Molecular tiltation and supramolecular interactions induced uniaxial NTE and biaxial PTE in bis-imidazole-based co-crystals

Abstract

Variable temperature single crystal X-ray diffraction (VT-SCXRD) study reveals an unusual thermal expansion behaviour of two new co-crystals prepared by solvent drop grinding using a flexible ligand [1,4-bis[(2-methylimidazol-1-yl)methyl]benzene] (BIMB) and aromatic carboxylic acids, viz. terephthalic acid (TA) and 4,4′-biphenyl dicarboxylic acid (BPDCA). The analysis of the crystal structures of the co-crystals using VT-SCXRD shows that the tiltation of the carboxylic acid molecules along a certain crystallographic axis induces uniaxial negative thermal expansion (NTE). In this study, supramolecular interactions, such as O–H⋯N, C–H⋯O, C–H⋯π, π⋯π, and van der Waals interactions, play an important role in the biaxial positive thermal expansion (PTE) in both the co-crystals.