Exploring non-covalent interactions in binary aromatic complexes

Crystal structure prediction for systems governed by weak non-covalent interactions remain a significant challenge due to the complex energy landscapes involved. Herein, we have experimentally investigated the impact of systematic halogen substitution in fluorinated aromatic co-formers on the formation, structure, and phase behaviour of donor-acceptor adducts and co-crystals with <i>p</i>-xylene (<i>p</i>-C₆H₄Me₂). Using a combined approach of differential scanning calorimetry (DSC), variable-temperature powder X-ray diffraction (VT-PXRD), and single-crystal X-ray diffraction (SXD), we have characterized a series of co-crystals formed by <i>p</i>-C₆H₄Me₂ with C₆F₅X (X = Cl, Br, I) and <i>p</i>-C₆F₄X₂ derivatives. Our results revealed a clear evolution from columnar π-stacked adducts in the Cl-substituted systems to halogen-bonded structures with the heavier halogens (Br, I). The columnar 1 : 1 adducts exhibit complex solid-state phase behaviour linked to molecular dipole and steric effects, whereas co-crystals involving Br and I show simpler behaviour, with discrete η² and η⁶ halogen-π interactions both being observed. In one instance, a 1 : 2 co-crystal was formed with antiferroelectric ordering requiring halogen bonding to <i>p</i>-C₆H₄Me₂ from two C₆F₅I molecules. The results underscore the tunability of solid-state architectures through targeted halogen substitution to probe subtle non-covalent interactions. In summary, this work advances our understanding of weak intermolecular forces in crystalline materials and provides data for the predictive design of functional co-crystals.