ELECTROCHEMICAL AND 2ND-HARMONIC GENERATION STUDIES OF 2,2'-BIPYRIDINE ADSORPTION AT THE AU(111) ELECTRODE SURFACE

Adsorption of 2,2'-bipyridine (22BPY) at the Au(lll)-solution interfac e has been studied using cyclic voltammetry, ac voltammetry, chronocou lommetry, and second harmonic generation spectroscopy (SHG). The therm odynamic quantities describing the energetics of 22BPY adsorption at t he Au(lll) surface such as the Gibbs surface excess, Gibbs energy of a dsorption, and the electrosorption valency were determined from electr ochemical measurements. The effect of 22BPY on the crystallographic an d electronic structure of the Au(lll) surface was assessed with the he lp of SHG experiments. It was found that 22BPY adsorbs at the Au(lll) surface in a number of states corresponding to different orientations and/or conformations of this molecule. The 22BPY molecule assumes a fl at orientation in which the two aromatic rings are parallel to the gol d surface at the negatively charged interface. At the positively charg ed interface, the molecule assumes a vertical orientation with a copla nar cis configuration in which the two nitrogen atoms of the molecule face the metal surface. The transition from the flat to the vertical o rientation is gradual and goes through a series of intermediate states , Adsorption of 22BPY has little effect on the crystallographic struct ure of the Au(lll) surface. A significant change of the phase angle be tween the real and imaginary components of the isotropic term of the S HG signal indicates, however, that mixing of the molecular orbitals of the 22BPY with the electronic states of the metal affects the electro nic structure of the surface.