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

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 [NO₂]^-, [COOH]^-, methylguanidine [C₂N₃H₈]⁺, and guanylurea [C₂N₄H₇O]⁺ groups with large anisotropic polarization. Driven by hydrogen bonding, two crystals, [C₂N₃H₈]NO₂ and [C₂N₄H₇O]COOH, with ideal linear ordering were synthesized with the birefringence markedly enhanced to 0.331 and 0.413@546 nm, respectively. In particular, the [C₂N₄H₇O]COOH 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.