Halogen Bond Unlocks Ultra‐High Birefringence

Abstract Anisotropy is crucial for birefringence (Δ n ) in optical materials, but optimizing it remains a formidable challenge (Δ n >0.3). Supramolecular frameworks incorporating π‐conjugated components are promising for achieving enhanced birefringence because of their structural diversity and inherent anisotropy. Herein, we first synthesized (C 6 H 6 NO 2 ) + Cl − ( NAC ) and then constructed a halogen‐bonded supramolecular framework I + (C 6 H 4 NO 2 ) − ( INA ) by halogen aliovalent substitution of Cl − with I + . The organic moieties are protonated and deprotonated nicotinic acid ( NA ), respectively. The antiparallel arrangement of birefringent‐active units in NAC and INA leads to significant differences in the bonding characteristics between the interlayer and intralayer domains. Moreover, the [O⋅⋅⋅I + ⋅⋅⋅N] halogen bond in 1D [I + (C 6 H 4 NO 2 ) − ] chain exhibits stronger interactions and stricter directionality, resulting in a more pronounced in‐plane anisotropy between the intrachain and interchain directions. Consequently, INA exhibits exceptional birefringent performance, with a value of 0.778 at 550 nm, twice that of NAC (0.363 at 550 nm). This value significantly exceeds those of commercial birefringent crystals, such as CaCO 3 (0.172 at 546 nm), and is the highest reported value among ultraviolet birefringent crystals. This work presents a novel design strategy that employs halogen bonds as connection sites and modes for birefringent‐active units, opening new avenues for developing high‐performance birefringent crystals.