Kitaigorodsky Revisited: Polymorphism and Mixed Crystals of Acridine/Phenazine

Non‐stoichiometric molecular mixed crystals have potential as functional materials, the properties of which can be tailored by rationally changing their composition. The guidelines for their preparation were summarized over thirty years ago by Alexander Kitaigorodsky. Here those principles are revised in light of new studies on the acridine/phenazine system, and solvent‐assisted grinding is presented as a convenient synthetic procedure to afford a more homogeneous product than traditional solvent‐evaporation methods. Finally, the proposed prerequisite of crystal isostructurality/isomorphicity for the pure compounds, which seems to be violated in the present case, is discussed. Just revisiting: Kitaigorodsky's principle for preparing a crystalline solid solution of organic molecules is revised in light of new studies on the acridine/phenazine system. The system studied here suggests that crystal isostructurality for the pure compounds is not required for the formation of the mixed phase (see figure).