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

Abstract Birefringent and nonlinear optical (NLO) crystals are crucial for advanced optical applications, yet achieving large birefringence (Δ n ), 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 Δ n and suitable second–harmonic generation (SHG) responses. Notably, II [C 5 H 6 NO] + (HF 2 ) − exhibits the largest Δ n (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.