Odd–Even Effects in Electroactive Self-Assembled Monolayers Investigated by Electrochemical Surface Plasmon Resonance and Impedance Spectroscopy

Abstract

Subtle conformational-based distinctions in the supramolecular structure of self-assembled monolayers (SAMs) of ω-ferrocenylalkanethiolates on gold (FcCnSAu) have been shown to significantly impact their electrical and redox properties. We investigate the effect of differences in the intermolecular van der Waals interactions and molecular packing energy of FcCnSAu SAMs comprised of an odd (SAModd) versus even (SAMeven) number of methylenes on the electrochemical behavior for n = 6–16. Redox-induced thickness changes are investigated by electrochemical surface plasmon resonance (ESPR). Electrochemical impedance spectroscopy (EIS) under non-Faradaic conditions is used to evaluate the dielectric properties of the reduced (neutral) state of the FcCnSAu SAMs. Oxidation of the SAM-bound ferrocenes yields an effective film thickness change of 0.19 ± 0.01 nm, which is attributed to untilting of the alkyl chains. The SAMs manifest odd–even differences in the apparent redox potential, and hence in the thermodynamic...