Halide-Dependent Structure and Pressure-Induced Electronic Evolution in DABCO-Stabilized Polyiodide Frameworks

Abstract

We report a combined experimental and computational investigation of four polyhalide salts based on 1,4-diazabicyclo[2.2.2]octane (DABCO) that highlights the structural and electronic tunability of polyiodide materials through halide substitution and external pressure. Three isomorphous framework compounds [DABCOH2][I3][I2X] (X = I, Br, Cl) and one molecular compound [DABCOH][I2Cl] were obtained and characterized by X-ray diffraction, Raman spectroscopy, high-pressure crystallography, and periodic DFT calculations. In the framework series, halogen bonding and hydrogen bonding govern the assembly of the polyhalide sublattice, while selective replacement of the terminal halide modulates the nature of the I2X– unit. Electron density analyses and vibrational data show that X···I interaction evolves from predominantly noncovalent (X = Cl) to increasingly covalent (X = Br, I). By contrast, the formation of monoprotonated DABCO disrupts the extended framework and gives rise to a distinct 1D supramolecular architecture sustained by hydrogen, halogen, and tetrel bonding. Compounds are predicted to be small gap semiconductors, with band edges dominated by iodine-derived states. Upon compression, progressive band gap narrowing and enhanced band dispersion reveal increasing electronic delocalization, and calculations predict pressure-induced metallization above ca. 50 GPa. These results identify polyhalide frameworks as responsive solids whose bonding and electronic structure can be finely controlled through composition and pressure.