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 (Simmons type behavior) I-V curves were observed, with analysis of the pseudolinear (ohmic) region giving a conductance value G = 3.9 × 10-5 G0, which is relatively high for an OPE derivative and may indicate a beneficial role of metal atom incorporation within the wire-like system. © 2014 American Chemical Society.