Abstract
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, (C(4)H(12)N(2))[Zn(2)(HPO(3))(2)Cl(2)]·2H(2)O and (C(5)H(7)N(2))[Zn(H(2)PO(3))Cl(2)], have been successively synthesized. (C(4)H(12)N(2))[Zn(2)(HPO(3))(2)Cl(2)]·2H(2)O exhibits a three-dimensional (3D) structure constructed by unique hydrogen-bonded {[Zn(2)Cl(2)(HPO(3))(2)(H(2)O)(2)](∞)}(2-) lamellar, with non-π conjugated piperazine (PIP) cations (C(4)H(12)N(2))(2+) serving as counter cations. (C(4)H(12)N(2))[Zn(2)(HPO(3))(2)Cl(2)]·2H(2)O 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 (C(5)H(7)N(2))[Zn(H(2)PO(3))Cl(2)]. It contains [Zn(H(2)PO(3))Cl(2)](-) anions constructed by [ZnO(2)Cl(2)](4-) and [H(2)PO(3)](-) units, which form a one-dimensional (1D) structure with the isolated π-conjugated organic cations (C(5)H(6)N(2))(+) ((4AMP)(+)). As a favourable result, the birefringence of (C(5)H(7)N(2))[Zn(H(2)PO(3))Cl(2)] is enhanced to 0.378@546 nm based on experimental testing results, which is 37.8 times higher than that of (C(4)H(12)N(2))[Zn(2)(HPO(3))(2)Cl(2)]·2H(2)O, 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 (C(5)H(7)N(2))[Zn(H(2)PO(3))Cl(2)] indicates that organic-inorganic hybrid phosphite with planar π-conjugated groups has potential advantages in birefringent materials.