Memory effects in functionalized conducting polymer films: titanocene derivatized polypyrrole in contact with THF solutions

Films with a polypyrrole matrix and pendant titanocene dichloride centers, p(Tc3Py), have been obtained by potentiostatic electropolymerization of the titanocene-pyrrolyl derivative, Tc3Py=Tc(CH2)3NC4H4 (Tc=Cl2TiCpCp′, Cp=C5H5, Cp′=C5H4), in acetonitrile (AN) solutions on a Pt surface. The redox activity has been studied after the transfer of the film-coated electrode into the monomer-free solution of the same electrolyte, TBAPF6, in THF. Contrary to the case of AN or dichloromethane solutions, one can achieve a stable redox activity of the immobilized Tc centers in THF solutions during a series of cycles. A parallel between the redox properties of the Tc complex in solution and inside the film for different solvents has been established. Various ‘memory’ effects have been observed, i.e. the dependence of the film response on the previous treatment of the film: (a) a prepeak at the onset of the polymer matrix oxidation (observed earlier for other functionalized conducting polymer films); (b) an ‘enhanced plateau’ within the range of the polymer matrix electroactivity; (c) a strong increase of the current within the reduction peak of titanocene centers after the previous passage through the range of the matrix electroactivity (compared to the response when the potential variation is limited to the titanocene potential range); (d) a shift of all oxidation waves in the positive direction as a result of the film exposure to sufficiently high negative potentials as well as their backward shift if the film is held at a potential in the ‘window’ between the two ranges of electroactivity. Possible interpretations of this phenomenon have been discussed. A non-equilibrium origin of this phenomenon has been proven. An attribution of the observed effects to redox processes in the polymer matrix and immobilized titanocene centers has been proposed.