En route to a motorized nanocar

Article abstract of DOI:10.1021/ol060445d

With the eventual goal of demonstrating a motorized nanocar, the key structure has been synthesized which bears a light-activated unidirectional molecular motor and an oligo(phenylene ethynylene) chassis and axle system with four carboranes to serve as th

Full text of DOI:10.1021/ol060445d

ORGANIC
LETTERS
2006
Vol. 8, No. 8
1713-1716
En Route to a Motorized Nanocar
Jean-Franc¸ois Morin, Yasuhiro Shirai, and James M. Tour*
Departments of Chemistry and Mechanical Engineering and Materials Science, and
the Smalley Institute for Nanoscale Science and Technology, Rice UniVersity,
Houston, Texas 77005
tour@rice.edu
Received February 20, 2006
ABSTRACT
With the eventual goal of demonstrating a motorized nanocar, the key structure has been synthesized which bears a light-activated unidirectional
molecular motor and an oligo(phenylene ethynylene) chassis and axle system with four carboranes to serve as the wheels. Kinetics studies
in solution show that the motor indeed rotates upon irradiation with 365 nm light, and the fullerene-free carborane wheel system is an essential
design feature for motor operation.
Recent efforts have been devoted to the synthesis of
nanomachines that could perform specific tasks at the
molecular level and ultimately construct molecular as-
semblies using a bottom-up approach.1 In this regard, the
first nanocar bearing a chassis, axles, and fullerene wheels
was recently synthesized by our group to exemplify the
structurally controlled directional movement on a surface due
to rolling of the wheels rather than the common stick-slip
motion2 of molecules on a substrate surface.3 The motion
could be thermally (heated substrate surface) or electrically
(STM-tip field) induced, thus opening the way to molecular-
structure-defined motion at the nanoscale. We describe here
the synthesis of a proposed nanocar that bears a light-
powered molecular motor in its central portion for an
eventual paddlewheel-like propulsion action along a substrate
surface for motion of the vehicle as shown in Figure 1. Also
described is the light-powered behavior, in solution, where
the kinetic parameters of motor rotation were measured.
Figure 1. Proposed propulsion scheme for the motorized nanocar
1 where (a) 365 nm light would impinge upon the motor which in
conjunction with a heated substrate (at least 65 °C) (b) affording
motor rotation and (c) sweeping across the surface to (d) propel
the nanocar forward.
(1) (a) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and
Machines: A Journey into the Nanoworld; Wiley-VCH: Weinheim, 2003.
(b) Ozin, G.; Manners, I.; Fournier-Bidoz, S.; Arsenault, A. AdV. Mater.
2005, 17, 13011. (c) Balzani, V. Small 2005, 1, 278. (d) Kinbara, K.; Aida,
T. Chem. ReV. 2005, 105, 1377. (e) Balzani, V.; Credi, A.; Raymo, F. M.;
Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348. (f) Muraoka, T.;
Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003, 125, 5612.
(2) Rosei, F.; Schunak, M.; Jiang, P.; Gourdon, A.; Laegsgaard, E.;
Stensgaard, I.; Joachim, C.; Besebbacher, F. Science 2002, 296, 328.
(3) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Kelly, K. F.; Tour, J. M. Nano
Lett. 2005, 5, 2330.
Various molecular motors have been developed.4 Among
them, unidirectional motors developed by Feringa et al.5 are
most applicable here because they (i) perform repetitive
10.1021/ol060445d CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/24/2006

rotary movement, (ii) use light and mild heating (35-65 °C)
as the power input, (iii) precisely perform unidirectional
rotation, (iv) can be functionalized without disturbing rotation
allowing them to be introduced into more complex structures,
and (v) can operate even when assembled atop metal
surfaces. The target nanocar bearing a Feringa motor is
shown in Figure 2. Unlike the former nanocar that bore
The axle was synthesized in six steps from 4-iodoaniline
(Scheme 1). After bromination to give compound 2, the
Scheme 1. Synthesis of p-Carborane-Containing Axle
Figure 2. (a) Structure of motorized nanocar 1. The p-carborane
wheels have BH at every intersection except at the top and bottom
vertexes which represent C and CH positions, ipso and para,
respectively, relative to the alkynes. (b) The space-filling model
of 1.
fullerene wheels,3 we opted for p-carborane wheels because
we observed that the Feringa motor is completely inoperative
in the presence of fullerenes, probably because of rapid
energy transfer, prior to rotation, from the excited state of
the motor to the fullerenes.6
All components of the nanocar 1 were chosen to obtain
translational movements on an atomically flat surface al-
lowing surface characterization using scanning tunneling
microscopy (STM).3 First, p-carboranes have been used as
wheels because they have a three-dimensional, near-spherical
structure7 that does not absorb light at 365 nm8 which is the
motor’s operational wavelength, and as we demonstrate here,
they do not prematurely quench the motor’s photochemical
rotary process. Second, alkynes have been used as axles
because their energy barrier to rotation is sufficiently low.3
The synthesis of the motorized nanocar 1 is presented in
Schemes 1 and 2.
amino group was replaced by an iodide using diazonium
chemistry followed by iodination to afford 3.9 Sonogashira
coupling introduced the alkynyl axles. After halogen inter-
conversion (4 to 5), in situ desilyl bromination10 was carried
out leading to compound 6 in excellent yield. p-Carborane
was then coupled to bromoalkyne 6 to afford the axles
bearing the two carborane wheels, and no side reaction
involving the aromatic iodide was noted at room temperature.
To build the motor (Scheme 2), we chose to use the
thioxanthene unit as the lower half and a naphtha[2,1-b]-
thiopyran as the upper half because the resulting motor
(unsubstituted) shows a good photostationary state (PSS)
ratio at room temperature.5 Moreover, functionalized thiox-
anthene can be easily prepared from commercially available
9-thioxanthenone by regioselective electrophilic substitu-
tion.11 Following the Feringa strategy,5 the thioketone 8 was
synthesized by heating 2,7-dibromothioxanthenone11 in tolu-
ene containing phosphorus pentasulfide. The racemic hy-
drazone 95b was oxidized to the unstable diazo compound
(4) (a) Badjic´, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F.
Science 2004, 303, 1845-1849. (b) Garcia-Garibay, M. A. Proc. Natl. Acad.
Sci. 2005, 102, 10771. (c) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti,
F.; Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540. (d) Kwon,
K.-Y.; Wong, K. L.; Pawin, G.; Bartels, L.; Stolbov, S.; Rahman, T. S.
Phys. ReV. Lett. 2005, 95, 166101. (e) Kottas, G. S.; Clarke, L. I.; Horinek,
D.; Michl, J. Chem. ReV. 2005, 105, 1281.
(5) (a) van Delden, R. A.; ter Wiel, M. K. J.; Pollard, M. M.; Vicario,
J.; Koumura, N.; Feringa, B. L. Nature 2005, 437, 1337. (b) Koumura, N.;
Geertsema, E. M.; van Gelder, M. B.; Meetsma, A.; Feringa, B. L. J. Am.
Chem. Soc. 2002, 124, 5037. (c) van Delden, R. A.; Hurenkamp, J. H.;
Feringa, B. L. Chem.-Eur. J. 2003, 9, 2845.
(6) Schuster, D. I.; Nuber, B.; Vail, S. A.; ManMahon, S.; Lin, C.;
Wilson, S. R.; Khong, A. Photochem. Photobiol. Sci. 2003, 2, 315.
(7) (a) Valliant, J. F.; Guenter, K. J.; King, A. S.; Morel, P.; Schaffer,
P.; Sogbein, O. O.; Stephenson, K. A. Coord. Chem. ReV. 2002, 232, 173.
(b) Kaszynski, P.; Pakhomov, S.; Tesh, K. F.; Young, V. G. Inorg. Chem.
2001, 40, 6622.
(9) Maya, F.; Flatt, A. K.; Stewart, M. P.; Shen, D. E.; Tour, J. M. Chem.
Mater. 2004, 16, 2987.
(10) Nishikawa, T.; Shibuya, S.; Hosokawa, S.; Isobe, M. Synlett 1994,
485.
(8) Kunkely, H.; Vogler, A. Inorg. Chim. Acta 2004, 357, 4607.
(11) Coleman, M. P.; Boyd, M. K. J. Org. Chem. 2002, 67, 7641.
1714
Org. Lett., Vol. 8, No. 8, 2006

3 h to obtain only the thermally stable conformation of the
motor. A ratio of >99:1 was thus obtained. The NMR tube
was then irradiated at room temperature with a UV lamp
using a 365 nm filter at a light intensity of ca. 10 mW/cm2.
The PSS was obtained after 60 min with an equilibrium ratio
of 18:82. Increasing the temperature to 65 °C has a dramatic
effect on the unstable isomer as shown in Figure 3. At that
Scheme 2. Synthesis of Motorized Nanocar 1
Figure 3. Graph of the relative concentration of the unstable isomer
vs heating time at different temperatures.
temperature, 90% of the unstable isomer was converted to
the stable isomer after 50 min. This is encouraging for future
applications because the motor can be switched on and off
over a very short range of temperatures.
Kinetic parameters (in solution) of the thermal conversion
of the unstable to stable isomer have been determined by
1H NMR at different temperatures and are shown in Table
1. All the values are very similar to those obtained by Feringa
with activated manganese dioxide in dichloromethane at 0
°C. In our hands, this oxidant gave better results than the
silver(I) oxide method formerly used.5 Thioketone 8 was then
added to the diazo compound leading to an isomeric mixture
of episulfides 10. The desulfurization was then accomplished
using trimethylphosphite to give 11 in excellent yield. In
our case, triphenylphosphine gave only partial conversion
to the alkene. This can be attributed to steric hindrance
induced by the bromine atoms in the 2- and 7-positions of
the thioxanthene unit. The alkynes were introduced using
standard Sonogashira couplings with TMSA and then the
axle/wheel assembly 7 to give the motorized nanocar 1 in
an overall yield of 5% for 12 steps.
Table 1. Kinetic Parameters of Motorized Nanocar 1 in
1
Toluene-d8 as Determined by H NMR
θ
Ea
kcal/mol kcal/mol cal/mol K kcal/mol
∆H*
∆S*
∆*Gθ
kθ
s-1
t1/2
h
Motorized nanocar 1, because of the symmetrical lower
part of the motor, is obtained as a mixture of two pairs of
enantiomers meaning that only two motor configurations
(stable and unstable) can be seen by 1H NMR (see Supporting
Information). As we expect to address 1 on surfaces as
individual entities, every nanocar is homochiral, hence it was
unnecessary to resolve the enantiomers. For this reason, we
can perform kinetics studies in solution by 1H NMR on the
racemic material. To avoid any interference with trace solvent
or the p-carborane-related peaks, we monitored the rotation
of the motor in toluene-d8 using the doublets at 8.01 ppm
(stable isomer) and 7.95 ppm (unstable isomer) (see Sup-
porting Information). Before starting the photoisomerization
study, the sample was heated in an NMR tube at 65 °C for
23.75 23.11 -4.49 24.82
1.91 × 10-6 101
for the motor bearing methoxy moieties (rather than alkynes)
at the 2,7-positions.5b Encouragingly, the presence of the
relatively bulky p-carborane wheels does not alter the rotation
of the motor, implying that the chassis and axle-bearing
alkynyl moieties are long enough to prevent steric interac-
1
tions. Moreover, the H NMR spectrum of 1 in toluene-d8
remains intact after >10 h of irradiation at 10 mW/cm2; no
significant decomposition products were observed.
Finally, to evaluate the influence of the presence of C60
(used as wheels in our first nanocar design3) on photoiso-
Org. Lett., Vol. 8, No. 8, 2006
1715

merization of the motor, we synthesized a system in which
the motor is covalently linked to the fullerenes as shown in
Scheme 3. To ensure good solubility of the resulting
coupling conditions to give 15. The alkynes were then
deprotected yielding 16, and fullerenes were introduced using
lithium hexamethyldisilazide (LHMDS)12 with an excess of
C60 to give 17.13
Compounds 15 and 17 were subjected to the same
photoisomerization procedure to which nanocar 1 had been
exposed. Although 15 underwent ca. 80% conversion upon
irradiation for 120 min, no isomerization was detected in
17. The intramolecular quenching of the photoexcited state
of the motor moiety by the fullerene wheels mitigates motor
operation. Thus, we cannot use our previous fullerene-wheel-
based nanocar design3 with the Feringa motor; use of the
carborane wheels is essential.
Scheme 3. Synthesis of C60-Containing Motor
Whether energy transfer to the metallic surface during
photoisomerization will be problematic needs to be further
evaluated. If there is such energy transfer to the metal surface,
use of a short alkanethiol monolayer as an insulating barrier,
as in Feringa’s gold-surface-bound isomerization study,5a or
change to a nonconductive surface will be necessary; the
latter requires a non-STM-based probing technique such as
atomic force microscopy or fluorescence microscopy. Fur-
thermore, whether the motor will have sufficient energy to
rotate and thus propel the nanocar on a surface remains to
be determined.
Acknowledgment. We thank the NSF (Penn State MR-
SEC), the Welch Foundation, and Honda for funding and
FAR Research (Dr. I. Chester) and Petra Research (Dr. R.
Awartani) for TMSA. J.-F. Morin thanks NSERC and
FQRNT for a postdoctoral fellowship.
Supporting Information Available: Synthetic proce-
dures, 1H NMR, 13C NMR, IR, and mass spectra of
compounds 1, 3-8, 10-13, and 15-17 and details of the
kinetics study of thermal isomerization of the motorized
nanocar. This material is available free of charge via the
Internet at http://pubs.acs.org.
molecule for NMR analysis, a didecyloxybenzene unit
needed to be introduced between the motor and the two
fullerene moieties. Starting from hydroquinone, 14 was
synthesized in three steps as we described previously.12
Compound 14 was coupled to 13 using standard Sonogashira
OL060445D
(12) Shirai, Y.; Zhao, Y.; Chen, L.; Tour, J. M. Org. Lett. 2004, 6, 2129.
(13) Komatsu, K.; Murata, Y.; Takimoto, N.; Mori, S.; Sugita, N.; Wan,
T. S. M. J. Org. Chem. 1994, 59, 6101.
1716
Org. Lett., Vol. 8, No. 8, 2006