Sustainable mechanochemical approach for the selective synthesis of multicomponent organic solids: real-time in situ insights

Crystalline multicomponent organic solids (MOSs) such as cocrystals and ionic cocrystals hold immense potential in diverse functional applications, ranging from pharmaceuticals to optoelectronics. However, conventional solution-based crystallization methods often result in polymorphic mixtures and lack precise control over product composition. Herein, we report a comparative investigation of solution crystallization versus mechanochemical synthesis for constructing MOSs from 9-anthracenecarboxylic acid (ACA) and 4,4′-bipyridine (BPY). Solution-based approaches consistently yielded concomitant formation of neutral cocrystal (CC) and ionic cocrystal (ICC) forms, regardless of the solvent used. The resulting multicomponent solids were comprehensively characterized using a combination of single crystal X-ray diffraction, powder X-ray diffraction, Fourier-transform infrared spectroscopy, differential thermal analysis, and thermogravimetric analysis. In contrast, mechanochemical methods, including neat grinding (without solvent) and liquid-assisted grinding (with minimum solvent), enabled selective formation of either a phase pure CC or ICC form. Less polar and nonpolar organic solvents favor the kinetic CC, while polar water promotes formation of the thermodynamically stable ICC. Time-resolved in situ powder X-ray diffraction (TRIS-PXRD) captures the dynamic evolution of solid-state phases and reveals the complete transformation of the CC into ICC under neat grinding or water-assisted conditions. This study highlights the powerful role of mechanochemistry and in situ monitoring in steering solid-state reactivity and offers a sustainable pathway for the targeted and scalable synthesis of pure multicomponent organic materials.