Unveiling the Superior Birefringence Property of a Record-Birefringent Crystal with Unique Building Block

Despite being crucial for various advanced optical components, obtaining optically anisotropic crystals with giant birefringence (e.g., Δn ≥ 1.0) remains a formidable challenge. Prevailing strategies typically rely on enhancing the π-delocalization of molecular building blocks by cyclic polymerization. Here, we demonstrate a new approach-simply connecting the π-conjugated rings along a specific direction via linear polymerization. This concept is well exemplified by a novel hybrid crystal, BIQCdCl₄ (BIQ = C₁₈H₁₄N₂, the protonated form of 2,2'-biquinoline). Although in accordance with Hückel's rule, the π-electrons within two quinoline subunits are not fully delocalized over the entire BIQ molecule, this compound exhibits a record-high birefringence value of Δn_exp = 1.017@546 nm, surpassing all commercial birefringent crystals and many newly developed birefringent crystals, thereby setting a new benchmark for such materials. Detailed electronic structure investigation indicates that while the C─C linkage limits π-delocalization extension, linear polymerization enhances the disparity in the in-plane (X, Y) π-electron distribution, thereby maximizing the polarizability contrast between two specific polarization directions (i.e., Y and Z). This leads to exceptional polarizability anisotropy of the BIQ molecules, which, combined with their uniform alignment directed by [CdCl₄] tetrahedra and hydrogen bonding, ultimately results in the giant birefringence of BIQCdCl₄.