Machinery has propelled human civilisation into another level by helping us to perform tasks beyond our capability. In decades, human has made different type of machines with various of sizes; from enormous machine into molecular-size machinery. The ideas of molecular-size machinery are inspired from the nature in the cellular level such as ATP synthase motor and kinesin transporter.
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| ATP synthase motor (right, J. Weber, Nat. Chem. Biol., 2010, 6, 794-795) and Feringa's nanocar (left, ©Johan Jarnestad/The Royal Swedish Academy of Sciences) |
One of the prominent examples of the molecular machines is Feringa's
nanocar where the molecule is propelled by four rotating molecular motors.
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| Nanocar 1 |
Feringa's
nanocar is designed to allow the molecule moves upon electronic excitation in a preferred and linear direction across the surface. The molecule has four chiral units which can move in unidirectional motor. The synthesis of the motor can be described as two-phase assembly, the formation of the 'wheel' then forming the chassis to join the front and rear wheels.
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| Synthesis of nanocar 1: The synthesis of the front and rear axles |
The wheels component was synthesised in 8 steps which started by installing
n-hexyl group using Sonogashira-type coupling between
2 and hex-1-yne followed by hydrogenation to give
4. The wheels were connected to the 'axle'
7 by converting the ketone into thioketone by Lawwesson's reagent then
9 was reacted to give
10 with sulphur-heterocycle. The reaction with PPh3 would form the desired alkene
11, possibly via Wittig-type reaction, then deprotection to give
12 which is a racemic mixture. The desired compound for the nanocar is the
meso configuration which will give the movement of the
1.
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| The Synthesis of nanocar 1: building the chassis |
The next job would be building the chassis to connect the front and rear wheels which is made from two phenyl groups with butadiyne spacer. The construction of the chassis was done by joining
13 to
12 then substituting Br with I for Sonogashira-type coupling with TMS-ethyne. The deprotection with TBAF followed by another coupling Pd-Cu catalysed coupling reaction finished the synthesis of nanocar
1.
The main goal of making
1 is to see if it can move in a controlled manner on a surface. Similar to the normal car, it also needs 'fuel' to drive it. In this sense, the nanocar moves upon electronic transition. Firstly,
1 was placed onto Cu after sublimation and images were taken using scanning tunneling microscope (STM) at low temperature. Then, an STM tip was placed above the centre of the molecule and a voltage pulse that is larger than 500 mV was applied. After ten excitations, it was observed that
1 moved as far as 6 nm.
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| (a) The schematic representation of the experiment, (b) The structural detail of the motor unit, (c) Schematic representation of the rotation of the motor, and (d) molecular model representation of the paddlewheel-like motion of the four-wheeled molecule. |
The movement of nanocar on the surface occurs via paddlewheel type movement where all four motors move in the same direction. This movement occurs as
1 is electronically excited, double bond isomerisation occurs which causes rotation of the 'wheel'. It is noteworthy that to be able to move the adsorption energy should be less than the energy released for isomerisation. As the environment around the wheel is sterically crowded due to
n-hexyl group, interconversion of helical conformers happens via vibrational excitation. The combination between electronic and vibrational excitations creates the movement.
Furthermore, only the correct configuration of the nanocar would give correct propulsion as the configuration dictates the rotation of the wheels.
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| Sketch of directionality of movement induced by concerted rotation of the motor units. |
In
meso isomer, there are two possibilities of the configuration as there is still rotation around bis-alkyne bond which needs to be considered because both configuration would be locked by physisorption. The 'correct landing' configuration would give the forward movement but the 'wrong landing' would give no propulsion as the the wheels cancel out the rotation.
From this study, it lays the main principles on designing molecular motor as only 'correct landed'
meso isomer will give the movements. Furthermore, this study also shows how a single molecule is capable converting the input energy into kinetic energy, i.e. movement, as it has been observed from the nature's molecular motor.
Reference
T. Kudemac, N. Ruangsupapichat, M. Parschau, B. Maciá, N. Katsonis, S. R. Harutyunyan, K.-H. Ernst, and B. L. Feringa,
Nature,
479, 2011, 208-211.
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