Imperial College London

Henry Rzepa

One of my chemical heroes is [William
Perkin](http://en.wikipedia.org/wiki/Sir_William_Henry_Perkin){target="_blank"},
who in 1856 famously (and accidentally) made the dye
[mauveine](http://en.wikipedia.org/wiki/Mauveine){target="_blank"} as an
18 year old whilst a student of August
von [Hofmann](http://en.wikipedia.org/wiki/August_Wilhelm_von_Hofmann){target="_blank"},
the founder of the Royal College of Chemistry (at what is now  Imperial
College London). Perkin went on to found the British synthetic dyestuffs
and perfumeries industries. The photo below shows Charles Rees, who was
for many years the Hofmann professor of organic chemistry at the very
same institute as Perkin and Hofmann himself, wearing his mauveine tie.
A colleague, who is about to give a talk on mauveine,
[asked](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3604){target="_blank"}
if I knew why it was, well so very **mauve**. It is a tad *bright* for
today's tastes!

::: {#attachment_3508 .wp-caption .aligncenter style="width: 230px"}
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/220px-Rees1.jpg "220px-Rees1"){.size-full
.wp-image-3508 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3508" width="220"
height="291"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/220px-Rees1.jpg)

Charles Rees, wearing a bow tie dyed with (Perkin original) mauveine and
holding a journal named after Perkin.
:::

The first thing to note about mauveine is that it is not a single
compound; [actual samples](10.1002/chem.200800718){target="_blank"} can
contain up to 13 different forms! These all vary in the number of methyl
groups present which range from none up to four, in various positions.
These compounds all have absorption maxima λ~max~ in the range
540-550nm, the colour of purple. The structure of one of these, known as
mauveine A, is shown below.

::: {#attachment_3512 .wp-caption .aligncenter style="width: 347px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine.jpg "mauveine"){.size-full
.wp-image-3512 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3512"
onclick="jmolInitialize('../Jmol/',true);jmolSetAppletColor('purple');jmolApplet([600,600],'load wp-content/uploads/2011/02/mauveine.cml;set measurementUnits Angstroms;measure 29 60;measure 55 54;measure 54 37;measure 54 56;measure 42 34;measure 57 42;');"
width="337" height="255"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine.jpg 1010w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-300x227.jpg 300w"
sizes="(max-width: 337px) 100vw, 337px"}

Mauveine A. Click to load 3D
:::

You can see from this that something is missing. The so-called
chromophore is a cation, and an anion needs to be provided to balance
the charge. We will now attempt to predict the color of purple using
purely the power of quantum mechanics (for many years, accurate
prediction of colour was a holy grail amongst dye chemists for obvious
reasons). The anion can be chloride, and the colour is often measured in
methanol as solvent. So the first task is to calculate this ion-pair.
This used to be easier said than done (and in the past, the anion was
often simply neglected). But using the ωB97XD density functional
procedure (to get the van der Waals interactions modelled correctly) and
a 6-311++G(d,p) basis set, coupled with a smoothed-cavity continuum
solvation procedure, and two molecules of water (standing in for
methanol, which is a bit bigger) as explicit solvent molecules, we get
the structure apparent when you click on the diagram above (DOI:
[10042/to-7320](http://hdl.handle.net/10042/to-7320)). Application of
time-dependent density function theory (TD-DFT) gives a measure of the
[UV-optical
spectrum](http://hdl.handle.net/10042/to-7321){target="_blank"} (below,
loaded as a scaleable SVG image. If you are using a modern browser, it
should display. If not, try the latest FireFox, Chrome, Safari etc).  
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine.svg "mauveine"){.aligncenter
.size-full .wp-image-3530 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine.svg)

 

 

This has several noteworthy aspects.

1.  The visible (right hand side) part of the spectrum is very
    monochromatic, with λ~max~ \~440nm. In other words, mauveine has a
    pure and intense colour.
2.  This λ~max~ is hardly affected by the presence of the counterion.
3.  The electronic transition responsible for this band is a simple HOMO
    (highest-occupied-molecular-orbital) to LUMO
    (lowest-unoccupied-molecular-orbital) excitation of an electron.
4.  These orbitals are shown below.  
    <table data-border="0">
    <colgroup>
    <col style="width: 50%" />
    <col style="width: 50%" />
    </colgroup>
    <tbody>
    <tr class="header">
    <th>LUMO</th>
    <th>HOMO</th>
    </tr>
    &#10;<tr class="odd">
    <td><div id="attachment_3522" class="wp-caption aligncenter"
    style="width: 224px">
    <img
    src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-mo123.jpg"
    title="mauveine-mo123" class="size-full wp-image-3522" loading="lazy"
    decoding="async" aria-describedby="caption-attachment-3522"
    onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/02/mauveine123_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-mo123.jpg 641w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-mo123-300x183.jpg 300w"
    sizes="(max-width: 214px) 100vw, 214px" width="214" height="131" />
    <p>Mauveine A. LUMO. Click for 3D</p>
    </div></td>
    <td><div id="attachment_3523" class="wp-caption aligncenter"
    style="width: 224px">
    <img
    src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-mo122.jpg"
    title="mauveine-mo122" class="size-full wp-image-3523" loading="lazy"
    decoding="async" aria-describedby="caption-attachment-3523"
    onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/02/mauveine122_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-mo122.jpg 641w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/02/mauveine-mo122-300x183.jpg 300w"
    sizes="(max-width: 214px) 100vw, 214px" width="214" height="131" />
    <p>Mauveine A. HOMO. Click for 3D</p>
    </div></td>
    </tr>
    </tbody>
    </table>
5.  Note how the excitation involves the central region of the molecule,
    and one of the pendant aryl groups, but not the other. One might
    presume that tuning the colour would only work if changes are made
    to the first of these aryl groups.
6.  There is a real mystery about the calculated value of λ~max~, which
    differs from the observed value by about 100nm (the wrong colour,
    making mauveine orange rather than purple). Normally, this sort of
    time dependent density functional theory has errors no greater than
    15-20nm. The calculated value of λ~max~ is not sensitive to the
    basis set, or the presence or not of the counter ion and solvent.
    Clearly, a discrepancy of this magnitude must have some other
    explanation. Watch this space!

So this post ends with [a bit of a
mystery](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3641){target="_blank"}.
The fanciest most modern computational theory gets the colour of
mauveine wrong by \~100nm. Why?

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


One of my chemical heroes is William Perkin, who in 1856 famously (and accidentally) made the dye mauveine as an 18 year old whilst a student of August von Hofmann, the founder of the Royal College of Chemistry (at what is now  Imperial College London). Perkin went on to found the British synthetic dyestuffs and perfumeries industries.

Messages de Rogue Scholar

The colour of purple