Electrochemical and surface enhanced infrared absorption spectroscopy studies of TEMPO self-assembled monolayers

Abstract

The electron transfer kinetics of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) monolayers have been studied using both a chronocoloumetric method and surface sensitive infrared spectroelectrochemistry (attenuated total reflection surface enhanced infrared absorption spectroscopy, ATR-SEIRAS). Redox active self-assembled monolayers (SAMs) are constructed by covalently attaching 4-amino-TEMPO to preformed monolayers of different chain length alkanoic acid thiol monolayers. The density of TEMPO groups on the resulting Cn-TEMPO (n = 6, 8, 11, 12, 16) SAMs is approximately 20% of a full monolayer coverage and the resulting SAMs exhibit very little kinetic heterogeneity. Standard heterogeneous rate constants are measured as a function of SAM chain length. Tafel plots constructed for n = 12 show asymmetry between the forward (oxidation of the radical to the nitrosonium cation) and reverse (reduction to the TEMPO radical) reactions. The reorganization energy of the nitrosonium cation is estimated to be 1.0 ± 0.1 eV for the C16-TEMPO SAM. ATR-SEIRAS studies reveal that the oxidation of the TEMPO radical leads to a conformational change (chair-chair flip) in the piperdinyl ring. The nitrosonium conformer pushes the N=O+ center away from the electrode and into a polar environment. Conformational differences in the reduced and oxidized species are cited as the reason for kinetic asymmetry in the TEMPO/TEMPO+ redox system. Time-resolved ATR-SEIRAS is used to extract the standard rate constant for C12-TEMPO on gold island films deposited on an indium tin oxide (ITO) surface. Standard rate constants for SAMs formed on Au@ITO are found to be higher than those formed on gold beads due to greater kinetic heterogeneity caused by the larger roughness of the gold islands.