Two-Component Supramolecular Organic Hosts as Colorimetric Indicators for Aromatic Guests: Visual Molecular Recognition via Cation−π Interactions

Supramolecular two-component organic host systems composed of 2,2′,6,6′-tetracarboxybiphenyl (H₄1)/4,4′-bipyridylethylene (bpe) and (H₄1)/4,4′-bipyridine (bpy) were shown to recognize various aromatic guest molecules. The inclusion of the guest molecules in the host systems is evident from the remarkable exhibition of different colors in their corresponding inclusion complexes. The host networks are formed by the self-assembly of two (H₃1) anions and dication (H₂bpe or H₂bpy) via O–H···O and N–H···O hydrogen bonding. The pyridinium ions have exhibited greater tendency to interact with aromatic guest molecules via strong cation−π interactions. The host system (H₃1)₂(H₂bpe) was found to recognize anthracene, phenanthrene, and pyrene, while (H₃1)₂(H₂bpy) was found to recognize several aromatic guests such as phenanthrene, pyrene, phenol, o-cresol, m-cresol, p-cresol, and chlorobenzene. The crystal structures of all these materials were determined and analyzed in terms of hydrogen bonding between the two host components and host–guest interactions. The extent of cation−π interactions is reflected in the color of these complexes. Solid-state diffusion reflectance spectral (DRS) studies have been carried out for all these different colored complexes.