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 (FcC n SAu) 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 FcC n SAu SAMs comprised of an odd (SAM odd ) versus even (SAM even ) 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 FcC n SAu 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 SAM odd and SAM even . The chain length dependencies of the measured parameters are discussed in the context of the SAM structure.

Square Wave Voltcoulommetry Analysis of the Influence of the Electrostatic Environment on the Electrochemical Functionality of Redox Monolayers

“…Therefore, these parameters should include contributions from both coulombic (long range), and other non‐covalent short range interactions (i. e., odd‐even, , etc.) ,,. In this sense, for the simpler case of a single charged species and low values of the surface excess, it is reasonable to assume that, if interactions are present, they are mainly of electrostatic nature, with higher prevalence of those of repulsive character through coefficients or .…”

“…These coulombic interactions would give rise to negative or null values of G . The increase of the surface excess of the electroactive species could provoke the appearance of new attractive interactions (for example, Van der Waals attractions as those relative to alkanethiols, also called odd‐even,), which would cause an increase of the values of parameter G that could achieve positive values . We will analyse the validity of such assumption below.…”

“…Therefore, these parameters should include contributions from both coulombic (long range), and other non-covalent short range interactions (i. e., odd-even, p À p, etc.). [21,22,42] In this sense, for the simpler case of a single charged species and low values of the surface excess, it is reasonable to assume that, if interactions are present, they are mainly of electrostatic nature, with higher prevalence of those of repulsive character through coefficients a OO or a RR . These coulombic interactions would give rise to negative or null values of G. The increase of the surface excess of the electroactive species could provoke the appearance of new attractive interactions (for example, Van der Waals attractions as those relative to alkanethiols, also called odd-even [21,42] ), which would cause an increase of the values of parameter G that could achieve positive values.…”

Odd‐Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

“…In addition, there are other intrinsic physico‐chemical properties of the α,ω,‐alkanedicarboxylic acids due to the odd‐even effect, which is quite well established in the literature . This effect plays an important role in the regular variations of different properties of compounds: melting points, solubility, nanocalorimetry, electrochemical, the emulsifying ability of some surfactants or structural self‐assembling . It is noteworthy that the odd‐even effect arises from a difference in the packing mode between odd‐odd or even‐even carbon members of a given homologous series …”

“…[3][4][5] This effect plays an important role in the regular variations of different properties of compounds: melting points, [3,5,6] solubility, [3] nanocalorimetry, [7] electrochemical, [8,9] the emulsifying ability of some surfactants [9] or structural selfassembling. [8,[10][11][12] It is noteworthy that the odd-even effect arises from a difference in the packing mode between odd-odd or even-even carbon members of a given homologous series. [4][5][6]13] According to the "parallelogram-trapezoid model" proposed by Thalladi et al, [3] the even carbon members are arranged in such a way that the repulsive interactions between chains are decreased, leading to a more stable packing pattern.…”