Hydrogen-Bonded Cyclic Tetramer Formation of a Pyridylethynylimidazole-Based Fluorophore

Supramolecular have promising application prospects owing to their catalytic, self-healing, and stimulus-responsive functions. Pyridylethynylimidazole-based supramolecular synthons with fluorescent properties have been developed. 4-(2-Pyridylethynyl)-5-arylethynyl-1H-imidazole derivatives 1 with different substituents at the 4- and 5-positions of the imidazole skeleton were prepared using a step-by-step Sonogashira coupling reaction. The fluorescence maxima of phenyl- and methoxyphenyl-substituted derivatives 1a and 1c were independent of solvent polarity. On the other hand, ester- and cyano-substituted 1b and 1d showed bathochromically shifted fluorescence maxima in highly polar solvents. In crystals, the 4-(2-pyridylethynyl)-5-arylethynyl-1H-imidazole derivatives formed a supramolecular cyclic tetramer structure via two hydrogen bonds: one was between N─H in imidazole and N in pyridine, and the other was between C─H in imidazole and N in imidazole. Introducing the pyridine ring as a hydrogen bond acceptor, in addition to the imidazole ring, promoted the unique cyclic tetramer structure formation.