Main-chain oligomers from Ni^II- And Cu^II-centered unsymmetrical N₂O₂ Schiff-base complexes: Synthesis and spectral, structural, and second-order nonlinear optical properties

Oligomeric main-chain Schiff-base compounds 4 and 5 have been synthesized by the trans-esterification of their respective doubly functionalized monomeric precursors [M{(η⁵- C₅H₅)Fe(η⁵-C₅H₄)C(=O)CH=C(4-C₆H₄OH)NCH₂CH₂N= CH-(2-O,4-CO₂H-C₆H₃)}] [M = Ni (2), Cu (3)] in dmf. The new compounds were characterized by elemental analysis, FTIR and multidimensional NMR spectroscopy, mass spectrometry, and cyclic voltammetry. Compounds 2, 3, and 6, the corresponding amide of 2, were characterized by X-ray structural investigation. Gel permeation chromatography estab- lished that oligomers 4 and 5 are formed of approximately six monomeric units, and differential scanning calorimetry and thermal gravimetric analysis indicated that they are thermally stable with decomposition temperatures exceeding 230 °C. Harmonic light scattering measurements showed that compounds 2-5 exhibit rather high second-order nonlinear responses, between 224×10^-30 and 399×10^-30 esu, with hyperpolarizability β_1.91 values that increase significantly on passing from the monomeric to the oligomeric species.