Single Gold Atom Containing Oligo(phenylene)ethynylene: Assembly into LB Films and Electrical Characterization

Monomolecular films of an oligo(phenylene)ethynylene, OPE, derivative [2-isocyano-1,3-dimethylbenzene][4-(4′-aminophenylethynyl)phenylethynyl]gold, 1, containing a gold atom in the molecule backbone have been prepared by the Langmuir–Blodgett (LB) method in order to study how the electrical properties can be modulated in monolayers of OPEs by incorporation of a gold center in their structures. UV–vis reflection spectra of Langmuir monolayers of 1 at the air–water interface reveal strong aurophilic interactions between neighboring molecules that increase upon compression. Monolayer Langmuir–Blodgett (LB) films were readily fabricated by the transfer of Langmuir films of 1 onto solid substrates. Quartz crystal microbalance (QCM) experiments conclusively demonstrate formation of monolayer LB films with a high surface coverage. The morphology of these films was analyzed by atomic force microscopy (AFM), revealing formation of homogeneous layers with an optimum surface pressure of transference of 6 mN·m–1. Film homogeneity and integrity was confirmed by cyclic voltammetry, with efficient blocking of gold electrodes by these well-formed monolayers of 1. The electrical properties of LB films of 1 were investigated by scanning tunneling microscopy (STM) using a “tip-to-contact” method. Characteristic shape for a tunneling junction (Simmon’s type behavior) I−V curves were observed, with analysis of the pseudolinear (ohmic) region giving a conductance value G = 3.9 × 10–5G0, which is relatively high for an OPE derivative and may indicate a beneficial role of metal atom incorporation within the wire-like system.