Strong conductance variation in conformationally constrained oligosilane tunnel junctions.

The effects of molecular conformation on conductance in oligosilane-bridged metal-molecule-metal junctions are studied theoretically using density functional theory combined with a nonequilibrium Green's function approach. Varying the internal SiSiSiSi dihedral angles in hexasilane diamine chains changes the conductance by up to 3 orders of magnitude. This conformational dependence is due to the effects of sigma-delocalization on the positions of the highest occupied molecular orbital (HOMO) energies. The conductance values for the different conformations are related to electron transfer rates in donor-bridge-acceptor systems, and the effect of shifting the injection energy is examined. The transport properties are found to be extremely sensitive to the alignment between the HOMO energies and Fermi level of the gold electrodes.