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

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 (Δn > 0.3) and short ultraviolet (UV) cut‐off edge (λcut‐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, Li3(C9N13)·6H2O (LCN), in which the Li atoms and water molecules act as linkers to connect the birefringence‐active group [C9N13]. This crystal material exhibits a giant optical anisotropy (Δnexp = 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 [C9N13] 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.