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

초록

Two melamine-based metal halides, (C3N6H7)(C3N6H6)HgCl3 (I) and (C3N6H7)(3)HgCl5 (II), are synthesized by incorporating the heavy d(10) 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 pi-conjugated system of melamine, I exhibits a strong second-harmonic generation efficiency of 5 x 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.

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