Dual‐Sided Multidentate Coordination Strategy Enables Record Birefringence in UV‐Transparent Antimony‐Based Hybrid Crystals

Abstract Achieving ultrahigh birefringence in UV‐transparent materials remains fundamentally constrained by the trade‐off between strong optical anisotropy and wide bandgap transparency. Herein, we report a dual‐sided multidentate coordination (DMC) strategy to construct two butterfly‐shaped UV organic–inorganic hybrid crystals—(C 6 H 4 NO 2 ) 2 SbF (PCSF) and (C 10 H 6 NO 2 ) 2 SbF (QCSF)—in which planar π‐conjugated bidentate ligands symmetrically chelate stereochemically active lone pair (SCALP) Sb 3+ centers. This coordination architecture enforces coplanar alignment of optical functional units and promotes dense π–π stacking, thereby significantly enhancing macroscopic birefringence. Notably, QCSF achieves a record‐high birefringence of Δ n = 0.87 at 546 nm, surpassing all previously reported lone‐pair‐containing halide crystals with UV transparency. The exceptional optical performance is attributed to the unique [SbN 2 O 2 F] coordination geometry, near‐planar molecular configuration, and extended π‐electron delocalization. First‐principles calculations reveal that the observed anisotropy stems from synergistic orbital coupling between the Sb centers and the π‐conjugated organic ligands. This work introduces a broadly applicable molecular design paradigm for next‐generation birefringent crystals that simultaneously offer high optical anisotropy and UV transparency.