Common Occurrence of Twisted Molecular Crystal Morphologies from the Melt

Abstract

Two books that describe the forms of thin films of many molecular crystals grown from the melt in polarized light, “Gedrillte” Kristalle (1929) by Ferdinand Bernauer and Thermomicroscopy in the Analysis of Pharmaceuticals (1971) by Maria Kuhnert-Brandstätter, are analyzed. Their descriptions, especially of curious morphologies consistent with helicoidal twisting of crystalline fibrils or narrow lamellae, are compared in the aggregate with observations from our laboratory collected during the past 10 years. According to Bernauer, 27% of molecular crystals from the melt adopt helicoidal crystal forms under some growth conditions even though helicoids are not compatible with long-range translational symmetry, a feature that is commonly thought to be an a priori condition for crystallinity. Bernauer’s figure of 27% is often met with surprise if not outright skepticism. Kuhnert-Brandstätter was aware of the tell-tale polarimetric signature of twisting (rhythmic interference colors) but observed this characteristic morphology in less than 0.5% of the crystals described. Here, the experience of the authors with 101 arbitrarily selected compounds—many of which are polymorphous—representing 155 total crystal structures, shows an even higher percentage (ca. 31%) of twisted crystals than the value reported by Bernauer. These observations, both positive (twisting) and negative (no twisting), are tabulated. It is concluded that twisting is not associated with molecular structure or crystal structure/symmetry. Rather, these nonclassical morphologies are associated with certain habits with exaggerated aspect ratios, and their appearance is strongly controlled by the growth conditions. Comments are offered in an attempt to reconcile the observations here, and those of Bernauer, the work of seekers of twisted crystals, with those of Kuhnert-Brandstätter, whose foremost consideration was the characterization of polymorphs of compounds of medicinal interest.