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Conformational switch in the crystal states of a calix[4]arene

We report the discovery of a novel metastable polymorph of a propyloxy substituted calix[4]arene molecule which undergoes two distinct transformations to form different conformers and packing arrangements. The first transition was found as a reversible thermal-induced rotation (180°) in two propyloxy moieties of the closed-I form upon cooling (from 298 K to 100 K) which resulted in a new polymorph (closed-II) and is an example of single-crystal-to-single-crystal transformation (SCSC). The second transition is an irreversible and remarkable change in packing and conformation (from close to open) at room temperature which was observed after keeping the mother crystals for few days in the liquor which resulted in an open conformer; this transformation is following the Ostwald's rule of stages and is the outcome of dissolvation/recrystallization of the metastable conformer (closed-I).