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

Abstract Despite being crucial for various advanced optical components, obtaining optically anisotropic crystals with giant birefringence (e.g., Δ n ≥ 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, BIQCdCl 4 (BIQ = C 18 H 14 N 2 , 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 Δ n 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 [CdCl 4 ] tetrahedra and hydrogen bonding, ultimately results in the giant birefringence of BIQCdCl 4 .