Exceptional Enhancement of Optical Anisotropy Achieved via the Strategy of Combining Rigid Groups with High Symmetry and π‐Conjugated Organic Groups in Hybrid Fluorides

Abstract Birefringent crystal materials used for modulating light polarization are of great significance in optical communication applications. According to the numerous reported organic–inorganic hybrid optical crystal materials, adopting π‐conjugated groups with high polarizability anisotropy is deemed to an effective strategy for constructing excellent birefringent crystals, while how to make the organic groups align in parallel to produce superimposed optical anisotropy remains a challenge. Here, an effective strategy of combining the inorganic rigid spherical group MF 6 (M = Si, Ge, Ga, Al) is implemented with the organic π‐conjugated group to promote coplanar alignment, and obtained two organic–inorganic hybrid fluorosilicate birefringent crystals, (C 6 H 5 N 2 ) 2 SiF 6 and (C 10 H 10 N 2 )SiF 6 . It's worth noting that their crystal structures are constructed by organic cations and [SiF 6 ] 2− anions with N‐H···F hydrogen bonds as bridges. The organic cations in both compounds are arranged in parallel in multiple directions under the guidance of the rigid group SiF 6 units. This optimal arrangement enables them to exhibit excellent optical anisotropy. (C 10 H 10 N 2 )SiF 6 shows the largest experimental birefringence (Δ n ) of 0.583@546 nm among known fluorides containing rigid groups. (C 6 H 5 N 2 ) 2 SiF 6 also exhibits a superior birefringence (Δ n exp. = 0.505@546 nm), far outperforming all current commercial birefringent crystals, which make them excellent candidates for birefringent crystal materials.