Influence of Spatial Redox Distribution on the Electrochemical Behavior of Electroactive Self-Assembled Monolayers

Abstract

Packing restrictions and hydrophobic interactions are likely to lead to a spatial distribution of redox centers in electroactive monolayers. A mean field analysis of the electrochemical implications of spatial redox dispersion in SAMs, including the possibility of surface ion pair formation, has been carried out. The boundary value problem associated with a layered distribution 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 voltammograms, reflecting a layer-by-layer redox conversion. It is also shown that the voltammetric shape is sensitive to the specific features of the spatial redox distribution, and theoretical requirements for the appearance of asymmetric broadening are examined in terms of the electrostatic properties of the monolayer. It is suggested that this type of spatial inhomogeneity may cause some of the broad and asymmetric voltammetric shapes that often characterize the electrochemical behavior of electroactive SAMs, and that some structural information can be gained from the analysis of these voltammograms, as long as electrolyte ions do not permeate the organic monolayer. The effect of surface ion association on the voltammetric features is also examined, and it is interpreted in terms of the distinct sensitivity of the potential at each redox plane with respect to the local counterion concentration. Comparison is made with the experimental results of Chidsey et al. (J. Am. Chem. Soc. 1990, 112, 4301) for the oxidation of FcCO 2 (CH 2 ) 11 SH/CH 3 (CH 2 ) 9 SH and Fc(CH 2 ) 16 SH/CH 3 (CH 2 ) 15 SH mixed monolayers.

Redox Actuation of a Microcantilever Driven by a Self-Assembled Ferrocenylundecanethiolate Monolayer: An Investigation of the Origin of the Micromechanical Motion and Surface Stress

“…This calculated Φ Fc + onset is consistent with the minimum surface concentration required for electrochemically interacting ferrocenes in mixed SAMs. 72,75,94 More importantly, it coincides with the experimentally observed Φ Fc + range in which the onset of microcantilever deflection occurs ( Figure 5B), suggesting that there is a correlation between the extent of the microcantilever deflection or bending and the number of neighboring ferroceniums.…”

“…The presence of multiple voltammetric waves is attributable to the existence of electrochemically distinct ferrocene microenvironments. 72,75,[93][94][95] The ferrocene surface coverage of 4.7 ((0.3) × 10 -10 mol cm -2 (Q Fc + ) 45 ((3) µC cm -2 ), determined experimentally on macroscopic Au-coated slides prepared and functionalized in the same manner as the microcantilevers (see Supporting Information), is close to the theoretical value (4.5 × 10 -10 mol cm -2 ) expected from the close packing of ferrocene spheres of 6.6 Å diameter. 96 Surface Stress Measurements.…”

“…72,75 In single-component FcRSAu, the oxidation of a ferrocene next to an already oxidized ferrocenium cation is unfavorable because of electrostatic or Coulombic repulsion between the charged moieties, so that a critical number of electrogenerated ferroceniums may be needed for neighboring interactions. 94,95 A simulation of the ferrocenium distribution as a function of Φ Fc + provides an approximate idea of the extent of neighboring ferrocenium interactions with the applied potential ( Figure 5A), which we have used to rationalize the % deflection vs Φ Fc + behavior obtained experimentally ( Figure 5B). Our crude modeling exercise consisted of placing ferroceniums in single fashion at random in a 36 × 36 grid (1296 molecules) and, for a given Φ Fc + , counting the number of ferroceniums which have more than one nearest neighbor.…”

Microcantilevers Modified with Ferrocene-Terminated Self-Assembled Monolayers: Effect of Molecular Structure and Electrolyte Anion on the Redox-Induced Surface Stress

“…This greater conformational mobility and an increased accessibility to the electrolyte solution favor a redox reaction at lower voltage potentials (i.e., shoulder peaks observed in Figure 1a,b). 19,24,54 Positively shifted voltammetric peaks are a consequence of domains or clusters of FcRSAu because electrostatic repulsion from ferrocenium cations and steric constraints renders the oxidation of neighboring ferrocene molecules less favorable (i.e., main anodic peaks in Figure 1a,b). 16,19,24 Single asymmetric peaks with no apparent The Journal of Physical Chemistry C ARTICLE shoulders are observed for Fc(CO)C 11 SAu (Figure 1c).…”

The effect of supporting electrolyte on the electron transfer at mixed self-assembled monolayers containing ferrocene moieties

“…In this study, we have elucidated the mechanism by applying the theory of the interfacial potential distribution proposed by S&W [16]. This theory formed the basis for the more detailed and profound discussion by several authors [23][24][25][26], and was also used to discuss the electric double-layer effects [9,10].…”