An Ion-Activated Molecular Electronic Device

Abstract

Several molecular devices based on the electronic switching properties of conducting polymers have been described previously and involve an addition/removal of a proton/electron to/from the polymer chain. This “doping” process creates charged topological defects such as polarons or bipolarons, which act as charge carriers. In the device presented here, we have attempted to produce a topological defect without effecting an electron/proton transfer. Such a topological defect has been produced by incorporating an ion-binding cavity in the polymer phase, which undergoes a conformational change on occupation of the cavity by the appropriate ion. Thus, anchoring 18-crown-6 in polyaniline results in the electronic switching of the polymer device in the presence of as low as 10-7 M K+ ion concentration. That this switching is accompanied by a conformational change was confirmed by measuring the mean molecular area of a Langmuir monolayer of polyaniline−crown film in the presence and absence of K+. It was found that there is an increase and then a saturation in the mean molecular area in response to increasing concentrations of K+ ions in the subphase, a trend which is similar to the electronic conductivity changes in the polymer film. This mechanism of switching makes it possible to design molecular devices that respond to a wide class of ionic species which need not undergo electron transfer to trigger the device.