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 thermodynamics of the redox reaction, across the chain lengths n = 6–16 investigated. By contrast, odd–even alternations in the oxidation-induced resonance angle change measured by ESPR and SAM double-layer capacitance determined by EIS are only observed beginning at n = 9 or 10. We show that the odd–even effects in the resonance angle and SAM capacitance are related to a difference in the dielectric constants of SAModd and SAMeven. The chain length dependencies of the measured parameters are discussed in the context of the SAM structure.