Adsorption of Carboxyl-Terminated Dithiophene and Terthiophene Molecules on ITO Electrodes and Their Electrochemical Coupling to Polymer Layers. The Influence of Molecular Geometry

Abstract

Carboxypentyl and carboxyhexyl bithiophenes and terthiophenes, in which the oligothiophene tail is either perpendicularly or linearly linked to the tethering carboxyalkyl chain, were adsorbed on ITO electrodes. The obtained stable monolayers are anodically oxidized in acetonitrile to produce polymer layers (storing a reversible charge of 15 μC cm-2) in the case of perpendicular adsorbates whereas linear bithiophene and terthiophene are able to couple only with oligothiophenes in solution. Electrochemical and UV−vis spectroscopic analysis indicate that the polymer layers are in fact constituted by thiophene hexamers. The adsorbed linear terthiophene may be coupled with the terthiophene itself in solution to produce a one-end surface-grafted sexithiophene monolayer (reversible charge 40 μC cm-2), whereas a two-end surface-grafted sexithiophene with the same coverage is produced by adsorption of the α-coupled terthiophene. The one-end and two-end grafted sexithiophene monolayers display different electrochemical and spectral patterns. Chronoamperometry and cyclic voltammetry show that the adsorbed linear bithiophene may be oxidatively coupled with 2,2‘-bithiophene in solution to produce surface-anchored polythiophene chains.