Synthesis of a porphyrin-fullerene pinwheel

Article abstract of DOI:10.1021/ol7029917

(Chemical Equation Presented) We disclose the synthesis of a porphyrin-fullerene pinwheel that was subsequently observed by scanning tunneling microscopy. The molecule was designed to further our understanding of fullerene-surface interactions, directional control, and surface-rolling versus pivoting capabilities of this class of nanomachines. The inner porphyrin provides the square planar configuration that might lead to realization of the pinwheel spiraling motion on surfaces.

Full text of DOI:10.1021/ol7029917

ORGANIC
LETTERS
2008
Vol. 10, No. 7
1377-1380
Synthesis of a Porphyrin-Fullerene
Pinwheel
Takashi Sasaki,† Andrew J. Osgood,‡ J. L. Kiappes,† Kevin F. Kelly,‡,§ and
James M. Tour*,†,§
Departments of Chemistry and Mechanical Engineering and Materials Science,
Department of Electrical and Computer Engineering, and The R. E. Smalley Institute
for Nanoscale Science and Technology, Rice UniVersity, MS 222, 6100 Main Street,
Houston, Texas 77005
tour@rice.edu
Received December 12, 2007
ABSTRACT
We disclose the synthesis of a porphyrin-fullerene pinwheel that was subsequently observed by scanning tunneling microscopy. The molecule
was designed to further our understanding of fullerene-surface interactions, directional control, and surface-rolling versus pivoting capabilities
of this class of nanomachines. The inner porphyrin provides the square planar configuration that might lead to realization of the pinwheel
spiraling motion on surfaces.
There have been recent advancements in the field of imaging
techniques, such as scanning tunneling microscopy (STM),1
applied to the field of molecular machinery.2 The two fields
have inspired us to work with individually accessible single-
molecular nanomachines. Of particular interest for our group
is the development of nanocars.3 In our design of the
nanocars, fullerenes served both as molecular components
easily identified by STM as well as wheels that enabled
(2) (a) Balzani, V.; Credi. A.; Venturi, M. Molecular DeVices and
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A.; Lubomska, M.; Mendoza, S. M.; Pe´rez, E. M.; Rudolf, P.; Teobaldi,
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V.; Credi, A.; Silvi, S.; Stoddart, J. F. Science 2004, 303, 1845-1849. (e)
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† Departments of Chemistry and Mechanical Engineering and Materials
Science.
‡ Department of Electrical and Computer Engineering.
§ The R. E. Smalley Institute for Nanoscale Science and Technology.
(1) (a) Gross, L.; Rieder, K.-H.; Moresco, F.; Stojkovic, S. M.; Gourdon,
A.; Joachim, C. Nat. Mater. 2005, 4, 892-895. (b) Grill, L.; Rieder, K.-
H.; Moresco, F.; Rapenne, G.; Stojkovic, S.; Bouju, X.; Joachim, C. Nat.
Nanotechnol. 2007, 2, 95-98. (c) Serreli, V.; Lee, C.-F.; Kay, E. R.; Leigh,
D. A. Nature 2007, 445, 523-527. (d) Wong, K. L.; Pawin, G.; Kwon, K.
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10.1021/ol7029917 CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/04/2008

controlled rolling motion of the molecules on surfaces.
Because of the friction-like (charge transfer at the molecular
level) interactions between the fullerenes and the gold
surface, a new motion mechanism was demonstrated for the
first time, directionally restricted rolling instead of sliding
on a surface. After establishing that rolling motion did occur,
we directed our attention to the study of functionalized
nanocars with varying properties that could produce different
surface movements.4 Here we report the synthesis and the
initial images of a fullerene pinwheel molecule that was
synthesized to further distinguish between translation and
pivoting in molecular surface-rolling molecules.
solar cells,5 photosynthetic antennas,6 and nonlinear-optics7
for their interesting photoinduced electron-transfer properties.
The goal of this synthetic project, however, was solely
directed at understanding nanocar directionality on metallic
surfaces. Since porphyrins and metal-complexed porphyrins
have been observed with STM,8 it was thought that the
addition of this feature to the nanocar design would make
imaging more facile. With the wheels projecting from the
body at 90° angles, the molecule is designed to show a
different rolling behavior than that of the nanocar. The
motion we predict may be similar to that of the trimer
molecule (Figure 1), a control experiment from our past work
in which the molecule rolled through pivoting about its
central axis without any translational motion on gold
surfaces.3 Whether or not the porphyrin-based square con-
formation will permit this pivoting rolling motion is an area
of intense interest in future STM experiments.
The pinwheel 1 has four fullerene wheels, comparable to
that of the nanocars (Figure 1), with copper inserted into
The synthesis began (Scheme 1) with three Pd-catalyzed
Sonogashira couplings, the first two followed by the selective
Scheme 1. Synthesis of Trimer 5
removal of the TMS-protecting group using K2CO3, to obtain
4.3,9 Note that for this design, long chain hydrocarbon
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(c) Sasaki, T.; Morin, J.-F.; Lu, M.; Tour, J. M. Tetrahedron Lett. 2007,
48, 5817-5820. (d) Morin, J.-F.; Sasaki, T.; Shirai, Y.; Guerrero, J. M.;
Tour, J. M. J. Org. Chem. 2007, 72, 9481-9490.
(5) (a) Sgobba, V.; Giancane, G.; Conoci, S.; Casilli, S.; Ricciardi, G.;
Guldi, D. M.; Prato, M.; Valli, L. J. Am. Chem. Soc. 2007, 129, 3148-
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Y.; Yoshida, K.; Isoda, S.; Araki, Y.; Ito, O. J. Phys. Chem. B. 2006, 110,
11399-11405.
Figure 1. The nanocar showed directionally controlled translational
motion on a surface while the trimer showed only pivoting. The
pinwheel (1) prepared here, also bearing four wheels like the
nanocar, should only undergo pivoting as observed with the trimer.
(6) (a) Kodis, G.; Terazono, Y.; Liddell, P. A.; Andreasson, J.; Garg,
V.; Hambourger, M.; Moore, T. A.; Moore, A. L.; Gust, D. J. Am. Chem.
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the porphyrin core. Porphyrin-fullerene combinations have
attracted a considerable amount of interest in the fields of
(3) (a) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Kelly, K. F.; Tour, J. M.
Nano Lett. 2005, 5, 2330-2334. (b) Shirai, Y.; Osgood, A. J.; Zhao, Y.;
Yao, Y.; Saudan, L.; Yang, H.; Yu-Hung, C.; Sasaki, T.; Morin, J.-F.;
Guerrero, J. M.; Kelly, K. F.; Tour, J. T. J. Am. Chem. Soc. 2006, 128,
4854-4864. (c) Shirai, Y.; Morin, J.-F.; Sasaki, T.; Guerrero, J. M.; Tour,
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Org. Lett., Vol. 10, No. 7, 2008

substituents were included to offset the low solubilities of
the fullerene and porphyrin moeities. An extended dimer axle
was used to increase the solubility of the final target.
Compound 4 was coupled at elevated temperature with
4-bromobenzaldehyde to afford 5, the precursor for the
porphyrin reaction.10
by STM (Figure 2).14 From cross-sectional analysis, the
diagonal distance from fullerene to fullerene was ∼4.8 nm.
Compound 5 was condensed with pyrrole and then
oxidized with DDQ to afford porphyrin 6.11 The copper metal
was inserted using Cu(OAc)2‚H2O,12 to obtain porphyrin 7.13
The deprotection step with TBAF was carried out after the
metal insertion to avoid possible homocoupling of the free
alkynes with exogenous copper. In addition, copper was the
metal of choice due to its stability in the porphyrin complex
and to avoid destruction of the porphyrin molecule in the
presence of excess lithium hexamethyldisilazide (LHMDS)
that would be used in the next step. Four fullerenes were
successfully coupled via the in situ ethynylation method to
complete the synthesis of the fullerene pinwheel 1 (Scheme
2).3,9 The solubility of 1 was sufficient in CHCl3/CS2 or
Scheme 2. Synthesis of Pinwheel 1a
Figure 2. (a) Materials Studio (MS) modeling of C60-poryphyrin
pinwheel molecule 1. The alkyl groups are ommitted for clarity.
(b) STM image of scattered individual molecules 1. (c,e,f) Higher
resolution images with (d) cross-section analysis of 1 pictured in
(e). (f) Lateral bend of OPE legs of 1 brings two fullerenes closer
together on a step edge. (Bias voltage ) -100 mV, tunneling
current ) 3 pA).
This is slightly smaller than ∼5.4 nm calculated in PC
Spartan (relative structure was calculated using empirical
molecular mechanics based on Tripos 5.1 force field). The
discrepancy may have resulted from the long oligo(phenylene
ethynylene) (OPE) legs bending normal to the surface due
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698.
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Hegmann, F. A.; Tour, J. M. Chem.-Eur. J. 2005, 11, 3643-3658. (b)
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a TBAF ) tetrabutylammonium flouride, DDQ ) 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone.
toluene to enable dosing valve14 deposition on a gold surface
and subsequent STM analysis.
Although some of the molecules were clustered together,
clear images of individual pinwheel molecules were observed
(11) Jagessar, R. C.; Tour, J. M. Org. Lett. 2000, 2, 111-113.
(12) NMR of molecules 1, 7, and 8 were braodened due to the inserted
copper metal.
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Org. Lett., Vol. 10, No. 7, 2008
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to a strong gold-fullerene interaction (evidence of lateral
bending is seen in Figure 2f). Another interesting feature
of this molecule is the lack of imaging of its inner core.
Our previous fullerene series3,9 gave similar images via
STM; however, with 1, it was expected that the copper in
the porphyrin would enable imaging of the core. The
electronic nature of the fullerene-OPE interaction may be
the reason for the unexpected failure to image the core;
this aspect of the molecule is under investigation.17 It should
be noted, however, that the precise nature of the imaged
moieties is highly structure dependent and is a topic that
has generated much discussion.18 Demetallation during the
deposition process can be ruled out because of the mild
dropcasting conditions used for the deposition technique, as
previously reported by Yoshimoto et al., where metallo-
prophyrins were intact on similar Au(111) surfaces for
imaging.19
Figure 3. UV/vis spectra of compounds 1 (blue), 6, (yellow), and
8 (pink).
The electronic absorption characteristics of the porphyrin-
OPE-fullerene backbone were studied by UV/vis spec-
troscopy in chloroform of compounds 1, 6, and 8
(Figure 3). The peak observed for 1 at 319 nm is indicative
of the fullerene peak.9 The distinct λmax, Soret band, is
observed in all three compounds 1, 6, and 8, indicating
the presence of the porphyrin. The central molecular re-
gion (690 nm for 6) is blue-shifted, consistent with metal
insertion in 1 and 8.20 The distinctively enhanced absorption
tail of 1, above 450 nm, is due to the weak electronic
interactions between fullerenes and the central OPE-
porphyrin backbone. This is consistent with the periconju-
gation effect observed in our past fullerene-OPE
hybrids.3,9
In summary, we have successfully designed and observed
a new fullerene-based molecule 1 for potential directional
control of surface-pivoting nanomachines. Further study is
currently underway using a heated substrate STM stage to
observe the molecular motion.
(14) Sample Preparation and Data Collection for STM Study.
A toluene solution of pinwheel 1 (5 µM) was dosed in high vacuum
using a fast-actuating, small orifice solenoid valve15,16 onto argon-
sputtered and annealed Au(111) on mica substrates and was imaged using
an RHK variable temperature UHV-STM. A range of tunneling para-
meters (-1 to +1 V, 3-20 pA) was explored while attempting to resolve
the inner structure. The dosing technique was chosen over sub-
limation in vacuum, as it appeared decomposition occurs at ∼300 °C in
thermal decomposition studies using a thermogravimetric analyzer in our
previous work.3 When imaging C60 and its derivatives with STM, their
appearance is distorted by two effects, the first being the charge trans-
fer between the fullerene and the underlying metal surface. Since STM is
a reflection of both topography and electronic structure, the effect of
the charge transfer causes the fullerenes to appear between 2.5 and 4 Å
tall. The second effect is convolution with the finite size of the STM tip
causing the lateral size of the fullerenes to appear much larger than ex-
pected. Thus, the combination of the two phenomenon results in the
fullerenes being somewhat more flattened in their appearance than expected.
For the case of bare fullerenes, this has been previously discussed by
Yamachika, R.; Grobis, M.; Wachowiak, A.; Crommie, M. F. Science 2004,
304, 281.
Acknowledgment. We thank the Welch Foundation,
C-1489, American Honda Motor Co., the NSF NIRT (ECCS-
0708765), and the NSF Penn State MRSEC for financial
support. The NSF, CHEM 0075728, provided partial funding
for the 400 MHz NMR. We thank Drs. I. Chester of FAR
Research, Inc. and R. Awartari of Petra Research, Inc. for
providing trimethylsilyacetylene.
(15) Kanno, T.; Tanaka, H.; Nakamura, T.; Tabata, H.; Kawai, T. Jpn.
J. Appl. Phys. 1999, 38, L606-L607.
Supporting Information Available: Experimental details
1
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(16) Terada, Y.; Choi, B. K.; Heike, S.; Fujimori, M.; Hashizume, T.
Nano Lett. 2003, 3, 527-531.
1
and mass spectrometry for compounds 5 and 6; H NMR,
(17) Fullerenes attached to a conjugated system via an alkyne bond
produce weak electronic interactions called periconjugation. In a through
space p-orbital overlapping mechanism, fullerene π-electrons may possibly
interact with the alkynyl π-electrons. Hamasaki, R.; Ito, M.; Lamrani, M.;
Mitsuishi, M.; Miyashita, T.; Yamamoto, Y. J. Mater. Chem. 2003, 13,
21-26 and references therein.
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Science 1996, 271, 181-184. (d) Gimzewski, J. K.; Joachim, C. Science
1999, 283, 1683-1688.
FTIR and mass spectrometry for 1; FTIR and mass spec-
trometry for 7 and 8). This material is available free of charge
via the Internet at http://pubs.acs.org.
OL7029917
(19) Yoshimoto, S.; Sato, K.; Sugawa, S.; Chen, Y.; Ito, O.; Sawaguchi,
T.; Niwa, O.; Itaya, K. Langmuir 2007, 23, 809-816.
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Org. Lett., Vol. 10, No. 7, 2008