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.
Qiu et al. (Thu,) studied this question.