Polymorphism in 3,4-bis(2-benzimidazolyl)pyridine: Controlled synthesis, crystal structures and photophysical properties

Two polymorphs of 3,4-bis(2-benzimidazolyl)pyridine (1) were prepared and characterized by infrared, nuclear magnetic resonance spectroscopy and single-crystal X-ray crystallography. The metastable greenish polymorph 1a (hexagonal space group P6522) was solvothermally formed first, which can be irreversibly transformed into the stable yellow polymorph 1b (orthorhombic space group Pbcn) on heating or recrystallization. The structure of 1a shows a dihedral angle of 45.3° between the benzimidazolyl/pyridyl rings and a dihedral angle of 52.2° between the benzimidazolyl/benzimidazolyl rings, while the corresponding dihedral angles in the structure of 1b are 48.0° and 67.9°, respectively. The two polymorphs exhibit different combination of intermolecular interactions (N–H⋯N hydrogen bonds, π–π stacking or C–H⋯π interaction), leading to significantly different supramolecular assemblies. The photophysical properties of 1a and 1b have also been studied.