Influence of spatial redox distribution on the electrochemical behavior ofelectroactive self-assembled monolayers

Packing restrictions and hydrophobic interactions are likely to lead to a s patial distribution of redox centers in electroactive monolayers. A mean fi eld analysis of the electrochemical implications of spatial redox dispersio n in SAMs, including the possibility of surface ion pair formation, has bee n carried out. The boundary value problem associated with a layered distrib ution of potential-induced charges has been solved by using the orthogonal collocation technique under equilibrium conditions. Spreading of the redox centers into a 3D dielectric slab results in broader and asymmetric Voltamm ograms, reflecting a layer-by-layer redox conversion. It is also shown that the voltammetric shape is sensitive to the specific features of the spatia l redox distribution, and theoretical requirements for the appearance of as ymmetric broadening are examined in terms of the electrostatic properties o f the monolayer. It is suggested that this type pf spatial inhomogeneity ma y cause some of the broad and asymmetric voltammetric shapes that often cha racterize the electrochemical behavior of electroactive SAMs, and that some structural information can be gained from the analysis of these voltammogr ams, as long as electrolyte ions do not permeate the organic monolayer. The effect of surface ion association on the voltammetric features is also exa mined, and it is interpreted in terms of the distinct sensitivity of the po tential at each redox plane with respect to the local counterion concentrat ion. Comparison is made with the experimental results of Chidsey et al. (J. Am. Chem. Soc. 1990, 112, 4301) for the oxidation of FcCO(2)(CH2)(11)SH/CH 3(CH2)(9)SH and Fc(CH2)(16)SH/CH3(CH2)(15)SH mixed monolayers.