  In this work, we have implemented the Fisher-Lee formalism to couple
non-equilibrium Green functions with tight-binding Density Functional to tackle
large molecular systems. This method is used to determine the decay constant of
a set of oligomers based on seven different monomers taken from the literature
in the non-resonant tunneling regime. Results show good agreement with
experimental measurements. The approach is then applied to explore the
conformational pattern effect as well as the asymmetry and the strength of
coupling with the electrode of bisthienylbenzene oligomers sandwiched between
gold (Au),titanium (Ti)and graphene (G)) electrodes The results indicate that
conformational patterns have low impact on the conductance, since the
delocalization of {\pi}-electrons exhibits similar behavior for all the
conformations explored. The calculated attenuation factor is found to be
comparable with the strength of contact coupling (Ti > Au = G). Electronic
analysis of metal-molecule interactions reveals the ionic nature of Ti-C bonds,
through the emergence of a local dipole contributing to the work function
variation of 0.35 eV. In addition, the Ti d-orbitals are found to be strongly
coupled with the lowest unoccupied orbital (LUMO) of BTB, thus facilitating
charge transfer from Ti to the molecule, at the origin of this strong
interfacial dipole. However, the Au-C bond is found to be similar to the C-C
bond, with pure covalent character. The results confirm the hole transport
mechanism observed experimentally in the cases of Au-(BTB)n-Au and
Au-(BTB)n-Ti, and predict possible combined mechanism of both hole and electron
transport in the case of Ti(BTB)n-Ti.

Mesoscale and Nanoscale Physics;Materials Science;Chemical Physics