Quantum Interference in Molecular Wires: Electron Propagator Calculations

In this work, we study the effect of electron correlations on transport in molecular tunnel junctions where a 1,4-benzenedithiol (BDT) molecule is placed between two gold electrodes. By employing electron propagator computational methods with geometry optimization in an electric field, we analyze different spatial configurations of a bridge attachment to metal electrodes and find that the configuration where the molecule is both stretched and tilted with respect to the surfaces by the angle of 28.5° provides the best agreement with the experimental data at εf = −1.35 eV and thus determines the most probable attachment geometry. In addition, we investigate current−voltage characteristics vs Fermi energy and find strong negative differential resistance at εf = 0.4 eV. To explain this phenomenon, we provide a detailed quantum mechanical analysis indicating that an applied field causes the breaking of aromaticity of a π molecular orbital of a benzene ring, resulting in the sharp drop in the electric current. The participation of the benzene π molecular orbital in the conductivity is surprising because one would expect that the main contribution is due to the overlap between the gold and the adjacent sulfur orbitals. Such a prediction provides an insight into how novel molecular devices can be constructed with desirable properties by a suitable modification of the surfaces.