IN-SITU GROWN FLEXIBLE SINGLE CRYSTALS OF CAFFEINE CO-CRYSTAL POSSESS DISPARATE THERMAL EXPANSION PROPERTIES THAN THE SOLVO-THERMAL GROWN MATERIAL

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. Although the in-situ obtained crystals are isomorphous to the prototype with orthorhombic space group symmetry Fdd2 and show a similar habit, the cell axis parallel to the crystal needle axis is significantly enlarged while the orthogonal directions are shrunk. Moreover, the volumetric thermal expansivity is at least ~73% lower than that of the prototype. Crystal structure solutions of the new form obtained employing synchrotron radiation suggest that the asymmetric unit contains an additional ~½ methanol molecule leading to the expansion of the needle axis that in turn tilts the molecules further. This results in compression of the orthogonal axes as compared to the prototype while the larger concentration of the solvent leads to suppression of the thermal expansion. The results depict a “breathing property” of these elastic crystals similar to zeolites, MOFs and COFs.