REDOX SWITCHING KINETICS OF POLYTHIONINE FILMS IN AQUEOUS ACETIC-ACIDSOLUTIONS

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