In-situ modified lattice and thermal expansion of a flexible molecular single crystal system governed by densification of solvent inclusion

The discovery of an in-situ method to grow flexible single crystals of methanol solvated cocrystals of caffeine and 4-chloro-3-nitrobenzoic acid by desolvation-resolvation of the solvent-grown material is reported. The in-situ obtained crystals are isomorphous to the starting Form with orthorhombic space group symmetry Fdd2, show similar habit and demonstrate macroscopic flexibility but possess greater unit cell volume. Crystal structures of the in-situ Form obtained employing synchrotron radiation suggest that the solvent channels are enlarged by an additional ~½ methanol molecule which leads to elongation of the unit cell axis parallel to the crystal growth direction. The new in-situ Form conserves the hydrogen bond geometries of the solvent grown material, while caffeine–coformer dimers along π···π stacks parallel to methanol channels are re-oriented by increased intermolecular distances and tilt of their mean planes. The most striking impact of the greater density of solvent molecules is in the thermal expansion of the in-situ crystals which is ~ 73 % lower than that of the solvent grown Form. The present results demonstrate the strength of in-situ techniques which not only depicts an unprecedented “breathing property” of these elastic crystals similar to zeolites, MOFs and COFs but also the potential of tuning their thermal response.