Nonlinear-optical properties of α-diiminedithiolatonickel(II) complexes enhanced by electron-withdrawing carboxyl groups.

We report the synthesis, characterization, nonlinear- optical (NLO) properties, and density functional theory (DFT) calculations for three nickel diiminedithiolate complexes [Ni(4,4′-R2carboxy-bpy)(L)] [R = methyl, L = 1,2-benzenedithiolate (bdt), 1; R = ethyl, L = 5,6-dihydro-1,4-dithine-2,3- dithiolate (dddt), 2; R = ethyl, L = 1-(N-methylindol-5-yl)ethene- 1,2-dithiolate (mi-5edt), 3]. The crystal structure of 1 shows a square-planar coordination for the nickel ion and bond distances consistent with a diiminedithiolate description for the complex. For all complexes, the cyclic voltammetry measurements show two reversible reduction processes (−1.353/−1.380 V and −0798/−0.830 V, respectively) and an anodic wave (+0.372/ +0.601 V). The UV−vis spectra present a band around 600−700 nm (ε = 4880−6000 dm3 mol−1 cm−1) mainly attributed to a charge-transfer highest occupied molecular orbital (HOMO)− lowest unoccupied molecular orbital (LUMO) transition, which shows a large negative solvatochromic shift, characteristic of push−pull complexes, and is responsible for the NLO properties of these molecules. The charge-transfer character of this electronic transition is confirmed by DFT calculations, with the HOMO mainly centered on the dithiolate moiety and the LUMO on the bpy ligand, with important contribution given by the carboxyl groups (≈13%). Small contributions from the nickel(II) ion are present in both of the frontier orbitals. The carboxyl groups enhance the optical properties of this class of complexes, confirmed by comparison with the corresponding unsubstituted compounds. The second-order NLO properties have been measured by an electric-field-induced second-harmonic-generation technique using a 10−3 M solution in N,Ndimethylformamide and working with a 1.907 μm incident wavelength, giving for μβ1.907 (μβ0) values of −1095 (−581), −2760 (−954), and −1650 (−618) × 10−48 esu for 1−3, respectively. These values are among the highest in the class of square-planar push−pull complexes, similar to those found for dithionedithiolate compounds. Moreover, spectroelectrochemical experiments demonstrate the possibility of using these complexes as redox-switchable NLO chromophores.