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,N-dimethylformamide 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.