All starting materials and anhydrous solvents were obtained from Wako Pure Chemical Industries (Osaka, Japan), Tokyo Chemical Industry (Tokyo, Japan), and Sigma-Aldrich (USA) and were used without further purification unless otherwise noted. 1-Ethyl-4-(4′-pyridyl)pyridinium hexafluorophosphate (Et-Py) was prepared by mixing 4,4′-bipyridine and bromoethane at room temperature, followed by ion exchange of Br- for PF6- using an aqueous solution of NH4PF633. Gold substrates were prepared either by thermal evaporation or physical deposition onto chromium-deposited silicon wafers (Cr layer thickness: approximately 100 Å; Au layer thickness: approximately 500 Å) immediately before use. For the AFM observations, Au substrates deposited onto cleaved mica were purchased from Agilent Technologies (California, USA) and cleaned using a UV/ozone method immediately prior to use.