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.