Imperial College London

Henry Rzepa

The world's [smallest nano
car](http://dx.doi.org/10.1038/nature10587){target="_blank"} was
recently driven a distance of 6nm along a copper track. When [I saw
this](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5441){target="_blank"},
I thought it might be interesting to go under the hood and try to
explain what makes its engine tick and its fuel work.

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine.svg "engine"){.aligncenter
.size-full .wp-image-5564 loading="lazy" decoding="async" width="306"
height="230"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine.svg)

The molecule above represents (I think) the essentials of the engine.
Its resting geometry in the S~0~ electronic state is shown below.

::: {#attachment_5568 .wp-caption .aligncenter style="width: 190px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine1.jpg "engine1"){.size-full
.wp-image-5568 loading="lazy" decoding="async"
aria-describedby="caption-attachment-5568"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('green');jmolApplet([450,450],'load wp-content/uploads/2011/11/engine1.mol;measure 39 32;measure 26 32;');"
width="180" height="157"}

The resting geometry of the engine. The green arrow points to the axle,
currently planar. Click for 3D model.
:::

1.  Its journey is started by irradiation with light. This has the
    effect of injecting around 58.6 kcal/mol worth of "fuel" into the
    molecule, enough to excite the π-electrons in the axle (green arrow
    above) into a biradical excited S~1~ state, in the process turning
    the double into a single bond and hence a properly rotatable axle.
2.  [![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine1.svg "engine1"){.aligncenter
    .size-full .wp-image-5570 decoding="async"
    width="300"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine1.svg)The
    minimum energy shape is different from the S~0~ ground state,
    involving a \~90° rotation around the axle. The crucial part here is
    that the axle is freely rotatable ONLY in a clockwise direction. In
    the *anti *direction, the benzo group on the right is prevented from
    moving up by a bump into the alkyl group above it. The energy
    profile for the clockwise rotation is shown below as downhill all
    the way, the driving force being a 13 kcal/mol drop in energy
    (calculated as a triplet rather than singlet state,
    ωB97XD/6-31G(d).[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine2.svg "engine"){.aligncenter
    .size-full .wp-image-5591 loading="lazy" decoding="async"
    width="278"
    height="150"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine2.svg)
3.  The rotation passes the point of no return, so that when the
    short-lived excited S~1~ state decays back to the S~0~ state
    (*via *a [conical
    intersection](http://en.wikipedia.org/wiki/Conical_intersection){target="_blank"}),
    the now restored C=C double bond finds it easier to continue the
    clockwise rotation a further \~60° (making 139° in total thus far),
    rather than to reverse its motion back to its original starting
    point. It is probably helped in this by its momentum as it emerges
    from the conical intersection. This new pose is 4.1 kcal/mol higher
    in free energy than the original starting one, a rather poor fuel
    economy for a half rotation of the wheel, given the 59 kcal/mol
    injection at the start!

    ::: {#attachment_5582 .wp-caption .aligncenter style="width: 207px"}
    ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/engine2.jpg "engine2"){.size-full
    .wp-image-5582 loading="lazy" decoding="async"
    aria-describedby="caption-attachment-5582"
    onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('orange');jmolApplet([450,450],'load wp-content/uploads/2011/11/engine2.log;frame 129;zoom 100;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;');"
    width="197" height="244"}
    The next resting position for the engine. Click for 3D.
    :::
4.  Back on the S~0~ state, the now un-rotatable double-bond axle acts
    as a [ratchet
    mechanism](http://en.wikipedia.org/wiki/Ratchet_(device)){target="_blank"},
    meaning that the next stage in the rotation of the wheel must occur
    elsewhere. This involves persuading the two groups indicated with a
    green arrow above to rotate past each other. At this stage the alkyl
    group is located underneath the benzo group, and so the latter has
    to rotate clockwise (and not anticlockwise) to achieve this.
5.  This step is accomplished with help from the tip of the STM
    microscope. When a high voltage is applied to the tip, electrons
    pass from the tip to the copper atoms on the surface, and experience
    what is called inelastic scattering as they tunnel through the
    molecule. As they do so, they lose energy to the molecule, which is
    presumed to pump up specific vibrations to help the molecule
    surmount what is [calculated to
    be](http://hdl.handle.net/10042/to-10281){target="_blank"} a 20.0
    kcal/mol free energy barrier to rotation (the molecule has little
    vibrational thermal energy of its own since the experiment is
    conducted at low temperatures). If this barrier were much higher, it
    it might be that no amount of such vibrational excitation could
    induce the molecule to surmount the barrier.
    - The experiments indicate a vibration of around 1600 cm^-1^ is
      excited, and the suggestion is that the axle C=C stretch is
      excited.
    - However, I cannot help feeling that this might simply undo the
      molecular ratchet, and lose the accumulated clockwise specificity
      of the wheel rotation.
    - A vibration is surely needed which would help the benzo group slip
      past the alkyl group, thus helping preserve the sense of
      direction. The calculated transition mode (click below) shows a
      fair degree of motion by the alkyl (methyl) group, as it contrives
      to avoid the benzo group.
    - Clearly, these are questions that need investigation. Thus
      deuterating parts of the molecule (say the alkyl group) to see if
      rolling is inhibited might be an interesting experiment?

    ::: {#attachment_5597 .wp-caption .aligncenter style="width: 244px"}
    ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/vib-ts.jpg "vib-ts"){.size-full
    .wp-image-5597 loading="lazy" decoding="async"
    aria-describedby="caption-attachment-5597"
    onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('orange');jmolApplet([450,450],'load wp-content/uploads/2011/11/vib-ts.log;frame 55;zoom 100;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;');"
    width="234" height="188"}
    The rotational transition state. Click for 3D.
    :::
6.  Once past this barrier, the wheel has completed a total of 180°
    worth of rotation since starting, in two stages of 139 and 41°.

Two pairs of light pulses + vibrational excitations are required to make
the wheel complete one whole rotation. But it is the mechanism whereby
the light and the vibration co-operate to roll the vehicle forwards is
what I find fascinating. What has not been addressed in the above
analysis (of a single wheel) is how all four wheels cooperate to move
the car forward. Does the light excitation rotate each wheel axle
separately or simultaneously, thus involving one or four photons? Does
the inelastic scattering due to electron tunneling excite just a single
in-phase vibration of all four wheels?  There are still mysteries (at
least to me) for which I continue to seek an explanation.

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


The world’s smallest nano car was recently driven a distance of 6nm along a copper track. When I saw this, I thought it might be interesting to go under the hood and try to explain what makes its engine tick and its fuel work. The molecule above represents (I think) the essentials of the engine. Its resting geometry in the S
<sub>
 0
</sub>
electronic state is shown below. The resting geometry of the engine.

Messaggi di Rogue Scholar

Under the hood of a nano car: the chemistry behind a molecular motor.