Probing molecular recognition in the solid-state by use of an enolizable chromophoric barbituric acid

Complex formation of the enolizable chromophor 1-n-butyl-5-(4-nitrophenyl)barbituric acid 1 with multiple binding sites for supramolecular assemblies and its corresponding adducts produced with the Proton Sponge (1,8-bis(dimethylamino)naphthalene), PS) and the adenine-mimetic 2,6-diacetamidopyridine (DAC) have been studied by means of solid-state proton NMR spectroscopy under fast magic-angle spinning, X-ray analysis, and UV/vis spectroscopy. Both NMR data and X-ray results reveal that the enolic chromophor undergoes self-aggregation to hydrogen-bonded dimers which are involved in stacked arrangements. Depending on the nature of the added base, this dimeric assembly is preserved in the formed enolate anion but can be broken in the presence of complementary hydrogen-bonding pattern leading to supramolecular complexes. Molecular recognition of these structural different bases significantly influences the chromophoric pi-system of 1.