Synergistic Enhancement of Birefringence in Metal Benzoxazolium Complexes by π-Conjugated [C ₉ H ₁₀ NO] ⁺ and Triangular [HgI ₃ ] ⁻ Motifs

Birefringent crystals can manipulate polarized light for lasers and precision instrumentation, and the excellent properties of typical ultraviolet birefringent crystals, e.g., BBO and CaCO₃, benefit from triangular planar π-conjugated groups such as (BO₃)³⁻ and (CO₃)³⁻. In order to expand the applicable wavelength range of birefringent crystals to the mid-infrared region, d¹⁰ configuration metal (Zn, Cd, Hg) iodides were combined with planar π-conjugated benzoxazolium (C₉H₁₀ON)⁺ groups, and three new hybrid birefringent crystals, i.e., (ZnI₄)(C₉H₁₀NO)₂ (I), (CdI₄)(C₉H₁₀NO)₂ (II), and (HgI₃)(C₉H₁₀NO) (III), were successfully synthesized by solution methods. Attributable to the synergistic enhancement of conjugated (C₉H₁₀ON)⁺ and near-parallel alignment of the triangular [HgI₃]⁻ units, compound III exhibits an obviously larger birefringence of 0.154 at 1060 nm than compounds I and II constructed from tetrahedrally coordinated [ZnI₄]²⁻/[CdI₄]²⁻. Further, the origin of anisotropic optical properties was elucidated via first-principles calculations. This study establishes that integrating the planar [HgI₃]⁻ unit through an organic-inorganic hybrid approach can effectively enhance birefringence, providing a feasible design strategy for mid-infrared birefringent crystals.