Impact of hydrogen bonding on the redox properties of 1-amino-2-sulfonic-4-hvdroxyanthraquinone monolayers

Monolayers of 1-amino-2-sulfonic-4-hydroxyanthraquinone, 1,2,4-AQASH, have been formed on mercury electrodes by spontaneous adsorption. Films were dep osited from solutions containing either quinone or hydroquinone forms where the bulk concentration, C-B, was between 0.1 and 5 muM These films have be en used as model systems to probe the effect of intersite separation and re dox composition on hydrogen bonding interactions in two-dimensions. Both re sonance Raman spectroscopy and surface coverage data indicate that at the o pen-circuit potential the adsorbates are coplanar with the electrode surfac e. For surface coverages, Gamma, less than approximately 1.4 x 10(-10) mol cm(-2) (C-B less than or equal to 1.5 muM) monolayers formed from the quino ne exhibit close to ideal voltammetric responses with a peak-to-peak splitt ing of 17 mV being observed. Also, the full widths at half-maximum are 53 a nd 63 mV for the cathodic and anodic branches, respectively. Capacitance da ta reveal that the adsorbates interact only very weakly within fully oxidiz ed or reduced films. The free energy of adsorption is larger for the quinon e (-35.3 +/- 0.2 kJ mol(-1)) than for the hydroquinone (-33.5 +/- 0.3 kJ mo l(-1)) monolayer. In contrast to fully oxidized or reduced monolayers, the dependence of the surface coverage as determined from the area under the vo ltammetric peak, i.e., where the film is of mixed redox composition, on the bulk concentration is best modeled by the Frumkin adsorption isotherm. The interaction parameters,g, are negative for monolayers formed from both qui none (-0.29 +/- 0.05) and hydroquinone (-1.78 +/- 0.22) forms indicating st abilizing lateral interactions. Where the intersite separation is less than approximately 3 Angstrom, i.e., when Gamma exceeds 1.4 x 10(-10) mol cm(-2 ), the anodic branch of the voltammogram becomes distorted and a current sp ike is observed. These data are interpreted in terms of lateral hydrogen bo nding between the carbonyl and hydroxy moieties when the film is in a quinh ydrone form.