Polymorphism and Host–Guest Chemistry of a Phenylpyridine-Functionalized Zwitterion

The supramolecular chemistry of a phenylpyridine-functionalized zwitterion, formed by a simple reaction between acetylenedicarboxylic acid and 4-phenylpyridine, has been investigated. Two polymorphs and two solvates have been identified in this study, and the particular conditions required to obtain these, as well as two previously reported forms of this zwitterion, have been investigated. In addition, it was found that conversion between many of the forms is possible by immersion in solvents and, in some cases, is reversible. The hydrated form of the zwitterion and one of the polymorphs (1α) appear to be the two most favorable structures, as all known forms can be converted to either of these two structures. The hydrated form of the zwitterion can also be converted to either of the two polymorphs, and both of these conversions are reversible. Solvent was identified as playing a key role in the structures obtained and conversions between them.