Structural requirements for producing solvent-free room temperature liquid fullerenes

Article abstract of DOI:10.1021/la400969f

A new class of solvent-free room temperature liquid fullerenes was synthesized by attaching a single substituent of 1,3,5-tris(alkyloxy)benzene unit to C₆₀ or C₇₀ under the Prato conditions. Although the C₆₀ monoadducts we

Full text of DOI:10.1021/la400969f

Article
pubs.acs.org/Langmuir
Structural Requirements for Producing Solvent-Free Room
Temperature Liquid Fullerenes
Tsuyoshi Michinobu,*,†,‡,§ Kensuke Okoshi,‡ Yoshihiko Murakami,‡ Kiyotaka Shigehara,‡
Katsuhiko Ariga,† and Takashi Nakanishi*,†
†National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan
‡Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
§Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552,
Japan
S
* Supporting Information
ABSTRACT: A new class of solvent-free room temperature
liquid fullerenes was synthesized by attaching a single
substituent of 1,3,5-tris(alkyloxy)benzene unit to C60 or C70
under the Prato conditions. Although the C60 monoadducts
were single components after chromatographic purification,
the C70 monoadducts were isomeric mixtures due to the
prolate spheroidal π-chromophore. The alkyl chain length of
the substituents significantly affected both melting points and
rheological behavior of the fullerene derivatives. When the alkyl chains were short, the intermolecular π−π interactions of
adjacent fullerene cores led to a melting point higher than room temperature. In contrast, in the case of exceedingly long alkyl
chains, such as eicosyl (−C20H41) and docosanyl (−C22H45) groups, the van der Waals interactions among neighboring alkyl
chains became dominant. Accordingly, only medium alkyl chain lengths could provide solvent-free fluidic fullerenes with low
melting points. The rheological measurements of the liquid fullerenes at 25 °C revealed their unique liquid characteristics;
molecular-level friction (or viscosity) and nanometer-scale clustering were noticed. It is generally thought that alkyl chains serve
as a stabilizer of the fullerene core units. Thus, a longer chain or higher plasticity of the stabilizers would promote the disturbance
of the core−core interactions. It was indeed shown that longer alkyl chains resulted in a lower fluid viscosity. It was also found
that metastable solid phases were produced by the noticeable van der Waals interaction between the long alkyl chains especially
when a symmetric C60 core was adopted. This interesting finding enabled the comparison of electrochemical activities of the C60
unit between the solvent-free liquid and metastable solid form, which revealed a superior electrochemical activity in the liquid
state.
sometimes become a glassy or wax-like material at room
temperature.17−20 This tendency is significant when the
substituents are flexible long chains, such as oligo(ethylene
oxide), oligosiloxane, and alkyl chains, with some such
fullerenes having been reported that were viscous oils or fluids
at room temperature.21−25 However, besides the pioneering
work26 in which it was reported that the weak long-range order
of C60 symmetrically substituted with six pairs of stearyl chains
resulted in the liquid state, the isotropic liquid phase remains
rather unexplored.
Recently, other nanocarbon fluids with unique opto- and
electroactive properties are also getting much attention due to
their potential for large-scale production at low cost and facile
solvent-free processing for use in practical applications.27−30 In
these cases, nanocarbon materials including carbon nanotubes
and graphenes were first functionalized with charged
INTRODUCTION
■
The phase diagram of fullerenes, particularly concerning the
existence of a liquid phase of fullerenes, has been controversial
for a long time.1,2 Because of the limited availability of harsh
experimental conditions, theoretical approaches have played an
important role to predict the existence of a liquid phase and its
narrow temperature range (e.g., 1880−1980 K for C60).3 In the
meantime, organic chemists have opened the door to
fascinating fullerene chemistry by using conventional synthetic
methods, such as cyclopropanation and several kinds of
cycloaddition reactions, making it possible to introduce a
wide variety of functional groups onto fullerenes.4−7 For
example, many fullerene derivatives containing mesogenic
groups have been prepared and their thermotropic liquid
crystalline phases characterized.8−12 Regio- and stereoselective
multiple functionalization of fullerenes has also raised particular
interest to tune the physical properties of fullerene derivatives
and construct supramolecular architectures.13−16 As the
number of amphiphilic substituents increases, the melting
points of functionalized fullerene derivatives decrease, and they
Received: March 14, 2013
Revised: April 1, 2013
Published: April 3, 2013
© 2013 American Chemical Society
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a
Scheme 1. Synthesis of 2,4,6-Tris(alkyloxy)benzaldehyde Derivatives and Their Fullerene (C60 or C70) Adducts
a
Reagents and conditions: (a) CnH2n+1Br, K2CO3, KI, DMF, 70 °C; (b) POCl3, DMF, 70 °C; (c) C60 or C70, N-methylglycine, toluene, 110 °C.
substituents, such as carboxylate and sulfonate groups. This was
followed by the complexation with poly(ethylene glycol)−
ammonium salts through ionic interactions. The produced
ionic-nanocarbon materials showed excellent solubility or
dispersive abilities in various solvents, and their melting points
were significantly lowered. This route successfully provided
solvent-free room temperature nanocarbon ionic fluids.
tris(alkyloxy)benzene derivatives 1a−1c in a 25−69% yield.
This was followed by the introduction of a formyl group under
the modified Vilsmeier conditions in 28−79% yield. Alter-
natively, the direct etherification of the commercially available
2,4,6-trihydroxybenzaldehyde also provided the target com-
pounds. It was found that the use of excess bromoalkanes was
the key to achieve high reaction yields in the direct
etherification.
We have shown that the bulk morphology of fullerene
derivatives can be controlled by utilizing two different
intermolecular forces of C60 and alkyl chain, namely π−π
interactions between C60 moieties and van der Waals
interactions among the alkyl chains.31−38 The N-methyl[60]-
fulleropyrrolidine derivatives with 3,4,5-tris(alkyloxy)phenyl
group exhibit a variety of solvent-dependent hierarchical
architectures in the volume phase as well as perfectly straight
nanowires on a graphite substrate.39,40 In the course of these
studies, we serendipitously found that some N-methyl[60]-
fulleropyrrolidines with 2,4,6-tris(alkyloxy)phenyl group show
solvent-free liquid phase at room temperature (at 25 °C).41
Here we report detailed structural studies on the full-
eropyrrolidine derivatives. The relationship between alkyl
chain length and melting point suggests that room temperature
liquid [60]fullerene derivatives are produced when the alkyl
chain lengths are in the range from dodecyl (−C12H25) to
docosanyl (−C22H45). The liquid characteristics were evaluated
by rheological measurements. A similar correlation between the
alkyl chain length and melting point was also revealed for
higher [70]fullerene derivatives. This is the first comprehensive
report on the room temperature liquid fullerenes, which could
be utilized in various optoelectronic applications.42,43
A mixture of the 2,4,6-tris(alkyloxy)benzaldehyde, N-
methylglycine, and C60 was refluxed in dry toluene. After
cooling to room temperature, the crude products were
thoroughly purified by preparative gel permeation chromatog-
raphy (GPC) followed by silica gel column chromatography,
yielding the fullerene monoadducts C60a−C60f in moderate
yields (30−64%). Conventional analytical techniques, such as
1H and 13C NMR, FT-IR, and UV/vis spectroscopy as well as
MALDI-TOF mass spectrometry, confirmed the structures of
the derivatives and their high purity (see Supporting
Information). The same synthetic methodology was applied
to the preparation of C70 monoadducts C70a−C70c. In
contrast to the highly symmetric C60, a higher fullerene of
C70 has four different types of the reactive 6−6 bonds.45−47
Several isomers were thus obtained as monoadducts after
chromatographic purification. For example, the 1H NMR
spectrum of C70a showed four peaks ascribed to the methyl
groups of the pyrrolidine ring (Figure 1). Their integration
ratios of 5:2:2:1 suggested that at least four isomers were
produced.48 The 13C NMR spectrum of C70a also indicated a
mixture of isomers, which was also the case for C70b and C70c.
However, the MALDI-TOF MS spectrometry of the C70
monoadducts clearly showed the single molecular ion peaks
ascribed to the exact molecular weights, and the elemental
analysis exhibited fairly good agreement with the theoretical
values.
RESULTS AND DISCUSSION
■
Synthesis of Fullerene Derivatives. Fullerene derivatives
with different alkyl chain lengths (from n-C8H17 to n-C22H45 for
C60 derivatives and from n-C18H37 to n-C22H45 for C70
derivatives) were synthesized by the Prato method44 using
2,4,6-tris(alkyloxy)benzaldehyde, N-methylglycine, and C60 or
C70 in refluxing dry toluene (Scheme 1). In order to prepare
2,4,6-tris(alkyloxy)benzaldehyde derivatives 2a−2f, two routes
were adopted. Starting from the commercially available 1,3,5-
trihydroxybenzene, triple etherification with 1-bromoalkane
under basic conditions afforded the corresponding 1,3,5-
Thermal Stability and Phase Behavior. The thermal
stability of both C60 and C70 derivatives was evaluated by
thermogravimetric analysis (TGA) under nitrogen flow at the
heating rate of 10 °C min−1. The representative TGA curves are
shown in Figure S1. All the fullerene derivatives were thermally
stable with the 5% decomposition temperatures (Td95%
)
exceeding 300 °C (Table S1). This indicates the high thermal
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temperature was observed. Therefore, they are recognized as
a liquid state at room temperature. This fact suggested a
correlation of the melting points with the alkyl chain length
surrounding C60. Based on molecular modeling, the length of
an octyl chain in an all-trans conformation is estimated to be
9.0 Å, whereas that of a dodecyl chain is 14.0 Å. Judging from
the diameter of C60 (7.1 Å),49,50 three dodecyl chains might be
expected to serve as an effective stabilizer to somewhat prevent
C60 aggregation and lower the melting point. However, when
the alkyl chain length was further elongated, the melting points
started to increase. The melting (glassy transition) points of
C60d, C60e, and C60f were −7.9, 4.5, and 21.5 °C,
respectively. These results indicated that the liquid phase of
C60 derivatives can be produced by controlling the length of
alkyl chains. Assuming a constant temperature of 25 °C, room
temperature liquid C60 derivatives are obtained when the alkyl
chain lengths are in the range from dodecyl to docosanyl
groups (Figure 2a). It should be noted that clear transition
peaks ascribed to the melting (glassy transition) points were
detected in the second heating scan of the DSC curves when
the alkyl chain length was longer than an eicosyl group (Figure
S2). This was most likely because the alkyl chains dominated
the phase behaviors of the fullerene derivatives (vide infra). The
C70 derivatives displayed phase behaviors similar to the C60
derivatives. When the alkyl chain length attached to the C70
unit increased from octadecyl to docosanyl, the melting (glassy
transition) point gradually increased from −10.5 to 20.6 °C
(Figure 2b and Table 1). Despite the extended π-system of the
C70 unit, the observed melting (glassy transition) points are
similar to those of the corresponding C60 derivatives. This is
probably due to the isomeric mixtures of the C70 monoadducts,
which prevented better molecular packing. In other words, a
trade-off between the presumably increased π−π interaction
and the effect of isomers is the cause of similar melting (glassy
transition) points.
It is significant to find out a general rule for reducing melting
points of organic π-systems.51 We noted the simple π-system of
benzene derivatives with three alkyl chains. A literature search
on 1,3,5-tris(alkyloxy)benzene or 2,4,6-tris(alkyloxy)-
benzaldehyde derivatives revealed an interesting fact (Tables
S2 and S3). The melting points of these derivatives also
depended on the alkyl chain length. When alkyl chain lengths
are short, the π−π interaction between benzene rings leads to
an increase in the melting points. In contrast, when alkyl chain
lengths are very long, the main intermolecular forces are van
der Waals interactions between adjacent alkyl chains, resulting
in a gradual increase in the melting point with the increasing
alkyl chain length. Only when these two different intermo-
lecular forces are competing, the melting points considerably
decreased and liquid phases appeared at room temperature
1
Figure 1. H NMR spectrum of C70a in CDCl3 at 25 °C. The peaks
ascribed to the methyl group of the pyrrolidine ring are enlarged.
stability of the fulleropyrrolidine derivatives even in a liquid
form (vide infra).
The phase transition temperatures were investigated by
differential scanning calorimetry (DSC) under nitrogen flow at
a scanning rate of 10 °C min−1 (Figures S2 and S3). The C60
derivative C60a with three octyl chains in the 2,4,6-positions of
the phenyl group was a dark brown solid at room temperature.
The melting point of this compound was 147−148 °C (Table
1). Interestingly, when the alkyl chain length increased, the
Table 1. Summary of the Melting (Glassy Transition) Points
of Fullerene Derivatives
a
n
M.p. of [C60 derivatives] (°C)
mp of [C70 derivatives] (°C)
b
8
147−148 [C60a]
c
12
16
18
20
22
13.7 [C60b]
c
−36.5 [C60c]
c
c
−7.9 [C60d]
−10.5 [C70a]
c
c
4.5 [C60e]
14.6 [C70b]
c
c
21.5 [C60f]
20.6 [C70c]
a
The carbon number (n) of alkyl chains attached to the fullerenes.
Determined by melting point measurements. Determined by the
second heating scan of DSC measurements.
b
c
melting points dramatically decreased. For example, C60b with
three dodecyl chains and C60c with three hexadecyl chains
showed their melting points (glassy transition) of 13.7 and
−36.5 °C, respectively. No transition above the melting
Figure 2. Relationship between the melting points and alkyl chain lengths of the fullerene ((a) C60 and (b) C70) derivatives.
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(Figure 3). This literature search supports a correlation
between the melting point and alkyl chain length of the
fulleropyrrolidine derivatives found in this study.
derivatives. However, these solvent-dependent morphology
changes (solidification) did not occur in the case of the C70
derivatives. This is probably because the C70 monoadducts are
isomeric mixtures, which prevents better packing of the
molecules for producing template-induced solid phases.
X-ray diffraction (XRD) analysis at 25 °C substantiated the
solvent-dependent phase behavior of C60e and C60f. The
liquid form of these C60 derivatives exhibited a very broad halo
at 2θ ≈ 7.4° (d = 1.2 nm) like other liquid C60 derivatives
C60b−C60d (Figure S5). This halo is likely to indicate the
average spacing of the adjacent C60 moieties in the liquid state.
However, the solid form showed several well-defined, complex
peak patterns with the most intense diffraction peak at 2θ ≈
3.5° (d = 2.5 nm) for C60e and at 2θ ≈ 3.3° (d = 2.7 nm) for
C60f (Figure 4). These d spacing values are approximately
equal to the lengths of fully extended eicosyl or docosanyl
chains, respectively. It is thought that the volatile alkanes, i.e., n-
hexane and n-heptane, help these long alkyl chains to order,
leading to crystalline, solid fullerene assemblies in which the
alkyl chains are likely to be interdigitated. Conversely, the XRD
halos of C70 derivatives were broad, as expected from the liquid
phases at 25 °C (Figure S6). The broad peak at 2θ ≈ 3−4° may
indicate some underlying nanocrystallinity, while that at 2θ ≈
6−8° could originate from the average spacing of adjacent C70
moieties.
Comparing the DSC data of the solid and liquid samples
presented more meaningful outcomes. As shown in Figure S2,
the DSC curves of the liquid C60e and C60f revealed well-
defined melting transition at 4.5 and 21.5 °C, respectively. In
contrast to these transition behaviors, the first cooling trace of
the solid samples of C60e and C60f did not display the distinct
transitions in the temperature range of −50 to 20 °C, but the
subsequent heating scan exhibited a very sharp endothermic
peak at 55 and 59 °C, respectively (Figure 5a,b). It should be
noted that these transition peaks did not exist in the DSC trace
of the liquid forms (Figure S2). The enthalpy and entropy at
the transition temperature were calculated as ΔH = 92.1 kJ
Figure 3. Relationship between the melting points and alkyl chain
lengths of (a) 1,3,5-tris(alkyloxy)benzene and (b) 2,4,6-tris(alkyloxy)-
benzaldehyde compounds.
Very interestingly, it was found that the C60 derivatives with
three eicosyl or docosanyl chains, C60e41 and C60f, formed a
solid phase when n-hexane or n-heptane solutions are slowly
evaporated at room temperature. Other organic solvents such
as CHCl3 and CH2Cl2 did not produce this solid phase. This
fact implied a templating effect of the alkane solvents. Both the
1H NMR spectra and the TGA curves indicated the absence of
residual solvents in these solid samples within their detection
limits. The FT-IR spectra of the solid samples of C60e and
C60f were measured, and the peak top positions were
compared to those of the liquid samples (Figure S4). The
solid sample of C60e exhibited asymmetric and symmetric CH2
stretching vibrations at 2920.7 and 2850.3 cm−1, while the
liquid C60e showed the corresponding vibrations at 2922.6 and
2851.3 cm−1, respectively. The high energy shifts by 1−2 cm−1
indicate a more disordered conformation of the alkyl chains in
the liquid sample.36 Similar to this result, the liquid C60f also
displayed a high-energy shift by 1 cm−1 relative to the solid
C60f (Figure S4). The FT-IR results revealed clear interactions
between the alkyl chains in the solid states of these C60
Figure 4. XRD patterns of the liquid forms of (a) C60e and (b) C60f and the solid forms of (c) C60e and (d) C60f prepared by slow evaporation of
an n-hexane solution.
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Figure 5. DSC thermograms of a sample of (a) C60e and (b) C60f prepared by slow evaporation of an n-hexane solution. Optical images of (c, d)
C60e in the liquid form at 25 °C. (e) Optical and (f) SEM images of C60e in the solid form at 25 °C. Scale bars represent (d, e) 100 μm and (f) 10
μm.
Figure 6. Rheological behavior of liquid fullerenes at 25 °C. Angular frequency (ω) dependence on (a) storage (G′) and loss moduli (G″) of C60b,
C60c, and C60e at an applied strain amplitude (γ) of 0.01 and (b) storage and loss moduli of C60e at γ values of 0.01, 0.1, and 1.0.
mol−1 and ΔS = 166.8 J mol−1 K−1 for C60e and as ΔH = 95.8
kJ mol−1 and ΔS = 171.4 J mol−1 K−1 for C60f. The entropy
values are in good agreement with the total entropy of phase
transition for alkyl chains (ΔS = 115.8 and 127.4 J mol−1 K−1
are estimated for the three eicosyl chains and three docosanyl
chains based on ΔS = 1.93 J mol−1 K−1 per methylene unit52)
and the C60 part (ΔS = 30−40 J mol−1 K−1).53,54 Although a
small endothermic peak at around 40 °C before melting
appeared at the heating trace in the DSC of C60f (Figure 5b),
it was difficult to assign its origin due to such a small transition
peak from the first cycle in DSC. In the second cycle of DSC
measurements, the corresponding exothermic peaks did not
appear upon cooling, and the entire transition behaviors were
the same as those of the liquid forms. These results suggested
that the solids induced by volatile alkanes as a template are
metastable states, and the liquids are the most stable form.
Note that these metastable states are only observed for C60e
and C60f with among the longest alkyl chains in this study. It
was further confirmed by the analyses of optical microscopy
and scanning electron microscopy (SEM) that these transition
events are ascribed to the melting points of each solid sample.
As an example, the optical micrograph and SEM images of
C60e are shown. The optical images of the bulk liquid sample
were taken at 25 °C (Figure 5c,d). The thin plate sample
sandwiched by two glass plates showed brown color liquid. The
solid sample prepared by slow evaporation of an n-hexane
solution displayed needle-shaped crystalline-like texture in
micrometer scale (Figure 5e). The SEM images of the solid
sample exhibited similar morphology (Figures 5f and Figure
S7). These solid samples became isotropic liquids when heated
to 80 °C, and their optical images were unchanged after the
repeated cooling and heating cycles. In addition, no
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birefringence appeared under polarized light irradiation to the
liquid samples, indicating their isotropic states.
initial cluster size could not be estimated for the liquid C70
series. However, it is interesting to compare the rheological
properties between the C60 and C70 derivatives. The complex
viscosity η*of all measured samples are plotted in Figure 8. The
Rheology. The rheological behavior of selected liquid
fullerenes was investigated at 25 °C. Storage (G′) and loss
moduli (G″) were measured at an applied strain amplitude (γ)
of 0.01. The G″ values were higher than the G′ for the liquid
C60 derivatives (C60b, C60c, and C60e) in the measured
frequency range, suggesting liquid-like behavior (Figure 6a).
This was also the case for the liquid C70 derivatives (C70a−
C70c) (Figure 7). The angular frequency (ω) dependency of
Figure 8. Complex viscosity (η*) of liquid C60 and C70 derivatives at a
γ of 0.01 at 25 °C.
constant η* values of each sample in the measured frequency
range suggested the reliable data. It was found that the complex
viscosity has a clear correlation with the alkyl chain length. The
η* value decreased with the increasing alkyl chain length in
both C60 and C70 series. In other words, the frictional
coefficient between molecules and/or clusters decreased as an
increase in the alkyl chain length. Consequently, it was thought
that the friction was mainly derived from the strong interaction
between fullerene (C60 or C70) moieties. A comparison of the
η* values between C60e and C70b with the eicosyl chains
indicated the greater η* value of C70b. This result is a good
indication of the stronger π−π interaction for C70b due to the
extended π-system.55 It is noted that an attachment of various
branched alkyl chains onto the similar N-methyl[60]-
fulleropyrrolidines with 2,4,6-, 3,5-, or 2,5- alkyloxyphenyl
group was also shown the formation of low-viscosity liquid
fullerenes.43 Therefore, the balance of π−π interaction of
fullerene units and inter- and/or intra-van der Waals
interactions of attached alkyl chains plays a significant role to
produce the liquid fullerenes.
Electrochemistry. In order to pursue application possibil-
ities of liquid fullerenes, it is important to demonstrate
attractive properties of pristine fullerenes. For example, the
photophysical properties of one of liquid C60 derivatives
(C60e) were recently reported.42 Another significant feature of
fullerenes is potent redox activities in the cathodic direction. In
this regard, the monoadduct approach is more advantageous
than previously reported multiadduct examples. Multiple
addition of substituents results in a decrease in the conjugated
fullerene chromophore and consequently in the loss of the rich
features such as a strong electron accepting ability and
reversible reductions incorporating up to six electrons. Cyclic
voltammetry (CV) of all the fullerene derivatives was recorded
in CH2Cl2 containing 0.1 M (nC4H9)4NClO4 at 25 °C. Three
well-resolved, reversible reduction steps were observed in both
cases of C60 and C70 derivatives (Figures S8 and S9). These CV
curves are similar to those of pristine C60 or C70 measured
under the same conditions. Note that strict conditions under
inert atmosphere at low temperatures are required for the
detection of six reduction steps.56 In comparison to pristine C60
or C70, the reduction potentials of the fullerene monoadducts
were slightly shifted in the cathodic direction due to the
electron-donating pyrrolidine substitution (Table S4). More-
over, the liquid form exhibited higher electrochemical activity.
The solvent-free liquid form of C60e, prepared on a glassy
Figure 7. Rheological behavior of liquid fullerenes at 25 °C. Angular
frequency (ω) dependence on storage (G′) and loss moduli (G″) of
C70a, C70b, and C70c at an applied strain amplitude (γ) of 0.01.
both series of C60 and C70 derivatives indicated that a number
of components with a long relaxation time exist for each
sample, which would reflect the formation of large clusters due
to the specific intermolecular interactions and their complicated
movements. The presence of such clusters in a bulk liquid
sample of C60e was fortunately confirmed by the varied strain
amplitudes. When the γ value increased from 0.01 to 1.0, the G′
and G″ curves of C60e deviated downward at the high-
frequency region, implying that the initially existing structure
was broken (Figure 6b). Note that a γ value of 0.1 did not cause
any deviation. Using the onset frequency of this deviation
(0.628 rad s−1) as well as the absolute value of the complex
viscosity (η*) (1400 Pa s) (vide infra), the effective
hydrodynamic radius (a) of the preformed structure of C60e
was estimated from the Stokes−Einstein equation (eq 1).
kT
6πηa
D =
(1)
Here, D is the diffusion coefficient, k is Boltzmann constant, T
is the temperature, and η is the viscosity. The diffusion
coefficient can also be expressed as eq 2, which is derived from
Fick’s law.
a2
6τ
D =
(2)
Here, τ is the time in which the clusters can traverse a distance
a. Combination of eqs 1 and 2 leads to eq 3, giving the a value
of the spherical cluster.
1/3
⎛
⎜
⎞
⎟
τkT
a =
⎝ πη ⎠
(3)
The determined a value was ∼21 nm, which corresponds to
the size of a cluster consisting of several hundreds of the C60e
molecule.
In contrast to the C60 series, the strain amplitude dependency
of the C70 derivatives did not show such a clear change,
probably due to the mixture of isomers. For that reason, an
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(3) Hasegawa, M.; Ohno, K. Monte Carlo Simulation Study of the
High-Temperature Phase Diagram of Model C60 Molecules. J. Chem.
Phys. 1999, 111, 5955−5963.
carbon electrode from a CH2Cl2 solution, showed a reversible
two-electron reduction (Figure S10). However, the correspond-
ing solid form of C60e displayed only a single redox peak under
the same conditions.
(4) Taylor, R.; Walton, D. R. M. The Chemistry of Fullerenes. Nature
1993, 363, 685−693.
(5) Hirsch, A. The Chemistry of the Fullerenes: An Overview. Angew.
Chem., Int. Ed. 1993, 32, 1138−1141.
CONCLUSIONS
■
Room temperature liquid fullerenes, including both C60 and
C70, were successfully produced by adjusting the length of the
linear-saturated three alkyl chains. By tuning the length of the
alkyl chains, a balance between their ability to disrupt the
interactions between π-chromophores while retaining low
alkyl−alkyl van der Waals interactions could be achieved. In
the case of C60 derivatives, the alkyl chain length required to
reduce the melting point below room temperature was between
dodecyl (−C12H25) and docosanyl (−C22H45) chains. The
rheology of the liquid fullerenes suggested that viscosity is
mainly governed by the alkyl chains. An increase in the alkyl
chain length thus lowered the frictional coefficient between
molecules. In contrast, the expansion of π-system from C60 to
C70 enhanced the molecular friction. The electrochemical
measurements revealed that all fullerene derivatives, especially
in the liquid form, show active reduction peaks, keeping the
intrinsic properties of pristine fullerenes. Taking into account
the advantage that liquid samples do not considerably suffer
from the influence of domain or grain boundaries, they might
look promising novel carbon materials for applications in
carbon electrodes of secondary batteries and electrochemical
capacitors as well as catalyst supports. The molecular design for
reducing melting temperatures could be applied to other π-
systems,43,57 such as oligophenylenevinylenes (OPVs)58 and
porphyrins.51 Furthermore, our finding would be useful in
facilitating solubilization or homogeneous dispersion of carbon-
based functional nanomaterials.59,60
(6) Diederich, F.; Thilgen, C. Covalent Fullerene Chemistry. Science
1996, 271, 317−323.
(7) Guldi, D. M.; Martin, N. Fullerenes: From Synthesis to
Optoelectronic Properties; Kluwer Academic Publishers: Dordrecht,
Netherlands, 2002.
(8) Zhong, Y.-W.; Matsuo, Y.; Nakamura, E. Lamellar Assembly of
Conical Molecules Possessing a Fullerene Apex in Crystals and Liquid
Crystals. J. Am. Chem. Soc. 2007, 129, 3052−3053.
(9) Li, W.-S.; Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.;
Tagawa, S.; Masunaga, H.; Sasaki, S.; Takata, M.; Aida, T. Amphiphilic
Molecular Design as a Rational Strategy for Tailoring Bicontinuous
Electron Donor and Acceptor Arrays: Photoconductive Liquid
Crystalline Oligothiophene-C60 Dyads. J. Am. Chem. Soc. 2008, 130,
8886−8887.
(10) Guillon, D.; Donnio, B.; Deschenaux, R. Liquid-Crystalline
Fullerodendrimers and Fullero(Codendrimers). In Fullerene Polymers:
Synthesis, Properties and Applications; Martin, N., Giacalone, F., Eds.;
Wiley-VCH: Weinheim, 2009; pp 247−270.
(11) Vergara, J.; Barbera,
́
J.; Serrano, J. L.; Ros, M. B.; Sebastian
́
, N.;
de la Fuente, R.; Lopez, D. O.; Fernan
́
́
dez, G.; Sanchez, L.; Martín, N.
́
Liquid-Crystalline Hybrid Materials Based on [60]Fullerene and Bent-
Core Structures. Angew. Chem., Int. Ed. 2011, 50, 12523−12528.
(12) Mamlouk-Chaouachi, H.; Heinrich, B.; Bourgogne, C.; Guillon,
D.; Donnio, B.; Felder-Flesch, D. Nematic Self-organization of
Regioselectively Polyfunctionalized [60]Fullerene. J. Mater. Chem.
2011, 21, 9121.
́ ́
(13) Diederich, F.; Gomez-Lopez, M. Supramolecular Fullerene
Chemistry. Chem. Soc. Rev. 1999, 28, 263−277.
(14) Nierengarten, J.-F. Chemical Modification of C60 for Materials
Science Applications. New J. Chem. 2004, 28, 1177−1191.
(15) Hirsch, A.; Brettreich, M. Fullerenes: Chemistry and Reactions;
Wiley-VCH: Weinheim, 2005.
ASSOCIATED CONTENT
* Supporting Information
■
S
Characterization of all compounds, TGA and DSC curves,
summary of melting points, XRD patterns, FT-IR spectra, SEM
images, and electrochemistry. This material is available free of
charge via the Internet at http://pubs.acs.org.
(16) Beuerle, F.; Hirsch, A. Synthesis of Amphiphilic [60]Fullerene
[3:3]Hexakisadducts with Four Spherically Defined Addend Zones.
Chem.Eur. J. 2009, 15, 7447−7455.
́
(17) Felder, D.; del Pilar Carreon, M.; Gallani, J.-L.; Guillon, D.;
Nirengarten, J.-F.; Chuard, T.; Deschenaux, R. Amphiphilic Fullerene-
Cholesterol Derivatives: Synthesis and Preparation of Langmuir and
Langmuir-Blodgett Films. Helv. Chim. Acta 2001, 84, 1119−1132.
AUTHOR INFORMATION
Corresponding Author
*E-mail: michinobu.t.aa@m.titech.ac.jp and NAKANISHI.
Takashi@nims.go.jp.
■
́
(18) Felder, D.; Nava, M. G.; del Pilar Carreon, M.; Eckert, J.-F.;
Luccisano, M.; Schall, C.; Masson, P.; Gallani, J.-L.; Heinrich, B.;
Guillon, D.; Nierengarten, J.-F. Synthesis of Amphiphilic Fullerene
Derivatives and Their Incorporation in Langmuir and Langmuir-
Blodgett Films. Helv. Chim. Acta 2002, 85, 288−319.
Notes
The authors declare no competing financial interest.
(19) Braun, M.; Hartnagel, U.; Ravanelli, E.; Schade, B.; Bottcher, C.;
̈
Vostrowsky, O.; Hirsch, A. Amphiphilic [5:1]- and [3:3]-Hexakisad-
ducts of C60. Eur. J. Org. Chem. 2004, 1983−2001.
ACKNOWLEDGMENTS
■
This work is partially supported by a Challenging Exploratory
Research from MEXT, Japan (Nos. 22655030 (T.M.) and
23685033 (T.N.)). The authors thank Dr. J. P. Hill, Dr. M. J.
Hollamby, and Dr. M. Takeuchi (NIMS), Prof. M. Funahashi
(Kagawa University), and Prof. T. Sagawa (Kyoto University)
for useful discussions.
(20) Nierengarten, J.-F.; Hahn, U.; Trabolsi, A.; Herschbach, H.;
Cardinali, F.; Elhabiri, M.; Leize, E.; Van Dorsselaer, A.; Albrecht-
Gary, A.-M. Synthesis of Fullerodendrons with an Ammonium Unit at
the Focal Point and Their Cooperative Self-Assembly on a Fluorescent
Ditropic Crown Ether Receptor. Chem.Eur. J. 2006, 12, 3365−3373.
(21) Hetzer, M.; Bayerl, S.; Camps, X.; Vostrowsky, O.; Hirsch, A.;
Bayerl, T. M. Fullerenes in Membranes. Structural and Dynamic
Effects of Lipophilic C60 Derivatives in Phospholipid Bilayers. Adv.
Mater. 1997, 9, 913−917.
REFERENCES
■
(1) Hagen, M. H. J.; Meijer, E. J.; Mooij, G. C. A.; Frenkel, D.;
Lekkerkerker, H. N. W. Does C60 Have a Liquid Phase? Nature 1993,
365, 425−426.
(2) Cheng, A.; Klein, M. L.; Caccamo, C. Prediction of the Phase
Diagram of Rigid C60 Molecules. Phys. Rev. Lett. 1993, 71, 1200−1203.
(22) Delpeux, S.; Beguin, F.; Benoit, R.; Erre, R.; Manolova, N.;
Rashkov, I. Fullerene Core Star-like Polymers-1. Preparation from
Fullerenes and Monoazidopolyethers. Eur. Polym. J. 1998, 34, 905−
915.
5343
dx.doi.org/10.1021/la400969f | Langmuir 2013, 29, 5337−5344

Langmuir
Article
(23) Miller, M. L.; West, R. Platinum-Catalyzed Hydrosilylation of
C60: Synthesis of a Novel Fullerene-Siloxane Polymer. Chem. Commun.
1999, 1797−1798.
(42) Kramer, T. J.; Babu, S. S.; Saeki, A.; Seki, S.; Aimi, J.; Nakanishi,
T. CdSe Nanocrystal/C60-Liquid Composite Material with Enhanced
Photoelectrochemical Performance. J. Mater. Chem. 2012, 22, 22370−
22373.
(24) Brettreich, M.; Burghardt, S.; Bottcher, C.; Bayerl, T.; Bayerl, S.;
̈
(43) Li, H.; Babu, S. S.; Turner, S. T.; Neher, D.; Hollamby, M. J.;
Hirsch, A. Globular Amphiphiles: Membrane-Forming Hexaadducts of
C60. Angew. Chem., Int. Ed. 2000, 39, 1845−1848.
(25) Bar-Shir, A.; Engel, Y.; Gozin, M. Synthesis and Water Solubility
of Adamantyl-OEG-Fullerene Hybrids. J. Org. Chem. 2005, 70, 2660−
2666.
Seki, T.; Yagai, S.; Deguchi, Y.; Mohwald, H.; Nakanishi, T. Alkylated-
̈
C60 Based Soft Materials: Regulation of Self-Assembly and
Optoelectronic Properties by Chain Branching. J. Mater. Chem. C
2013, 1, 1943−1951.
(44) Prato, M.; Maggini, M. Fulleropyrrolidines: A Family of Full-
Fledged Fullerene Derivatives. Acc. Chem. Res. 1998, 31, 519−526.
(45) Bellavia-Lund, C.; Wudl, F. Synthesis of [70]Azafulleroids:
Investigations of Azide Addition to C70. J. Am. Chem. Soc. 1997, 119,
943−946.
(46) Thilgen, C.; Herrmann, A.; Diederich, F. The Covalent
Chemistry of Higher Fullerenes: C70 and Beyond. Angew. Chem., Int.
Ed. 1997, 36, 2268−2280.
(47) Thilgen, C.; Diederich, F. Structural Aspects of Fullerene
Chemistry-A Journey through Fullerene Chirality. Chem. Rev. 2006,
106, 5049−5135.
(48) Langa, F.; de la Cruz, P.; de la Hoz, A.; Espíldora, E.; Cossío, F.
P.; Lecea, B. Modification of Regioselectivity in Cycloadditions to C70
under Microwave Irradiation. J. Org. Chem. 2000, 65, 2499−2507.
(49) Kobayashi, K.; Yamada, H.; Horiuchi, T.; Matsushige, K.
Investigations of C60 Molecules Deposited on Si(111) by Noncontact
Atomic Force Microscopy. Appl. Surf. Sci. 1999, 140, 281−286.
(50) Bonifazi, D.; Spillmann, H.; Kiebele, A.; de Wild, M.; Seiler, P.;
(26) Hetzer, M.; Gutberlet, T.; Brown, M. F.; Camps, X.;
Vostrowsky, O.; Schonberger, H.; Hirsch, A.; Bayerl, T. M.
̈
Thermotropic Behavior of Lipophilic Derivatized [60]Fullerenes
Studied by Deuterium NMR, X-ray Diffraction, and Microcalorimetry.
J. Phys. Chem. A 1999, 103, 637−642.
(27) Bourlinos, A. B.; Georgakilas, V.; Tzitzios, V.; Boukos, N.;
Herrera, R.; Giannelis, E. P. Functionalized Carbon Nanotubes:
Synthesis of Meltable and Amphiphilic Derivatives. Small 2006, 2,
1188−1191.
(28) Lei, Y.; Xiong, C.; Guo, H.; Yao, J.; Dong, L.; Su, X. Controlled
Viscoelastic Carbon Nanotube Fluids. J. Am. Chem. Soc. 2008, 130,
3256−3257.
(29) Li, Q.; Dong, L.; Sun, F.; Huang, J.; Xie, H.; Xiong, C. Self-
Unfolded Grapheme Sheets. Chem.Eur. J. 2012, 18, 7055−7059.
(30) Tang, Z.; Zhang, L.; Zeng, C.; Lin, T.; Guo, B. General Route to
Graphene with Liquid-Like Behavior by Non-Covalent Modification.
Soft Matter 2012, 8, 9214−9220.
(31) Nakanishi, T.; Ariga, K.; Michinobu, T.; Yoshida, K.; Takahashi,
Cheng, F.; Guntherodt, H.-J.; Jung, T.; Diederich, F. Supramolecular
̈
H.; Teranishi, T.; Mohwald, H.; Kurth, D. G. Flower-Shaped
̈
Patterned Surfaces Driven by Cooperative Assembly of C60 and
Porphyrins on Metal Substrates. Angew. Chem., Int. Ed. 2004, 43,
4759−4763.
Supramolecular Assemblies: Hierarchical Organization of a Fullerene
Bearing Long Aliphatic Chains. Small 2007, 3, 2019−2023.
(32) Nakanishi, T.; Shen, Y.; Wang, J.; Yagai, S.; Funahashi, M.; Kato,
́
(51) Nowak-Krol, A.; Gryko, D.; Gryko, D. T. Meso-Substituted
T.; Fernandes, P.; Mohwald, H.; Kurth, D. G. Electron Transport and
̈
Liquid Porphyrins. Chem.Asian J. 2010, 5, 904−909.
(52) Chapman, D. Biomembrane Structure and Function; Verlag
Chemie: Weinheim, 1984; pp 1−42.
Electrochemistry of Mesomorphic Fullerenes with Long-Range
Ordered Lamellae. J. Am. Chem. Soc. 2008, 130, 9236−9237.
(33) Nakanishi, T.; Michinobu, T.; Yoshida, K.; Shirahata, N.; Ariga,
(53) Matsuo, T.; Suga, H.; David, W. I. F.; Ibberson, R. M.; Bernier,
P.; Zahab, A.; Fabre, C.; Rassat, A.; Dworkin, A. The Heat Capacity of
Solid C60. Solid State Commun. 1992, 83, 711−715.
K.; Mohwald, H.; Kurth, D. G. Nanocarbon Superhydrophobic
̈
Surfaces Created from Fullerene-Based Hierarchical Supramolecular
Assemblies. Adv. Mater. 2008, 20, 443−446.
(54) Murakami, H.; Watanabe, Y.; Nakashima, N. Fullerene Lipid
Chemistry: Self-Organized Multibilayer Films of a C60-Bearing Lipid
with Main and Subphase Transitions. J. Am. Chem. Soc. 1996, 118,
4484−4485.
(34) Babu, S. S.; Mohwald, H.; Nakanishi, T. Recent Progress in
̈
Morphology Control of Supramolecular Fullerene Assemblies and Its
Applications. Chem. Soc. Rev. 2010, 39, 4021−4035.
(35) Nakanishi, T. Supramolecular Soft and Hard Materials Based on
Self-Assembly Algorithms of Alkyl-Conjugated Fullerenes. Chem.
Commun. 2010, 46, 3425−3436.
(55) Ghosh, H. N.; Sapre, A. V.; Mittal, J. P. Aggregation of C70 in
Solvent Mixtures. J. Phys. Chem. 1996, 100, 9439−9443.
́
(56) Xie, Q.; Perez-Cordero, E.; Echegoyen, L. Electrochemical
6−
(36) Nakanishi, T.; Shen, Y.; Wang, J.; Li, H.; Fernandes, P.; Yoshida,
Detection of C60 and C706−: Enhanced Stability of Fullerides in
Solution. J. Am. Chem. Soc. 1992, 114, 3978−3980.
K.; Yagai, S.; Takeuchi, M.; Ariga, K.; Kurth, D. G.; Mohwald, H.
̈
(57) Li, H.; Choi, J.; Nakanishi, T. Optoelectronic Functional
Materials Based on Alkylated-π Molecules: Self-Assembled Architec-
tures and Non-Assembled Liquids. Langmuir 2013, DOI: 10.1021/
la400202r.
Superstructures and Superhydrophobic Property in Hierarchical
Organized Architectures of Fullerenes Bearing Long Alkyl Tails. J.
Mater. Chem. 2010, 20, 1253−1260.
(37) Fernandes, P. A. L.; Yagai, S.; Mohwald, H.; Nakanishi, T.
̈
(58) Babu, S. S.; Aimi, J.; Ozawa, H.; Shirahata, N.; Saeki, A.; Seki, S.;
Molecular Arrangement of Alkylated Fullerenes in the Liquid
Crystalline Phase Studied with X-ray Diffraction. Langmuir 2010, 26,
4339−4345.
Ajayaghosh, A.; Mohwald, H.; Nakanishi, T. Solvent-Free Luminescent
̈
Organic Liquids. Angew. Chem., Int. Ed. 2012, 51, 3391−3395.
(59) Lintinen, K.; Efimov, A.; Hietala, S.; Nagao, S.; Jalkanen, P.;
Tkachenko, N.; Lemmetyinen, H. Cationic Photopolymerization of
Liquid Fullerene Derivative under Visible Light. J. Polym. Sci., Part A:
Polym. Chem. 2008, 46, 5194−5201.
(38) Li, H.; Hollamby, M. J.; Seki, T.; Yagai, S.; Mohwald, H.;
̈
Nakanishi, T. Multifunctional, Polymorphic, Ionic Fullerene Supra-
molecular Materials: Self-Assembly and Thermotropic Properties.
Langmuir 2011, 27, 7493−7501.
(60) Yan, X.; Li, L.-s. Solution-Chemistry Approach to Graphene
Nanostructures. J. Mater. Chem. 2011, 21, 3295−3300.
(39) Nakanishi, T.; Schmitt, W.; Michinobu, T.; Kurth, D. G.; Ariga,
K. Hierarchical Supramolecular Fullerene Architectures with Con-
trolled Dimensionality. Chem. Commun. 2005, 5982−5984.
(40) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.;
Tsuruoka, T.; Ariga, K.; Kurth, D. G. Perfectly Straight Nanowires of
Fullerenes Bearing Long Alkyl Chains on Graphite. J. Am. Chem. Soc.
2006, 128, 6328−6329.
(41) For a preliminary communication of this work, see: Michinobu,
T.; Nakanishi, T.; Hill, J. P.; Funahashi, M.; Ariga, K. Room
Temperature Liquid Fullerenes: An Uncommon Morphology of C60
Derivatives. J. Am. Chem. Soc. 2006, 128, 10384−10385.
5344
dx.doi.org/10.1021/la400969f | Langmuir 2013, 29, 5337−5344