Organic Halogen Substitution-Driven Halogen Bonding Induces Polar 3D Supramolecular Framework and Second-Harmonic-Generation Response

The development of nonlinear optical (NLO) crystals with strong second-harmonic-generation (SHG) efficiency, large birefringence, and wide bandgaps is crucial for next-generation ultraviolet photonics. Here we report two new organic-inorganic metal halides, HPyXB(OH)₂CdCl₃ (X = Cl, Br; Py = pyridyl), that demonstrate how halogen substitution on organic ligands can effectively tune crystal symmetry and optical performance. Both compounds feature one-dimensional [CdCl₃]_∞^- chains constructed from face-sharing CdCl₆ octahedra. The chlorine-containing HPyClB(OH)₂CdCl₃ crystallizes in the centrosymmetric space group <i>P</i>1̅, forming a two-dimensional supramolecular structure with antiparallel π-conjugated cations. It shows a large birefringence of 0.22 at 546 nm and a wide bandgap of 4.08 eV. In contrast, bromine substitution induces C-Br···O halogen bonding and O-H···O hydrogen bonding, which drive HPyBrB(OH)₂CdCl₃ into a polar three-dimensional supramolecular framework in the noncentrosymmetric space group <i>Pna</i>2₁. This symmetry breaking unlocks a phase-matchable SHG efficiency of 1.0× KH₂PO₄ while retaining a large birefringence (0.16 at 546 nm) and a wide bandgap (3.81 eV). These results highlight halogen substitution as an effective strategy for designing high-performance ultraviolet NLO crystals.