We suggest a design for a light-driven molecular motor, which is different from the existing designs. It is a rotaxane molecule, having identical 'stations' and an asymmetric 'shuttle'. We argue that the molecule would exhibit unidirectional rotational/translational motion continuously, upon shining with light of just one frequency. With this design, it should be possible to synthesizea light-driven single-molecular motor in the near future

Sebastian, K. L.

English

Publications of the IAS Fellows

RESEARCH COMMUNICATIONS 
CURRENT SCIENCE, VOL. 87, NO. 2, 25 JULY 2004 232
e-mail: kls@ipc.iisc.ernet.in 
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ACKNOWLEDGEMENTS. We are grateful to Dr R. P. Bajpai,  
Director, Central Scientific Instruments Organization, Chandigarh for 
constant encouragement, support and permission to publish this work. 
We thank Department of Science and Technology, New Delhi for  
financial support. A.K.S. thanks CSIR, New Delhi for a fellowship. We 
also thank Mr H. K. Sardana and Mr H. N. Bhargaw for help in record-
ing the experimental results. 
Received 2 January 2004; revised accepted 25 March 2004 
Theoretical design for a light-driven 
molecular motor based on rotaxanes 
K. L. Sebastian 
Department of Inorganic and Physical Chemistry, Indian Institute of 
Science, Bangalore 560 012, India 
We suggest a design for a light-driven molecular motor, 
which is different from the existing designs. It is a rota-
xane molecule, having identical ‘stations’ and an asym-
metric ‘shuttle’. We argue that the molecule would 
exhibit unidirectional rotational/translational motion 
continuously, upon shining with light of just one fre-
quency. With this design, it should be possible to syn-
thesize a light-driven single-molecular motor in the 
near future.  
THERE has been considerable interest in the design and 
fabrication of molecular devices1–16. Of particular interest 
are molecular motors in which the components are forced 
to move past each other by external stimuli1–11. Recent 
insights into the working of biological motors10,17,18 have 
stimulated a surge of papers on such artificial molecular 
motors. Rotaxanes are the most widely studied as they offer 
the possibility of long range translational motion of a 
threaded ‘shuttle’ along a molecular wire or ‘rail track’. 
The shuttle can be driven chemically, electrochemically 
or photochemically. 
Natural molecular motors are usually driven by chemi-
cal energy and are believed to have high efficiency, often 
close to unity. One of the most efficient natural motors, 
the ATP synthase, rotates 15–20 times per second. 
Though chemically driven motors are plentiful in bio-
logy, their components are rather largish molecules and it 
does not look as if an artificial molecular motor of compa-
rable efficiency satisfying the three desirable features will 
be made in the near future. Any synthetic molecular  
motor should have the following desirable characteristics: 
(a) It should draw energy from a source and produce me-
chanical work, with high efficiency, hopefully compara-
ble to those of natural molecular motors. (b) The motion 
produced should be on a time scale, faster or at least 
comparable to those of natural molecular motors. (c) Its 
operation should not, if possible, lead to the formation of 
wast products. 
In an interesting paper, unidirectional rotational motion 
in a mechanically interlocked catenane molecule has been 
reported8. The authors have studied a ‘three-station’ 
[2]catenane and a ‘four-station’ [3]catenane. ‘Stations’ 
are the binding sites in the macrocyclic ring of the catenane 
assembly. In these systems, there is a large macrocycle 
with the binding sites and smaller macrocyles which are 
bound to these sites. It was found that in response to exter-
nal stimuli, the small ring in the three-station [2]catenane 
moves sequentially between the binding sites. In the four-
station [3]catenane there are two smaller macrocycles and 
one of them blocks the backward Brownian movement of 
the other macrocycle, effectively making the rotation uni-
directional. 
The chemically driven motors that have been synthe-
sized recently1–8 are of great interest, though they have 
rather low efficiency and rotate by 360 degrees on a rather 
long time scale (for example, one day). They are in no 
way near the goals that one would like to achieve. The 
existing attempts based on rotaxanes have not put the 
ideas from Brownian motors to maximum use to get an 
efficient molecular motor. 
Here, we suggest a design that we believe will lead to 
machines that satisfy all the three requirements above. It 
is to be stressed that the ideas that we use are well known 
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CURRENT SCIENCE, VOL. 87, NO. 2, 25 JULY 2004 233
from papers on the working of Brownian motors17,18. 
However, as yet, there does not yet exist an efficient arti-
ficial molecular level Brownian motor. The only ones that 
exist are the natural ones. Our design is very simple and 
we argue that by just shining light of appropriate frequency, 
it is possible to generate net unidirectional mo (rota-
tional or translational), a goal that has been found to be 
extremely difficult to achieve. As the energy comes from 
light, the response time of the motor can be easily made 
faster than would be the situation if it were chemically 
driven. In fact, our motor is simpler than the ones that ex-
ist in nature, whose workings are not well understood. 
With the expertise available in making [2]rotaxanes, it 
should be possible to synthesize a molecular motor based 
on the design, in the near future.
 Our design is a [2]rotaxane, which uses the photochemi-
cal unbinding known in rotaxanes, shown in Figure 1. 
The design consists of a ring (the shuttle), which is 
threaded with an oligomer, which has identical stations 
(sites) where the shuttle could be bound. The oligomer
could be a ring (so that one has a catenane) and then the 
shuttle would go around the ring, exhibiting rotational 
motion. Alternatively, the oligomer could be a very long 
chain molecule and the shuttle would then exhibit transla-
tional motion. 
 We imagine that the shuttle, when it is in the ground 
state (G) prefers to bind to the site, S. The state of the 
shuttle can be changed photochemically to a state E and 
in this state, it prefers to unbind from the site S (see ref. 
19 for an example where this happens readily). So on 
photo-excitation, it would leave the site S and move to 
the region between the two sites. However, left to itself, 
the state E would decay back to G and the shuttle would 
get back to the site S. So, if the whole assembly is put in 
presence of light of appropriate frequency, the cycles of 
 
 
 
 
Figure 1. Schematic picture of unbinding of a symmetric [2]rotaxane, 
where there is a no net (directional) rotational motion. 
 
 
Figure 2. The symmetric case. a, Variation of the potential energy of 
the shuttle-oligomer system, as the position of the shuttle is varied
along the oligomer. It also shows the probability distribution (shaded) 
for the position of the shuttle on a site to which it is initially bound. b, 
Effect of exciting the shuttle. As a result of the excitation, the potential 
energy curve is changed drastically and the initial probability distribu-
tion, which was sitting in a minimum of the ground state potential en-
ergy curve now sits on top of a barrier. It can now go forward or 
backward with equal likelihood, as a result of which it splits up into 
two parts as shown in c. This is the unbinding step. Then, the shuttle 
would get de-excited and rebind to a site. When the rebinding happens, 
it can bind to the original site, as well as the one preceding it, or to the 
one that follows it. The probabilities of the latter two are equal and 
hence there is no net rotation. 
 
 
 
 
Figure 3. Schematic picture of a [2]rotaxane, where there is a net mo-
tion in one direction. 
a 
b 
c 
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CURRENT SCIENCE, VOL. 87, NO. 2, 25 JULY 2004 234
excitation, unbinding, de-excitation and rebinding would 
go on (see Figure 2)17,18. 
 With a symmetric shuttle, the motion can be either in 
the forward or backward direction wi h equal likelihood 
and hence there would not be any net unidirectional mo-
tion. If one can prevent it from occurring with equal like-
lihood, then there will be a net motion in one direction. 
This however is easy to achieve – all that is required is to 
have a shuttle that is asymmetric. For example, if the 
shuttle has pawls (P) as in Figure 3 on one side, then its 
motion towards the left will be less likely than that to- 
 
 
 
 
Figure 4. The unsymmetric case. , Variation of the potential energy 
of the shuttle-oligomer system, as the position of the shuttle is varied 
along the oligomer. The probability distribution is seen to be asymmet-
ric, due to the asymmetry of the shuttle. b, Effect of exciting the shut-
tle. Unbinding occurs in the forward/backward directions with unequal 
probabilities as shown in (c). The shuttle would get de-excited and re-
bind to a site, as shown in (d) and (e). The net result is more probability 
that the shuttle moves forward, rather than backward, resulting in net 
rotation. 
wards the right. This will cause a net unidirectional mo-
tion towards the right. It may be asked whether such uni-
directio al motion is really possible, in view of the fact 
that a molecular ratchet, kept at constant temperatur , 
does not show any net rotational motion, as that would 
violate the second law of thermodynamics20–23. The an-
swer to this question is that, it is possible in this case, as 
he system is driven by an external stimuli and hence u i-
directional motion does not violate the second law of 
thermodynamics. In Figure 4, we show pictorially the de-
ailed mechanism. It is not necessary to have an extra 
group acting as a pawl, if the shuttle itself is a moleculelike 
calixarene, as in ref. 24. Alternatively, one can have a 
symmetric shuttle and have asymmetry built into the sta-
tions, as in the example given in Figure 5. 
 We give in Figure 5 a system in which the suggested 
mechanism is likely to work. Photoexcitation of the elec-
tron donor D would cause electron transfer to the shuttle 
A, which is assumed to be an electron acceptor. The A– 
so formed has no affinity towards the station S and hence 
would unbind from it. Typical examples of D, A and S 
are suggested in Figure 5. 
 Typically, the unbid ng would need a time of the or-
der25 of 10–6 s. Asymmetry of S would cause the unbind-
ing to be more in one direction than in the other. The  
 
 
 
 
 
Figure 5. The case where unbinding is due to electron transfer. Note 
that the station S is asymmetric. Typical examples for A, D and S are 
shown. 
a 
b 
c 
d 
e 
RESEARCH COMMUNICATIONS 
CURRENT SCIENCE, VOL. 87, NO. 2, 25 JULY 2004 235
*For correspondence. (e-mail: kamalagarwal73@hotmail.com) 
major problem would be that back electron transfer is 
usually fast25 (~ 10–9 s). One would then have to tune the 
donor acceptor system in such a manner that the back 
electron transfer takes place in a time longer than ~ 10–5 s. 
This may be achieved by varying the free energy change 
for the back electron transfer to be in the Marcus inverted 
regime and by varying the solvent polarity. The time 
could be made longer also by increasing the physical 
separation between the donor and the acceptor. 
Finally, we note: (1) the net rotational motion would 
occur in presence of steady light and that the net motion 
would stop when the light is switched off. (2) Our motor 
combines the ideas of devices that have already been used 
in the literature rotaxanes[2] with idea of a ratchet21–23. 
(3) In the earlier motors that have been suggested, it is 
the site (station) that is excited/reacted, while for our mo-
tor, it is the shuttle that is changed externally. (4) Further, 
all the earlier suggestions have stations that are different 
while in our model, all the sites are the same and there-
fore synthesizing the motor should be easier. (5) In our 
opinion, the suggested change in design, though simple, is 
the easiest way to get an efficient working molecular mo-
tor. (6) The synthesis of the motor poses a challenge, but 
it should be possible in the near future, given the abilities 
that organic chemists have attained in synthesizing ele-
gant structures. 
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Received 23 February 2004; revised accepted 21 May 2004 
Effects of erosion on stable thrust  
wedges: A new perspective in sandbox  
analogue modelling 
K. K. Agarwal* and G. K. Agrawal 
Department of Geology, University of Lucknow, Lucknow 226 007, 
India 
Scaled sandbox analogue models have been used to 
simulate the growth of Coulomb thrust wedges in iso-
tropic cohesionless and anisotropic cohesionless mate-
rials. The internal and surface geometry of such  
wedges is controlled mainly by parameters like the 
coefficient of basal friction and the physical properties 
of the deforming materials. The effects of erosion on a 
stable Coulomb wedge have been studied and are de-
scribed here. In the experiment carried out for this 
study, the uppermost 2 cm material of a developed 


Theoretical design for a light-driven molecular motor based on rotaxanes

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Theoretical design for a light-driven molecular motor based on rotaxanes

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