagged aldehydes. Under
MW-assisted conditions in IL phases, fluorous-tagged CNS
Therefore, the best solvent composition was set for the
synthesis of model FPs 1 by changing the relative IL/o-DCB
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Chem. Eur. J. 2009, 15, 12837 – 12845

Functionalization of Carbon Nanostructures
FULL PAPER
Table 1. Effect of [omim]BF4/o-DCB ratio on the yield and selectivity of
mono- and poly-FPs 1, obtained by MW-assisted reaction of [60]fullerene
with sarcosine and heptaldehyde.
Table 2. Effect of the IL nature on the yield and selectivity of mono- and
poly-FPs 1, obtained by MW-assisted reaction of [60]fullerene with sarco-
sine and heptaldehyde.
Entry[a]
IL/o-DCB
Conv. [%][b]
Mono-FP [%][c]
Selectivity[d]
Entry[a] IL[b]
Conv.
[%][c]
Mono-FP 1
Selectivity[e]
[%][d]
1[e]
2[e]
3[e]
4[e]
5[e]
6[e]
7[f]
1/0
4/1
2/1
1/1
1/2
1/3
1/3
1/9
0/1
–
–
6
9
22
21
30
44
38
15
–
18
21
64
58
60
64
52
15
0.5
0.8
0.5
0.6
1
2.2
3.0
–
1[f]
N
25
15
9
46
58
56
52
35
44
44
17
15
9
2.1
–
–
2[g]
3[f]
4[f]
18
43
13
38
19
34
33
41
0.6
2.9
0.3
2.7
1.2
3.4
3.0
2.2
5[g]
6[f]
8[f]
7[g]
8[f]
9[f,g]
9[g]
10[g]
11[g]
[a] In all reactions: [60]fullerene 0.14 mmol; sarcosine 0.28 mmol; heptal-
dehyde 0.56 mmol; MW irradiation at 12 W, 10 min, Tbulk =1008C, under
magnetic stirring and simultaneous cooling by compressed air.
[b] [60]Fullerene conversion monitored by HPLC analysis. [c] % of
mono-FP 1 monitored by HPLC analysis. [d] Mono-FP/poly-FP ratio.
[e] Reaction performed in 0.5 mL. [f] Reaction performed in 1.0 mL.
[g] MW irradiation at 20 W.
ACHTUGNTNERN[UNG (CF3SO2)2N] 60
[a] In all reactions: [60]fullerene 0.07 mmol; sarcosine 0.14 mmol; heptal-
dehyde 0.28 mmol; MW irradiation at 12 W, 3–5 min, Tbulk =1008C, under
magnetic stirring and simultaneous cooling by compressed air. [b] 1-
Methyl-3-n-butyl imidazolium [bmim], 1-methyl-3-n-octyl imidazolium
[omim], 1-methyl-3-n-decyl [dmim], 1-methyl-3-n-hexadecyl [hmim], 1-
vinyl-3-n-hexadecyl [hvim]. [c] [60]Fullerene conversion monitored by
HPLC analysis. [d] % of mono-FP
[e] Mono-FP/poly-FP ratio. [f] Reaction performed in 0.5 mL. [g] Re-
action performed in 1.0 mL.
1 monitored by HPLC analysis.
content and optimizing the overall [60]fullerene conversion
(Table 1). In this preliminary screening, 1-methyl-3-n-octyl
imidazolium tetrafluoroborate ([omim]BF4) was used be-
cause it gives finer fullerene dispersions than other imidazo-
linium-based ILs.[19] Interestingly, under MW irradiation at
12 W, Tbulk =1008C for 10 min, the change in solvent compo-
sition in favor of o-DCB leads to a steady increase of the
overall yield and of the mono-substitution selectivity (en-
tries 1–6, Table 1). This latter turned out to be remarkably
affected by the relative solubility of reagents and products
in the solvent system, which is mainly controlled by the IL
content and by the [60]fullerene concentration. Indeed, an
unexpected prevalence of polyadducts (mono-FP/poly-FP<
1) was registered at IL>25% (entries 2–6, Table 1). In addi-
tion, the selectivity reverts in favor of the monoadduct upon
dilution (compare entries 6 and 7, Table 1). For the IL-free
reaction, which lacks the ionic absorbing component, the ac-
celerating effect of MW irradiation (even at MW power=
20 W) is very poor (Table 1, entry 9). The results reported in
Table 1 indicate that the phase in which IL/o-DCB=1:3
guarantees both a fast reaction and a convenient selectivity
control, which, possibly, depends on clustering/aggregation
phenomena. Further insight into the interplay of such fac-
tors for selectivity tuning has been obtained from kinetic
studies (vide infra).
3–5 min with ILs bearing long alkyl chains (entries 4–11,
Table 2). Selectivity, on the other hand, seems once again
governed by the solubility of fullerenes in the reaction
medium. Values of 3.4 for the mono-FP/poly-FP ratio were
obtained (entry 9, Table 2). This compares well with the re-
sults recorded under diluted conditions (Table 1). Apparent-
ly, the IL counteranion plays a minor role in this chemistry.
From the data reported in Table 2, [omim]BF4 is the best hit
in terms of conversion yield and selectivity for FPs 1 synthe-
sis. Figure 1 shows the time evolution profile of mono- and
poly-FPs 1 in [omim]BF4/o-DCB=1:3, and the comparison
with the IL-free reaction setup. The combined use of IL and
MW, even at applied power as low as 12 W, is responsible
for a remarkable acceleration of the reaction, leading to
With the aim of optimizing the reaction protocol, a series
of ILs were screened as co-solvents for the MW-assisted FP
1 reference reaction (Table 2). Alkyl imidazolium ILs, butyl
([bmim]+), octyl ([omim]+), decyl ([dmim]+), hexadecyl
([hmim]+) and hexadecylvinyl ([hvim]+) (Scheme 1), were
evaluated to highlight the impact of the alkyl chain length
on FP
1
synthesis. Different counteranions, [BF4]À,
[CF3SO3]À, [PF6]À, and [(CF3SO2)2N]À were also considered
as their role in controlling the hydrophilicity/hydrophobicity
properties of IL media is well known.[7]
Figure 1. Kinetics of [60]fullerene conversion (circles) to mono- and poly-
FPs 1 (triangles and squares, respectively) by 1,3-dipolar cycloaddition of
sarcosine, and heptaldehyde in 1 mL of o-DCB (empty symbols) or in
[omim]BF4:ODCB=1:3 (filled symbols) under MW irradiation (see con-
ditions in Table 1).
Both [60]fullerene conversion and addition selectivity
(mono- versus polyaddition) depend on the IL nature. In
particular, conversions in the 40–60% range are achieved in
Chem. Eur. J. 2009, 15, 12837 – 12845
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12839

M. Bonchio, M. Maggini, A. Fontana, M. Prato et al.
Table 4. MW-assisted azomethine ylide cycloaddition to [60]fullerene in
[omim]BF4/o-DCB=1/3. Reaction scope explored in the presence of sar-
cosine and different aldehydes.
mono-FP 1 with a plateau yield >40%, after only 3 min
under MW irradiation. This finding is important for the po-
tential upgrade of the azomethine ylide synthesis under con-
tinuous flow conditions using microfluidic technology.[20]
Yield optimization was also investigated by varying the rela-
tive ratio of the azomethine ylide precursors, sarcosine and
heptaldehyde, and their additional recharge during the reac-
tion that, in turn, was carried out at different [60]fullerene
concentrations (Table 3). Under the optimal conditions for
Entry[a] Aldehyde Ratio[b] Conv.
[%][c]
Mono-FP
[%][d]
Selectivity[e]
1[f]
2[f]
3[f]
4[f]
5[g]
6[g]
7[g]
8[g]
1
2
3
4
1
2
3
4
2/4
2/4
2/4
2/4
10/20
10/20
10/20
10/20
61
90
61
62
95
98
81
69
41
23
51
46
4
–
5
23
2.1
0.3
4.9
2.9
0.04
–
0.07
0.5
Table 3. Effect of sarcosine/heptaldehyde equivalent ratio on the yield
and selectivity of mono- and poly-FPs 1, obtained by MW-assisted cyclo-
addition to [60]fullerene.
[a] In all reactions: [60]fullerene 0.14 mmol; [omim]BF4/o-DCB=1/3;
MW irradiation at 12 W, Tbulk =1008C, under magnetic stirring and simul-
taneous cooling by compressed air. [b] Added equivalents with respect to
[60]fullerene. [c] [60]Fullerene conversion monitored by HPLC analysis.
[d] % of mono-FP monitored by HPLC analysis. [e] Mono-FP/poly-FP
ratio. [f] Reaction performed in 1.0 mL for 2 min. [g] Reaction performed
in 0.5 mL for 10 min.
Entry[a] Sarcosine/
heptaldehyde[b]
Conv.[%][c] Mono-FP [%][d] Selectivity[e]
1[f]
1/1
2/4
10/20
2/4
4/8
34
60
95
58
69
72
16
30
4
42
44
41
0.9
1
0.04
2.9
1.8
1.3
2[f]
3[f]
4[g]
5[g]
6[g,h]
The MW-assisted protocol has been also applied to the
synthesis of the novel fluorous-tagged FP 4. Due to the pres-
ence of a C2H4 aliphatic spacer, the fluorinated aldehyde
shows a similar reactivity to the parent heptaldehyde.
Mono-FP 4 has been isolated in 30% yield and character-
ized by 1H-, 19F-, 13C-NMR, ESIMS, FTIR, and UV/Vis
spectroscopies. It is worth mentioning that the poly-FP 4
mixture displays a remarkable solubility in fluorinated alco-
hols, such as trifluoroethanol and hexafluoropropanol,
which makes it interesting for photosensitized oxygenations
in fluorous environments, as well as for applications in sur-
face and membrane chemistry.[21]
2/4+2/4
[a] In all reactions: [60]fullerene 0.14 mmol; [omim]BF4/o-DCB=1/3;
MW irradiation at 12 W, 10 min, Tbulk =1008C, under magnetic stirring
and simultaneous cooling by compressed air. [b] Added equivalents with
respect to [60]fullerene. [c] [60]Fullerene conversion monitored by HPLC
analysis. [d] % of mono-FP 1 monitored by HPLC analysis. [e] Mono-FP/
poly-FP ratio. [f] Reaction performed in 0.5 mL. [g] Reaction performed
in 1.0 mL, in 3 min. [h] Added in two portions in 4 min.
monoaddition, the yield of mono-FP 1 levels off at ꢀ40%,
with no major effects of reagent recharging (entries 4–6,
Table 3). Such limiting yield is probably attributable to equi-
librium constraints, which dictate the observed product dis-
tribution, as a result of the thermally activated retro-cyclo-
addition involving both mono- and poly-FP.[17,18]
Functionalization of carbon nanotubes in IL media under
MW irradiation: The combined use of MW activation and
ILs provides a new strategy for the functionalization of as-
produced, non-oxidized SWNTs by the 1,3-dipolar cycload-
dition of azomethine ylides. In the present work, we explore
the use of the so-called “bucky gels”, as reaction media,
when exposed to reagents and to MW irradiation. We tested
three different ILs, namely [bmim]BF4, [omim]BF4, and
[hvim]ACTHUNGTRENUNG[(CF3SO2)2N], to evaluate the influence of the length
of the alkyl chain (4, 8, or 16 carbon atoms, respectively)
and of the presence of an additional p bond ([hvim]-
AHCTUNGTREN[GNUN (CF3SO2)2N]) on the SWNT exfoliation ability and cycload-
dition promotion. The exfoliation ability was investigated
through rheological measurements at 25 and 608C in order
to study the changes in stability, consistency, and degree of
the IL coating on the SWNT by varying the IL nature and
temperature. Indeed, the affinity of the ILs for the SWNT
surface is fundamental for the formation of the gels, as pure
ILs do not possess any elastic component. Rheological
measurements, in the dynamic mode at a constant shear
stress, indicate a viscoelastic gel-like behavior for the inves-
tigated IL/SWNT mixtures with the measured storage
moduli (elastic, G’) dominating the loss moduli (viscous,
G’’) by one order of magnitude and exhibiting little frequen-
cy dependence over the range of angular frequencies tested.
A fast and quantitative conversion of pristine [60]fuller-
ene to FP 1 polyadducts became accessible at a fullerene
concentration equal to 0.28m (see the kinetic plot in the
Supporting Information, Figure S1) and excess (10/20-fold)
of sarcosine and heptaldehyde (entry 3, Table 3). The reac-
tion scope is highlighted in Table 4. In all cases, monosubsti-
tution occurs with good to excellent selectivities in just
2 min of MW irradiation, with a top performance registered
for the p-methoxyphenyl derivative (entry 3 in Table 4). No-
tably, in the reaction with pyrene aldehyde, poly-FPs 2 are
the major products regardless of the sarcosine/aldehyde
excess and of the MW irradiation time (entries 2 and 6,
Table 4). This latter observation can be addressed on the
basis of the poor cycloreversion tendency exhibited by
pyrene-substituted FPs in IL phases, under MW-activated
conditions.[17] The thermal stability of such cycloadducts pre-
vents the back-release of [60]fullerene and its equilibration
to both mono- and poly-FPs in the reaction mixture. The
analysis of the poly-FP mixtures by electrospray source ioni-
zation mass spectrometry with positive mode, ESIMS (+),
accounts for the formation of mono-, bis-, tris-, and tetra-
substituted derivatives (see Experimental Section).
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Functionalization of Carbon Nanostructures
FULL PAPER
These results are indicative of systems that possess elastic
and permanent networks. However, these gels lose their
elastic properties at a relatively low value of shear stress (ty)
(see Table 5) and show, during measurements in the static
extra double bond), which renders it capable of interacting
favorably with the CNT surface through stronger dispersion
forces, which help to overcome the inter-SWNT attraction.
Functionalization of bucky gels:
The gel phase provides the
CNT with a uniform coating of
IL film, which favors exfolia-
tion and assists the modification
Table 5. Rheological characterization of the IL-SWNT gels.
IL[a]
G’ [Pa][b]
at 258C
G’’ [Pa][b]
at 258C
h* [cP][b,c]
at 258C
ty [Pa]
at 258C
(at 608C)
h [cP][d]
at 258C
ACHTUNGTRENN(UNG at 608C)
A
E
N
of
the
carbon
surface
G
550 (300)
800 (200)
A
55 (50)
80 (50)
8.9ꢄ105 (4.3ꢄ105)
12 (5)
25 (4)
(9)
5000 (1000)
7200 (1800)
ACHTUNGTRNE(NUNG 2400)
(Scheme 2).[23] In this scenario,
the pseudo-supported IL layer
ACHTUNGTRENNUNG
13ꢄ105 (3.6ꢄ105)
G
G
(400)
(43ꢄ105)
acts as
a sort of extracting
AHCTUNGTRENNG[UN (CF3SO2)2N] have a viscosity of 80 cP at 258C and 70 cP at
608C, 240 cP at 258C and 200 cP at 608C, and 200 cP at 608C, respectively. [b] Shear stress (t) 1.0 Pa satisfying
the linear viscoelasticity; angular frequency (w) varied from 0.1 to 10 Hz. [c] Calculated at the frequency of
medium to: 1) solubilize and
transfer the starting reagents;
2) promote the in situ forma-
tion of active intermediates;
and 3) stabilize uphill pathways
0.1 Hz. [d] Calculated at g=10 sÀ1
.
mode (see Figure S8 in the Supporting Information), a shear
thinning behavior with a leveling off of the viscosity (h) to a
minimum plateau value at shear rates of about 10 sÀ1. This
piece of evidence highlights the existence of weak interac-
tions within the gel, and excludes the possibility that the
rheological behavior is governed by the high intermolecular
cohesive forces (as high as 0.5 eVnmÀ1) between SWNTs
characterizing their entanglement.[11] On the other hand,
such interactions point to a large number of weak physical
crosslinks between the SWNT bundles mediated by ILs. As
a matter of fact, cation–p interactions, and also weaker p–p
interactions, between alkylimidazolium cations and the sp2
carbon framework of SWNTs may orient and trigger cluster-
ing of the surrounding imidazolium ions and consequently
interconnect neighboring SWNTs. In addition, an increase
in the temperature (from 25 to 608C) generates a decrease
of the viscosity and also a further reduction of the shear
stress. As far as the comparison of the investigated gels is
concerned, the trend of the complex viscosity h*, of the elas-
tic modulus and of the steady shear viscosity, h, follow ap-
and transition states.
Furthermore, the IL matrix sets forward a molecular
heater array, under MW irradiation, to foster a flash and
highly efficient thermal activation for endothermal process-
es. With this aim, we have investigated the 1,3-dipolar addi-
tion of azomethine ylides to SWNTs in IL-structured gels
(Table 6). In the first instance, the degree of CNT function-
alization (i.e. the number of attached functions per carbon
atom of the nanotube) was established by thermogravimet-
ric analysis (TGA), which enables evaluation of the total
mass loss as derived from the functional groups covalently
attached to the CNT surface, after subtraction of the pris-
tine SWNT contribution. The functionalized SWNTs were
further characterized with a variety of techniques, including
Raman, UV/Vis/NIR spectroscopies, and transmission elec-
tron microscopy (TEM). The MW-assisted protocol was
compared with the conventional synthesis in either molecu-
lar solvent (DMF) or [bmim]BF4, performed for five days at
1208C (entries 1 and 2 in Table 6).
proximately the order [hvim]
[(CF3SO2)2N]> [omim]BF4
ꢁ
Notably, MW irradiation, for 1 h, and with an applied
power of 20 W, is instrumental for good to excellent func-
tionalization of SWNTs, yielding group coverages up to
[bmim]BF4. The higher the values of G’ and G’’, the greater
the number of interactions and/or the stronger the interac-
tions within the gel.[22] These re-
sults point to the fact that, be-
sides the prevailing cation–p in-
teractions as already highlight-
ed elsewhere,[11] Van der Waals
interactions between the ILs
and the SWNT surface, due to
the presence of the alkyl chains
on the imidazolyl group, do
affect the viscosity of the bucky
gels and the ability of the inves-
tigated ILs to exfoliate SWNTs.
The better performance of
[hvim]ACHTUNGTRENNUNG[(CF3SO2)2N] at SWNT
debundling is probably due to
its higher polarizability (thanks
to both its long alkyl chain and Scheme 2. Azomethine ylide cycloadditions to IL-containing bucky gels under MW irradiation.
Chem. Eur. J. 2009, 15, 12837 – 12845
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12841

M. Bonchio, M. Maggini, A. Fontana, M. Prato et al.
Table 6. MW-assisted azomethine ylide cycloaddition to SWNTs in IL.
interactions. Therefore, in the latter case, the mobility of
one or more reactants might be particularly reduced, for ex-
ample, by selective entrapment in the gel network, to an
extent adversely affecting the reaction rate.
In agreement with what is observed for the parent
[60]fullerene, the addition of o-DCB as a co-solvent brings
about a substantial improvement of the SWNT functionali-
zation yield (entry 6, Table 6). Besides the TGA result, this
is unequivocally confirmed by the time evolution of the
Raman spectra, collected with excitation at 632.8 nm, of the
treated SWNT by MW irradiation under the reaction condi-
tions (Figure 2). Comparison with the Raman features of
Entry[a] Aldehyde
Solvent, T[b]
TGA
wt loss
[%][c]
N[d]
1[e]
2[e]
3[f]
CH3-(CH2)5CHO DMF, 1208C
7
7
9
6
1/164
1/156
1/120
1/191
0
CH3-(CH2)5CHO
CH3-(CH2)5CHO
CH3-(CH2)5CHO
CH3-(CH2)5CHO
CH3-(CH2)5CHO
C8F17-(CH2)2CHO
C8F17-(CH2)2CHO
C8F17-(CH2)2CHO
G
R
4[f]
E
5[f]
N
U
0
6[f,g]
7[f,h]
8[f,i]
9[h,j]
N
16
14
16
27
1/60
C
1/239
1/211
1/108
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] In all reactions: SWNT Carbon Nanotechnologies Incorporated, 5–
10 mg; sarcosine 1.0 mmol; aldehyde 2.0 mmol. [b] Solvent or IL gel and
conditions with conventional heating (8C) or MW irradiation under mag-
netic stirring and simultaneous cooling by compressed air (MW). [c] %
Weight loss determined by thermogravimetric analysis. [d] SWNT func-
tionalization coverage per carbon atom. [e] Sarcosine 0.2 mmol, aldehyde
0.2 mmol, 5 day reaction. [f] 20 W, Tbulk =1208C, 1 h. [g] [omim]BF4/o-
DCB=1/3. [h] Aldehyde 0.2 mmol, sarcosine 0.2 mmol. [i] aldehyde
0.5 mmol, sarcosine 0.5 mmol. [j] 50 W, Tbulk =1408C, 1 h.
1/60. This result represents one step forward in the chemis-
try of CNTs, as far as product yields and reaction time are
concerned.[6]
Moreover, under the conditions adopted, the bucky gels
respond properly to MW absorption, with a well-behaved
and reproducible temperature/pressure rise profile, so that
the scaling up of the synthesis is feasible with a controlled
and safer reaction setup. It is known that pristine SWNTs,
under solvent-free MW irradiation, undergo intense heat re-
lease in conjunction with uncontrolled spark emission and
ignition phenomena. In a dense/viscous solvent environ-
ment, such highly effective MW absorption is prevented, but
also the flash MW-induced thermal activation is sup-
pressed.[6] In this light, the bucky-gel matrix does represent
a powerful alternative, as the IL medium shields the strong
SWNT response, while providing a safer source of instant
heating, by virtue of its fully ionic character.
The data in Table 6 allow us to address the impact of the
IL nature and of o-DCB as cosolvent on the reaction effi-
ciency. In particular, an increased performance is registered
in low viscosity bucky gels. Indeed, the functionalization
coverage decreases in the order [bmim]BF4 > [omim]BF4 >
[hvim]TF2N (entries 3–5, Table 6), thus ranking the ILs in
the reverse order with respect to the viscosity determination
(see previous paragraph). The different abilities of the inves-
tigated bucky gels to function as solvents for the dipolar cy-
cloaddition probably stem from the different mesoscopic
structures of the gels formed. Indeed, many gels contain do-
mains with microviscosities close to those of the neat liquid
components (the ILs in this case),[24] leading to relatively
short translational diffusion coefficients despite a very high
macroscopic viscosity. Indeed, in the present cases, the mea-
sured G’ and G’’ moduli clearly highlight increased interac-
tions within the gel on increasing the length of the alkyl
Figure 2. Time evolution of Raman spectra collected for pristine
SWCNTs (CNI) (solid line) and for CNT derivatives obtained as report-
ed in Table 6: entries 3 (dash-dot line) and 6 (dashed line). In the inset,
the increase of the D band is visible: the higher the functionalization, the
higher the D band in the Raman spectrum.
the starting material, which show a small defect mode at
1314 cmÀ1, points to a gradual increase of this band, corre-
sponding to the functionalization progress. Inspection of the
radial breathing mode (RBM) spectral region (<400 cmÀ1)
in the Raman spectrum provides direct evidence that the ap-
plied MW-assisted protocol is not affecting the overall distri-
bution of the nanotube types. As a further remark, the par-
tial loss of the characteristic interband transitions between
van Hove singularities of SWNTs is also consistent with the
occurrence of their functionalization (Figure 3). TEM
images indicate that the nanotube bundles are highly disper-
sible in polar organic solvents such as DMF (Figure 4 and
Figure S10 in Supporting Information).
The rational explanation for the enhanced efficiency ob-
served with the molecular cosolvent is probably twofold,
and not only related to solubilization/diffusion effects, but
also due to the parallel inhibition of the competitive retro-
cycloaddition process. Indeed, pyrrolidine cycloreversion has
been demonstrated also in the case of covalently modified
SWNTs, under thermal and catalytic conditions.[6a]
As in the case of [60]fullerene, the MW-assisted protocol
has been applied to the introduction of fluorous-tagged pyr-
rolidine moieties onto the SWNT surface (entries 7–9,
chain, with [hvim]
ACHTUNGTRENNUNG[(CF3SO2)2N] being the IL characterized
by the greatest number of interactions and/or the strongest
12842
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ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 12837 – 12845

Functionalization of Carbon Nanostructures
FULL PAPER
The easy access to novel CNS-based materials with a
broader range of properties may be envisaged, and, in the
present work, we have proven that interesting fluorous-
tagged derivatives can be prepared in a very simple way.
Experimental Section
General: 1H and 19F NMR spectra were recorded at 298 K on a Bruker
Avance DRX 400 spectrometer, operating, respectively, at 400.13 and
376.50 MHz.
d values in ppm are relative to SiMe4 and C6H5CF3.
13C{1H} NMR spectra were recorded at 298 K on a Bruker AC250 spec-
trometer operating at 62.9 MHz. d values in ppm are relative to SiMe4.
ESIMS (+) spectra were collected with a Agilent LC/MSD Trap SL in-
strument. UV/Vis spectra were recorded on a Perkin–Elmer Lambda 45
spectrophotometer using quartz cells with 1 mm path length. Micro-
Raman spectra were recorded with an inVia Renishaw spectrometer
equipped with a He–Ne (632.8 nm) laser. UV/NIR spectra were recorded
in 1 cm quartz cuvettes on a Varian
Figure 3. UV/NIR spectra in DMF of SWCNTs (CNI) (solid line) and of
CNT derivatives obtained as reported in Table 6: entries 1 (dotted line),
3 (dash-dot line), and 6 (dashed line).
Cary 5000 spectrophotometer. FTIR
(KBr) spectra were collected by using
a Nicolet 5700 instrument. The ther-
mogravimetric analyses were per-
formed with a TGA Q500 (TA Instru-
ments) at 108CminÀ1 under N2. For
the TEM analyses, a small amount of
the functionalized SWNTs was sus-
pended in DMF, and a drop of the sus-
pension was placed on a copper grid
(3.00 mm, 200 mesh, coated with
carbon film). After being air-dried, the
sample was investigated by TEM Phi-
lips EM 208, with accelerating voltage
of 100 kV. Continuous microwave irra-
diation was carried out in a CEM-Dis-
cover monomode microwave appara-
tus, with simultaneous monitoring of
irradiation power, pressure, and tem-
perature. Compressed air was applied
to improve the temperature control of
Figure 4. TEM images of functionalized CNT derivatives obtained as reported in Table 6 [entries 2 (a), 3 (b), 6
(c)] and dispersed in DMF.
Table 6). Interestingly a 1/108 functional coverage is ob-
the reaction mixtures. All reactions were monitored by HPLC, using a
Shimadzu LC-10AT VP pump system, equipped with a UV detector
SPD-10 A VP set at 340 nm. HPLC analysis was performed with Phe-
nomenex Luna and Cosmosil Buckyprep columns: (250ꢄ4.6 mm, 5 mm
particles).
tained by using a lower excess of the fluorous-tagged alde-
hyde, at an applied MW power of 50 W. The resulting mate-
rial features an unprecedented affinity for fluorinated
phases. Solutions of up to 1 mg in hexafluoroisopropanol
(1 mL) remain stable for weeks, without showing apprecia-
ble material precipitation. TGA data, Raman spectra, and
TEM images provide converging evidence of the expected
functionalization (see the Supporting Information).
Fulleropyrrolidines 1–3 and the fluorous-tagged aldehyde CF3-
AHCTUNGTRENNUNG
(CF2)7CH2CH2CHO were prepared following literature procedures.[2,25]
Generally, sarcosine (50 mg, 0.56 mmol) and the aldehyde (1.12 mmol)
were added to a solution of fullerene (200 mg, 0.28 mmol) in toluene
(150 mL). The reaction mixture was heated under reflux for 5 h. Then
the solution was reduced in volume and the crude product was purified
by flash column chromatography. All the spectral features were in agree-
ment with the expected structures.
Conclusions
Mono-FP 4: Yield: 60 mg, 30%. Soluble in C6H6, C6H5CH3, CHCl3. Rf =
1
2
0.84 (toluene); H NMR (400.13 MHz, C6D6, 258C, TMS): d=4.28 (d, J-
An innovative strategy for the functionalization of CNS has
been designed by the combined use of IL phases and MW ir-
radiation. Under the conditions explored, azomethine ylide
cycloaddition to [60]fullerene occurs smoothly in just 2–
10 min. The same procedure has been successfully applied
to SWNTs. Here, the use of IL-structured bucky gels as re-
action media allows a controlled flash thermal activation in-
duced by MW absorption, providing a straightforward tool
for the covalent functionalization of the SWNT surface.
N
ACHTUNGTRENNUNG
2.54 (m, 4H, CH2CH2), 2.43 ppm (s, 1H; CH3); 19F NMR (376.47 MHz,
A
2
N
(CH2CH2CF2) (signals for C-F nuclei, expected between 125 and
Chem. Eur. J. 2009, 15, 12837 – 12845
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12843

M. Bonchio, M. Maggini, A. Fontana, M. Prato et al.
105 ppm, cannot be detected due to their low intensity, resulting from
(C,F) scalar couplings); FTIR (KBr): n˜ =2961, 2922, 2851, 2783, 1261,
Acknowledgements
JACHTUNGTRENNUNG
1239, 1205, 1146, 1133, 1097, 1028, 804, 704, 574, 554, 527 cmÀ1; UV/Vis
(CDCl3): lmax (e)=254 (64960), 308 (21434), 427 (2079), 430 (3347),
700 nm (317 molÀ1dm3 cmÀ1); ESIMS (+) (CH2Cl2:CH3OH=1:1,
CF3CO2H 0.01%): m/z: calcd for C73H11F17N+: 1223.9; found: 1223.9.
We thank Prof. Moreno Meneghetti (Nanophotonics Laboratory, Univer-
sity of Padova, Dept. of Chemical Sciences) for assistance with Raman
spectroscopy. This work was carried out with support from the Universi-
ties of Padova (Progetto Strategico 2008, HELIOS, prot. STPD08RCX),
Trieste, and Chieti, ITM-CNR, INSTM, MIUR (PRIN 2006, prot.
200634372 and FIRB, prot. RBNE033 KMA).
Procedure for MW-assisted synthesis of FPs: Unless otherwise stated,
fullerene (5 mg, 0.07 mmol) was dispersed upon sonication in a mixture
of ionic liquid and o-dichlorobenzene, together with sarcosine (1.2 mg,
0.14 mmol) and the aldehyde (0.28 mmol), in a closed glass test tube.
Continuous microwave irradiation was carried out with simultaneous
monitoring of irradiation power, pressure, and temperature. Samples
were diluted using o-dichlorobenzene and analyzed by HPLC. Retention
times of the corresponding monoderivatives are the following: (HPLC,
Phenomenex Luna, toluene/n-hexane 4/1): 7.4 (1), 3.9 (2), 9.2 (3), 4.1
(4) min. Mono- and poly-FPs 2 have been characterized by ESIMS(+), in
CH2Cl2:CH3OH=1:1, CF3CO2H 0.01%, as follows: mono-FP 2: m/z:
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+
C98H31N2+: 1236.2; found: 1236.2; tris-FP 2: m/z: calcd for C117H46N3
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Bucky gel preparation: The proper amount of purified and unpurified
HiPCO SWNTs was poured in an agate mortar and ground for 30 min
with 1 mL (or 1 g in the case of solid IL) of the ionic liquid. The obtained
suspension was poured into a vessel and sonicated using an ultrasonic
bath sonicator (Bandelin Sonorex, 35 KHz) for 1 h. The sonication in-
creases the apparent viscosity of originally liquid samples. [Hvim]-
ACHTUNGTRENNUNG[(CF3SO2)2N] is solid at room temperature. Therefore, the grey powder
obtained by 30 min grinding in the mortar was poured into a vessel and
heated at 508C before sonication. The obtained sample was either used
for synthetic purposes or poured in the rheometer for the rheological
measurements. No centrifugation step was performed in order to remove
excess IL. The samples appeared to be quite stable for a few weeks, and
did not tend to separate into a black gel phase and a transparent liquid
phase within this time frame.
Rheology: Static and dynamic measurements were performed on
a
Thermo Scientific modular rheometer Haake M.A.R.S. II equipped with
an Electron Corporation thermocontroller system Haake Phoenix (data
evaluation: RheoWin software 3.61) using the cone plate geometry (cone
diameter 6 mm, angle 0.58). All samples, subjected to vacuum pumping
before measurements, were directly loaded onto the plate of the rheome-
ter and were allowed to equilibrate for only 6 min. As a matter of fact
we did not pre-stress the samples or allow a longer equilibration time, be-
cause this would have required the samples to be left undisturbed be-
tween the plates for around 24 h, and this delay would have favored the
undesired hydration of the ionic liquids. The shear stress sweep tests
showed that 1.0 Pa satisfies the linear viscoelasticity for all samples. The
dynamic storage and loss moduli were examined in the linear viscoelastic
regime at 258C unless otherwise stated.
Procedure for the MW-assisted functionalization of bucky gels: Unless
otherwise stated, 5 mg of HiPCO purified SWCNTs, used as received
from CNI, were dispersed in the IL and mixed with sarcosine (1 mmol)
and aldehyde (2 mmol). The reactions were carried out in closed glass
test tubes with stirring, under microwave irradiation, as detailed in the
Supporting Information. The reaction mixtures were cooled down to
room temperature and then suspended in DMF. Afterwards they were fil-
tered through a Millipore filter (JH 0.45 mm). The resulting material was
washed on the filter with DMF and diethyl ether and recovered, then it
was resuspended in DMF upon sonication for 20 min. It was refiltered,
rewashed on the filter with DMF and diethyl ether, and at this point the
collected black material was suspended in acetone and sonicated for
20 min in order to remove all the ILs. Finally, it was filtered again, and
washed on the filter with acetone and diethylether. Thermal reactions,
with and without ILs, were carried out for comparison purposes, by heat-
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Chem. Eur. J. 2009, 15, 12837 – 12845

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Chem. Eur. J. 2009, 15, 12837 – 12845
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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12845