[CON₃H₆BF₄]: An Efficient Metal-Free Borate with Optimized Birefringence Achieved by Structural Design Strategy

Deep-ultraviolet (UV) birefringent materials are urgently needed for advancing light polarization in deep-UV lithography. In crystal engineering, maximizing anisotropy by classical template alteration demonstrates its efficiency in achieving superior performance characteristics. Consequently, a novel planar deep-UV functional building unit (FBU), [CON₃H₅], is proposed, acquired by amino modulation within the urea template. This article presents a methodical investigation of novel fluoroborate [(CON₃H₆)BF₄] (AUBF), designed by unifying a planar π-conjugated functional cation with a fully fluorinated non-π-conjugated tetrahedron in a single system. The compound exhibits well-balanced properties, owing to its parallel molecular arrangement, and has been characterized as a high-performing deep-UV birefringent material. As is known to all, it is the first semi-organic urea-based compound to reach the deep-UV region with a cutoff edge at 196 nm, and a substantial birefringence of 0.127@546 nm, similar to that of the previously reported urea-based compounds as well as commercial birefringent crystal α-BBO (0.123@546 nm). This work not only identifies a novel birefringent gene with improved optical anisotropy but also opens the door for synthesizing novel deep-UV birefringent materials via a structural regulation design strategy.