SURFACE ELECTROCHEMISTRY AND MOLECULAR-ORIENTATION - STUDIES OF PYRIDYL HYDROQUINONES ADSORBED AT PT(111) BY CYCLIC VOLTAMMETRY, AUGER-ELECTRON SPECTROSCOPY AND ELECTRON-ENERGY LOSS SPECTROSCOPY

Reported here are surface electrochemical studies of three newly synthesized compounds: (4-pyridyl) hydroquinone (4PHQ), (3-pyridyl) hydroquinone (3PHQ) and (2-pyridyl) hydroquinone (2PHQ). These compounds possess essentially the same electrochemical reactivity in the dissolved form, but exhibit different electron transfer behavior in the adsorbed state. Each chemisorbs at Pt(111) electrode surfaces from aqueous solutions to form a close-packed and highly oriented monolayer which is stable in vacuum and in solution. Molecular packing density (in nanomoles per square centimeter) was measured by means of Auger electron spectroscopy and coulometry. Surface molecular vibrations were determined by use of high resolution electron energy loss spectroscopy (EELS). Electrochemical reactivity was investigated by use of cyclic voltammetry. Long range surface structure was monitored by low energy electron diffraction. The results indicate that 3PHQ and 4PHQ are attached to the platinum surface exclusively through the nitrogen atom with the pyridine ring in a tilted vertical orientation. Such a surface molecular orientation keeps the hydroquinone moiety pendant and thus reversibly electroactive. Since none of the molecular conformations of adsorbed 4PHQ permits direct contact between the hydroquinone moiety and the Pt(111) surface, electron transfer between the hydroquinone moiety and the surface evidently proceeds by electron hopping and/or tunneling through the chemisorbed pyridine ring. In contrast with the behavior of 3PHQ and 4PHQ, the HQ moiety of 2PHQ is directly attached to the Pt(111) surface in addition to the Pt-N surface bond of the pyridine ring, as required by the 2PHQ molecular structure. Accordingly, adsorbed 2PHQ possesses virtually no reversible electroactivity and only one adsorbed state in the electrode potential range from - 0.1 to 0.4 V, based upon EELS spectra.