Rational Construction of Fluoride Crystals With Record Birefringence via Water-Regulated Hydrogen-Bond Network...

Birefringent crystals are central to optical polarization modulation. However, combining wide bandgap and ultrahigh birefringence (Δn ≥ 1.0) is challenging due to the trade-off between optical anisotropy and transparency. Herein, we report three molecular crystals: (C₁₀H₁₀N₂)SiF₆, (C₁₂H₉N₂)₂SiF₆, and (C₁₂H₉N₂)₂SiF₆·2H₂O, which are constructed from organic π-conjugated units and rigid inorganic [SiF₆]^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 [SiF₆]^2- skeletons and hydrogen bonds. Notably, water incorporation in (C₁₂H₉N₂)₂SiF₆·2H₂O reconstructs the hydrogen-bond network into a binary N-H···O/O-H···F system, prompting [C₁₂H₉N₂]⁺ cations to align more coplanarly relative to the optical axis plane, which endows (C₁₂H₉N₂)₂SiF₆·2H₂O with an outstanding birefringence of 1.121@546 nm. Although the highly delocalized π-electron system of the [C₁₂H₉N₂]⁺ cation leads to smallest bandgap of (C₁₂H₉N₂)₂SiF₆·2H₂O among the three materials, its bandgap of 3.21 eV remains the highest among all hybrid halides with Δn ≥ 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.