Hybrid Zinc‒Based Halides with Giant Optical Anisotropy via Protonation‒Engineered <i>π</i> ‒Conjugated Organic Group

Abstract Birefringent crystals are indispensable for manipulating polarized light in optical technology; however, commercial ones exhibit relatively low birefringences, which restrict their widespread applications in diverse photonic fields. Herein, through protonation engineering strategy, two distinct birefringence “gene” with notable polarizability anisotropy, viz. monoprotonated organic (phenH) + cation featuring a slightly twisted spatial configuration and diprotonated (phenH 2 ) 2+ cation displaying perfect coplanarity, are identified. Their combination with the inorganic rigid [ZnCl 4 ] 2‒ tetrahedron yields two new organic–inorganic hybrid metal halides (phenH) 2 ZnCl 4 ·C 2 H 6 O 2 ( 1 ) and (phenH 2 )ZnCl 4 ( 2 ) (phen = C 12 H 8 N 2 ). In contrast to the monoprotonated organic (phenH) + cation in 1 , the diprotonated (phenH 2 ) 2+ cation in 2 displays an optimal spatial parallel arrangement, thereby effectively maximizing polarization anisotropy and exhibiting outstanding birefringence of 0.84 at 546 nm, surpassing that of 1 (0.80@546 nm) as well as all commercial birefringent crystals and previously reported ionic compounds containing d 10 metal cations. Theoretical and structural analyses reveal that the enhanced birefringences in both compounds originate from the cooperative effect of the ordered [ZnCl 4 ] 2− tetrahedra and the aligned π ‒conjugated organic FBUs. This study provides an effective strategy for developing high–performance birefringent materials.