Epitaxial relation of carbamazepine and its precursor template extracted from rotating grazing incidence X-ray diffraction

Directed crystal growth often requires considerable experimental effort to achieve epitaxial control. In this work a simple solution step is shown to yield crystals of two different molecules simultaneously within a thin film and with defined epitaxial relations to each other. This was achieved by dissolving simultaneously carbamazepine (CBZ), a drug molecule, and iminostilbene (ISB), its precursor, in a single solution. When the sample is prepared using slow solvent evaporation rates, both molecules grow into needle-shaped crystals. The ISB crystals contact the substrate surface and extend for hundreds of μm. CBZ tends to minimize the contact with the substrate and assembles on top or is ledge-directed. In both cases a defined inclination of CBZ to the underlying ISB crystals is seen with optical microscopy showing a 36° inclination of the elongated crystals. In a similar way to pole figure measurements, rotating grazing incidence X-ray diffraction allows the determination of crystallographic properties and epitaxial relation, i.e. how the unit cells of both molecules align with respect to each other in terms of contact plane and azimuthal orientation. A point-on-line coincidence was identified, which can be related to the herringbone packing in the ISB crystals. Furthermore, changing the solute content as well as the processing times/kinetics, the appearance and the quality of crystal growth changes, but with the epitaxial relations remaining unaffected; crystals grow more frequent on top when processed slowly and when processed fast, growth takes place more often at facet inclined 90° from the top one.