What Makes a Branched Aromatic Compound a Crystallization Chaperone? Insights from a Comparison of Three Organic Scaffolds

Some tetraaryladamantane (TAA) octa- and tetraethers have the ability to crystallize into well-ordered lattices without full desolvation. In many cases, the solvates then yield high-resolution X-ray crystal structures of the encapsulated liquids. To shed light on this unusual effect of TAAs as crystallization chaperones, we have synthesized a series of spirobiflourene and porphyrin derivatives with four phenyl arms also found in TAA chaperones. Despite the structural similarity, neither of the non-TAA compounds showed promising crystallization properties. Six new X-ray crystal structures were obtained, but neither gave a high-resolution structure of an encapsulated guest. Quantum chemical computations suggest that conformational changes have low activation barriers for the TAAs, which may help to adapt to the structures of guest molecules in tightly packed arrangements. These findings on supramolecular chemistry in the crystalline state may help to design new chaperones with improved properties.