Syntheses, Crystal Structures, and Spectral Properties of a Series of 3,8-Bisphenyl-1,10-phenanthroline Derivatives: Precursors of 3,8-Bis(4-mercaptophenyl)-1,10-phenanthroline and Its Ruthenium(II) Complex for Preparing Nanocomposite Junctions with Gold Nanoparticles between 1 μm Gap Gold Electrodes

A multistep synthesis was achieved to obtain 3,8-bis(4-mercaptophenyl)-1,10-phenanthroline, which has two free
thiol end groups with a molecular length of 1.89 nm, and its ruthenium(II) complex. Five single-crystal structures
and UV−vis spectra of related intermediates in methanol and the solid state were studied in order to obtain additional
information on the molecules as well as on the supramolecular interactions in the structures. Thermal and electrochemical properties of related Ru(II) complexes were also involved. 3,8-Bis(4-mercaptophenyl)-1,10-phenanthroline
and one of its ruthenium(II) complexes were then treated with gold nanoparticles (Au-NPs) via in situ thiol-to-thiol
ligand exchange in the presence of two facing Au electrodes with a 1 × 1 μm2 gap. Stable molecular junctions
composed of self-assembled films were produced as revealed by an obvious increase of the conductivity between
the Au electrodes, wherein dithiols-bridged Au-NPs were attached to the electrodes by means of Au−S-bonded
contacts. Temperature-dependent current−voltage (I−V) measurements for the junctions were performed in the
temperature range of 7−300 K, and classical Arrhenius plots and their linear fits were obtained to determine the
average activation energies (ΔE) of these devices. It is found that when the Ru(II) ion is introduced, the conductivity
of the nanodevice is increased and the energy barrier is lowered under the same experimental conditions.