ELECTROCHEMICAL CHARACTERISTICS OF SELF-ASSEMBLED MONOLAYERS OF A NOVEL NICKEL(II) PENTAAZAMACROCYCLIC COMPLEX ON A GOLD ELECTRODE

Ethyl disulfide possessing two nickel(II) pentaazamacrocyclic redox ce nters at its end carbon atoms, 1, adsorbs on gold electrodes from meth anol solutions and yields stable, electroactive self-assembled monolay ers (SAMs). Cyclic voltammetry of SAMs of 1 shows a stable redox wave at 0.55 V in aqueous 0.1 M Na2SO4 (pH 2), corresponding to the Ni2+/3 redox reaction. For SAME of 1 with the surface coverage (Gamma) of ni ckel(II) redox centers of(1.41 +/- 0.04) x 10(-10) mol cm(-2) the cycl ic voltammograms (CV) show very small peak-to-peak separations (Delta E-p < 20 mV) with the Delta E-fwhm (full-width at half-maximum of CV p eaks) values ranging from 140 to 150 mV, indicating electrostatic repu lsive interactions among the surface-immobilized nickel(II) redox cent ers. The constructed SAM of 1 was defined as a square close-packed mon olayer with the polyazamacrocyclic ring of the nickel(II) redox center oriented perpendicular to the electrode surface. Coadsorption of 1 wi th electroinactive alkyl/aryl disulfides furnishes mixed monolayers, a nd the Gamma of the electroactive nickel(II) redox centers can be vari ed by the mole ratio of 1 and coadsorbate in the adsorption solution. In mixed monolayers, the Delta E-fwhm decreases as the Gamma of 1 decr eases from the limiting monolayer surface coverage, suggesting a homog eneous distribution of 1 and coadsorbate in the constructed mixed mono layers. The formal potentials (E degrees') of the solution-dissolved 1 and the self-assembly of 1 with different Gamma values, equivalent to 0.3-1 monolayer, are nearly equal, evidencing the same microenvironme nt around the nickel(II) redox centers in both the self-assembly and t he solution phase. Exchange between the surface-immobilized gold-alkan ethiolates with disulfides in solution has been observed and was found to accompany ordering of the monolayer. The apparent rate constant fo r the heterogeneous electron-transfer reaction, k(app), has been deter mined by fast-scan cyclic voltammetry to be 1.3 x 10(3) s(-1) for the SAM of 1. An increase in the k(app) with a decrease in the Gamma of 1 in mixed monolayers was observed.