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

Abstract

AbstractTwo melamine‐based metal halides, (C3N6H7)(C3N6H6)HgCl3 (I) and (C3N6H7)3HgCl5 (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 × KH2PO4, 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.