Structural insight into imidazopyridines and benzimidazoles: the importance of the hydrogen bond, π-stacking interactions and intramolecular charge transfer effect for fluorescence

Abstract

Imidazopyridines, benzimidazoles and their derivatives are important heterocyclic substances with a wide range of future applications in the field of fluorescent sensors due to their excellent solid-state luminescence properties, long fluorescence lifetimes, and large Stokes shifts. Two 3-phenylimidazo[1,5-a]pyridines and two 2-phenyl-1H-benzo[d]imidazoles were synthesized and spectroscopically characterized by NMR spectroscopy and fluorescence spectroscopy analysis. The molecular structures and crystal stacking properties of three types of compounds, namely, imidazo[1,2-a]pyridine, imidazo[1,5-a]pyridine and benzimidazole, have been investigated by single-crystal X-ray diffraction analysis, and the results show that the different positions of the N atoms affect the stacking of the crystals, whereas the molecules are connected mostly through intermolecular π–π stacking and C–H⋯N and C–H⋯π interactions. Tight π-stacking contacts lead to a decrease in the fluorescence quantum yield of the molecules. Also, the shortened distance between imidazolepyridine or benzimidazole units and the strong aromatic interactions from polar substituents lead to solid-state fluorescence quenching. With the help of Hirshfeld surface analysis and electrostatic potential analysis, the roles of various noncovalent forces including hydrogen bonding, π-stacking, and C–H⋯π interactions in the formation of solid-state molecules are elucidated. The redshift of the emission spectra in solution with increasing solvent polarity and the apparent separation of the electron cloud densities between the HOMO and LUMO orbitals suggest that all seven compounds are typical intramolecular charge transfer (ICT) molecules. The HOMO–LUMO energy gap results are consistent with the fluorescence emission behavior of the compounds in solution and solid states.