Halogen and Hydrogen Bonding Interplay Triggered Reversible Phase Transformations through Water Release/Uptake: X-ray Powder Diffraction Studies on Halogen-Substituted 1,4-Bis(4,5-diphenyl-imidazolyl)benzene

The inclusion of water in organic crystals is more frequent compared to other solvents, given its smaller size and excellent ability of hydrogen bonding with organic molecules. This work addresses reversible water absorption and desorption by halogen-substituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene (Cl-BDPI and Br-BDPI) in its as-synthesized crystalline forms. The as-synthesized hexahydrates of these molecules were found to convert to di- or monohydrates upon heating, which can revert back to hexahydrates upon exposure to water vapors. Further heating of these di- or monohydrates resulted in amorphous anhydrates, which cannot be revived to the crystalline hydrates. The reversible transformations between the hydrates were further characterized by variable-temperature powder X-ray diffraction (VT-PXRD), variable-temperature Fourier transform infrared (VT-FTIR) spectroscopy, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The analyses of the crystal structures of the hydrates reveal that the water molecules in hexahydrates bind to the X-BDPI via hydrogen bonds, which can be released upon heating in favor of the formation of bifurcated halogen bonding between the X-BDPI molecules.