Modulating Elasticity and Plasticity through Halogen Substitution: Insights from Crystal Engineering

Abstract

Modulating the mechanical properties of molecular crystals via crystal engineering is gaining increasing attention in materials research. In this study, three single crystals of pyridine derivatives, 2,3,5-trichloropyridine (Cry-B), 2,5-dichloro-3-bromopyridine (Cry-E), and 2,3-dibromo-5-chloropyridine (Cry-P), were prepared, and their mechanical properties were systematically evaluated. These crystals possess highly similar molecular structures but totally distinct macroscopic mechanical responses. Specifically, Cry-B exhibits brittle behavior, while Cry-E and Cry-P display distinct elastic and plastic deformations, respectively. Single-crystal X-ray diffraction (SC-XRD) and powder X-ray diffraction (P-XRD) analyses revealed differences in molecular packing, which correlate with mechanical performance. Energy framework analysis and micro-Raman spectroscopy were utilized to understand the molecular interactions and structural changes during deformation. It is found that the variations in mechanical behavior correlate with their respective primary structural mechanisms such as the interlocking structure necessary for elastic bending and the slip plane structure for plastic deformation. Furthermore, nanoindentation tests quantified the hardness and elastic moduli, with Cry-B showing the highest stiffness and Cry-P being the most deformable. Hirshfeld surface analysis highlighted the significant role of halogen interactions in controlling crystal flexibility. This study examined the relationship between crystal structure and mechanical properties and highlights the significant potential of crystal engineering to tune mechanical properties via weak intermolecular interactions.