Synthesis of a new photoactive nanovehicle: A nanoworm

Article abstract of DOI:10.1021/ol703027h

A nanovehicle with a new photoactive moiety has been synthesized. The incorporation of the azobenzene chassis allows for potential wormlike movement accompanying the rolling behavior of the wheels. Two versions, the fullerene-wheeled and carborane-wheeled nanoworms, were synthesized to examine the solution-based photoisomerization yields of the chassis.

Full text of DOI:10.1021/ol703027h

ORGANIC
LETTERS
2008
Vol. 10, No. 5
897-900
Synthesis of a New Photoactive
Nanovehicle: A Nanoworm
Takashi Sasaki and James M. Tour*
Departments of Chemistry and Mechanical Engineering and Materials Science and
The R. E. Smalley Institute for Nanoscale Science and Technology, Rice UniVersity,
MS 222, 6100 Main Steet, Houston, Texas 77005
tour@rice.edu
Received December 17, 2007
ABSTRACT
A nanovehicle with a new photoactive moiety has been synthesized. The incorporation of the azobenzene chassis allows for potential wormlike
movement accompanying the rolling behavior of the wheels. Two versions, the fullerene-wheeled and carborane-wheeled nanoworms, were
synthesized to examine the solution-based photoisomerization yields of the chassis.
Advancements in the field of molecular machinery have
afforded various bottom-up approaches toward performing
macroscopic-like tasks at the molecular level.1 In combina-
tion with imaging tools such as scanning tunneling micros-
copy (STM),2 our group developed individually accessible
nanocars, each bearing a chassis, axles, and fullerene wheels
for a restricted rolling motion on a metallic surface.3 The
movement is induced either thermally (surface heating) or
electrically (STM-tip field). Recently, a nanocar with a light
powered motor was introduced to provide a third source of
external stimuli.4 A cis-trans photoisomerization of azoben-
zene chromophores (Figure 1) to potentially generate inch-
worm-like motion on a surface is described. It is widely
accepted that the photoirradiation step from trans to cis has
a rotational pathway, whereas the heating step from cis to
trans has an inversion pathway.5 The combination of these
dissimilar pathways might help propel the nanoworm move-
ment.
(2) (a) Gross, L.; Rieder, K.-H.; Moresco, F.; Stojkovic, S. M.; Gourdon,
A.; Joachim, C. Nature Mater. 2005, 4, 892-895. (b) Grill, L.; Rieder,
K.-H.; Moresco, F.; Rapenne, G.; Stojkovic, S.; Bouju, X.; Joachim, C.
Nature Nanotechnol. 2007, 2, 95-98. (c) Serreli, V.; Lee, C.-F.; Kay, E.
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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,
J. M. Chem. Soc. ReV. 2006, 35, 1043-1055.
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10.1021/ol703027h CCC: $40.75
© 2008 American Chemical Society
Published on Web 02/09/2008

nanocar showed that the motor unit was inactive in the
presence of fullerene.4 The rapid intramolecular energy
transfer to the fullerene wheels quenched the photoexcited
state of the motor. The photoisomerization of azobenzene
has been studied extensively and used in devices and
photoinduced systems for the large geometrical change
accompanying the isomerization.6 However, there are few
reports on the photoisomerization properties of azobenezene/
fullerene hybrids.7 p-Carboranes have been used as wheels
because they have three-dimensional, near-spherical struc-
tures8 that do not absorb at 365 nm. Other favorable
properties from carboranes such as enhanced solubility and
Pd-catalyzed compatible chemistry have allowed our group
to produce several new-generation nanocars with this wheel.4,9
The synthesis of nanoworm 1 consists of repetitive steps
of Pd-catalyzed Sonogashira couplings and deprotection of
alkynes. It begins with diiodoazobenzene 310 being coupled
with 4,3,11 followed by the removal of the triisopropyl
protecting group to afford 5.12 Another coupling between 5
and 6,3c,4,9 then deprotection, afforded the chassis 7. The axles
were then elongated via coupling of 83,11 to the chassis
followed by desilylation to afford 9, the backbone of the
fullerene nanoworm. The four fullerene wheels were then
Figure 1. (A) Side view model of the nanoworm. The proposed
photoactivated rotational pathway on a surface away from the
reference point (noted by the star) by irradiating at 365 nm or 435
nm to convert A to B (trans to cis isomerization) by rotation. (B)
Model of the nanoworm (cis) irradiated at >495 nm (or heated) to
induce the inversion pathway to C which constitutes a cis to trans
conversion, thus further propelling the nanoworm from the starred
reference location. (C) The nanoworm again in the trans conforma-
tion, after completing the cycle of trans to cis to trans conversion,
that is further displaced from the starred reference point. (D) Top
view; rotation about the alkyne bonds is possible in order to achieve
favorable conformations.
Scheme 1. Synthesis of Nanoworm 1
Two targets, fullerene-wheeled, and carborane-wheeled
nanoworms, were synthesized (Figure 2). The latter was
synthesized because the previous study of a motorized
Figure 2. (A) C60-wheeled nanoworm 1. Extended dodecyl-
substituted axles were added to ensure solubility. (B) p-Carborane-
wheeled nanoworm 2. The p-carboranes have BH at every
intersection except at the points denoted by black spheres, which
represent C and CH positions, ipso and para, respectively.
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Org. Lett., Vol. 10, No. 5, 2008

installed via the in situ ethynylation method3,11 to complete
the synthesis of nanoworm 1 (Scheme 1).
As summarized in Table 1, two noteworthy results were
obtained. First, as expected, starting material 9 underwent
Compounds 1 and 9 were subjected to photoisomerization
experiments and monitored using UV-vis spectroscopy. As
seen in Figure 3, the azo absorption peak was red-shifted
Table 1. Relative % Cis Yieldsa with Irradiation at 436 or 365
nm for 10 min to Reach PSS Determined by UV-vis
Spectroscopy
nanoworm 1
9
9/C60 (5:1) 9/C60 (1:1) nanoworm 2
12 24
% cis
8
23
8
a Change in absorption (∆abs) for the main absorption band of the trans
isomer (π-π*) (λmax ∼440 nm) reflects the minimum concentration of cis
isomer at the PSS. Values are solely based on estimated yields due to
limitations in UV-vis spectroscopy instrumentation as well as overlapping
peaks.14 1 and 9 were excited at 436 nm, and 2 was excited at 365 nm.
cis-trans isomerization with cis conversion ∼23% upon
irradiation at 436 nm for 10 min, at which time the
photostationary state (PSS) was reached. The low conversion
yields are due to steric interactions caused by the alkoxy
groups and from the increased conjugation effect.12 It was
surprising that any switching behavior was observed from
nanoworm 1. Fullerenes attached via alkynes on conjugated
systems introduce a weak electronic interaction called
periconjugation,13 where the fullerene π-electrons com-
municate with the alkynyl π-electrons in a through-space
p-orbital overlapping mechanism. This causes disruption in
the electronic communication of the chromophore that has
produced ∼0% conversions in systems with two fullerenes
in prior work.4,12 However, despite having four fullerene
wheels, 1 showed slight conversion,13 a result that suggests
that the distances between the azo and fullerene moieties
are important despite their conjugated systems. Interaction
of free fullerenes with compound 9 was also measured to
illustrate the intermolecular effect of C60 on photoisomer-
ization yields. From the UV-vis spectroscopy data, a mixture
of 5:1 of 9 to free C60 decreased the cis conversion to 12%.
Further addition of free C60 to produce a 1:1 ratio decreased
the cis conversion to 8%. This large intermolecular effect
was not observed in the previous work.12 The intermolecular
photoquenching may be assisted through a supramolecular
complex between free fullerenes and the flexible alkyl
groups. Extensive studies on other factors such as relaxation
Figure 3. UV-vis absorption experiments of (A) nanoworm 1 in
CHCl3, 7.0 × 10-6 M, irradiated at both 365 and 436 nm for 10
min; (B) 9 in CHCl3, 1.8 × 10-5 M, irradiated at 436 nm for 10
min; (C) 5:1, 9 in CHCl3, 1.8 × 10-5 M, to C60 in CS2, 1.8 × 10-5
M, irradiated at 436 nm for 10 min (D) 1:1, 9 in CHCl3, 1.8 ×
10-5 M, to C60 in CS2 1.8 × 10-5 M, irradiated at 436 nm for 10
min.
(when compared to the typical azo π-π* absorption peak
around 360 nm), due to the increased conjugation of the
molecule; therefore the maximum concentration of cis isomer
was obtained when 1 and 9 were irradiated at 436 nm rather
than 365 nm.
(6) (a) Shimoboji, T.; Larenas, E.; Fowler, T.; Kulkarni, S.; Hoffman,
A. S.; Stayton, P. S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 16592-16596.
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2000, 288, 1624-1626.
Scheme 2. Synthesis of Nanoworm 2
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Reed, M. A.; Tour, J. M. Tetrahedron 2003, 59, 8555-8570.
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T.; Hegmann, F. A.; Tour, J. M. Chem. Eur. J. 2005, 11, 3643-3658.
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Org. Lett., Vol. 10, No. 5, 2008
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lene (TMSA) and then deprotected to afford 10.15 The
carborane axle/wheel system 113c,4,9 was then coupled to
afford the target nanoworm 2 (Scheme 2).
From the photoisomerization experiment (Figure 4), the
highest concentration of the cis isomer of 2 was obtained
when the system was irradiated at 365 nm for 10 min, at
which time the PSS was reached. Cis to trans switching
occurred both photochemically when irradiated at >495 for
5 min (the temperature increase was <5 °C) as well as
thermally when heated at 40 °C for 15 min.
In summary, two new photoactive nanovehicles with
azobenzene chromophores have been synthesized. Prelimi-
nary solution-based photoisomerization showed conversion
of trans to cis in nanoworm 2 and slight changes in
nanoworm 1. The results obtained from both spatially distant
intramolecular interactions as well as intermolecular effects
produced insights on chromophore-fullerene interactions.
Studies are underway to image these nanoworms by micros-
copy and thereby study the photoactivated motion on
surfaces. This might further necessitate nonconductive
surfaces and or a switch to AFM or confocal microscopies.
Figure 4. Isomerization experiments of 2 in CHCl3, 1.4 × 10-5
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.
M, at different wavelengths and temperatures.
rates and nonconductive linker energy transfers have recently
been summarized.12
Because of the small switching activity observed with 1,
another wheel system was introduced in order to overcome
the photoquenching effect, and as before,4 carboranes were
chosen. The same starting material diiodoazobenzene 3 was
coupled via Sonogashira coupling with trimethylsilylacety-
Supporting Information Available: Experimental details
as well as spectroscopic data (FTIR, 1H NMR, and 13C NMR)
for compounds 1, 2, 7, and 9. This material is available free
of charge via the Internet at http://pubs.acs.org.
(13) Hamasaki, R.; Ito, M.; Lamrani, M.; Mitsuishi, M.; Miyashita, T.;
Yamamoto, Y. J. Mater. Chem. 2003, 13, 21-26 and references cited
therein.
OL703027H
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