V-Shaped Pseudo-Chains Coupled with Lone-Pair-Distorted [GeX ₃ ] ⁻ Units Enable Large Birefringence in Ge(II) Hybrid Halides

Birefringent crystals are key materials for polarization and phase control, yet achieving large Δn remains challenging. Here we report two Ge(II)-based organic–inorganic hybrid halides, (H4biimCl)GeCl3 and (H4biimBr)GeBr3 (H2biim = 2,2′-biimidazole). Single-crystal X-ray diffraction shows that protonated (H4biim)2+ units form V-shaped hydrogen-bonded cationic pseudochains via N–H···X hydrogen bonds to a noncoordinating halide anion, while lone-pair-distorted trigonal-pyramidal [GeX3]− units reside between the chains, creating pronounced anisotropy. The compounds exhibit large birefringence values of Δn = 0.263 (Cl) and 0.302 (Br) at 546 nm, in good agreement with first-principles calculations (0.285 and 0.336). Halide substitution (Cl → Br) enhances the polarizability anisotropy of [GeX3]− and increases Δn. These results highlight a viable motif for birefringent crystals by coupling hydrogen-bond-directed π organization with Ge(II) lone-pair distortion.