Hydrogen-Bond-Directed Assembly of Urea Derivatives: Exceptional Birefringence Achieved by π-Conjugated Unit...

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(NH₂)₂H₂C₂O₂ (UCO), C(OH)(NH₂)₂NO₃ (UNO), [C(OH)(NH₂)₂]₂SiF₆ (USF-1), and [C(OH)(NH₂)₂]₂CO(NH₂)₂SiF₆ (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 ([H₂C₂O₂], [NO₃]^-, [SiF₆]^{2 -}) reconstruct the hydrogen-bonding network, steering the planar [CO(NH₂)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.