Controlled rotary motion of light-driven molecular motors assembled on

Controlled rotary motion of light-driven molecular motors assembled on
a gold film†
Gregory T. Carroll,a
Michael M. Pollard,b
Richard van Deldena
and Ben L. Feringa*a
Received 2nd February 2010, Accepted 26th March 2010
First published as an Advance Article on the web 24th May 2010
DOI: 10.1039/c0sc00162g
Using circular dichroism (CD) spectroscopy, we show that light-driven rotary molecular motors based
on overcrowded alkenes can function in a self-assembled monolayer on semi-transparent gold films.
Molecular motors are ubiquitous in the nanoscale machines of
nature and play a key role in nearly every important biological
process.1
A current challenge is to make use of nanoscale motions
in synthetic systems to develop molecular machines.2–5
The
majority of studies have focused on measurements in solution,
however surface-mounted motors6,7
provide nanoscale rotary
systems that are interfaced with the macroscopic world and offer
the most potential for developing advanced molecular
machinery. Pertinent applications include probes for micro-
viscosity,8
signal processing,9–12
tunable dielectrics,13–15
directed
nano- and microscale translational16–18
and rotational19
motion
and possibly the exertion of controlled nanomechanical forces20
and ultimately nanopumps and valves. Moreover, such systems
are fertile ground for uncovering new paradigms in molecular
motion at interfaces. One of the most widely used strategies for
assembling monolayers of oriented organic molecules on solid
surfaces is the adsorption of thiols on metals.21
Rotary move-
ment at macroscopic gold surfaces has previously been investi-
gated with STM.22–25
The motion in the systems studied involved
thermally driven rotation around thiol–gold bonds or carbon–
carbon single bonds. Although important aspects of rotary
motion at surfaces were uncovered, a key requirement to fulfil the
criteria of a rotary motor and apply the rotary motion to ulti-
mately perform work at the molecular level is control over the
direction of rotation.
Among the few synthetic systems that have demonstrated
controlled unidirectional rotary motion,26–29
photoactive over-
crowded alkenes are the best-studied.30
Advantages of the motor-
systems based on overcrowded alkenes include the range of
synthetic protocols for introducing functionality into the struc-
ture of the motor-molecule (i.e. for surface attachment) and their
activation with photons, which provides a clean and traceless fuel
that can be delivered with spatial control. With the long-term aim
of harnessing work from the collective action of nano-motors,
the simplicity of the fabrication of monolayers from organic
thiols self-assembled on gold make this a very attractive
approach to construct arrays of light-powered rotors. Despite
many studies and advances applied to these light-driven unidi-
rectional rotary motors,4
their assembly and rotary function on
macroscopic gold films has not been reported. Although these
motors were previously shown to function while grafted to the
surface of gold nanoparticles dispersed in solvent,31,32
the gold
particles themselves were subject to Brownian motion, thus
making the application of these motor molecules more difficult.
Here we report unidirectional rotary motion of light-driven
rotary molecular motors in self-assembled monolayers (SAMs)33
on gold surfaces as shown conceptually in Fig. 1. We prepared
‘‘second-generation molecular motors’’34
bearing thiols for self-
assembly into monolayers on a semi-transparent gold film
deposited on quartz. The use of a thin (5 nm) layer of gold
minimized the optical absorbance of the metal layer so as to
allow the characterization of the surface-bound chiroptical
motors using circular dichroism (CD) spectroscopy, and to
follow the photochemical and thermal steps in the rotary cycle.
Fig. 1 Self-assembly of light-driven molecular motors on a solid surface
provides a monolayer of nanoscale motors. The four stages of the 360
rotary cycle can be addressed with light and heat.‡
a
Centre for Systems Chemistry, Stratingh Institute for Chemistry and
Zernike Institute for Advanced Materials, University of Groningen,
Nijenborgh 4, Groningen, 9747 AG, The Netherlands. E-mail: B.l.
feringa@rug.nl
b
Department of Chemistry, York University, 4700 Keele St., Toronto,
Ontario, Canada M3J 1P3
†
Electronic supplementary information (ESI) available: Preparation of
surfaces, UV-Vis spectrum of semi-ransparent Au, additional CD
spectra, synthesis of motors and characterization including 1
H and 13
C
NMR spectra. See DOI: 10.1039/c0sc00162g
This journal is ª The Royal Society of Chemistry 2010 Chem. Sci., 2010, 1, 97–101 | 97
EDGE ARTICLE www.rsc.org/chemicalscience | Chemical Science

We found that when the molecule was linked to the surface by
shorter tethers (8 atoms), the gold interfered with the motors’
excited state processes, preventing any photoisomerization of the
central overcrowded alkene. However, when longer tethers (16
atoms) were used, the molecules regained the ability to undergo
photochemical isomerization and unidirectional rotation.
The motors employed for assembly on thin gold films are
based on the parent-overcrowded alkene 1 (Fig. 2), a recently
designed light-driven molecular motor that undergoes unidirec-
tional rotary motion upon irradiation with UV light.35
The
stereogenic center is crucial in achieving unidirectional rotation.
The methyl-substituent at the stereogenic center adopts a pseu-
doaxial conformation in the stable isomer, and a disfavoured
pseudoequatorial conformation in the unstable isomer. The
relative stabilities of the two isomers originate from differences in
the degree of steric strain imposed by crowding of the methyl
group with the adjacent aromatic ring in the lower-half. The
configuration of the stereogenic center dictates the helicity of the
chromophore, (P) or (M), in both the stable and unstable
isomers. The R configuration of 1 will induce an M helicity in the
stable isomer and a P helicity in the unstable isomer, whereas the
S configuration of 1 induces a P helicity in the stable isomer and
an M helicity in the unstable isomer. Consequently, the config-
uration at the stereogenic center controls the direction of
rotation.
In the first step of the rotary cycle (Fig. 2), absorption of
a photon by (20
R)-(M)-1 results in an isomerization of the central
overcrowded alkene to give unstable (20
R)-(P)-1. The photo-
chemical isomerization leaves the molecule in a high-energy
conformation of the opposite helicity wherein the methyl group
at the stereogenic center is forced to adopt a pseudo-equatorial
orientation, which results in unfavorable steric crowding between
the methyl substituent and the adjacent aromatic moiety from
the lower-half of the molecule. The strain imposed by steric
crowding is relieved in a thermodynamically favorable helix
inversion where both the methyl group and the naphthalene ring
of the rotor moiety slip past the aromatic moieties of the lower-
half stator, thus regenerating the stable conformation with
a pseudoaxial methyl group. The large free energy gain from the
thermal isomerization step provides an effectively irreversible
conformational change that completes a 180
rotation of the
upper-half rotor relative to the lower-half stator, regenerating
(20
R)-(M)-1, and is the origin of the unidirectionality of the
rotary cycle. Subsequent photo- and thermal isomerization steps
complete the rotary cycle.
Parent motor 1 was selected because it has a large barrier to
thermal helix inversion, making thermal isomerization at room
temperature very slow and thus facilitating characterization of
the separate states of the rotary cycle. Overcrowded alkene 1 was
designed with two ester groups in the lower-half of the molecule
to allow the incorporation of two linkers for its subsequent
attachment to a surface. Both points of attachment to the surface
are required to prevent uncontrolled thermal rotation of the
entire system with respect to the surface, and thus transforming
relative rotary motion of the two ‘‘halves’’ (rotor and stator) of
the molecule into absolute rotary motion of the rotor relative to
the stator. In order to enable self-assembly onto a gold surface,
1 was elaborated with tethers (or ‘‘legs’’) that terminate with thiol
moieties (Fig. 3). The diester 1 was saponified to give the corre-
sponding diacid, which was in turn converted to the diacid
chloride upon treatment with oxalyl chloride in benzene in the
presence of a catalytic amount of DMF. The diacid chloride was
Fig. 2 The rotary cycle of the parent motor 1 employed in surface
attachment consists of four steps. A full cycle is achieved by a combina-
tion of light-driven and thermal helix inversion steps.
Fig. 3 Modification of parent motor 1 with thiol-terminated linkers
provides a precursor for the self-assembly of a monolayer of motors on
a gold surface. The CD spectrum of (2R)-(M)-2 (black) in dodecane is
shown. Irradiation produces the enantiomer (20
R)-(P)-2, which gives
a CD spectrum (red) that is a pseudo-mirror image of the original due to
incomplete conversion at the PSS.
98 | Chem. Sci., 2010, 1, 97–101 This journal is ª The Royal Society of Chemistry 2010

then treated with the bromoalcohol (n ¼ 1 or 9) to give the
dibromide, which was then converted to the dithioester by
displacement with potassium thioacetate in DMF. The molecules
were resolved with chiral HPLC at this stage. The thioesters in
the resulting enantiopure materials were then hydrolyzed under
an inert atmosphere immediately prior to surface attachment by
either treatment with piperidine in THF at 50
C for 2 h or, by
treatment for 20 min with MeOH that was pre-saturated with
NH3 (g). Thioacetate-protected (20
R)-2 has a signal that is similar
to structurally related molecules, while its enantiomer clearly
displays a spectrum that is its mirror-image (Fig. 3).
In order to study SAMs composed of 2 or 3 using CD
spectroscopy, a semi-transparent gold film of approximately 5 nm
was prepared (see ESI†).36,37
Quartz slides were cleaned with
piranhax+?/^!string(//article/@type), EDG) and then treated
with aminopropylmethyldiethoxysilane to generate an amine-
functionalized surface that promotes adhesion of gold. The
organic layer was used as the adhesive coating rather than chro-
mium or titanium to maximize the optical transparency of the
substrate. Afilmofapproximately 5nmofgoldwasdepositedonto
both sides of the substrate using vapor deposition. The resulting
faint blue film was semi-transparent to light of both visible and UV
wavelengths (see S1†) and could be handled without noticeable
flaking. The absorption spectrum shows a broad band centered
above 600 nm as previously reported and attributed to a localized
gold surface plasmon.38
Atomic force microscopy (AFM) images
show a typical island morphology (with diameters in the range of
50 nm), closely resembling previously reported films with a thick-
ness of 5 nm (see S2†).38
The gold-coated substrateswere immersed
in a 10 mM solution of enantiomerically pure dithiol 2 or 3 in
toluene for 15 h (Fig. 4). After removal the substrates were rinsed
thoroughly and sonicated (3Â 1–2 min) to remove any remaining
material that was not bound to the gold surface. Diluted mixed
monolayers containing enantiomerically pure dithiol 2 or 3 were
prepared in an identical fashion in the presence of 10 equivalents of
dodecanethiol. The CD spectra of the substrates clearly show the
presence of the motor on the surface and that the coverage can be
diluted by mixing with dodecanethiol (Fig. 5, S3†). The major
absorption bands of the CD spectrum of the monolayer coincide
with the major absorption bands observed for thioacetate-pro-
tected 3 in dodecane solution. When the enantiomer of the motor
chromophore was assembled the resulting spectrum showed an
approximate mirror image. CD spectra of materials in monolayers
are in generalrare, andare madepossiblehere inpart bythe intense
CD absorption of these helicene-like chromophores.35,39,40
Addi-
tionally, the IR spectrum contains a band at 1734 cmÀ1
corre-
sponding to the carbonyl of the ester in the ‘‘legs’’ of the surface-
bound motor (S4†).
Although the CD spectra indicate that the motor assembles on
the gold film, photochemical and thermal isomerization
processes of the rotary cycle cannot be assumed to work a priori.
Irradiation of (20
S)-(P)-2-SAM (Hg lamp, l 280 nm or lmax ¼
365 nm) for 1 h under an inert N2 atmosphere generated no
change in the CD spectrum of the sample. Note that irradiation
of the SAMs in air was found to irreversibly diminish the
intensity of the absorptions in the CD spectrum of the sample,
implying that it had destroyed the motor-molecules. Previous
studies on the photophysical behaviour of azobenzenes and
stilbenes have shown that their photoisomerization was partially
or completely suppressed in the solid state or when confined at
interfaces.41–43
One study showed that although photo-
isomerization of azobenzenes was completely suppressed in
densely packed monolayers, photoisomerization was restored in
mixed monolayers comprising both an azobenzene-thiol and
a ‘spacer’ thiol.42
The change in behavior was attributed to an
additional ‘free-volume’ in the mixed monolayers that allowed
the photoisomerization to occur. In an attempt to relieve steric
congestion between the motor-molecules in the monolayer,
mixed monolayers from a solution containing 10 : 1 dodeca-
nethiol : (20
S)-(P)-2 were prepared as noted above. Unfortu-
nately, irradiation of the monolayer led to no changes in its CD
spectrum, hinting that intermolecular crowding effects may not
exclusively prevent photoisomerization. However, it cannot be
assumed that the motor-molecules co-self-assemble with the
dodecanethiol into a homogeneous monolayer rather phase-
separate into domains, limiting the possible influence of mixing
a second molecule into the layer.44
In any case, both surfaces
showed no change under irradiation conditions that have been
shown to produce geometric isomerization of the central over-
crowded alkene in similar motors attached to gold nano-
particles31
or confined at the surface of silanated quartz.45,46
Fig. 4 Chiral thiol-terminated molecular motors for self-assembly on
gold. Note that the motor is depicted perpendicular to the surface for ease
of presentation and not meant to indicate a completely perpendicular
orientation.
Fig. 5 CD spectra of (20
R)-(P)-2-SAM (red) and (20
S)-(M)-3-SAM
(black).

Assembly of 2 on thinner gold films (approximately 1.3 nm) gave
monolayers that had a CD spectrum comprised of absorptions of
both similar intensity and profile, suggesting that the available
surface area for binding was unchanged. Unfortunately, this
monolayer also remained unaffected by irradiation. In sharp
contrast, preparation of gold nanoparticles protected with (20
R)-
(M)-2 provided a surface-bound system that was able to undergo
photoisomerization and thermal helix inversion as evident from
the CD spectra corresponding to the four stages of the rotary
cycle (S5†). The fact that 2 preserved its light-driven rotary cycle
while grafted on the gold nanoparticles was encouraging because
it showed that the parent motor was not intrinsically unable to
function in the presence of a gold surface, although it was still
unclear whether the malfunction was due to competing photo-
physical processes such as energy transfer, or to the rigidity of the
monolayer.
A plausible explanation for the absence of photochemical
isomerization is energy transfer from the excited state of 2 to the
gold substrate. Energy transfer from chromophores to nearby
metal particles and films has been observed.47–52
For example, we
showedthat in a gold break junction reversibility of singlemolecule
isomerization was blocked.53
Some reports have shown that
photochemical and photophysical processes of chromophores are
more severely quenched by gold films in comparison to gold
nanoparticles.Ithasbeenreportedthatthe quantumyieldoftrans–
cis isomerization of azobenzenes was diminished by immobiliza-
tionongold colloids, andthenfurther diminished byincorporation
into SAMs on gold.42
Fluorescence from lissamine (a rhodamine
dye) was visible when the dye was grafted to gold nanoparticles, yet
completely quenched when grafted to gold films.52
Since the interaction of the gold layer with the excited state of
the chromophores in the SAM should be distance dependent,
increasing the length of the tethers of the chromophore to the
metal surface should diminish the quenching, and restore the
photoisomerization. We chose to explore (20
R)-(M)-3, which
contains an additional 8 atoms of space between the motor and
the surface (for synthesis and resolution see ESI†). CD analysis
of a semi-transparent gold film after formation of a monolayer of
(20
R)-(M)-3 showed a spectrum similar to (20
R)-(M)-2 (Fig. 6).
Irradiation of the resulting SAM, (20
R)-(M)-3-SAM, with UV
light (365 nm) for 1 h under an inert N2 atmosphere gave
inversion of the major absorptions in the CD spectrum (black to
red, Fig. 6) analogous to the spectra obtained for the motor in
solution and on Au nanoparticles, indicating successful
completion of the first step of the rotary cycle: conversion of
stable (20
R)-(M)-3-SAM to a monolayer comprised of predomi-
nantly unstable (20
R)-(P)-3-SAM. Moreover, subsequent heating
of (20
R)-(P)-3-SAM at 70
C for 2 h resulted in an almost
complete regeneration of the original CD spectrum, which is
consistent with a thermal induced helix inversion comprising
a 180
rotation of the upper-half rotor relative to the lower-half
stator, and hence, relative to the gold surface. A second irradi-
ation (lmax ¼ 365 nm) of the sample resulted in the inversion of
the major bands in the CD spectrum of the monolayer. Heating
the sample a second time regenerated the original CD spectrum
completing a full 360
unidirectional rotation of the rotor relative
to the gold surface. Similar results were obtained with a mixed
monolayer prepared by the self-assembly of (20
R)-(M)-3 in the
presence of 10 equivalents of 1-dodecanethiol (S6†). As before,
the intensity of the absorptions in the CD spectrum of the mixed
monolayer were weaker than was observed for (20
R)-(M)-3-
SAM. Irradiation of the monolayer (280 nm) required $1 h to
reach its PSS, which is similar to what was observed for the dense
SAM containing only motor 3 (S6†).
We have shown that self-assembly of motor-molecules 2 and 3
on semi-transparent gold surfaces allows the characterization
and analysis of the substrate with CD spectroscopy, an invalu-
able technique in addressing the stages of the rotary cycle at the
Fig. 6 Irradiation of (20
R)-(M)-3-SAM results in a photochemical
isomerization. The initial CD spectrum (black) shows an inversion of the
signal as the enantiomer is generated (red). Thermal helix inversion
restores the original spectrum (green). A second photochemical isomer-
ization (blue) generates the second unstable enantiomer in the rotary
cycle. Subsequent thermal helix inversion regenerates the original
conformation of the motor.
100 | Chem. Sci., 2010, 1, 97–101 This journal is ª The Royal Society of Chemistry 2010

surface. Provided that the tethers between the chromophore and
gold film are sufficiently long to minimize quenching of the
excited state by the gold film, the molecular motor can undergo
photochemically and thermally induced isomerizations that are
consistent with a 360
rotary motion of the upper-half rotor
relative to the gold surface. The ability to assemble functional
unidirectional rotary motors on gold surfaces offers exciting
opportunities in developing dynamic systems that allow for
control of interfacial properties and movement by exploiting the
collective motion of the nanomotors. The widespread availability
and expertise in the preparation of SAMs on gold films and the
ability to obtain atomically flat gold terraces expands the
possible repertoire of experimental techniques to pattern,
address, control and detect processes at the motor interface. The
design parameters uncovered will be used to develop and
assemble more advanced motors on metallic films.
Acknowledgements
Financial support from the Netherlands Foundation for Scien-
tific Research (NWO-CW), ERC (advanced grant 227897) and
NanoNed is gratefully acknowledged.
Notes and references
‡ We note that, while all the motor molecules rotate in one direction, the
number of rotations performed by each may vary.
x Piranha is a 7 : 3 sulfuric acid : H2O2 solution. Caution must be used in
preparing and handling as it reacts violently with organic material.
Mixing is very exothermic and must be performed slowly.
1 D. S. Goodsell, The Machinery of Life, Springer, New York, 1994.
2 P. Ball, Chem. World, 2010, 7, 56–60.
3 V. Balzani, A. Credi, F. M. Raymo and J. F. Stoddart, Angew. Chem.,
Int. Ed., 2000, 39, 3348–3391.
4 B. L. Feringa, J. Org. Chem., 2007, 72, 6635–6652.
5 W. R. Browne and B. L. Feringa, Nat. Nanotechnol., 2006, 1, 25–35.
6 V. Balzani, A. Credi and M. Venturi, ChemPhysChem, 2008, 9, 202–
220.
7 J. Michl and E. C. H. Sykes, ACS Nano, 2009, 3, 1042–1048.
8 M. A. Haidekker and E. A. Theodorakis, Org. Biomol. Chem., 2007,
5, 1669–1678.
9 J. J. de Jonge, M. A. Ratner, S. W. de Leeuw and R. O. Simonis,
J. Phys. Chem. B, 2004, 108, 2666–2675.
10 V. M. Rozenbaum, Phys. Rev. B, 1996, 53, 6240–6255.
11 S. L. Gould, D. Tranchemontagne, O. M. Yaghi and M. A. Garcia-
Garibay, J. Am. Chem. Soc., 2008, 130, 3246.
12 E. B. Winston, P. J. Lowell, J. Vacek, J. Chocholousova, J. Michl and
J. C. Price, Phys. Chem. Chem. Phys., 2008, 10, 5188–5191.
13 R. D. Horansky, L. I. Clarke, E. B. Winston and J. C. Price, Phys.
Rev. B, 2006, 74, 054306.
14 R. D. Horansky, L. I. Clarke, J. C. Price, T. A. V. Khuong,
P. D. Jarowski and M. A. Garcia-Garibay, Phys. Rev. B, 2005, 72,
014302.
15 L. I. Clarke, D. Horinek, G. S. Kottas, N. Varaksa, T. F. Magnera,
T. P. Hinderer, R. D. Horansky, J. Michl and J. C. Price,
Nanotechnology, 2002, 13, 533–540.
16 Y. Shirai, A. J. Osgood, Y. M. Zhao, K. F. Kelly and J. M. Tour,
Nano Lett., 2005, 5, 2330–2334.
17 A. V. Akimov, A. V. Nemukhin, A. A. Moskovsky, A. B. Kolomeisky
and J. M. Tour, J. Chem. Theory Comput., 2008, 4, 652–656.
18 S. Khatua, J. M. Guerrero, K. Claytor, G. Vives, A. B. Kolomeisky,
J. M. Tour and S. Link, ACS Nano, 2009, 3, 351–356.
19 R. Eelkema, M. M. Pollard, J. Vicario, N. Katsonis, B. S. Ramon,
C. W. M. Bastiaansen, D. J. Broer and B. L. Feringa, Nature, 2006,
440, 163–163.
20 T. Hugel, N. B. Holland, A. Cattani, L. Moroder, M. Seitz and
H. E. Gaub, Science, 2002, 296, 1103–1106.
21 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and
G. M. Whitesides, Chem. Rev., 2005, 105, 1103–1169.
22 X. L. Zheng, M. E. Mulcahy, D. Horinek, F. Galeotti, T. F. Magnera
and J. Michl, J. Am. Chem. Soc., 2004, 126, 4540–4542.
23 A. E. Baber, H. L. Tierney and E. C. H. Sykes, ACS Nano, 2008, 2,
2385–2391.
24 P. Maksymovych, D. C. Sorescu, D. Dougherty and J. T. Yates,
J. Phys. Chem. B, 2005, 109, 22463–22468.
25 L. Gao, Q. Liu, Y. Y. Zhang, N. Jiang, H. G. Zhang, Z. H. Cheng,
W. F. Qiu, S. X. Du, Y. Q. Liu, W. A. Hofer and H. J. Gao, Phys.
Rev. Lett., 2008, 101, 197209-1–197209-4.
26 N. Koumura, R. W. Zijlstra, R. A. van Delden, N. Harada and
B. L. Feringa, Nature, 1999, 401, 152–155.
27 D. A. Leigh, J. K. Y. Wong, F. Dehez and F. Zerbetto, Nature, 2003,
424, 174–179.
28 S. P. Fletcher, F. Dumur, M. M. Pollard and B. L. Feringa, Science,
2005, 310, 80–82.
29 S. Fournier-Bidoz, A. C. Arsenault, I. Manners and G. A. Ozin,
Chem. Commun., 2005, 441–443.
30 M. M. Pollard, M. Klok, D. Pijper and B. L. Feringa, Adv. Funct.
Mater., 2007, 17, 718–729.
31 R. A. van Delden, M. K. J. ter Wiel, M. M. Pollard, J. Vicario,
N. Koumura and B. L. Feringa, Nature, 2005, 437, 1337–1340.
32 M. M. Pollard, M. K. J. ter Wiel, R. A. van Delden, J. Vicario,
N. Koumura, C. R. van den Brom, A. Meetsma and B. L. Feringa,
Chem.–Eur. J., 2008, 14, 11610–11622.
33 A. Ulman, Chem. Rev., 1996, 96, 1533–1554.
34 N. Koumura, E. M. Geertsema, A. Meetsma and B. L. Feringa,
J. Am. Chem. Soc., 2000, 122, 12005–12006.
35 M. M. Pollard, M. Lubomska, P. Rudolf and B. L. Feringa, Angew.
Chem., Int. Ed., 2007, 46, 1278–1280.
36 P. A. Dimilla, J. P. Folkers, H. A. Biebuyck, R. Harter, G. P. Lopez
and G. M. Whitesides, J. Am. Chem. Soc., 1994, 116, 2225–2226.
37 M. Wanunu, A. Vaskevich and I. Rubinstein, J. Am. Chem. Soc.,
2004, 126, 5569–5576.
38 I. Doron-Mor, Z. Barkay, N. Filip-Granit, A. Vaskevich and
I. Rubinstein, Chem. Mater., 2004, 16, 3476–3483.
39 C. Nuckolls, T. J. Katz, T. Verbiest, S. Van Elshocht, H. G. Kuball,
S. Kiesewalter, A. J. Lovinger and A. Persoons, J. Am. Chem. Soc.,
1998, 120, 8656–8660.
40 N. Keegan, N. G. Wright and J. H. Lakey, Angew. Chem., Int. Ed.,
2005, 44, 4801–4804.
41 F. H. Quina and D. G. Whitten, J. Am. Chem. Soc., 1975, 97, 1602–
1603.
42 S. D. Evans, S. R. Johnson, H. Ringsdorf, L. M. Williams and
H. Wolf, Langmuir, 1998, 14, 6436–6440.
43 A. Manna, P.-L. Chen, H. Akiyama, T.-X. Wei, K. Tamada and
W. Knoll, Chem. Mater., 2003, 15, 20–28.
44 S. J. Stranick, A. N. Parikh, Y. T. Tao, D. L. Allara and P. S. Weiss,
J. Phys. Chem., 1994, 98, 7636–7646.
45 J. Areephong, W. R. Browne, N. Katsonis and B. L. Feringa, Chem.
Commun., 2006, 3090–3092.
46 G. London, G. T. Carroll, T. F. Landaluce, M. M. Pollard, P. Rudolf
and B. L. Feringa, Chem. Commun., 2009, 1712–1714.
47 A. Adams, R. W. Rendell, W. P. West, H. P. Broida, P. K. Hansma
and H. Metiu, Phys. Rev. B, 1980, 21, 5565.
48 P. Andrew and W. L. Barnes, Science, 2000, 290, 785–788.
49 H. Imahori, H. Norieda, Y. Nishimura, I. Yamazaki, K. Higuchi,
N. Kato, T. Motohiro, H. Yamada, K. Tamaki, M. Arimura and
Y. Sakata, J. Phys. Chem. B, 2000, 104, 1253–1260.
50 E. Dulkeith, A. C. Morteani, T. Niedereichholz, T. A. Klar,
J. Feldmann, S. A. Levi, F. van Veggel, D. N. Reinhoudt,
M. Moller and D. I. Gittins, Phys. Rev. Lett., 2002, 89, 203002-1–
203002-4.
51 K. G. Thomas and P. V. Kamat, Acc. Chem. Res., 2003, 36, 888–
898.
52 T. Kudernac, S. J. van der Molen, B. J. van Wees and B. L. Feringa,
Chem. Commun., 2006, 3597–3599.
53 D. Dulic, S. J. van der Molen, T. Kudernac, H. T. Jonkman,
J. J. D. de Jong, T. N. Bowden, J. van Esch, B. L. Feringa and
B. J. van Wees, Phys. Rev. Lett., 2003, 91, 207402.

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