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 Δnexp = 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.