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.