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.

The simple planar molecule acridine is polymorphically promiscuous, with, at latest count, eight distinct unsolvated forms, and one hydrate. This makes it a compelling model system to study its polymorphism, as this represents a challenge to our understanding of crystallization.

We report a new polymorph of acridine, C13H9N, denoted form IX, obtained as thin needles by slow evaporation of a toluene solution. The structure was solved and refined from powder X-ray data. The structures of five unsolvated forms were previously known, but this is only the second with one molecule in the asymmetric unit. The melting point [differential scanning calorimetry (DSC) onset] and heat of fusion are 108.8 (3) °C and 19.2 (4) kJ mol−1, respectively.

The title compound, C13H9N·0.75H2O was obtained during a study of the polymorphic system of acridine, by slow evaporation from an ethanol–water solution. There are two acridine mol­ecules (indicated by I and II, respectively) and one and a half water mol­ecules in the asymmetric unit. The half-mol­ecule of water is located on a crystallographic twofold axis. The crystal structure is built up from two threads of mol­ecule II sewn together with water mol­ecules through O—H⋯O and O—H⋯N hydrogen bonds from one side and with π–π inter­actions [centroid–centroid distance = 3.640 (3) and 3.7431 (3) Å] between overlapping mol­ecules II on the other side. Mol­ecule I is attached to this thread from both sides by C—H⋯O hydrogen bonds. The threads are connected to each other by π–π inter­actions [centroid–centroid distances = 3.582 (3) and 3.582 (3) Å] between the inner side of mol­ecule I and stabilized by a C—H⋯π inter­action on the other side of mol­ecule I. This thread with rows of mol­ecule I hanging on its sides is generated by translation perpendicular to the a axis.

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