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