Redox switching kinetics of polythionine films in aqueous acetic acid solutions

The kinetics of polythionine film redox switching in aqueous acetic acid solutions were studied. The thionine/ leucothionine half-reaction is a three proton/two electron process. Dissociation of a coordinated acetic acid molecule within the film provides one proton and the charge-balancing acetate counterion. The overall two electron reduction process also involves the entry of two protons into the film from the aqueous solution and the expulsion from the film to the solution of one water molecule per redox site. The process can be described by using a 1 X 1 scheme of squares mechanism involving electron, proton, and solvent transfer. Electrochemical quartz crystal microbalance (EQCM) data show that an intermediate species is involved in each direction of redox conversion. During oxidation the intermediate, Int(ox), is a dehydrated species. During reduction the intermediate, Int(red), is a hydrated species. In both the oxidation and reduction electrode processes, solvent transfer is rate limiting. The barrier for water entry into Int(ox) is considerably larger than that for water exit from Int(red). The relative barrier heights for solvent transfer determine the mechanistic pathway through the scheme of squares.