Studies in the acridine series

Abstract

In the Annual Reports on the Progress of Chemistry for 1918 under the heading "chromoisomerism" the following passage may be found: but it seems tine to protest against one halfpennyworth of practice to this intolerable deal of theory, and to demand something more than assertion in proof of the constitutional formulae which are drawn up so lavishly." It is fairly clear that the reviewer is referring principally to Hantzsch who originated the term "chromoisomeric" to describe compounds which exist in two or more solid modifications with distinctly different colours. Hantzsch (1) supposed that all such cases are to be explained in terms of isomerism and he proposed many structures.Hantzsch, however, was right in looking for an explanation and his reviewers were wrong where they dismissed the phenomena as being "merely polymorphism" (2). Polymorphism perhaps; but where, as in some cases, the colour change is pronounced - say from green to red - this is hardly a sufficient explanation.In the sequel the purely descriptive term polychromism will be used,in place of Hantzsch's "chromoisomerism'' to avoid the implication that isomerism is necessarily involved.We know that electronic absorption is not exclusively a function of molecular structure; it may be affected by the polar environment of the molecule and also by more specific intermolecular forces such as hydrogen bonding or charge transfer interaction. A crystal lattice may be regarded as a structure of intermolecular interactions; to change the lattice is to change these interactions and, in so far as some of these may influence light absorption, to change the colour.But the problem remains. If there is no difference in the structure or conformation of the molecules in Lifferent polychromic states then there must be a difference in one or other of those forms of intermolecular interaction which can have a significant effect on colour and it should be possible to specify which of these is involved.Most polychromic organic compounds for which no evident explanation in terms of isomerism exists, are either salts or aromatic nitro -compounds (usually polynitro-compounds and usually phenols or amines). The origin of polychromism in these cases has not been fully established, but it may well be that comparatively recent work on charge transfer phenomena will provide at least a partial answer. Charge transfer might be expected to occur in the solid state with both of these classes of compounds.It is well known that aromatic polynitro-compounds can form coloured molecular compounds with aromatic hydrocarbons, amines,phenols etc. The formation of such compounds has been attributed to charge transfer interaction between the components one of which must be a Lewis acid (e.g. a nitro - compound) and the other á Lewis base (e.g. an aromatic hydrocarbon or amine) (3 a - e).This type of interaction gives rise to a new absorption band, which has been called the charge transfer spectrum, in the visible or near ultra -violet regions. It appears to be due to the occurrence of electronic transitions between rather than within molecules. Charge transfer spectra are always broad and without fine structure. This is probably due (3a) to the weakness of the charge transfer bonding (2 - 4 K.cals. /mole (4)) allowing thermal vibration to provide a considerable range of displacements and orientations between the interacting groups. If the energy required for these intermolecular transitions does indeed depend upon the orientation of the interacting groups in the crystal - and this seems probable - then the colour of the solid will be a function of crystal structure. Thus where a compound, which shows charge transfer interaction in the solid, is polymorphic it will in general be polychromie.This charge transfer hypothesis for polychromism must be regarded as a more modern version of a theory put forward by Pfeiffer (8) as early as 1915 in terms of residual valency. He proposed that the orange and yellow forms of nitromethoxystilbenes result from the orientations A and B respectively in the solids.No work appears to have been done in order to decide whether charge transfer can occur in organic salts: its occurrence in inorganic salts appears to be very common. It is too early therefore to judge whether an extension of the charge transfer idea to cover the numerous cases of polychromism in organic salts is justifiable. A purely electrostatic theory has been proposed by Lucas and Kemp (9) to explain the polychromism of organic and inorganic salts. Their general conclusion is that the electronic absorption of an ion in a crystal lattice will depend on its electrostatic environment, created by the surrounding ions of opposite charge, and that this will depend on the crystal structure.Although at present it is not possible to decide which, if any, of these theories is true, one thing is clear: where a compound exists in more than one differently coloured solid form it is not possible to conclude directly from this that different molecular structures must be assigned to these forms. On the other hand a knowledge of the general classes of compounds which show polychromism and an appreciation of the factors which may influence light absorption in a crystal will help in deciding, in specific cases, whether it is worth looking for an explanation in terms of isomerism. For example if the compound in question is a salt or a polynitrocompound, or indeed if it is a betaine or contains both Lewis acid and Lewis base functions, any "isomeric" theory will be somewhat unconvincing. But the converse also is true.According to Kehrmann and Matusinsky (10) 2- hydroxy5- phenylacridine crystallises from hot benzene as fine yellow needles with a melting point of 264 °C. On crystallising slowly from cold benzene red prisms are obtained which may be converted to the yellow form by heating at 135 °C. The red modification is formed from the yellow slowly on standing and rapidly by crushing and powdering. From these observations it was concluded that the yellow is the form stable at higher temperatures and the red the lower temperature stable form.In view of the difference of more than 100 °C between the melting points of the two forms and in view of their strong difference in colour, Kehrmann suggested that this was a case of tautomerism between the structures.Neither argument is very convincing and the great ease with which the red modification can be formed from the yellow would seem to weigh heavily in favour of an explanation in terms of polymorphism. But if this example is considered in the context of polychromism in general and in the light of work carried out more recently by John (11) and by Albert and Short (12) on the tautomerism of analogous compounds, Kehrmann's theory becomes distinctly more probable.In the first place 2- hydroxy- 5- phenylacridine does not fall into any of the general classes of polychromic compounds: 8. this appears to be the only published example of a polychromic free acridine (although polychromism in acridine salts is very common). On the other hand John has studied a group of 2- hydroxy -phenazine derivatives.This compound exists in a yellow and a deep violet modification,these colours corresponding to the colours of the 0- Methyl and N- Methyl derivatives respectively. In solution 1:3:4 trimethyl- 2- hyäroxyphenazine is present as an equilibrium mixture of the structures(IIIa) and (IIIb). John concluded that the yellow and violet modifications were to be identified with (IIIa) and (IIIb) respectively. In this case however interconversion between the solids can only be brought about by recrystallising from solution or by heating to 135 °C at which temperature sublimation can be seen to occur.Albert and Short (12) have shown that 2- hydroxyacridine also is tautomeric (although not polychromic) existing in solution as an equilibrium mixture of lactim and lactam structures analogous to (I) and (II). Here the lactim structure is yellow and the lactam structure red.Kehrmann and P+iatusinsky's theory might now seem to be so reasonable as to be hardly worth questioning. But there remains one difficulty: the yellow crystals of 2- hydroxy- 5- phenylacridine can be converted to the red modification simply by rub-uing. If this is a tautomeric change it occurs with remarkable facility.It was therefore decided to investigate more thoroughly the colour changes of 2- hydroxy -5- phenylacridine.