Imperial College London

Henry Rzepa

The ultimate reduction in size for an engineer is to a single molecule.
It's been [done for a
car](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5615 "The chemistry behind a molecular motor. The four wheels?"){target="_blank"};
now it has been reported for the pixel
(picture-element).\[cite\]10.1021/ja404256s\[/cite\]

[![pixel](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/08/pixel.svg){.aligncenter
.size-full .wp-image-10991 decoding="async"
width="250"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/08/pixel.svg)

The molecule above (X=O, NR, R=aryl, etc) has been shown to be capable
of acting as a molecular pixel. To give some idea of the reduction in
size, computer displays currently only squeeze 400 or so pixels into an
inch (the archaic, but common units used to measure pixel sizes). The
secret to engineering this is to prevent energy transfer occurring
between adjacent pixels (= molecules on this scale), and this has been
done using quite simple
chemistry\![![pixel-excitation](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/08/pixel-excitation.svg){.aligncenter
.size-full .wp-image-10992 decoding="async"
width="350"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/08/pixel-excitation.svg)

The concept is to allow a molecule to reach an excited state by photon
absorption, but to prevent emission from occurring (which would result
in energy transfer to adjacent molecules) by inducing a rapid change in
the molecular structure of the excited state. This reaction has to be
very fast, and one of the fastest reactions is the intramolecular proton
transfer. In this example it converts the enol form of the oxazole **E**
to the keto form **K**. On the ground state surface, prior to
excitation, the enol form is the lower in free energy (retaining the
aromaticity of the phenyl ring). The basis of the molecular design is to
find a molecule where it is the keto form that is lower in energy on the
excited state surface, such that the *excited state intramolecular
proton transfer* is both fast and in effect irreversible. In the keto
form, any emission down to the ground state is now incapable of energy
transfer to adjacent molecules (which are presumed to be still in the
ground state and hence the **enol** form).

This sort of system is perfect for designing with the help of quantum
calculations, and to give just a hint of how this could be done, I
thought I would illustrate how the energetics of the ground and excited
states could be quickly obtained to show that the above energy diagram
really does apply to these molecules (R=H, X=O). At the
ωB97XD/6-311G(d,p)/SCRF=chloroform level, the
enol\[cite\]10.6084/m9.figshare.769259\[/cite\]^‡^ is 11.4 kcal/mol
**lower** in free energy than the keto
form\[cite\]10.6084/m9.figshare.769260\[/cite\]. A vertical
(non-adiabatic) excitation to the first excited singlet now produces a
system where the enol\[cite\]10.6084/m9.figshare.769248\[/cite\] is 3.9
kcal/mol **higher** than the keto
form\[cite\]10.6084/m9.figshare.769230\[/cite\], which reflects the
above diagram exactly.^†^

It is easy to see now that variation in R, X or other parts of the
molecule could be rapidly scanned computationally to find out how such
variation alters these relative energies. Computational tuning could
then be used to *e.g.* optimize avoidance of energy transfers between
adjacent molecules (pixels) and no doubt to also predict the actual
absorption energies (*i.e.* colours) of new candidate molecules.

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^‡^[Here I introduce the use of the so-called "**short-doi**". The [data
citations](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10972 "The Amsterdam Manifesto on Data Citation Principles"){target="_blank"}
above refer to the Figshare repository, the first citation of which
takes the long form `http://dx.doi.org/10.6084/m9.figshare.769259` By
invoking `http://shortdoi.org/10.6084/m9.figshare.769259` one can obtain
the short
form [http://doi.org/nd9](http://doi.org/nd9){target="_blank"}, of which
the essential part, **nd9**, is now just 3-characters long. This form
might be an alternative to
[QR-codes](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7311 "QR codes and InChI strings."){target="_blank"}
in **e.g.** lecture slides and other media where the human has to
remember the value. In a machine-sense of course, the short form offers
no advantage over the long form.]{.small}

^†^[Strictly speaking, one should locate the conical intersection for
proton transfer on the excited state, but the above calculations take
only minutes literally, whereas locating a conical intersection is a
rather more complex task.]{.small}

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


The ultimate reduction in size for an engineer is to a single molecule. It’s been done for a car; now it has been reported for the pixel (picture-element).[cite]10.1021/ja404256s[/cite]    The molecule above (X=O, NR, R=aryl, etc) has been shown to be capable of acting as a molecular pixel.

Messaggi di Rogue Scholar

Molecule-sized pixels.