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.