Chemisorption and Electrochemical Activity of Thiophenols at Well - Defined Pd ( 111 ) Surfaces : Studies by LEED , AES , HREELS , and

The chemisorption and electrochemical activity of 2,5-dihydroxythiophenol (DHT) and 2-(8mercaptooctyl)-1,4-benzenediol (DHOT) on well-defined Pd(111) surfaces were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energyloss spectroscopy (HREELS), and electrochemistry (EC). Results confirm that DHT is chemisorbed in two discrete orientations such that at low concentrations, DHT is oxidatively bound to the surface through the diphenol and mercapto groups as quinonoid and S moieties, respectively, whereas at high concentrations, the molecule is coordinated oxidatively through the –SH group in a vertical S-η orientation. When DHT is chemisorbed in a vertical orientation, the diphenol group is pendant and thus exhibits reversible quinone/diphenol redox activity. However, when the compound acts as a bidentate ligand (at low concentrations), the ring loses its redox activity. Iodine was then found to displace the coordinated quinone ring causing it to regain its electrochemical activity. Electrochemical results also show that by comparing the redox behavior of chemisorbed DHT and DHOT, interactions between DHT molecules on the Pd(111) surface are indeed found to be mediated by the surface.