Ultra‐High Optical Anisotropy with UV Transmission Achieved by Rational Arrangement of Extended π‐Conjugated Groups

Abstract Birefringent material serves as a cornerstone in photonic applications, including optical communications, polarization control, and laser technologies. The development of birefringent materials with large birefringence (Δ n > 0.3) and short ultraviolet (UV) cut‐off edge ( λ cut‐off edge < 400 nm) remains a significant challenge. Here, we demonstrate that properly aligned expanded π‐conjugated groups provide a solution to balance the birefringence and UV transmittance. We report a new birefringent material, Li 3 (C 9 N 13 )·6H 2 O ( LCN ), in which the Li atoms and water molecules act as linkers to connect the birefringence‐active group [C 9 N 13 ]. This crystal material exhibits a giant optical anisotropy (Δ n exp  = 1.031 @ 546 nm), which is one of the highest among bulk crystal materials known to date. In addition, LCN shows a band gap of 3.62 eV, indicating its applicability in the UV optical range. Owing to favorable Li cation linkers and hydrogen bonding, the [C 9 N 13 ] groups achieve a perfectly coplanar arrangement, thereby maximizing the optical anisotropy. This work offers a novel strategy for the rational design of advanced birefringent materials.