A remarkable variety of chiral and achiral solid morphologies is seen in the crystallisation of a series of achiral diketopyrrolopyrrole derivatives. The alkyl chain length of the N ‐substituent in the compounds determines their solid morphologies. Despite the lack of any chiral feature in the molecules, the crystals formed by the compounds had the shape of twisted objects with helical macroscopic features. Microscopy also revealed distinct dendritic structures for the hexyl‐derivative. The decyl‐appended compound produced a wider range of morphologies, which depended strongly on the crystallisation solvent. The X‐ray crystal structures of two of the compounds reveal hydrogen bonds that link the molecules together in supramolecular chains or dimers. Raman spectroscopy and scanning probe microscopies were used to further characterise the twisted crystals. These measurements indicate similar non‐covalent interactions and environments across the solid forms. The tendency of the compounds to form twisted crystals from a solution crystallisation is unusual, especially given the lack of stereogenic centres in the molecules that comprise the chiral objects. It is believed that auto‐deformation occurs, arising from the accumulation and partial relaxation of elastic stress during crystal growth, that depends on the length of the alkyl chains present in the compounds since all other molecular features are identical.
Killalea et al. (Wed,) studied this question.