Unveiling the Superior Optical Properties of Novel Melamine‐Based Nonlinear Optical Material with Strong Second‐Harmonic Generation and Giant Optical Anisotropy

Abstract

Two melamine-based metal halides, (C₃ N₆ H₇ )(C₃ N₆ H₆ )HgCl₃ (I) and (C₃ N₆ H₇ )₃ HgCl₅ (II), are synthesized by incorporating the heavy d¹⁰ cation, Hg²⁺ , and the halide anion, Cl^- . The noncentrosymmetric structure of I results from two unique attributes: large asymmetric secondary building units produced by direct covalent coordination of melamine to Hg²⁺ and a small dihedral angle between melamine molecules. The former makes inorganic modules locally acentric, while the latter prevents planar organic groups from forming deleterious antiparallel arrangement. The unique coordination in I results in an enlarged band gap of 4.40 eV. Due to the large polarizability of the heavy Hg²⁺ cation and the π-conjugated system of melamine, I exhibits a strong second-harmonic generation efficiency of 5 × KH₂ PO₄ , larger than any reported melamine-based nonlinear optical materials to date. Density functional theory calculations indicate that I possesses giant optical anisotropy, with a birefringence of 0.246@1064 nm.