Bridge effect of the C=C, C=N and N=N bonds on the long distance electronic charge transfer of para-substituted stilbenoid compounds

By means of C-13-NMR spectroscopy and ab initio molecular orbital theory calculations, we have analyzed the bridge effect of the C=C, C=N and N=N bonds on the long distance charge transfer of 4-dimethylamino-4'-nitrostilbenoid compounds in the ground electronic state. After a complete spectral assignment of the C-13-NMR signals in these molecular compounds, we have characterized the effect of the nitrogen centres on the molecular bridge by means of the chemical shifts of the carbon centres, the theoretical charge densities and the dipolar moments. From an electronic molecular point of view, our results describe two main properties of the double bond bridge. The first is related to the local charge accumulation capacity given by the type of the atomic centres and the structural orientation of the double bond bridge, and the second property is related to the modulation of the electronic charge distribution through the molecular system by the electrical polarization of the bridge. Other complementary experimental evidences have permit us to establish new local molecular domains of the bridge effect in these stilbenoid compounds by means of linear correlations between C-13-NMR chemical shifts of the aromatic carbon centres of the acceptor-phenyl group and the molecular polarity of the species under study.