Dual-Ligand-Oriented Design of Noncentrosymmetric Complexes with Nonlinear-Optical Activity

When the N- and O-donor ligands are combined as coligands, two noncentrosymmetric (NCS) complexes of [Ni(p-bdc)(tipa)(H₂O)₂]₂·H₂O (1) and Ni(npdc)(tipa)H₂O (2) [tipa = tris[4-(1H-imidazol-1-yl)phenyl]amine, p-H₂bdc = 1,4-benzenedicarboxylic acid, and H₂npdc = 2,6-naphthalenedicarboxylic acid] were achieved under solvothermal conditions. For both structures, N-donor ligands are responsible for the generation of a layered structure, while the O-donor ligands are hung on the layers and are responsible for enhancing the polarity, giving rise to the NCS structures. Because of the different connection modes between the metal centers and different carboxylate ligands (p-bdc^2- in 1 and npdc^2- in 2), 1 and 2 show some structural differences. The p-bdc^2- ligands in 1 are suspended on the upper and lower sides of the [Ni(tipa)] layers, while all of the npdc^2- ligands in 2 hang on one side of the [Ni(tipa)] layers and point in the same direction, which makes the two NCS complexes show phase-matchable behavior with different second-harmonic-generation (SHG) responses of about 0.9 and 1.5 times that of KH₂PO₄ (KDP), respectively. Theoretical studies reveal that charge transfers between Ni²⁺ and carboxylate ligands make the dominant contribution to the optical properties. It is expected that a dual-ligand strategy may guide the design of novel superior-performing NCS complexes.