Towards the design of effective multipodal contacts for use in the construction of Langmuir-Blodgett films and molecular junctions
As part of on-going efforts to optimize the electrical performance and stability of molecular electronic components, anchor groups that bind molecules to electrode surfaces via multiple points of connection (multipodal contacts) have begun to attract attention. Here an oligo(arylene)ethynylene (OAE) derivative with ‘tripodal’ 2,6-bis((methylthio)methyl)pyridine anchoring groups at both molecular termini has been prepared and used to form well-ordered monolayer Langmuir films at the air–water interface. These films were transferred onto solid supports (surface pressure of transference of 8 mN•m-1) to give homogeneous, densely packed, monolayer Langmuir-Blodgett (LB) films, which efficiently block a gold electrode surface. Within the surface-supported LB film, the molecules are oriented with a tilt angle of approximately 30° to the surface normal and contacted through both the ‘buttressed’ methylthio-ether groups and the pyridine nitrogen atom, as determined by X-ray photoelectron spectroscopy (XPS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Molecular junctions formed by contact to a single molecule within the film by the “STM touch-to-contact” method give a most probable molecular conductance of 4.4×10-5 G0. This compares well with the single molecule conductance of 1,4-bis(pyridin-4-ylethynyl)benzene determined by a variety of methods (3.2 – 5.4 x 10-5 G0), indicating that the addition of the buttressing groups do not perturb the favorable electrical characteristics of the pyridyl contacting group. Consistent with these conductance data, a relatively low transition voltage (Vtrans = 0.48 V) was observed for this ‘buttressed’, pyridine-contacted OAE derivative, indicating relatively good alignment of the metal electrode Fermi level and the frontier molecular orbitals.