Nonideal Electrochemical Behavior of Ferrocenyl-Alkanethiolate SAMs Maps the Microenvironment of the Redox Unit

We studied the electrochemical behavior of self-assembled monolayers (SAMs) of n-alkanethiolates with ferrocenyl (Fc) termini on gold (S(CH₂)_nFc, n = 0-15) in relation to their supramolecular structures (characterized by photoemission spectroscopy (PES) supported by molecular dynamics (MD) simulations) to elucidate the origin of nonideal features commonly observed in cyclic voltammograms (CVs) of these SAMs by systematically changing n from 0 to 15. For all SAMs the CV data are dominated by a main peak for Fc units directly in contact with the electrolyte solution and interacting with neighboring Fc units. A second peak is observed for SAMs with n ≥ 2 ascribed to partially buried Fc units as a result of lattice strain due to the different sizes of the lattices of the Fc units and the sulfur atoms. For thick SAMs with strong molecule-molecule interactions, the strain is large, resulting in a third peak due to Fc units that are well shielded from the electrolyte by other Fc units. Our results do not agree with widely used explanations to account for peak splitting involving isolated Fc units vs clustered Fc units, disordered vs ordered domains, or back bending (which requires the formation of Gauche defects). In contrast, we show that the peak splitting and peak broadening is an inherent property of densely packed SC_nFc SAMs with Fc units in different electrochemical environments to release the buildup of strain. On the basis of our results we propose a simple model to explain all the nonideal features observed in CV data. In addition, we show that this model can also explain the abnormal shapes of CV recorded on S(CH₂)₁₁Fc SAMs formed on very rough and defective electrodes or derived from the corresponding disulfide, i.e., (S(CH₂)₁₁Fc)₂.