Surface-rolling molecules

Article abstract of DOI:10.1021/ja058514r

Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely, nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic components that include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use of spherical wheels based on C₆₀ and freely rotating axles based on alkynes permits directed nanoscale rolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles the same motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A new synthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, and the latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provided definitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. The studies here underscore the ability to control directionality of motion in molecular-sized nanostructures through precise molecular design and synthesis.

Full text of DOI:10.1021/ja058514r

Published on Web 03/21/2006
Surface-Rolling Molecules
Yasuhiro Shirai,§ Andrew J. Osgood,† Yuming Zhao,§ Yuxing Yao,§ Lionel Saudan,§
Hanbiao Yang,§ Chiu Yu-Hung,§ Lawrence B. Alemany,§ Takashi Sasaki,§
Jean-Franc¸ois Morin,§ Jason M. Guerrero,§ Kevin F. Kelly,*,† and James M. Tour*,§
Contribution from the Departments of Chemistry, Mechanical Engineering and Materials
Science, and Smalley Institute for Nanoscale Science and Technology, and Department of
Electrical and Computer Engineering and Rice Quantum Institute, Rice UniVersity,
6100 Main Street, Houston, Texas 77005
Received December 15, 2005; E-mail: kkelly@rice.edu; tour@rice.edu
Abstract: Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely,
nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic components
that include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use of
spherical wheels based on C60 and freely rotating axles based on alkynes permits directed nanoscale
rolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles the
same motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A new
synthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-
wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, and
the latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provided
definitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. The
studies here underscore the ability to control directionality of motion in molecular-sized nanostructures
through precise molecular design and synthesis.
biomolecular machines6 and new challenges on the construction
and control of miniaturized machines7 on an unprecedented
Introduction
The current trend in the physical and biological sciences is
the continued miniaturization of machinery from the macro-
scopic to the microscopic world given that our current scientific,
medical, and technological disciplines will increasingly rely on
the ability to control the transport of materials, transmission of
power, and transfer of information at the microscopic level.1
The ultimate goal is to build machines at the nanoscopic level
from individual atoms.2 However, this bottom-up approach poses
a critical question of how to construct and control the molecular-
sized machines. Building such a diminutive machine is no longer
a matter of scaling down a macroscopic cousin.3 The manipula-
tion of nanomachines requires control of the motion and
performance at the molecular level.3-5 From the chemical
perspective, the synthesis of various functional molecules as
molecular-sized machines might provide insight regarding
level. Directed by this objective, and with the hope of directing
future bottom-up fabrication through bulk external stimuli, such
as electric fields, on nanometer-sized transporters, we have
designed and synthesized a class of single molecular vehicles,
namely, nanotrucks and nanocars, and sought to study controlled
molecular motion on surfaces through the rational design of
the surface-capable molecular structures (Figure 1). We report
here the synthetic details related to the preparation of these
nanomachines, while summarizing the initial imaging and
fullerene-based wheel-like rolling motions, as opposed to the
more common stick-slip or sliding8 translation.
At the outset, a definition of terms is helpful since we are
defining names of single-molecule entities by analogy to the
macroscopic view of vehicles, namely, cars and trucks. As with
any analogy, there are similarities and dissimilarities between
§ Departments of Chemistry, Mechanical Engineering and Materials
Science.
(6) (a) Kinbara, K.; Aida, T. Chem. ReV. 2005, 105, 1377-1400. (b) For recent
reviews on biomolecular machines, see: Cell (special issue on macromo-
lecular machines) 1998, 92, 291-390.
† Department of Electrical and Computer Engineering.
(1) (a) Timp, G. L. Nanotechnology; AIP Press/Springer: New York, 1999.
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Int. Ed. 2000, 39, 3348-3391. (c) Balzani, V.; Credi, A.; Venturi, M.
Molecular DeVices and MachinessA Journey into the Nano World; Wiley-
VCH: Weinheim, Germany, 2004. (d) Molecular Switches; Feringa, B.
E., Ed.; Wiley-VCH: Weinheim, Germany, 2001. (e) Kottas, G. S.; Clarke,
L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005, 105, 1281-1376. (f) Ozin,
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Surface-Rolling Molecules
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Figure 1. The structures of the fullerene-wheeled molecular machines developed in the present study. (a) Molecular structures of the molecular machines.
(b) Space-filling model of the nanotruck 1a (devoid of dodecyl groups for clarity) on a surface. The blue arrow beneath the molecule illustrates the expected
direction of fullerene-wheel-assisted rolling motion.
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Figure 2. Rotational barrier of the fullerene-wheel structures. Using HF/3-21G level of theory on the (a) fullerene-wheel model compounds with three
different substituents, (b) relaxed potential energy surface scans around the wheel rotation were performed (the least stable conformation of each model was
set to 0 kcal/mol). (c) Optimized structure of the fullerene-wheel model (R ) OMe) at 0° rotation angle.
the entities of comparison; if there were no dissimilarities, the
two entities would no longer be analogous. The highest-level
formal definition of a vehicle is “any device on wheels for
transporting people or objects”.9 We demonstrate here a
nanosized “device on wheels for transporting” itself (ca. 100
nm, as recorded), and on that basis, we use the term “nanocar”,
albeit obviously displayed with dissimilarities from the common
macroscopic objects. Likewise, a truck is a “vehicle for hauling
loads”,9 and when we constructed a nanosized “device on
wheels” that has a platform that might accommodate a load,
we used the term “nanotruck”.
As shown in Figure 1a, the designed nanotrucks and nanocars
consist of three basic molecular mechanical parts: fullerene
wheels, a chassis made of fused aromatic rings or oligo-
(phenylene ethynylene)s (OPEs), and alkynyl axles. The nan-
otrucks 1a and 1b have a potential loading bay (acid-base
bonding to the nitrogen atoms), unlike the nanocars 3a and 3b.
The new synthesis methodology, in situ ethynylation of
fullerenes,10 was developed in order to synthesize these mo-
lecular machines. Through the syntheses of several related
molecular structure versions, it was clear that the alkyl units
were critical for the requisite solubility of these multi-fullerene
structures. These highly flexible groups have minimal interac-
tions with surfaces, compared to the strong charge-transfer
interaction of the fullerene wheel on gold.11 The molecule-
surface interaction is likely to be dominated by the character-
istics specific to the fullerene wheels on gold surfaces; the alkyl
groups contribute comparatively little to the surface stiction.
We have successfully demonstrated that our nanomachines
exhibit fullerene-wheel-assisted rolling translation and rotation
on gold surfaces because of the spherical fullerene wheels
connected to a chassis via freely rotating alkynyl axles.12 The
observed rolling motion resembles that of macroscopic cars,
and this is the first observation of a molecular-sized machine
that incorporates mechanical components, such as wheels and
axles, with movement at the single molecule level. While the
thrust to design single-molecule-sized nanoscale machines with
controlled mechanical motion has yielded a variety of molecular
machinery resembling macroscopic motors, switches, shuttles,
turnstiles, gears, bearings, and elevators,7,13 these nanomachines
have been operated and observed only spectroscopically as
ensembles of molecules in the solution or the solid13a state.
Examples in which a molecule has a mechanical design and
the mechanism of movement can be probed at the single
molecular level are limited and include cyclodextrin necklaces,14
altitudinal molecular rotors,15 molecular barrows,16 molecular
landers,17 and nanowalkers.18
Results and Discussion
Modeling of Fullerene-Wheeled Nanostructures. One of
several promising molecular architectures explored in our group
for the synthesis of the surface-capable molecular structures is
based on the fullerene wheel and alkynyl axle. We expected
that the symmetrical spherical structure of C60 would allow the
smooth rotation of the molecular wheel. Free rotation of the
(12) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Kelly, K. F.; Tour, J. M. Nano Lett.
2005, 5, 2330-2334.
(13) (a) Garcia-Garibay, M. A. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10771-
10776. (b) Badjiæ, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F.
Science 2004, 303, 1845-1849. (c) Balzani, V.; Clemente-Leo´n, M.; Credi,
A.; Ferrer, B.; Venturi, M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad.
Sci. U.S.A. 2006, 103, 1178-1183.
(14) Shigekawa, H.; Miyake, K.; Sumaoka, J.; Harada, A.; Komiyama, M. J.
Am. Chem. Soc. 2000, 122, 5411-5412.
(15) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T. F.; Michl,
J. J. Am. Chem. Soc. 2004, 126, 4540-4542.
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technology 2002, 13, 330-335. (b) Rapenne, G. Org. Biomol. Chem. 2005,
3, 1165-1169.
(9) Webster’s New Twentieth Century Dictionary, 2nd ed.; Collins & World:
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(17) (a) Otero, R.; Hu¨mmelink, F.; Sato, F.; Legoas, S. B.; Thostrup, P.;
Laesgaard, E.; Stensgaard, I.; Galva˜o, D. S.; Besenbacher, F. Nat. Mater.
2004, 3, 779-782. (b) Rosei, F.; Schunack, M.; Jiang, P.; Gourdon, A.;
Legsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Science 2002,
296, 328-331.
(10) Shirai, Y.; Zhao, Y.; Cheng, L.; Tour, J. M. Org. Lett. 2004, 6, 2129-
2132.
(11) (a) Chase, S. J.; Bacsa, W. S.; Mitch, M. G.; Pilione, L. J.; Lannin, J. S.
Phys. ReV. B 1992, 46, 7873-7877. (b) Altman, E. I.; Colton, R. J. Surf.
Sci. 1993, 295, 13-33. (c) Tzeng, C.-T.; Lo, W.-S.; Yuh, J.-Y.; Chu, R.-
Y.; Tsuei, K.-D. Phys. ReV. B 2000, 61, 2263-2272.
(18) Kwon, K.-Y.; Wong, K. L.; Pawin, G.; Bartels, L.; Stolbov, S.; Rahman,
T. S. Phys. ReV. Lett. 2005, 95, 166101.
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Scheme 1. Synthesis of the Prototype: Half-Trucka
b
1
c
a Reagent: Pd/Cu ) PdCl2(PPh3)2, CuI. Yield determined by H NMR. Isolated yield.
alkynyl axles between the fullerene wheel and chassis should
facilitate the rolling motion of the entire molecular vehicle on
solid surfaces. To verify this design concept, we have examined
the rotational barrier of the wheel and axle architecture using
ab initio methods19 with HF/3-21G level of theory (Figure 2).
The result revealed that, even though the phenylene-ethynylene
and fullerene π systems have been shown to exhibit an
interaction called periconjugation,20 its contribution to the
rotational barrier was expected to be small since the largest
estimated barrier height of the three model compounds was only
about 1.0 kcal/mol.21 This low barrier height is actually
comparable to that of the triple bond rotation in a usual
phenylene-ethynylene system,22 affording the free rotation of
the fullerene-wheel structures at room temperature, or even
under cryogenic conditions.
Another key molecular architecture explored in our laboratory
for the synthesis of the nanomachines is the chassis design.
Working toward the synthesis of fullerene-wheeled nanostruc-
Figure 3. Flexibility of the semirigid chassis structure (nanocar 3a, devoid
of alkoxy groups for clarity). (a) The triple bonds in the OPE structure can
rotate until the fullerene wheels touch one another, which gives the nanocar
flexibility orthogonal to the surface plane. (b) One fullerene wheel is
elevated, while the other wheels remain on the surface to illustrate the
suspension concept.
tures, we have chosen two different chassis structures: a rigid
structure as in nanotrucks 1a and 1b, and a semirigid structure
as in nanocars 3a and 3b. These fullerene-wheeled nanostruc-
tures share several features worth mentioning. First of all, most
proposed structures weigh more than 4000 Da due to the high
molecular weight of each C60 (720 Da). Second, integration of
as many as four C60s in a well-defined molecular system is rare
and not previously known for a rigid molecular system due to
the synthetic difficulty associated with fullerene derivatives.23
This feature made it difficult to obtain highly pure samples of
the nanotrucks. The semirigid OPE chassis was introduced to
remedy those difficulties met in the rigid chassis system and
enabled us to perform successful single-molecule STM studies.
Better flexibility of OPE structure combined with the increased
number of alkyl units dramatically increased the processibility
of the massive fullerene-wheeled structures in organic solvents.
Furthermore, the rotation around alkyne connections between
the chassis and axle portions in the OPE system can cause it to
act as suspension, giving the nanocar flexibility orthogonal to
the surface plane (Figure 3), and thereby permitting it to climb
one-atomic-step high gold islands.24
Synthesis Effort toward Multi-Fullerene Structures: Half-
Truck Approach. The synthesis of the prototype nanotruck,
half-truck 2, is outlined in Scheme 1. The half-truck 2 was
designed and synthesized to develop a reliable synthetic
methodology for the attachment of multiple fullerenes to
terminal alkynes. The dibromo-diketone 525 was iodinated to
give compound 6. Protection of the diketone moiety of
compound 6 was achieved with ethylene glycol.26 Compound
(19) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004 (see Supporting Information for the full author list).
(20) Hamasaki, R.; Ito, M.; Lamrani, M.; Mitsuishi, M.; Miyashita, T.;
Yamamoto, Y. J. Mater. Chem. 2003, 13, 21-26 and references therein.
(21) It appears that the main contribution to this rotational barrier is the very
weak interaction between the acidic proton on the fullerene cage and the
lone pair electrons on the oxygen atom in the methoxy group because such
interaction is missing in the other two model compounds and ethyl and
methoxy groups should be similar in terms of sterics. Further confirmation
applying B3LYP/6-31G* level of theory to the 0 and 180° rotations of the
methoxy model predicted the barrier height to be only 0.54 kcal/mol.
(22) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 1998,
120, 3970-3974.
(23) For well-defined structures with more than four fullerenes attached, see:
(a) Nierengarten, J.-F.; Schall, C.; Nicoud, J.-F. Angew. Chem., Int. Ed.
1998, 37, 1934-1936. (b) Armaroli, N.; Boudon, C.; Felder, D.; Gissel-
brecht, J.-P.; Gross, M.; Marconi, G.; Nicoud, J.-F.; Nierengarten, J.-F.;
Vicinelli, V. Angew. Chem., Int. Ed. 1999, 38, 3730-3733. (c) Zhang, S.;
Gan, L.; Huang, C.; Lu, M.; Pan, J.; He, X. J. Org. Chem. 2002, 67, 883-
891.
(24) Effect of suspension-like behavior can be possibly observed by STM.
Manuscript on the detailed STM study is in preparation by Osgood, A. J.;
Shirai, Y.; Zhao, Y.; Kelly, K. F.; Tour, J. M.
(25) Bhatt, M. V. Tetrahedron 1964, 20, 803-821.
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Scheme 2. Synthesis of the Nanotrucksa
a Reagent: Pd/Cu ) PdCl2(PPh3)2, CuI.
7 underwent a Pd-catalyzed coupling reaction with trimethylsilyl
acetylene (TMSA) to give compound 8. After the lithium-
halogen exchange on the bromide followed by quenching with
n-dodecyl iodide, the TMS protecting group was removed using
tetrabutylammonium fluoride (TBAF) in THF to obtain com-
pound 9 in 69% yield over two reaction steps. The key reaction
step was achieved in 46% yield after testing numerous different
conditions. The breakthrough was afforded when the terminal
alkynes were deprotonated in THF using excess LHMDS in
the presence of excess C60. Deviation from any one of the
conditions dramatically affected reaction outcomes as listed in
Scheme 1. This in situ approach was applicable to other
substrates and resulted in the preparation of a wide variety of
multi-fullerene structures in our laboratory.27 Indeed, this
methodology was so effective that the attachment of as many
as four C60s was achieved on the nanocars and nanotrucks.
Eventually, all our nanotrucks and nanocars were synthesized
by applying this methodology to the tetra-terminal alkyne
precursors, as shown in Schemes 2 and 3.
(26) The formation of two fused 1,4-dioxane structures (below) could not be
spectroscopically ruled out; however, since deprotection was imminent,
no further effort was given to the determination.
Synthesis and Spectral Characterization of Nanotrucks.
The syntheses of nanotrucks 1a and 1b are outlined in Scheme
(27) Zhao, Y.; Shirai, Y.; Slepkov, A. D.; Cheng, L.; Alemany, L. B.; Sasaki,
T.; Hegmann, F. A.; Tour, J. M. Chem.sEur. J. 2005, 11, 3643-3658.
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Scheme 3. Synthesis of the Nanocarsa
a Reagent: Pd/Cu ) PdCl2(PPh3)2, CuI.
2. The half-truck 2 or its precursors, 8 and 9, could not be used
in the synthesis of nanotrucks 1a or 1b because deprotection
of the diketone moiety with various aqueous acids was not
possible in the presence of alkynes. Alternatively, protecting 5
with ethylene glycol under CSA catalysis afforded diacetal 10.26
Compound 10 underwent lithium-halogen exchange reaction
with t-BuLi to form a dilithiated species that was directly
alkylated by n-dodecyl iodide to yield compound 11. Depro-
tecting of 11 with TsOH in water afforded diketone 12.
Iodination of 12 with NIS in trifluoroacetic acid (TFA) thus
afforded diiodide 13. Compound 13 was condensed with 14
(prepared by bistosylation of 1,2-phenylenediamine, 4,5-dini-
tration with HNO3, and finally reduction with Sn/HCl)28 to
afford the half-chassis 15.29 Compound 15 was deprotected with
H2SO4 to give free diamine 16, which was subsequently
condensed with 13 to form the full chassis 17. Compound 17
was then cross-coupled with TMSA under Pd catalysis to afford
the chassis-axle component 18. Desilylation of 18 with KOH,
followed by the in situ ethynylation in THF with excess C60
using LHMDS, yielded nanotruck 1a. As mentioned earlier, the
use of excess LHMDS and C60 in THF is essential for the
successful reaction. Unfortunately, nanotruck 1a was insoluble
in all organic solvents, which prevented us from determining
the yield of a pure sample.
(28) Starnes, S. D.; Arungundram, S.; Saunders: C. H. Tetrahedron Lett. 2002,
43, 7785-7788 and references therein.
(29) Hedberg, F. L.; Arnold, F. E.; Kovar, R. F. J. Polym. Sci., Polym. Chem.
Ed. 1974, 12, 1925-1931.
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Figure 4. MALDI-TOF mass spectrum of nanotruck 1a. Peaks for one-,
two-, three-, and four-wheel nanotrucks (m/z 1972.5, 2693.0, 3412.3, and
4132.2, respectively) are observed (negative ion mode, dithranol matrix).
The complete purification of nanotruck 1a was hampered due
to its low solubility in most good organic solvents for fullerene
derivatives, such as CS2 and o-dichlorobenzene. Since the
fullerene-attached products are the only low solubility compo-
nents in the reaction mixture, washing the resultant solid from
the reaction mixture yielded a brownish solid of 1a. Although
it was impossible to directly determine the purity of the
nanotruck 1a by solution phase NMR or other solution phase
techniques, solid state 13C NMR and MALDI-TOF MS analyses
can be used to roughly estimate the purity. The formation of
nanotruck 1a was first confirmed by its MALDI-TOF MS
spectrum (Figure 4). The molecular ion peak of nanotruck 1a
is clearly observed at m/z 4132.2, while the signals at m/z 3412.3,
2693.0, and 1972.5 are assigned to the fragmentations by losing
one, two, and three fullerene moieties, respectively, since the
peak separations correspond to the mass of C60 (720 Da) (Figure
4). In the solid state 13C NMR experiments,30 aside from the
remarkably intense signals for fullerene and chassis aromatic
sp2 carbons at ca. 150-120 ppm, and the n-dodecyl carbons at
30.6, 23.6, and 14.4 ppm, distinguishable resonance peaks for
the sp3 carbons on the fullerene core are also observed (Figure
5a). The fullerene C-H carbon was assigned to the resonance
at δ 62.1 for several reasons. The signal intensity is significantly
Figure 5. Solid state 13C and 15N NMR spectra of nanotruck 1a. (a) Direct
1
13C pulse, MAS at 6.8 kHz. (b) H-13C CPMAS, MAS at 6.0 kHz, 3 ms
1
contact time. (c) H-15N CPMAS, MAS at 4.0 kHz, 10 ms contact time,
relative to glycine defined as 347.58 ppm relative to CH3NO2 at 0 ppm.35
experiments were conducted on nanotruck 1a and a model
compound dibenzo[a,c]phenazine that possesses a similar
nitrogen moiety. Nanotruck 1a shows a signal at -67.5 ppm,
which is consistent with that of the model compound at -66.7
ppm.34 Therefore, the analysis indicates that in the solid state
the fullerenes in nanotruck 1a remain neutral rather than in a
deprotonated state. Unfortunately, the 15N NMR spectrum could
not be improved. Even with a full 7 mm rotor, it took a week
of signal averaging (118 000 scans with a 5 s relaxation delay)
to acquire this spectrum; with a molecular formula of C330H114N4,
nanotruck 1a contains only 1.4 wt % nitrogen, corresponding
to just 0.005 wt % nitrogen-15.
1
increased in the H-13C CPMAS experiment (Figure 5b); the
signal disappears in a dipolar dephasing experiment,31 and the
chemical shift is consistent with literature data.27 The quaternary
fullerene carbon was assigned to the resonance at δ 55.8 in light
of literature data.27 In Figure 5a, two alkynyl sp carbons are
also discernible at δ 83.1 (C60H-CtC-) and δ 97.5 (C60H-
CtC-), respectively, consistent with the solution state 13C
NMR data obtained from other multi-fullerene structures.27
The proton attached to the fullerene is known to be
significantly acidic due to the stabilized aromatic anion structure
after its deprotonation,32 and the chassis contains four nitrogen
sites that could possibly be protonated. 15N NMR can clearly
differentiate between an imine and an iminium ion (protonation
results in a large upfield shift33), and thus, we carried out further
experiments to shed light on this issue. 1H-15N CPMAS NMR
As the four n-dodecyl functionalities failed to provide
sufficient solubility for nanotruck 1a, we next tried to alter the
axle structure by incorporating compound 1910,27 as a solubility
enhancer. The tetraiodo full chassis 17 was coupled with 19,
and then fullerene wheels were attached following the same in
situ ethynylation procedure to afford the nanotruck 1b with
better solubility. Nanotruck 1b could be well-purified and fully
characterized.
(30) See Supporting Information for more detail of solid state NMR experiments.
(31) Liang, F.; Alemany, L. B.; Beach, J. M.; Billups, W. E. J. Am. Chem. Soc.
2005, 127, 13941-13948.
Our next step was aimed at driving the nanotrucks on surfaces
in a controllable mode; however, our expectations were not
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D. D.; Copan, W. G.; Abrahamson, W. W.; Mateescu, G. D. Org. Magn.
Reson. 1984, 22, 121-124.
(34) See Supporting Information for the 1H-15N CPMAS spectrum of the model
compound dibenzo[a,c]phenazine.
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Figure 6. Comparison of thermally induced motions of (a) four-wheeled 3b and (b-f) its STM-imaged motions, and (g) three-wheeled 4a and (h-k) its
STM-imaged motions. All sequence images were taken during annealing at ∼200 °C (bias voltage Vb ) -0.95 V, tunneling current It ) 200 pA; image size
is 51 × 23 nm). The orientation of the nanocar 3b is easily determined by the fullerene-wheel separation, with motion occurring perpendicular to the axles.12
Acquisition time for each image is approximately 1 min, with (b-f) selected from a series spanning 10 min, which shows ∼80° pivot (b) followed by
translation interrupted by small-angle pivot perturbations (c-f). (h-k) A sequence of STM images acquired approximately 1 min apart during annealing at
∼225 °C shows the pivoting motion of 4a (both circled molecules) and lack of translation of any molecules (Vb ) -0.7 V, It ) 200 pA; image size is 34
× 27 nm). See the Supporting Information for the complete video file and interstitial images.38
fulfilled by either of the nanotrucks. Preliminary STM studies
on both insoluble 1a and soluble 1b nanotrucks revealed rather
complicated results. The contamination of the gold surfaces by
unreacted fullerenes and possible instability of nanotrucks on
gold surfaces inhibited detailed STM studies (vide infra).36
Synthesis of Nanocars. Because previously made trifullerene
compounds 4a10 and 4b27 with OPE structure are relatively
stable, soluble in common organic solvents, and can be purified
free of C60, we combined our fullerene-wheel design with a
Z-shaped OPE backbone to produce nanocars 3a and 3b
(35) Hayashi, S.; Hayamizu, K. Bull. Chem. Soc. Jpn. 1991, 64, 688-690.
(36) See Supporting Information for the STM images of nanotrucks 1a and 1b.
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(Scheme 3). Commercially available compound 21 was iodi-
nated and coupled with TMSA using Pd-catalyzed conditions,
and then the amino group was converted to an iodide to afford
compound 22. The subsequent coupling reaction with axle unit
19 gave bromide 23. After replacing the bromide with iodide
using a lithium-halogen exchange reaction, compound 24 was
coupled with the middle chassis 2510,27 to afford compound 26.
The exchange reaction of the bromide group with iodide was
required for a successful coupling reaction with 25. Surprisingly,
our attempts to get pure nanocar 3a at this stage failed due to
the limited solubility of the product. A brown crude solid
material was obtained after the removal of solvents; however,
it was not soluble enough for further purification and solution
phase characterization. To further improve solubility of the
nanocar, another solubility enhancing group 2810,27 was coupled
after chemoselectively removing the TMS protecting groups
using K2CO3. The coupling reaction with compound 28 yielded
full chassis 29. After the removal of all silyl-protecting groups
with TBAF, four fullerene wheels were successfully coupled
via the in situ ethynylation method to complete the synthesis
of the nanocar 3b. The nanocar 3b was an improvement over
nanotrucks 1a and 1b in terms of stability, solubility, and purity,
and it was well-suited for STM studies.
k). No significant translation or pivoting motion of trimer 4b
was observed due to the wheel orientation relative to the axles.
The pivoting behavior of trimer 4a continued even up to 300
°C,39 a temperature at which the four-wheeled nanocars 3b were
moving across the surface too quickly to be imaged by the STM.
One would expect the energy barrier for sliding or stick-slip
motion for 4a and 4b to be comparable or even less than that
for 3b given there is one less fullerene in 4a and 4b and thus
a weaker overall interaction between the molecule and the gold
surface. However, since 4a and 4b exhibited little of the
thermally induced translation over the temperature range
investigated, this further suggests that the motion of nanocar
3b is due to rolling of the fullerene wheels.12
Conclusion
In summary, we have presented the design, syntheses, and
testing of new, fullerene-wheeled nanostructures: nanocars and
nanotrucks. These single-molecule-sized nanovehicles are com-
posed of three basic nanocomponents that are spherical fullerene
wheels, freely rotating alkynyl axles, and a molecular chassis.
The use of spherical wheels based on fullerene-C60 and freely
rotating axles based on alkynes has enabled the directed
nanoscale rolling of a molecular structure. The rotational barriers
for the fullerene-wheel model compounds, determined by using
ab initio methods (HF/3-21G or B3LYP/6-31G* level of theory),
confirmed a free rotation of the axle in the designed fullerene-
wheel structure. A new synthesis methodology, the in situ
ethynylation of fullerenes,10 was developed for the realization
of the fullerene-wheel design. The first generation of our four-
fullerene-wheeled nanomachine, nanotruck 1a, composed of the
rigid chassis structure based on fused aromatic rings was not
soluble in any solvents we tested. However, its structure was
successfully characterized using solid state NMR30 and MALDI-
TOF mass spectroscopy, and preliminary STM imaging was
conducted.36 The second generation, nanotruck 1b, was soluble
due to the incorporation of solubility enhancing axle units and
was fully characterized spectroscopically. Detailed STM imaging
on these nanotrucks 1 was, however, hampered due to the
contamination of surfaces with unreacted fullerenes and possible
instability of the nanotruck structures on gold surfaces. In the
development of the third generation fullerene-wheeled nano-
structure, we incorporated a new chassis design that is based
on an OPE structure. The new design contains no nitrogen
moieties for cargo purposes and was thus designated as a
nanocar. Each monomer unit in the nanocar chassis contains
polar decyloxy groups that increase the solubility of the molecule
and facilitate the purification process for the removal of
unreacted C60. Surprisingly, however, the third generation
nanocar 3a was not soluble. The poor solubility is presumably
due to the strong intermolecular interactions caused by the four
fullerene wheels. Eventually, after a small modification, the
fourth generation nanocar 3b was synthesized. Nanocar 3b is
soluble, almost completely free from unreacted fullerenes, and
suitable for detailed STM studies, which have successfully
demonstrated the new type of fullerene-based wheel-like rolling
motion, as not being the usual stick-slip or sliding translation.
The studies here underscore the ability to control directionality
of motion in molecular-sized nanostructures through precise
Fullerene-Wheel-Assisted Motion of Nanocars. With nano-
car 3b and three-wheelers 4a and 4b (their syntheses being
previously described10,27), we have been able to demonstrate
the action of the fullerene-wheel architecture at the single-
molecule level. The evidence for the fullerene-wheel-assisted
rolling motion of the nanocars 3b on the gold surface was
obtained by the comparison of two different modes of thermally
induced motions: translation and pivoting (Figure 6).
The four-wheeled nanocars 3b remained relatively stationary
on the surface up to approximately 170 °C. As the temperature
increased above this point, the molecules began to move in two
dimensions through a combination of both translation and
pivotingsnot in the 1-D manner initially expected.37 At ap-
proximately 200 °C, the motion of the nanocars 3b is, on
average, slow enough to be followed through a series of 1 min
images. Pivoting motion can be seen in a sequence of images
(Figure 6b-f).38 The translational motion that occurred between
pivoting was perpendicular to the axles, illustrating a directional
preference relative to the molecular orientation (the length
differing from the width permitted the directional assignment).12
Above approximately 225 °C, the rapid and erratic motion of
the molecules could not be tracked due to the relatively slow
acquisition time necessary (approximately 1 min for a 90 × 90
nm scan) compared to the rate of surface diffusion of the
molecules.
When three-wheeled nanocars 4 were slowly heated to 225
°C (a higher temperature than needed to induce significant
translational motion in the nanocars 3b), only occasional surface
diffusion was observed. Translation in these cases was on the
order of only a few nanometers over 20-30 min for those that
translated at all. The majority of motion of these molecules was
of 4a pivoting in place around a central pivot point (Figure 6h-
(37) The observed 2-D motion of the molecules, instead of the expected 1-D
motion, can be explained by the ability of the fullerenes to rotate
independently of one another, giving rise to a pivoting motion of the
molecule on the small atomic corrugation of a Au(111) substrate.
(38) A supplemental movie for this series of images can be found at our
website: http//tourserver.rice.edu/movies/.
(39) The onset temperature for decomposition as studied by thermogravimetric
analysis on fullerene-wheel model compounds; see Supporting Information.
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A R T I C L E S
11 mmol) in H2O (2 mL). The mixture was stirred overnight. The
suspension was then diluted with CHCl3 and then washed with H2O
(3×). The organic fraction was heated until it became clear, and then
dried over MgSO4. After filtration, removal of solvent in vacuo gave
desilylated product (71 mg, 87%) as a reddish-orange solid: FTIR
molecular design and synthesis. Further studies are concentrating
on electric field-induced motion of nanocars and nanotrucks and
new construction of motorized nanocars integrating a light-
powered molecular motor unit as well as nanotrains. Investiga-
tion of the new molecular-wheel architectures is also in progress
using a variety of spherical molecules, such as carboranes and
cyclodextrin-shaft complexes; the inherent insolubility of
fullerene-wheeled structures and the necessity to attach fullerenes
at the final synthetic step, since they tend to inhibit Pd-catalyzed
reactions, makes fullerene wheel replacement welcome.
(KBr) 3303, 2954, 2923, 2851, 1605, 1467, 1436 cm-1 1H NMR
;
(CDCl3, 200 MHz) δ 9.09 (br s, 4H), 8.55 (br s, 2H), 7.83 (br s, 4H),
3.46 (s, 4H), 2.88 (br m, 8H), 1.79 (br m, 8H), 1.50-1.22 (m, 72H),
0.91 (t, J ) 6.9 Hz, 12H); meaningful 13C NMR measurement failed
due to the limited solubility. The desilylated product (88 mg, 0.070
mmol obtained from multiple desilylation reactions) was subjected to
the general in situ ethynylation procedure with C60 (504 mg, 0.700
mmol), THF (300 mL), and LHMDS (0.70 mL, 1 M in THF, 0.70
mmol). After adding LHMDS, the mixture was stirred at room
temperature for 30 min, then TFA (0.3 mL) was added dropwise.
Removal of the solvents in vacuo afforded a black sticky solid, which
was rinsed with copious amounts of toluene until the purple color
disappeared. Further, sequentially rinsing the solid with MeOH/H2O
(50 mL), chloroform (50 mL), and CS2 (50 mL) afforded a dark brown
solid. The solid was gently ground into fine powder and then sonicated
in CS2 (10 mL) to remove trace amounts of unreacted fullerene.
Filtration and drying under vacuum afforded nanotruck 1a (121 mg,
although insolubility precluded adequate purification and yield deter-
mination) as a dark brown solid: FTIR (KBr) 2919, 2848, 1682, 1428
cm-1; solid state 13C NMR (50 MHz) δ 160-120 (br), 97.5, 83.1, 62.1,
55.8, 30.6 (br), 23.6, 14.4; solid state 1H-15N CPMAS NMR (20 MHz)
δ -67.5; MALDI-TOF MS m/z (matrix, dithranol) calcd for C330H114N4
4134.5, found 4132.2 (M-).
Experimental Section
General Methods. All reactions were performed under an atmo-
sphere of nitrogen unless stated otherwise. Precursors 5,25 14,28 19,10
25,10 and 2810 were prepared according to literature procedures (see
Supporting Information), and trimers 4a and 4b were available from
previous studies.10,27 Reagent grade diethyl ether and tetrahydrofuran
(THF) were distilled from sodium benzophenone ketyl. Triethylamine
(TEA) was distilled over CaH2. Fullerene (99.5+% pure) was purchased
from MTR Ltd. and used as received. LHMDS (1 M solution in THF)
and TBAF (1 M solution in THF) were obtained from Aldrich. Flash
column chromatography was performed using 230-400 mesh silica
gel from EM Science. Thin layer chromatography was performed using
glass plates precoated with silica gel 40 F254 purchased from EM
Science. Solution state 1H and 13C NMR spectra were recorded on 400
and 500 MHz spectrometers. Solid state NMR spectra were acquired
1
at 200.13 MHz H, 50.33 MHz 13C, and 20.28 MHz 15N.30 Melting
Nanotruck (1b). To a solution of compound 20 (136 mg, 0.0386
mmol) dissolved in THF (5 mL) was added dropwise TBAF (0.20 mL,
1 M in THF, 0.20 mmol). The solution was stirred at room temperature
for 15 min, then the CH2Cl2 (20 mL) was added. The solution was
washed with 10% HCl, saturated NaHCO3, and brine, then dried with
MgSO4. Removal of the solvents under reduced pressure gave the crude
desilylated product as a reddish wax. This crude product was subjected
to the general in situ ethynylation procedure with C60 (277 mg, 0.385
mmol), THF (100 mL), LHMDS (0.38 mL, 1 M in THF, 0.38 mmol),
and TFA (0.2 mL). The crude product was purified by a silica column
chromatography with a gradient eluent (5:1 hexane/CS2 to 1:1:1 hexane/
CS2/CH2Cl2) to yield the nanotruck 1b as a dark reddish solid (57 mg,
points were uncorrected. Ultrasonicated fullerene slurry in THF was
prepared in general ultrasonic cleaners.
General Procedure for the Coupling of a Terminal Alkyne with
an Aryl Halide Using a Palladium-Catalyzed Cross-Coupling
(Sonogashira) Protocol. To an oven-dried round-bottom flask equipped
with a magnetic stir bar were added the aryl halide, the terminal alkyne,
PdCl2(PPh3)2 (ca. 2 mol % per aryl halide), and CuI (ca. 4 mol % per
aryl halide). A solvent system of TEA and/or THF was added depending
on the substrates. Upon completion, the reaction was quenched with a
saturated solution of NH4Cl. The organic layer was then diluted with
hexanes, diethyl ether, or CH2Cl2 and washed with water or saturated
NH4Cl (1×). The combined aqueous layers were extracted with hexanes,
diethyl ether, or CH2Cl2 (2×). The combined organic layers were dried
over MgSO4, filtered, and the solvent was removed from the filtrate in
vacuo to afford the crude product, which was purified by column
chromatography (silica gel). Eluents and other slight modifications are
described below for each compound.
General Procedure for the Addition of C60 to Terminal Alkynes
Using LHMDS, In Situ Ethynylation Method. To an oven-dried
round-bottom flask equipped with a magnetic stir bar were added the
terminal alkyne and C60 (2 equiv per terminal alkyne H). After adding
THF, the mixture was sonicated for at least 3 h. To the greenish-
brown suspension formed after the sonication was added LHMDS
dropwise at room temperature over 0.5-1.5 h. As the reaction
progressed, the mixture turned into a deep greenish-black solution.
During the addition of the LHMDS, small aliquots from the reaction
were extracted and quenched with trifluoroacetic acid (TFA), dried,
and redissolved in CS2 for TLC analysis (developed in a mixture of
CS2, CH2Cl2, and hexanes). Completion of the reaction was confirmed
by the disappearance of the starting materials. The reaction was usually
completed within 1.5 h from the beginning of LHMDS addition. Upon
completion, the reaction was quenched with TFA to give a brownish
slurry. Excess TFA and solvent were then removed in vacuo to afford
a crude product that was purified by flash column chromatography
(silica gel). Eluents and other slight modifications are described below
for each compound.
1
26%): FTIR (KBr) 2920, 2850, 1634, 1497 cm-1; H NMR (CDCl3/
CS2, 500 MHz) δ 9.67 (br s, 4H), 8.31 (br s, 4H), 7.14 (m, 14H), 4.14
(br m, 16H), 3.24 (br s, 8H), 1.98 (br s, 24H), 1.63 (br s, 24H), 1.50-
0.99 (m, 160H), 0.90 (t, J ) 6.62 Hz, 12H), 0.83 (t, J ) 6.80 Hz,
12H), 0.78 (t, J ) 6.72 Hz, 12H); 13C NMR (CDCl3, 125 Hz) δ 154.5,
153.7, 153.6, 151.6 (×2), 151.5, 147.7, 147.6, 147.4, 146.7, 146.5,
146.3, 145.9, 145.8, 145.7, 145.6, 145.5, 145.4, 144.8, 144.6, 144.2,
143.3, 142.7 (×2), 142.2, 142.1, 142.0, 141.4, 140.5, 140.4 (×2), 136.2,
135.2, 132.1, 131.5, 128.8, 123.6, 123.1, 116.6, 115.0, 113.2 (30 signals
from sp2-C in the C60 core and 15 signals from sp2-C in the aromatic
ring; 4 signals are missing due to overlapping), 97.7, 94.2, 91.3, 80.8,
69.5, 69.4, 62.1 (CH in the C60 core), 55.6 (quaternary sp3-C in the C60
core), 35.7, 32.2 (×2), 31.3, 31.2, 30.3, 30.2 (×2), 30.1 (×2), 30.0
(×2), 29.9, 29.8, 29.7 (×2), 27.0, 26.5, 23.1 (×2), 23.0, 14.5, 14.4
(×2); MALDI-TOF MS m/z (matrix, dithranol) calcd for C442H290N4O8
5785.1, found 5785 (M+).
Half-Truck (2). Compound 9 (0.068 g, 0.1 mmol) was subjected to
the general in situ ethynylation procedure with C60 (0.288 g, 0.4 mmol),
THF (100 mL), LHMDS (0.8 mL, 1.0 M in THF, 0.8 mmol), and TFA
(0.3 mL, 0.39 mmol). Flash column chromatography on silica gel with
CS2:hexane (1:1) (1-2 L) to recover all unreacted C60 (0.167 g, 58%
of starting material) and with CS2:CH2Cl2:hexane (1:1:3) to collect
product afforded the half-truck 2 as shiny brown powder (0.098 g, 46%).
Elution with CS2:CH2Cl2:hexane (1:4:5) can recover unreacted com-
pound 9 and mono-C60, if present: FTIR (KBr) 2918, 2848, 1427, 1384,
Nanotruck (1a). To a solution of 18 (0.10 g, 0.065 mmol) in THF
(25 mL) were added methanol (2 mL) and a solution of KOH (0.60 g,
1
1187, 1093 cm-1; H NMR (CS2/CDCl3, 400 MHz) δ 8.18 (s, 2H),
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A R T I C L E S
Shirai et al.
7.92 (s, 2H), 7.18 (s, 2H, C60-H), 4.37 (br s, 4H), 3.84 (br s, 4H),
3.20 (t, J ) 8.0 Hz, 4H), 1.98 (q, J ) 7.7 Hz, 4H), 1.62-1.23 (m,
36H), 0.87 (m, 6 H); 13C NMR (CS2/CDCl3, 100 MHz) δ 151.6, 151.5,
147.8, 147.63, 147.57, 146.9, 146.63, 146.61, 146.5 (×2), 146.0, 145.9,
145.77, 145.72, 145.66, 145.58, 144.9, 144.7, 143.4, 142.83, 142.80,
142.34, 142.27, 142.20, 142.1, 141.9, 141.8, 140.7, 140.6, 136.3, 135.3,
133.2, 131.5, 130.9, 124.9, 122.8 (30 signals from sp2-C in the C60
core and 6 signals from sp2-C in the aromatic ring), 97.1 (CtC), 92.5,
82.8 (CtC), 61.9 (CH in the C60 core), acetal peak was hidden, 55.6
(quaternary sp3-C in the C60 core), 35.9, 32.2, 31.8, 30.6, 30.24, 30.21,
30.1, 30.09, 30.0, 29.7, 23.0, 14.4; MALDI-TOF MS m/z (matrix,
dithranol) calcd for C166H64O4 2121, found 2121 (M+).
Nanocar (3a). To a solution of 26 (0.085 g, 0.036 mmol) in THF
(15 mL) was added dropwise TBAF (0.3 mL, 0.3 mmol). Ten minutes
after the addition of the TBAF, the reaction was quenched with saturated
aqueous NH4Cl and extracted twice with hexanes. The organic portion
was dried over MgSO4 and filtered. After concentration in vacuo, the
residue was purified by flash column chromatography with 30% CH2-
Cl2 in hexanes to give desilylated product (0.065 g) as a green-yellow
solid. Desilylated material (0.060 g, 0.031 mmol) was subjected to the
general in situ ethynylation procedure with C60 (0.20 g, 0.28 mmol),
THF (100 mL), LHMDS (0.6 mL, 0.6 mmol), and TFA (0.3 mL). A
brown solid crude material was obtained after the removal of solvents;
however, it was not soluble enough for further purifications and solution
phase characterizations.
61.98 (CH in the C60 core), 55.60, 55.59 (quaternary sp3-C in the C60
core), 32.04, 31.97, 31.95, 29.8, 29.74, 29.70, 29.66, 29.48, 29.45,
29.40, 22.77, 22.75, 22.73, 14.23, 14.21, 14.19; MALDI-TOF MS m/z
(sulfur as the matrix) calcd for C430H274O12 5632, found 5631 (M+).
Sample Preparation and Data Collection for STM Study. The
nanotrucks 1a and 1b were solution spin-coated in air on hydrogen-
flame-annealed gold on mica surfaces and then imaged under ambient
conditions and then in ultrahigh vacuum. Even in vacuum, these samples
appeared to rapidly degrade upon scanning. This might be due to the
electrophilicity of the nanotruck cores or to impurities introduced from
the deposition protocol. With this limited success, the design of the
molecule was changed to that of the nanocars. Following preliminary
investigation of the nanocar molecules, our apparatus was modified to
allow dosing of the molecules directly from solvent into vacuum onto
freshly sputtered and annealed gold on mica. A majority of the nanocar
data discussed in this work were obtained with this new sample
deposition technique. Given this new deposition method, it might be
possible to return to the nanotrucks and obtain better images of their
motion. However, owing to the increased ease of synthesis and
improved solubility of the nanocars over the nanotrucks, we focused
subsequent imaging efforts exclusively on the nanocars. Hence, nanocar
3b was initially suspended in toluene (5 µM) and initially spun-cast
on Au(111) on mica and imaged in an ambient, home-built STM.
Following that initial investigation in air, the toluene solution of 3b
was dosed in high vacuum using a fast-actuating, small orifice solenoid
valve40,41 onto argon-sputtered and annealed Au(111) on mica substrates
and was imaged using an RHK variable temperature UHV-STM. The
dosing technique was chosen over sublimation in vacuum, as it appeared
in thermal decomposition studies using a thermogravimetric analyzer
on related OPE-alkynyl-fullerenes that the fullerene-based wheels
began to cleave from the alkynyl axles at ca. 300 °C with rapid
decomposition occurring by 350 °C.39 A piece of silicon, placed directly
underneath the gold substrate, was resistively heated to perform variable
temperature studies in the STM. The sample temperature was measured
by a K-type thermocouple wire placed directly on the gold surface.
Nanocar (3b). To a solution of 29 (0.096 g, 0.030 mmol) in THF
(5 mL) was added dropwise TBAF (0.2 mL, 0.2 mmol). Ten minutes
after the addition of the TBAF, the reaction was quenched with saturated
aqueous NH4Cl and extracted twice with hexanes. The organic portion
was dried over MgSO4 and filtered. After concentration in vacuo, the
residue was purified by flash column chromatography with 30-42%
CH2Cl2 in hexanes to give desilylated product (0.072 g) as a yellow
oil. This material was pure enough to carry on to the next reaction.
The desilylated product (0.070 g, 0.025 mmol) was subjected to the
general in situ ethynylation procedure with C60 (0.15 g, 0.21 mmol),
THF (100 mL), LHMDS (0.7 mL, 0.7 mmol), and TFA (0.7 mL).
(Note: product spot was not clearly visible on TLC.) Crude products
were dissolved in CS2 and directly loaded onto a column. The column
was eluted with CS2/CH2Cl2 (100:1) to remove unreacted C60, and then
with CS2/CH2Cl2 (1:1) for complete removal of trace C60 and elution
of product. The product was further purified using another flash column
with graduate elution of CS2/CH2Cl2/hexanes (1:1:100), (3:2:5), then
(3:3:4) to afford nanocar 3b (0.028 g, 20%) as a brown solid: FTIR
Acknowledgment. The Welch Foundation, Zyvex Corpora-
tion, and the NSF Penn State MRSEC funded this work. The
Office of Naval Research funded the 200 MHz NMR spec-
trometer, and the National Science Foundation provided partial
funding of the 400 and 500 MHz NMR spectrometers. We thank
Drs. I. Chester of FAR Research Inc. and R. Awartani of Petra
Research Inc. for providing TMSA, Tomi Hashizume and
Yasuhiko Terada for their expertise in vacuum deposition with
the solenoid pulse valve, and Paul Weiss for his insight
concerning the STM investigations of these molecules.
1
(CH2Cl2 cast) 2922, 2850, 2203, 1502, 1463, 1214 cm-1; H NMR
(500 MHz, CDCl3) δ 7.77 (d, J ) 1.4 Hz, 2H), 7.58 (d, J ) 8.0 Hz,
2H), 7.51 (dd, J ) 8.0, 1.4 Hz, 2H), 7.30 (s, 2H), 7.25 (s, 2H), 7.18 (s,
2H), 7.15 (s, 2H), 7.14 (s, 2H), 7.10 (s, 2H), 7.07 (s, 2H), 7.03 (s, 2H),
4.19-4.14 (m, 8H), 4.09 (m, 4H), 4.00 (m, 4H), 3.90-3.88 (m, 8H),
1.95-1.13 (m, 192H), 0.88-0.80 (m, 36H); 13C NMR (125 MHz,
CDCl3) δ 154.7, 154.6, 153.9, 153.75, 153.67, 153.65, (6 signals from
aryloxy sp2-C in the aromatic ring), 151.65, 151.59, 151.497 (×2), 147.7
(×2), 147.44, 147.43, 146.74, 146.69, 146.50, 146.477 (×2), 146.46,
146.32 (×2), 146.31 (×2), 145.92, 145.89, 145.82, 145.77, 145.72,
145.69, 145.55, 145.529 (×2), 145.51, 145.45, 145.43, 144.78, 144.76,
144.60, 144.58, 143.28, 143.26, 142.69, 142.67, 142.66, 142.64, 142.21,
142.17, 142.13, 142.11, 142.07, 142.05, 141.98, 141.95, 141.77, 141.73,
141.69, 141.67, 140.47, 140.44, 140.40, 140.37, 136.18, 136.15, 135.3,
135.2 (30 × 2 signals from sp2-C in the C60 core), 134.4, 131.6, 130.9,
126.5, 125.8, 123.3, 117.6, 117.4 (×2), 117.2, 117.0, 116.9, 114.9,
114.7, 114.4, 114.2, 113.4, 113.2, 97.8, 96.1, 94.3, 94.1, 93.1, 92.2,
91.8, 91.0, 88.3, 80.2 (×2), 70.14, 70.10, 69.8, 69.6, 69.5, 69.3, 62.02,
Supporting Information Available: NMR spectra of all new
compounds, TGA spectra of model fullerene-wheel compounds,
1H-15N CPMAS spectra of model compound dibenzo[a,c]-
phenazine, detailed syntheses of the intermediate compounds,
and preliminary STM images of nanotrucks 1a and 1b are
available. Movie files for nanocar 3a and trimers 4 are available
at http://tourserver.rice.edu/movies/. This material is available
free of charge via the Internet at http://pubs.acs.org.
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