Studying the influence of the surface on long-range electron transfer across self-assembled monolayers using norbornylogous bridges

Abstract

This thesis discusses factors that govern long-range electron transfer across self- assembled monolayers (SAM) formed on gold surfaces. The main focus is directed towards studying the effects of the environment related to the position of a redox moiety with respect to the surface of the diluents. Understanding these effects, around ferrocene and anthraquinone redox moieties, is vital for designing electro-active devices and biosensors which employ such redox-active groups. The aim was achieved by the synthesis of rigid norbornylogous bridges (NB) with anthraquinone or ferrocene redox-active groups at the distal end, and thiol groups at the proximal end. The structural rigidity of the NB molecules allowed placing the redox moiety, at atomic scale distances, above the surface of the diluents in mixed alkanethiols-NB SAMs assembled on an in-house fabricated atomically flat Au (111) single crystal surfaces. The SAMs were characterized using in-situ infra red spectroscopy, ex-situ sum frequency generation spectroscopy, scanning tunnelling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy and AC voltammetry. Straight and L-shaped NB molecules were shown to situate the anthraquinone moiety close to the surface normal or close to the surface parallel, respectively. It was found that the redox reaction of the anthraquinone moiety is affected by the position of the carbonyl groups with respect to the surface of the diluents. H-bonding interactions of the carbonyl groups with hydroxyl-terminated diluents were found to decrease the rate constant of the redox reaction. In addition, the NB molecules were able to sense the hydrophobic environment near a methyl-terminated surface in which the anthraquinone reduction reaction involves multiple processes. The ferrocene-terminated NB molecules were used in mixed SAMs and the ferrocene moiety was situated at various heights above the surface of the diluents. This was performed by using various lengths of hydroxyl-terminated diluents. It was found that the rate constant and the apparent formal potential decrease as the distance between the ferrocene moiety and the surface of the diluents is increased. The results agree with the theory of interfacial potential distribution and the ion spatial distribution across the electrical double layer.