Unit Symmetry‐Breaking and Layered Linear Ordering Enabling Superior Short‐Wave Ultraviolet Birefringent Crystal

Abstract Short‐wave ultraviolet (200–280 nm) crystals with large birefringence are important but scarce for polarization state modulation. The optical anisotropy in crystals is significantly influenced by the spatial arrangement of microscopic functional units. Herein, a design concept of constructing two‐dimensional and linear arrangement structures is proposed to achieve large birefringence. By employing a symmetry‐breaking strategy, commonly used triangular π‐conjugated groups were extended to NO 2 − , COOH − , methylguanidine C 2 N 3 H 8 + , and guanylurea C 2 N 4 H 7 O + groups with large anisotropic polarization. Driven by hydrogen bonding, two crystals, C 2 N 3 H 8 NO 2 and C 2 N 4 H 7 OCOOH, with ideal linear ordering were synthesized with the birefringence markedly enhanced to 0.331 and 0.413@546 nm, respectively. In particular, the C 2 N 4 H 7 OCOOH crystal achieves a rare balance between a wide bandgap and superior birefringence in the short‐wave ultraviolet region. Moreover, centimeter‐sized single crystals were successfully obtained. This work not only provides new ideas for designing birefringent crystals but also offers promising candidate materials for optical modulation.