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

ABSTRACT

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 ·H ₂ O ₂ ( 4HPO2 , 4HP = 4‐hydroxypyridine), obtained by incorporating H ₂ O ₂ 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 H ₂ O ₂ 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 H ₂ O ₂ 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.