Direct measurement of the local field within alkyl-ferrocenyl-alkanethiolate monolayers: Importance of the supramolecular and electronic structure on the voltammetric response and potential profile

This paper describes the electrochemical behaviour of self-assembled monolayers (SAMs) of n-alkanethiolates with Fc groups inserted at 14 different positions along the alkyl chain (SC_nFcC_13-n, n = 0–13) studied by cyclic voltammetry. The electronic and supramolecular structures of the SAMs have been fully characterised and all molecules are standing up, allowing for precise control over the position of the Fc unit within the SAM as a function of n revealing the shape of the electrostatic potential profile across the SAMs. The potential profile is highly non-linear due to electronic changes in the nature of the Fc—electrode interaction for small values of n < 5, and supramolecular changes for large values of n = 11–13. For intermediate values of n = 5–11, the potential drop is linear and the data can be fitted to a model developed by White and Smith. The electrochemical behaviour was dominated by a one-step reversible redox-process, but the presence of a shoulder indicates that the Fc units are present in different microenvironments resulting from the mismatch in size between the Fc units and the alkyl chains. Other features, including peak splitting, peak broadening, and peak shifts, can be related to changes in the electronic and supramolecular structure of the SAM revealed by molecular dynamics simulations and spectroscopy. For small values of n < 5, electronic effects dominate and the peak oxidation waves are shifted anodically (∼150 mV) and broadened (full width at half maximum of up to 220 mV) because the Fc units hybridise with the Au electrode (for n < 3) or interact with the Au electrode via van der Waals interactions (n = 4, 5). For intermediate values of n = 5–11, supramolecular effects direct the packing structure of the SAMs and clear odd-even effects are observed. For large values of n = 11–13, the top alkyl chains are liquid-like in character and do not block the Fc units from the electrolyte.