Protonation‐Triggered Structural Transformations in 4‐Hydroxypyridinium Halides for Solar‐Blind UV Functional Crystals

Birefringent and nonlinear optical (NLO) crystals are crucial for advanced optical applications, yet achieving large birefringence (Δn), noncentrosymmetric (NCS) structures, and broad transparency from solar-blind ultraviolet (SUV) to near-infrared (NIR) regions remains challenging. Here, we report a 4HPX family of twelve 4-hydroxypyridinium halides, constructed from 4-hydroxypyridinium cations [4HPH]⁺ and halide anions [X]^- (X = F (I, II), Cl (III, VII, X), Br (IV, VIII, XI), I (V, VI, IX, XII)). Solution acidity dictates the structural transformations of this family, from centrosymmetric (I-IX) to NCS (X-XII) phases, where C─O bond lengths act as definitive structural fingerprints. Their optical bandgaps exceed those of all known halogen polyanion-based crystals due to the large HOMO-LUMO gaps of both [4HPH]⁺ and [X]^- and can be systematically tuned via hydrogen-bond strength. The large hyperpolarizability and polarizability anisotropy of [4HPH]⁺ endow this family with exceptional Δn and suitable second-harmonic generation (SHG) responses. Notably, II [C₅H₆NO]⁺(HF₂)^- exhibits the largest Δn (0.293) within the 4HPX family and among reported water-free SUV crystals. Meanwhile, I-V (0.222-0.293) also outperform commercial birefringent materials. NLO crystals X-XII show SHG responses. This work provides valuable insights into the rational design of next-generation birefringent and NLO crystals.