Ion Selection by Redox Changes in a Soluble Helical Polythiophene Channel

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Molecular dynamics are applied to simulate molecular motions in an aqueous solution of two soluble derivatives of a 100-ring polythiophene chain – one with sidechains terminating in a SO3– group, the other in NMe3+. Each chain is in a helical conformation defining a water-containing central channel along whose axis the dynamics of ions from the solution to the channel’s axial electrostatic potential is simulated. The profiles of these potentials distinguish the tendencies of the two channel species to occlude water molecules on their surfaces. Invoking the conductive polymer characteristics of polythiophene that can accomplish the transfer of electrons between the aromatic rings and redox reagents in the solution, the effect of this transfer on the axial potential and migration is followed. The electrostatic potential monitors differences in the association of the solvent molecules with the two species of helical polymer and shows that while Na+ and Cl– ions do not enter the channel in the absence of the redox changes, an ion with a selected charge does so spontaneously when appropriate electric charge is transferred to the channel. This enables the selected ion to travel about 10 – 20Å in the channel without the application of an external electric field.