Hydrogen‐Bond‐Directed Assembly of Urea Derivatives: Exceptional Birefringence Achieved by π ‐Conjugated Unit Alignment

ABSTRACT The rational design of optical crystals with large birefringence, deep‐ultraviolet (DUV) transparency, and strong nonlinear optical effects remains a significant challenge, primarily due to the difficulty in precisely controlling the alignment of functional anisotropic units within the crystal lattice. Herein, we propose and demonstrate a hydrogen‐bond‐directed assembly strategy to regulate the packing density and orientation of π ‐conjugated cations in urea derivatives, achieving a synergistic enhancement of optical properties. Four novel urea derivative crystals, CO(NH ₂ ) ₂ H ₂ C ₂ O ₂ (UCO), C(OH)(NH ₂ ) ₂ NO ₃ (UNO), [C(OH)(NH ₂ ) ₂ ] ₂ SiF ₆ (USF‐1), and [C(OH)(NH ₂ ) ₂ ] ₂ CO(NH ₂ ) ₂ SiF ₆ (USF‐2), were synthesized. Among them, UNO exhibits a record‐high birefringence of 0.372 at 546 nm within the urea family, while USF‐1 and USF‐2 achieve DUV transparency with cutoff edges below 200 nm (193 and 195 nm, respectively). Structural and theoretical analyses reveal that the introduced groups ([H ₂ C ₂ O ₂ ], [NO ₃ ] ⁻ , [SiF ₆ ] ^{2 −} ) reconstruct the hydrogen‐bonding network, steering the planar [CO(NH ₂ ) ₂ ] from the vertical and antiparallel arrangement found in pristine urea into a highly parallel alignment within layered structures. This significantly enhances macroscopic optical anisotropy. This work establishes a hydrogen‐bond engineering paradigm for the rational design of high‐performance UV/DUV optical crystals by controlling π ‐conjugated unit assembly.