Polymorphism in a Rotaxane Molecule: Intra- and Intermolecular Understanding

Polymorphs have been widely studied since different crystalline phases of the same compound may have distinct properties. Macromolecules are an area of major study with just a few polymorphs reported. This investigation presents the first case of polymorphism in a [2]rotaxane molecule. This rotaxane contains a succinamide station bearing nonsymmetric stoppers and a tetralactam macrocycle. A different polymorphic phase is achieved by varying the crystallization solvent. These polymorphs presented relevant differences at the molecular level and were classified as conformational polymorphs. At the supramolecular level, polymorph I is -21 kcal mol(-1) more stable than polymorph II. Crystallization mechanisms to assess the stages of the crystallization process were proposed. Similar stabilization of the first nuclei formed was observed to be responsible for the possibility of both forms to coexist. The large differences observed between the polymorphs were not significantly reflected in the types and contribution of the intermolecular interactions, in which a high resemblance was observed.