Rational Construction of Fluoride Crystals With Record Birefringence via Water‐Regulated Hydrogen‐Bond Network Reconfiguration

ABSTRACT Birefringent crystals are central to optical polarization modulation. However, combining wide bandgap and ultrahigh birefringence (Δ n ≥ 1.0) is challenging due to the trade‐off between optical anisotropy and transparency. Herein, we report three molecular crystals: (C 10 H 10 N 2 )SiF 6 , (C 12 H 9 N 2 ) 2 SiF 6 , and (C 12 H 9 N 2 ) 2 SiF 6 ·2H 2 O, which are constructed from organic π‐conjugated units and rigid inorganic [SiF 6 ] 2− units via a hydrogen‐bond‐mediated co‐assembly strategy. Structural analysis reveals the organic moieties of the three compounds adopt parallel arrangements via synergistic [SiF 6 ] 2− skeletons and hydrogen bonds. Notably, water incorporation in (C 12 H 9 N 2 ) 2 SiF 6 ·2H 2 O reconstructs the hydrogen‐bond network into a binary N–H···O/O–H···F system, prompting [C 12 H 9 N 2 ] + cations to align more coplanarly relative to the optical axis plane, which endows (C 12 H 9 N 2 ) 2 SiF 6 ·2H 2 O with an outstanding birefringence of 1.121@546 nm. Although the highly delocalized π‐electron system of the [C 12 H 9 N 2 ] + cation leads to smallest bandgap of (C 12 H 9 N 2 ) 2 SiF 6 ·2H 2 O among the three materials, its bandgap of 3.21 eV remains the highest among all hybrid halides with Δ n ≥ 1.0. First‐principles calculations confirm that this exceptional performance originates from the synergy between the organic π‐conjugated modules and the inorganic units, mediated by the multiple hydrogen‐bond networks. This work sets a new performance benchmark for fluoride birefringent materials and inspires the molecular engineering design of high‐performance ultraviolet optical crystals.