Supramolecular gelation as the first stage in Ostwald’s rule

Ostwald’s rule of stages describes the observation that a metastable polymorph of a particular compound will crystallise before the thermodynamically stable form under the same conditions. Whilst traditionally applied to crystalline systems, there have been recent examples in which non-crystalline supramolecular assemblies transform according to this rule. In this work we report the highly selective organogelation behaviour of a mono-iodinated 2,4,5-triphenylimidazole (lophine) derivative (I-TPI). The gel emerges as a kinetically trapped intermediate, prior to the crystallisation of a series of increasingly stable methanol solvates. As such, we describe the supramolecular gel as a pre-crystalline phase, representing the first of Ostwald’s stages for this system. Similar behaviour can be observed when the gel is used as a medium for pharmaceutical crystallisation of diatrizoic acid (DTA). In this case, two solid forms are produced, both containing molecules of DTA, I-TPI and methanol in different stoichiometries, and once again crystallising in sequence according to Ostwald’s rule. These results demonstrate that supramolecular gelation may be considered the first stage in the stepwise crystallisation of a gelator, as governed by Ostwald’s rule, and that this behaviour also applies to multi-component systems, facilitating the production of metastable pharmaceutical solid forms.