Adsorbed state of 4,4′-BiPy and BiPyH22+ on Au(111) electrode

We found using ATR-IR spectroscopy that 4,4 ' -diprotonated bipyridine (BiPyH22+) adsorbs at the second layer outside sulfate ions on Au(111) surface above ca. 0.0 V (vs Ag/AgCl) at pH < 2.65. In contrast, surface coverage of BiPyH22+ increases from -0.3 V to -0.1 V at pH approximate to 2.65 close to pK(a1) (dissociation constant for the protonated forms) of BiPyH22+, while BiPy coverage increases to +1.10 V as in neutral Na2SO4 solution (pH = 7.1). The same BiPyH22+ adspecies were observed in 0.1 M HClO4 solution. Noteworthy is that BiPyH22+ adsorbs at the first layer and ClO4- at the second layer in this case. Furthermore, the surface coverage of these species does not decrease with increasing the potential after the saturation at ca. +0.3 V. In contrast, the surface coverage of BiPyH22+ and ClO4- increases up to ca. +0.8 V in Cl--containing solution, because chemisorbed Cl- ions at the first layer stabilizes the BiPyH22+ and ClO4- adsorption at the second and third layer, respectively. Moreover, the adsorption of BiPyH22+ is suppressed in Br-- or I--containing solution due to discharge of these ions at the Au(111) surface. Pyridine and 2,2 ' -BiPy molecules in contrast to 4,4 ' -BiPy adsorb after a deprotonation even in a strong acidic solution with pH < pK(a). Thus, the quite unique adsorbed state of 4,4 ' -BiPy is attributed to the molecular structure in which the solution side is strongly hydrophilic for an end-on adsorption to the Au gold electrode.