In-situ recrystallization of elastically bendable cocrystals of caffeine post partialdesolvation

The system of 1:1:1 cocrystal solvate of caffeine, 4-chloro-3-nitrobenzoic acid and methanol isthe first reported example of molecular crystal that exhibits reversible elastic flexibility [1]. Thecompound crystallizes in orthorhombic space group symmetry Fdd2 [T = 100 K: a = 32.784(9) Å,b = 55.541(15) Å, c = 3.9564(12) Å, V = 7191(4) Å3]. Elastic bending in these crystals has beendetermined to be governed by changing distances between molecules within stacks andmolecular rotations [2]. The crystals have been demonstrated to lose flexibility at hightemperatures which is attributed to partial loss of solvent from their structure (irreversiblephase transformation at Tc = 388 K) [1]. While combination of weak dispersive interactions viz.C–H ···O hydrogen bonds, π-stacking and van der Waals forces between pseudo sphericalfunctional groups aids flexibility, permanent plastic deformation in these crystals has beenargued to be prevented by “interlocking”/ steric barriers in the supramolecular architecture[1,2].High temperature in-situ powder X-ray diffraction studies confirm the phase transformationaround Tc. Upon cooling down to room temperature, long acicular crystals were observed afterfew days. These crystals demonstrate excellent elastic flexibility. Single crystal X-ray diffractionexperiments employing synchrotron radiation revealed that the in-situ recrystallized crystalshave identical space group symmetry but a unit cell volume significantly larger than as growncrystals [T = 100 K: Vin-situ = 7444.7(3) Å3]. Interestingly, the large a and b axes are found to beslightly shorter as compared to the as grown crystals while the significantly larger c-axis [=4.15160(10) Å] accounts for such increased unit cell volume of the regrown crystals.Here we present the atomic level studies of the regrown crystals in comparison to the as growncrystals. The reversibility post partial desolvation is governed by the re-inclusion of methanolalbeit different electron counts at the site of the solvent that is suggested to be ~ 1.3-1.4 timeslarger than the as synthesized crystals from structure refinements. Increased content in thestructure enhances the solvent channels that concomitantly result in larger distances betweenmolecules within stacks thus stabilizing the lattice with a larger c-axis. The apparentreconstruction of the molecular stacks is accompanied by tilting of the molecules andreorientation between hydrogen bonded tapes different to as synthesized crystals. While in thepresent case, inherent disorder of the solvent possibly aids in such inclusion of molecularentities, such in-situ recrystallization could be employed to manipulate overall structure to tunein variable and/or revive mechanical response in molecular crystals