High Second-Order NLO Response Exhibited by the First Example of Polymeric Film Incorporating a Diimine–Dithiolate Square-Planar Complex: The [Ni(o‑phen)(bdt)]

A novel square-planar diimine–dithiolate nonlinear optical (NLO) chromophore, the complex [Ni­(o-phen)­(bdt)] (1), is reported in this paper (o-phen = dianion of 1,2-phenylendiamina; bdt =1,2-benzenedithiolate). This compound has been fully characterized by single-crystal X-ray diffraction, UV–vis–NIR spectroscopy, cyclic voltammetry, DFT, and TD-DFT calculations. 1 crystallizes in the P₂₁ space group and shows an almost planar molecule. The cyclic voltammetry measurements present an irreversible anodic peak at 0.87 V and reversible and quasi-reversible reduction waves at −0.35 and −1.13 V, respectively. A medium-intense (ε = 1.91 × 10⁴ mol^–1 dm³ cm^–1) absorption with a maximum at 728 nm appears in the UV–vis–NIR spectrum. The molecular second-order NLO properties of 1 have been measured by the electric field induced second-harmonic generation technique in DMF solution, giving values of −1000 × 10^–48 esu for μβ_1.907 and −356 × 10^–48 esu for μβ₀. Furthermore, remarkable solid-state responses have been measured for polymeric films of 1. Indeed, this complex has been embedded into poly­(methyl methacrylate) poled films showing high NLO response (d₃₃ = 1.90 ± 0.38 pm/V), higher than those previously reported for similar push–pull metal complexes. Moreover, 1 represents the first example of a diimine–dithiolate chromophore incorporated into a NLO-active film.