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

ABSTRACT 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 [C 7 N 2 H 11 ] + and [C 5 N 4 H] − 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: [(C 7 N 2 H 11 ) + (C 5 N 4 H) − ]·C 7 N 2 H 10 ·H 2 O (birefringence Δ n = 0.233 @ 546 nm, band gap 3.66 eV) and [(C 7 N 2 H 11 ) + (C 5 N 4 H) − ]·H 2 O (birefringence Δ n = 0.629 @ 546 nm, band gap 3.79 eV). [(C 7 N 2 H 11 ) + (C 5 N 4 H) − ]·H 2 O 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 [C 7 N 2 H 11 ] + and [C 5 N 4 H] − , combined with relatively small intermolecular dihedral angles between adjacent functional groups in the crystal, contribute synergistically to the excellent birefringence properties. Additionally, [C 5 N 4 H] − 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.