Various Stacking Patterns of Two-Dimensional Molecular Assemblies in Hydrogen-Bonded Cocrystals: Insight into Competitive Intermolecular Interactions and Control of Stacking Patterns

Control over stacking patterns in two-dimensional (2D) molecular assemblies is demonstrated by chemical modification. A target system is a hydrogen-bonded cocrystal (2:1) composed of 2-pyrrolidone (Py) and chloranilic acid (CA) (PyCA). X-ray crystallography revealed that weak intersheet interactions result in a variety of metastable overlapping patterns comprising 2D assemblies, mainly formed via hydrogen bonds, allowing reversible and irreversible structural phase transitions. Cocrystals of Py and anilic acids bearing different halogens were prepared, in which the 2D assemblies, isostructural to those observed in PyCA, exhibited various overlapping patterns. The order of stability for each overlapping pattern estimated using calculations of the intermolecular interactions did not completely coincide with those indicated by our experimental results. This can be explained by considering the entropic effect, that is, the molecular motion of Py, as detected using nuclear quadrupole resonance spectroscopy.