Enhancement of Birefringence in π‑Conjugated Oxyfluorides Achieved via van der Waals Layered Structural Optimization

The development of efficient methods for producing novel birefringent materials has become a significant and urgent undertaking. Herein, we propose to use organic units as a structural guide to obtain organic–inorganic hybrid structures, where oxyfluoride units are used to optimize the arrangement of organic motifs, resulting in an enhanced optical anisotropy of the structure. Two novel C­(NH2)3 +-containing oxofluorovanadates, [C­(NH2)3]3V2O4F5 (Gu3V2O4F5) and [C­(NH2)2NHCH2COOH]2VOF5 (guanidinoacetic acid-VOF5, GA2VOF5) with dimensions up to 5 × 4 × 0.1 and 5 × 3 × 1 mm3, have been successfully designed and grown via facile reactions. Remarkably, Gu3V2O4F5 features an optimized two-dimensional (2D) van der Waals layer structure. It shows a birefringence of 0.35 at 546 nm (cal.); this is larger than most previously reported birefringences of C­(NH2)3 +-based organic–inorganic compounds. Additionally, it possesses an ultraviolet cutoff edge. GA2VOF5 is the first GA-containing oxofluorovanadate with a birefringences of 0.066 at 546 nm (cal.), which was verified experimentally. Theoretical calculations show that the remarkable birefringence observed in the title compounds primarily arises from the incorporation of π-conjugated units and [VO x F6–x ] (x = 1 and 2) groups. Our study further confirms the effectiveness of structural optimization in the design and engineering of structural-driven functional materials.