Imperial College London

Henry Rzepa

The story of Monastral is not about a character in the Magic flute, but
is a classic of chemical serendipity, collaboration between industry and
university, theoretical influence, and of much else. Fortunately, much
of that story is actually recorded on film (itself a unique archive
dating from 1933 and being one of the  very first colour films in
existence!). Patrick Linstead, a young chemist then (he eventually rose
to become rector of Imperial College) tells the story himself
[here](http://www.ch.ic.ac.uk/video/linstead/){target="_blank"}. It is
well worth watching, if only for its innocent social commentary on the
English class system (and an attitude to laboratory safety that should
not be copied nowadays). Here I will comment only on its colour and its
aromaticity.  

::: {#attachment_3646 .wp-caption .aligncenter style="width: 302px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine.jpg "phthalocyanine"){.size-full
.wp-image-3646 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3646" width="292" height="292"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine.jpg 1170w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine-150x150.jpg 150w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine-1024x1020.jpg 1024w"
sizes="(max-width: 292px) 100vw, 292px"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine.jpg)

Copper phthalocyanine
:::

In 1933, Hückel was still thinking about his molecular orbital
electronic theory of benzene, but for \~15 years, there remained little
need for the rule we now know as
**[4n+2](http://dx.doi.org/10.1021/ja01146a537){target="_blank"}**,
because n was invariably equal to 1 for most known aromatic molecules!
It was only the discovery of so-called non-benzenoid aromatics in the
1940s (*e.g.* [Dewar's tropolone
structure](http://dx.doi.org/10.1038%2F155050b0){target="_blank"}) that
propelled chemists to identify aromatic molecules with other values of
n. And [Monastral
blue](http://dx.doi.org/10042/to-7479){target="_blank"} is a prime
example of n=4 (although it would be of interest to find out when it
became so associated with the Hückel rule). If you count the red bonds
above, there are eight, along with one lone pair of electrons located on
the highlighted (blue) nitrogen atom. This makes **18** π-electrons in
the ring, or 4×4+2 (there are paths other than the one shown, but they
give the same count). Part of the reason for the remarkable thermal
stability of this molecule must be its aromaticity.

So what about the colour? The visible spectrum is shown below, with
λ~max~ \~ 610 and 710nm.

::: {#attachment_3650 .wp-caption .aligncenter style="width: 302px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phtalocyanine.jpg "phtalocyanine"){.size-full
.wp-image-3650 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3650" width="292"
height="181"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phtalocyanine.jpg)

Visible absorption spectrum of copper phthalocyanine.
:::

Well, a [TD-DFT
ωB97Xd/6-31G(d)](http://hdl.handle.net/10042/to-7568){target="_blank"}
calculation reveals the following. This reproduces the band at 610nm
very nicely, but leaves the identity of the band at  710nm
mysterious. How does that originate? One might speculate that this could
arise from the presence of another species. Thus copper phthalocyanine
itself is neutral, but it could easily be oxidised to a cation, and this
could then form a  1:1 π-complex with a second molecule of the neutral
radical
(DOI:[10.1021/ja00238a021](http://dx.doi.org/10.1021/ja00238a021){target="_blank"}
)  
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine.svg "mauveine"){.aligncenter
.size-full .wp-image-3530 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/phthalocyanine.svg)

The electronic excitation at \~610nm arises from the following MOs:

<table class="aligncenter">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td><div id="attachment_3656" class="wp-caption aligncenter"
style="width: 204px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/147.jpg"
title="147" class="size-full wp-image-3656" loading="lazy"
decoding="async" aria-describedby="caption-attachment-3656"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/03/cu-147_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/147.jpg 581w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/147-300x291.jpg 300w"
sizes="(max-width: 194px) 100vw, 194px" width="194" height="188" />
<p>Orbital 147, the highest occupied MO (HOMO). Click for 3D</p>
</div></td>
<td><div id="attachment_3657" class="wp-caption aligncenter"
style="width: 204px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/148.jpg"
title="148" class="size-full wp-image-3657" loading="lazy"
decoding="async" aria-describedby="caption-attachment-3657"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/03/cu-148_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/148.jpg 581w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/148-300x291.jpg 300w"
sizes="(max-width: 194px) 100vw, 194px" width="194" height="188" />
<p>Orbital 148, the lowest unoccupied MO. Click for 3D</p>
</div></td>
</tr>
</tbody>
</table>

The unpaired electron in copper phthalocyanine occupies the following
rather interesting orbital, which appears not to be involved in its blue
colour.

::: {#attachment_3658 .wp-caption .aligncenter style="width: 204px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/146.jpg "146"){.size-full
.wp-image-3658 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3658"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2011/03/M_mo146.cub.xyz;isosurface colour red blue wp-content/uploads/2011/03/M_mo146.cub.jvxl translucent;');"
width="194" height="188"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/146.jpg 581w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/03/146-300x291.jpg 300w"
sizes="(max-width: 194px) 100vw, 194px"}

Orbital 146. The singly occupied MO. Click for 3D
:::

So, just as with
[mauveine](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3507){target="_blank"},
a mystery remains. The colour of Monastral blue is not monochromatic, in
that it appears to be caused by two bands in the 600-700 region.
Calculation however reveals it to have only one band at 610nm. What is
the other one?

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


The story of Monastral is not about a character in the Magic flute, but is a classic of chemical serendipity, collaboration between industry and university, theoretical influence, and of much else.

Messaggi di Rogue Scholar

Monastral: the colour of blue