Halogen‐Bond Dimensionality Controls π–π Packing to Unlock Ultrahigh Second‐Harmonic Generation and Giant Birefringence

Abstract

ABSTRACT 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 (IClPzNO 2 ) 2 ·H 2 O 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, (I 2 ImNO 2 ) 2 ·H 2 O ( Pna 2 1 ). The crystal exhibits a phase‐matchable powder SHG response 20 times that of KDP (KH 2 PO 4 ), 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.