Adsorbed State of 4,4‘-BiPy and BiPyH22+ on Au(111) Electrode

We found using ATR-IR spectroscopy that 4,4‘-diprotonated bipyridine (BiPyH 2 2+ ) 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 BiPyH 2 2+ increases from −0.3 V to −0.1 V at pH ≈ 2.65 close to p K a 1 (dissociation constant for the protonated forms) of BiPyH 2 2+, while BiPy coverage increases to +1.10 V as in neutral Na 2 SO 4 solution (pH = 7.1). The same BiPyH 2 2+ adspecies were observed in 0.1 M HClO 4 solution. Noteworthy is that BiPyH 2 2+ adsorbs at the first layer and ClO 4 - 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 BiPyH 2 2+ and ClO 4 - increases up to ca. +0.8 V in Cl - -containing solution, because chemisorbed Cl - ions at the first layer stabilizes the BiPyH 2 2+ and ClO 4 - adsorption at the second and third layer, respectively. Moreover, the adsorption of BiPyH 2 2+ 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 < p K 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.