Halide Ion-Directed Symmetry Control in π-Conjugated Molecular Crystals

Two halide-containing π-conjugated molecular crystals, (C₅H₇BNO₂)Cl and (C₅H₇BNO₂)Br, were synthesized using pyridine-4-boronic acid and halide ions of differing ionic radii. (C₅H₇BNO₂)Cl adopts a noncentrosymmetric (NCS) structure and exhibits SHG efficiency comparable to KH₂PO₄, along with a wide UV transparency window, making it promising for UV NLO applications. In contrast, (C₅H₇BNO₂)Br crystallizes in a centrosymmetric (CS) structure, exhibiting enhanced birefringence. Structural and theoretical analyses reveal that the halide ions influence molecular packing and coplanarity, thereby tuning the symmetry and optical properties. This work demonstrates a halogen-engineering strategy for symmetry control and optical property modulation in π-conjugated molecular crystals, offering a viable pathway for the rational design of high-performance UV NLO materials.