Comparison of CC Triple and Double Bonds as Spacers in Push-Pull Chromophores

We report the synthesis and properties of two series of homologous donor-acceptor (D-A) chromophores in which N,N-dimethylanilino (DMA) or N,N-dihexylanilino (DHA) donors and dicyanovinyl acceptors are separated by up to four C C triple-bond spacers or up to three C=C double-bond spacers. The intramolecular charge-transfer (CT) interactions of the new D-A oligoynes and the known all-trans D-A oligoenes were investigated by X-ray crystallography, electrochemistry, UV/Vis spectroscopy, and theoretical calculations. In both series, the optical and electrochemical HOMO-LUMO gaps decrease with increasing spacer length. The HOMO-LUMO gaps for the D-A oligoynes and oligoenes with a given spacer length are nearly identical. The effect of the spacer length was found to level-off for spacers with more than six carbon atoms. The third-order optical nonlinearity of both series of molecules was determined by measuring the rotational averages of the third-order polarizabilities gamma(rot) by degenerate four-wave mixing.