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. 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.
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, 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) that propelled chemists to identify aromatic molecules with other values of n. And Monastral blue 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 λmax ~ 610 and 710nm.
Well, a TD-DFT ωB97Xd/6-31G(d) 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 )
The electronic excitation at ~610nm arises from the following MOs:
The unpaired electron in copper phthalocyanine occupies the following rather interesting orbital, which appears not to be involved in its blue colour.
So, just as with mauveine, 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?