Electronic Factors Affecting Second-Order NLO Properties: a Case Study of four Different Push-Pull Bis-Dithiolene Nickel Complexes

The paper presents a detailed experimental and theoretical study of the four mixed nickel-bisdithiolene complexes [Ni(Pr(2)(i)pipdt)(dmit)] (1b, Pr(2)(i)pipdt = 1,4-diisopropyl-piperazine-3,2-dithione; dmit = 1,3-dithiolo-2-tione-4,5-dithiolato), [Ni(R(2)pipdt)(mnt)] (2b", R = 2-ethylhexyl; mnt = maleonitriledithiolato), [Ni(Pr(2)(i)timdt)(dmit)] (3b, Pr(2)(i)timdt = 1,3-diisopropyl-imidazoline-2,4,5-trithione), and [Ni(Pr(2)(i)timdt)(mnt)] (4b), and their models. All the complexes, with common (C2S2)Ni(C2S2) core and two different terminal groups, are uncharged and square-planar coordinated. Previous measurements of the first molecular hyperpolarizability indicated that some of the species are potential NLO chromophores due to the pi-delocalized character of two frontier levels (HOMO and LUMO) which is asymmetrically perturbed by the combination of one push (R(2)pipdt, R(2)timdt) with one pull ligand (dmit and mnt). The X-ray structure of complex 1b is presented and its geometry is compared with those available in the literature for the four types of complexes under study. The results of electrochemical and spectroscopic measurements (oxidation and reduction potentials, IR, dipole moment, molecular absorptivities, etc.) indicate rather different responses between the pairs of complexes 1-2 and 3-4. Hence, DFT calculations on the model compounds 1a-4a, where hydrogen atoms replace the alkyl groups of R(2)pipdt and R(2)timdt, have been carried out to correlate geometries and electronic structures. Moreover, the first molecular hyperpolarizabilities have been calculated and their components have been analyzed with the simplest two-level approximation. The derived picture highlights the different roles of the two push and pull ligands, but also the peculiar perturbation of the pi-electron density induced by the terminal CS3 grouping of the ligand dmit.