Bending of conjugated molecular wires and its effect on electron conduction properties

Abstract

The electronic structure and electron transport properties of simple conjugated molecularwires like oligophenylene ethynylene (OPE) and oligophenylene vinylene (OPV) are studiedunder compression. If artificially confined to a given shorter length, the oligomers tend tobend and bending causes a loss in the overlap of the conjugated molecular orbitals.Theoretical modeling of electronic transport has been carried out for all undistorted andcompressed OPE/OPV oligomers. OPV exists in step-like or V-like conformations andthey have the same stability with very similar frontier molecular orbitals. Theconductances of these molecular wires are calculated when inserted between twogold probes and the conductances for OPV are found to be comparable to OPEwhen the interfaces are same. The conductance decreases with bending due tothe gradual loss in overlap of the molecular orbitals. It is also found that theconductances of the molecular wires decrease very strongly if the terminal sulfur atom issimultaneously bonded to hydrogen and a gold surface, thus reflecting the importance ofthe interface in determining the conductance in two-probe systems. From theconductance studies it may be concluded that if one or more benzene rings ofOPE are rotated from coplanar conditions, the orthogonal molecular orbitalsmay completely block the electronic transport, rendering the molecule insulating.