Enhanced Birefringence in Hybrid Antimony Halides via π-Conjugated Organic Cation Design

A π-conjugated organic cation engineering strategy to enhance birefringence has been adopted in hybrid antimony halides. By leveraging the polarizability anisotropy of organic cations, we replaced a monocyclic [C6H7N2O]+ cation with a bicyclic [C8H6IN2O]+ species. The resulting compound, (C8H6IN2O)SbBr4 (II), exhibits a birefringence of 0.35 at 546 nm, which is more than 5 times higher than that of its monocyclic analogue (C6H7N2O)SbBr4 (I) (0.07@546 nm). Structural and theoretical analyses show that this improvement arises from the combination of stronger optical anisotropy and more favorable molecular alignment. This study demonstrates that organic cation screening is an effective strategy for the rational design of high-performance birefringent materials.