Halogen-Bond Dimensionality Controls π-π Packing to Unlock Ultrahigh Second-Harmonic Generation and Giant Birefringence.

Compact all-solid-state lasers require nonlinear optical crystals that combine strong second-harmonic generation (SHG) with large birefringence, yet planar π-conjugated molecules often pack antiparallel, canceling their macroscopic second-order response. Here, we break this trade-off through dimensional competition between halogen bonding and π-π stacking in two nitroazole-based organic frameworks. The parent (IClPzNO2)2·H2O forms a centrosymmetric two-dimensional framework with coplanar chromophores and a birefringence of 0.705 at 546 nm. Combined skeletal and halogen substitutional editing, from pyrazole (Pz) to imidazole (Im) and from an ortho-I/Cl to a meta-I/I substitution pattern, redirects in-plane hydrogen-bonding aggregation via relocating the N─H donor, and then extends C─I···O connectivity from two to three dimensions. The resulting halogen-bonded network governs chromophore orientation and produces a twofold-interpenetrated polar framework, (I2ImNO2)2·H2O (Pna21). The crystal exhibits a phase-matchable powder SHG response 20 times that of KDP (KH2PO4), the largest value among five-membered-ring and nitro/nitrate NLO crystals, as well as a giant birefringence of 0.554 at 546 nm. To the best of our knowledge, this combination occupies a previously unreported performance regime. These results identify interaction dimensionality as a practical design parameter for aligning polar π-conjugated molecules and combining strong SHG with giant optical anisotropy.