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 [VOxF6–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.