Peroxide-Assisted Solvate Engineering Enables Record-High Birefringence in a Solar-Blind Transparent Crystal

Developing birefringent materials that simultaneously exhibit short ultraviolet (UV) cutoff edges and large birefringence is highly desirable but remains challenging. Here, we report a peroxide-containing solvate crystal, 4HP·H₂O₂ (4HPO2, 4HP = 4-hydroxypyridine), obtained by incorporating H₂O₂ into the 4HP lattice through a mild aqueous-solution method. 4HPO2 exhibits a solar-blind UV cutoff edge at 278 nm together with a giant experimental birefringence of 0.609@546 nm, representing an approximately 14-fold enhancement compared with pristine 4HP. Structural analysis reveals that H₂O₂ induces a pronounced reorganization of the 4HP packing, converting a nearly orthogonal arrangement into an ordered, nearly parallel alignment through strengthened hydrogen-bonding interactions. Combined experimental and theoretical analyses indicate that the aligned 4HP sublattice serves as the dominant source of optical anisotropy, whereas H₂O₂ mainly acts as a hydrogen-bond-directed structural modulator with a secondary direct optical contribution. The reordered packing also creates an anisotropic local-field environment that further amplifies the molecular polarizability anisotropy (Δα). These findings highlight peroxide-assisted solvate engineering as an effective strategy for tuning packing and optical anisotropy and provide a promising design principle for developing ultraviolet birefringent materials.