ANODIC OVEROXIDATION OF POLYTHIOPHENES IN WET ACETONITRILE ELECTROLYTES

Three polythiophenes, the parent molecule (PT), poly(3)methylthiophene (PMT) and polybithiophene (PBT) were potentiodynamically overoxidized at potentials U-OO positive to the reversible doping/undoping redoxpe aks U-rev(+) up to 2.5 V vs see. The water concentration in the aceton itrile with 0.1 M QAS was varied in the range of 0.002 to 20 M. Large, irreversible overoxidation peaks are observed with peak potentials ra nging from 1.8 to 2.2 V vs see (at 0.1 M H2O). Coulometric evaluation with respect to the reversible charge, which is characterized by y = 0 .33, one charge per 3 monomer units, shows that a Y-factor of 2-14 is characteristic for the overoxidation process. At low water concentrati ons, up to 0.1 M H2O, Y corresponds to 4 F per 3 monomer units. It is attributed to the dioxide (sulphone) formation at one of those three m onomer units. FTIR reveals in addition that the SO, group is even pres ent just after the galvanostatic electropolymerization due to the clos e vicinity of the potentials U-rev(+), and U-OO. This partial overoxid ation has nearly no influence on the electronic conductivity. At highe r water concentrations, 1-10 M, a further oxidation proceeds with an o verall charge stoichiometry of 12 F/mole. This corresponds to an oxida tive SO, elimination, described previously for the monomer, and the fo rmation of carbonyls in the 2-, 3- and 5-position. At the vicinal dica rbonyl, anodic C-C cleavage occurs finally, and two carboxylic groups arise, which is compatible to the result of a leaching with 0.01 M NaO H in 50% isopropanole. The results open the possibility for a systemat ic anodic modification of the conducting polymer through organic elect rochemistry in the solid state. Transport processes are not limiting d ue to the porosity of the polymer layer. Swelling in MeCN seems to pla y some role in addition. Copyright (C) 1996 Elsevier Science Ltd