Synthesis of novel conjugated oligomers for second-order nonlinear optics: incorporation of a central spacer as a conjugation modulator

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Abstract
A new series of second-order nonlinear optical chromophores has been synthesized that consists of a conjugated segment end-capped with an electron acceptor and an electron donor, respectively, and a central spacer intended to modulate the electro-optical effect. Conjugated chains have been tailored with trans-vinylene-1,5-thienylene as the building unit and N,N-dimethylamino and nitro groups as the donor–acceptor pair. Four spacers have been incorporated into the central part of the conjugated oligomers, which range from saturated to totally unsaturated functions, i.e. from methylene to vinylene units. The general strategy relies upon two consecutive Wittig or Wittig–Horner reactions between the spacer precursor and an aromatic phosphonium or phosphonate bearing the strong electron donor and the acceptor, respectively. Two synthetic pathways have been studied. The first procedure is based on the use of a symmetric precursor for the spacer. However, a reaction byproduct is formed, which must be removed and decreases the reaction efficiency. The second approach requires an asymmetric precursor for the spacer, the synthesis of which is a multistep process. In order to evaluate the effect of the spacer, a completely conjugated oligomer has been prepared by the one-step coupling of two conjugated segments end-functionalized by the electron donor and the acceptor, respectively.

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