Overcoming the Bandgap-Birefringence Trade-Off: Proton-Transfer Engineering of High-Performance Ultraviolet-Transparent Organic Crystal.

Birefringent crystals are key materials for controlling the polarization state of light, and breakthroughs in their performance are crucial for advanced optical devices. In this study, π-conjugated groups [C₇N₂H₁₁]⁺ and [C₅N₄H]⁻ with high polarizability anisotropy (Δα), are constructed via a proton transfer strategy. The Δα of these groups is significantly higher than that of the corresponding neutral molecules. Based on this design, two centimeter-sized crystals are successfully prepared: [(C₇N₂H₁₁)⁺(C₅N₄H)⁻]·C₇N₂H₁₀·H₂O (birefringence Δn = 0.233 @ 546 nm, band gap 3.66 eV) and [(C₇N₂H₁₁)⁺(C₅N₄H)⁻]·H₂O (birefringence Δn = 0.629 @ 546 nm, band gap 3.79 eV). [(C₇N₂H₁₁)⁺(C₅N₄H)⁻]·H₂O exhibits the highest birefringence among organic crystals with a wide band gap (> 3.0 eV). Structural analysis and theoretical calculations indicate that the inherently high Δα of [C₇N₂H₁₁]⁺ and [C₅N₄H]⁻, combined with relatively small intermolecular dihedral angles between adjacent functional groups in the crystal, contribute synergistically to the excellent birefringence properties. Additionally, [C₅N₄H]⁻ stabilizes water molecules through hydrogen-bond networks, enhancing the material's air stability. This work provides a new strategy for designing high-performance birefringent crystals based on proton transfer and π-conjugated groups with high polarizability anisotropy.