Significant enhancement in the birefringence of metal phosphite halides via the introduction of π-conjugated cations

Birefringent crystals have received increasing attention due to their extensive applications in the field of modern optics. While rationally designing excellent birefringent materials still remains challengeable, the emergence of organic-inorganic hybrid birefringent crystals has opened up new possibilities for high-performance birefringent crystals by combining the advantages of organic and inorganic components. Herein, two organic-inorganic hybrid zinc phosphite halides, (C₄H₁₂N₂)[Zn₂(HPO₃)₂Cl₂]·2H₂O and (C₅H₇N₂)[Zn(H₂PO₃)Cl₂], have been successively synthesized. (C₄H₁₂N₂)[Zn₂(HPO₃)₂Cl₂]·2H₂O exhibits a three-dimensional (3D) structure constructed by unique hydrogen-bonded {[Zn₂Cl₂(HPO₃)₂(H₂O)₂]_∞}^2- lamellar, with non-π conjugated piperazine (PIP) cations (C₄H₁₂N₂)²⁺ serving as counter cations. (C₄H₁₂N₂)[Zn₂(HPO₃)₂Cl₂]·2H₂O reveals an ultraviolet (UV) cutoff edge below 200 nm and a small birefringence (0.01@546 nm) based on theoretical calculation. Considering the similar six-membered ring structures, we attempted to enhance birefringence performance by substituting non-π-conjugated piperazine with 4-aminopyridine featuring π-conjugation, and successfully obtained (C₅H₇N₂)[Zn(H₂PO₃)Cl₂]. It contains [Zn(H₂PO₃)Cl₂]^- anions constructed by [ZnO₂Cl₂]^4- and [H₂PO₃]^- units, which form a one-dimensional (1D) structure with the isolated π-conjugated organic cations (C₅H₆N₂)⁺ ((4AMP)⁺). As a favourable result, the birefringence of (C₅H₇N₂)[Zn(H₂PO₃)Cl₂] is enhanced to 0.378@546 nm based on experimental testing results, which is 37.8 times higher than that of (C₄H₁₂N₂)[Zn₂(HPO₃)₂Cl₂]·2H₂O, exceeding almost all phosphites and all commercially available birefringent crystals. Obviously, by converting the non-π conjugated piperazine to π-conjugated 4-aminopyridine units, a significant birefringence is achieved. First-principles theoretical studies confirm that the (4AMP)⁺ cation makes a major contribution to the enhanced birefringence effect. The discovery of (C₅H₇N₂)[Zn(H₂PO₃)Cl₂] indicates that organic-inorganic hybrid phosphite with planar π-conjugated groups has potential advantages in birefringent materials.