Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

Abstract

Metal complexes are receiving increased attention as molecular wires in fundamental studies of the transport properties of metal|molecule|metal junctions. In this context we report the single-molecule conductance of a systematic series of d⁸ square-planar platinum(II) trans-bis(alkynyl) complexes with terminal trimethylsilylethynyl (CCSiMe₃) contacting groups, e.g. trans-Pt{CCC₆H₄CCSiMe₃}₂(PR₃)₂ (R = Ph or Et), using a combination of scanning tunneling microscopy (STM) experiments in solution and theoretical calculations using density functional theory and non-equilibrium Green's function formalism. The measured conductance values of the complexes (ca. 3–5 × 10⁻⁵G₀) are commensurate with similarly structured all-organic oligo(phenylene ethynylene) and oligo(yne) compounds. Based on conductance and break-off distance data, we demonstrate that a PPh₃ supporting ligand in the platinum complexes can provide an alternative contact point for the STM tip in the molecular junctions, orthogonal to the terminal CCSiMe₃ group. The attachment of hexyloxy side chains to the diethynylbenzene ligands, e.g. trans-Pt{CCC₆H₂(Ohex)₂CCSiMe₃}₂(PPh₃)₂ (Ohex = OC₆H₁₃), hinders contact of the STM tip to the PPh₃ groups and effectively insulates the molecule, allowing the conductance through the full length of the backbone to be reliably measured. The use of trialkylphosphine (PEt₃), rather than triarylphosphine (PPh₃), ancillary ligands at platinum also eliminates these orthogonal contacts. These results have significant implications for the future design of organometallic complexes for studies in molecular junctions.