A molecular four-stroke motor

Article abstract of DOI:10.1002/anie.201101501

Round and round it goes: In the molecular four-stroke motor the forward and backward moving of the pushing blade of the motor is combined with the closing and opening of the blade in such a way that there is a net rotation about a virtual axis (see scheme). Thus, during one cycle, surrounding molecules should automatically be transported in a definite direction. Copyright

Full text of DOI:10.1002/anie.201101501

DOI: 10.1002/anie.201101501
Molecular Motor
A Molecular Four-Stroke Motor**
Gebhard Haberhauer*
Dedicated to Professor Rolf Gleiter on the occasion of his 75th birthday
One of the most challenging aspect in the construction of
molecular analogues of mechanical devices[1–4] is the creation
of synthetic molecular motors which utilize the unidirectional
movements of smaller parts and which thus should be able to
perform a physical task.[5,6] Almost all synthetic molecular
motors described to date are rotary motors. The axis of the
rotation in these motors is a single or a double chemical bond
or a mechanical bond, and is coincident with the center where
energy is transformed into mechanical work.[7,8] As their
flexible parts remain the same size during a cycle, they
perform a simple rotation like that found in the F1-ATPase in
nature.[9] Thus they are designed to transport an attached
molecule in a rotation movement of 3608, but they are not
able to transport surrounding molecules in one definite
direction—they just distribute them in a circular movement.
Herein we present a synthetic molecular motor that has a
motion sequence which resembles the movement of motile
cilia.[10] Motile cilia are widely found in nature and, through
their beating movement, are used either for the transport of
particles and surrounding medium or for the locomotion of
cells. As our motor works in a similar way, it should
automatically push surrounding molecules in a definite
direction during a four-stroke cycle. The essential principle
is the spatial separation of the area where chemical energy is
transformed into mechanical work, from the rotation axis.
The design and the concept of our molecular four-stroke
motor is illustrated in Figure 1. The central part of the motor
is a chiral clamp,[11] which we have already successfully used
for the control of unidirectional movements and for the
design of a molecular chirality pendulum.[12,13] Through the
chiral clamp, the clamp-bound pyridine rings of the bipyridine
units are fixed in a cycle, and thus adopt a P configuration
with respect to each other. One of the bipyridine units carries
a light-switchable azobenzene unit and is the chemically
driven pushing blade of the motor. The other bipyridine unit
acts as a stopper and controls the direction of the movement.
An alternating stimulation of the whole blade and the
azobenzene unit of the blade leads in sum to a 3608 rotation
of the phenyl group of the azobenzene unit around a virtual
axis.
Letꢀs consider the states of the rotation process in detail.
The molecule trans-(P)-1 corresponds to state I of the rotation
cycle. As non-complexed 2,2’-biypridine units have an N-C-C-
N dihedral angle of about 1808,[14] the arm of the blade
(azobenzene unit) and the arm of the stopper (bromine) in
trans-(P)-1 have a definite relative spatial configuration (the
P configuration).[13] The addition of salts containing metal
ions, such as Zn2+, leads to a complexation of the 2,2’-
bipyridines and thus the complex trans-(M)-1*Zn24+ is formed
which represents state IV of the rotation cycle. As metal-
complexed 2,2’-bipyridines have a N-C-C-N dihedral angle of
around 08, the transition from state I to state IV is
accompanied by a movement of the blade. The reversibility
of this movement—the backward movement of the blade—is
achieved chemically by the addition of cyclam which com-
plexes the Zn2+ ions better than the bipyridine units of 1. The
second important process for the cycle is the light-induced
switching of the azo group.[15] In the trans configuration of the
azo group (state I) the para-bonded hydrogen atom points
away from the bipyridine unit whereas in the cis configuration
(state II) it is orientated toward the bipyridine unit. Thus, the
trans!cis isomerization of the azobenzene group triggered
by UV irradiation (l = 355 nm) is accompanied by a folding of
the blade. This transition (I!II) is also reversible. The
reverse state change from II!I takes place on a brief
irradiation of the cis isomer with visible light. The whole
rotation cycle consists of an alternating combination of the
rotation movement of the blade and its light-induced folding
and opening: Exposure of trans-(P)-1 to UV light at 208C
induces trans!cis isomerization and results in a transition
from state I to state II (first stroke). The addition of Zn2+
leads to a movement of the blade (second stroke) and the
complex cis-(M)-1*Zn24+ representing state III in the cycle is
formed. This stroke resembles the recovery stroke of motile
cilia. The third stroke (III!IV; opening of the blade) is the
4+
cis!trans back isomerization to trans-(M)-1*Zn2 induced
by visible light. The backward movement of the opened blade
triggered by the addition of cyclam (fourth stroke) resembles
the effective stroke of motile cilia. As the opened blade can
push more molecules in one direction (fourth stroke) than the
closed blade in the other (second stroke) there should be a net
transport of the surrounding molecules in one direction.
An important requirement for the motor is that the
movement of the blade must proceed unidirectionally. To
check if this is true for the motor 1, the reference compound 2
was investigated. The only difference between 1 and 2 is the
absence of the azobenzene unit in molecule 2. In principle
there are two possible ways for the movement of the blade. In
[*] Prof. Dr. G. Haberhauer
Institut fꢀr Organische Chemie, Fakultꢁt fꢀr Chemie
Universitꢁt Duisburg-Essen
Universitꢁtsstrasse 7, 45117 Essen (Germany)
E-mail: gebhard.haberhauer@uni-due.de
[**] This work was generously supported by the Deutschen For-
schungsgemeinschaft (DFG). We thank Dr. Andreea Schuster for
helpful discussions and Helma Kallweit and Petra Schneider for
assistance in synthesis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101501.
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Figure 1. Design and concept of the molecular four-stroke motor. a) Structure formulas of the four states. b) Schematic representation of the four
states. The bipyridine unit which acts as a stopper is gray. The bipyridine unit to which the (red) pushing blade is bound, is light blue. The azo
group is the dark blue pivot and the hydrogen atom which performs a 3608 rotation during one cycle is green.
the first case (A in Figure 2) the sterically crowded diisopro-
pylphenyl group (red box) moves outside of the molecule. In
the second case (B) it must move through the other bipyridine
group (the stopper) which should lead to high steric
repulsions. This assumption can be confirmed by calculating
the energy profile as a function of the dihedral angle q(N-C-C-N)
using the B3LYP/6-31G* level of theory.[16] During the
movement
A only a small energy barrier of about
15 kJmolꢀ1 must be overcome. The energy barrier for path
B is much higher (37 kJmolꢀ1) as this movement causes strong
Figure 2. Unidirectional movement of the blade. a) Structure formula of reference compound 2 and the two possible ways A and B for the
movement of the diisopropylphenyl group (red box) around the N-C-C-N axis. b) Energy profile as a function of the dihedral angle q(N-C-C-N)
calculated at the B3LYP/6-31G* level.
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steric interactions. In the presence of Zn2+ ions in solution this
energetic discrimination would be enhanced, as the metal ions
wouldn’t affect the early transition state (path B) but would
stabilize the late transition state of path A. According to the
Boltzmann distribution between the two transition states, the
movement of the blade of reference compound 2 should be
almost completely (> 99.99%) unidirectional at 208C. As the
motor 1 contains a blade which is even more sterically
crowded than that of 2, the movement of the blade in 1 should
be definitively unidirectional.
The experimental demonstration of the whole four-stroke
cycle can be provided by UV and circular dichroism (CD)
spectroscopy. The trans!cis isomerization of the azobenzene
unit (I!II) caused by UV irradiation (l = 355 nm) can be
observed in the UV spectrum by the decrease of the
absorption band at 335 nm (Figure 3). This change in the
p!p* band is typical for isomerization of azobenzene
derivatives. The shape of the CD spectrum is mainly caused
by the relative configuration of the bipyridine arms which is
not changed during the trans!cis isomerization. Thus, the
isomerization does not lead to any significant change of the
CD spectrum. The 1H NMR spectra show that after synthesis,
the azo compound (P)-1 is present in a trans/cis ratio of 80:20.
At the photostationary state at l = 355 nm a trans/cis ratio of
42:58 is achieved. The reverse isomerization (II!I) triggered
by visible light leads back to the basic ratio which can be seen
in the change of the UV and the NMR spectra. This result
means that 38% of all the molecules of the motor are
involved in the light-induced switching.
the formation of metal complexes with more than one
bipyridine ligand per metal, the complexation experiments
must be carried out in high dilution and with an excess of
metal ions.[13] As a result of the metal complexation the UV
absorption in the area of the bipyridine band at around
330 nm increases. The complexation and thus the movement
of the blade can most clearly be detected in the change of the
CD spectrum: Compound trans-(P)-1 in which the bipyridine
arms are P configured exhibits a positive Cotton effect at
281 nm (De = + 25 molꢀ1 dm3 cmꢀ1) and a negative Cotton
effect at 253 nm (De = ꢀ29 molꢀ1 dm3 cmꢀ1).[13] The complex-
ation of the 2,2’-bipyridine units occurs not only very rapidly
and almost completely, but also leads to a metal complex in
which the bipyridine arms adopt the M configuration (I!IV).
As a result a negative Cotton effect at 269 nm (De =
ꢀ28 molꢀ1 dm3 cmꢀ1) and a strong positive one at 248 nm
(De = + 45 molꢀ1 dm3 cmꢀ1) is observed. The addition of
cyclam leads back to the original spectrum (IV!I).
An alternating combination of these two switching
processes leads to the desired four-stroke rotation. The first
stroke (I!II; folding of the blade) is the light-induced
trans!cis isomerization and leads only to a small decrease of
the UV absorption at 330 nm (Figure 4). The second stroke
(II!III; recovery stroke) is effected by the metal-triggered
movement of the folded blade. The configuration change of
the bipyridine arms during this stroke results in a dramatic
change in the CD spectrum which becomes most pronounced
at a wavelength of 250 nm (DDe = 68 molꢀ1 dm3 cmꢀ1). The
third stroke, which is the light-induced cis!trans back
isomerization (III!IV; opening of the blade), does not lead
to a significant change in the CD spectrum. In the UV
The second moving process is caused by the complexation
of the bipyridine units with Zn2+ ions (Figure 3). To prevent
Figure 3. a) CD (top) and UV (bottom) spectral changes of trans-(P)-1 (state I, blue) in dichloromethane/acetonitrile (95:5; c=2.0ꢂ10ꢀ5 m) at
208C upon UV irradiation (l=355 nm) for 15 (violet), 30 (green), and 60 s (state II, cis-(P)-1, red). b) CD (top) and UV (bottom) spectral changes
of trans-(P)-1 (state I, blue) in dichloromethane/acetonitrile (95:5; c=2.0ꢂ10ꢀ5 m) at 208C upon addition of 4.5 (violet), 5.0 (green), and
5.5 equivalents of Zn(OTf)2 (state IV, trans-(M)-1*Zn24+, red).
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Keywords: CD spectroscopy · chirality · molecular machines ·
.
molecular motors · stereo control
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Figure 4. CD and UV spectra of the four-stroke cycle. CD (top) and UV
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spectrum the absorption at 330 nm increases. The recorded
spectra of state IV are completely identical with those
obtained in the transition from state I to state IV. The
fourth stroke is the backward movement of the opened blade
(IV!I; effective stroke) and is caused by the addition of
cyclam. The spectra after the fourth stroke are identical to the
spectra of the starting state which provides confirmation of
the assumed cyclic process. This forward and backward
motion can be repeated several times.[17]
As the movement of the blade proceeds unidirectionally,
the circular path for all motors in all four-stroke cycles is the
same. Thus the physical task which is performed in the first
four-stroke cycle is not annihilated in one of the subsequent
four-stroke cycles. As the light-induced isomerization is not a
complete process, only 38% of all the motors perform
mechanical work during a cycle. In the other motor molecules
during this cycle the pushing blade is only moved forward and
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[16] All calculations were performed with the Gaussian 09 program
package: M. J. Frisch, et al., Gaussian 09, Revision A.1, Gaus-
sian, Inc., Wallingford, CT, 2009.
[17] The number of cycles is not limited by the system but by the
measurement technique: With increasing overall concentration
of all the components the absorption of the entire system is too
high to obtain meaningful CD signals in the desired part of the
spectra.
The simple design of this four-stroke motor resembling
the movement of motile cilia should allow the construction of
a variety of new motors designed for the directed transport of
molecules or for their locomotion in a medium.
Received: March 1, 2011
Revised: March 30, 2011
Published online: May 31, 2011
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Angew. Chem. Int. Ed. 2011, 50, 6415 –6418