Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency

Abstract

Ultrathin layers of thiophene derivatives were covalently attached to GC electrodes by electroreduction of diazonium salts. The films are densely packed structures and are able to mediate electron transfer above a threshold voltage tuned by the nature of the grafted molecules onto the electrode. We investigate the electron transfer properties of these layers by using electroactive probes having various redox potentials. Voltammetry and SECM measurements clearly show that for redox probes with low redox potentials diode-like behavior is observed and the current can flow in only one direction across these organic layers whereas, when the potential of the external redox probe increases, the transparency of the layers toward electron transfer in both direction increases (within the time range investigated). This behavior is not compatible with an ECcat mechanism, with electroactive immobilized centers characterized by a single redox potential E°Im, and clearly demonstrates the switching of the layer conductivity. In this sense, these layers are better modeled as covalently grafted conducting oligomers densely packed on the surface and capable of changing their charge transfer characteristics upon charge injection (i.e., doping of the grafted conjugated oligomers). The charge transfer mechanism of these ultrathin films is thus very different from that most of SAMs bearing an electroactive group, in which electron tunnelling through the monolayer is the main charge transfer mechanism.