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

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 MF6 (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, (C6H5N2)2SiF6 and (C10H10N2)SiF6. It's worth noting that their crystal structures are constructed by organic cations and [SiF6]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 SiF6 units. This optimal arrangement enables them to exhibit excellent optical anisotropy. (C10H10N2)SiF6 shows the largest experimental birefringence (Δn) of 0.583@546 nm among known fluorides containing rigid groups. (C6H5N2)2SiF6 also exhibits a superior birefringence (Δnexp. = 0.505@546 nm), far outperforming all current commercial birefringent crystals, which make them excellent candidates for birefringent crystal materials.