Electrochemical and Computational Studies on the Electrocatalytic Effect of Conducting Polymers toward the Redox Reactions of Thiadiazole-Based Thiolate Compounds

We have studied the electrocatalytic effects of polythiophene-based conducting polymers toward the redox reactions of the dilithium salt of the thiadiazole-based dithiol compound 2,5-dimercapto-1,3,4-thiodiazole (DMcT-2Li) via cyclic voltammetry (CV), rotating-disk electrode voltammetry, and electrochemical impedance spectroscopy (EIS). We have found that the electrocatalytic activity of the conducting polymers is strongly influenced by the potential range over which the polymers are electrically conductive (i.e., window of conductivity), which was tuned by employing different electron-donating groups at the 3- or 3,4-positions of polythiophene (PTh). Both poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-propylenedioxythiophene) (PProDOT), whose windows of conductivity exhibited a good overlap with the formal potential for the dimerization process of DMcT-2Li; E 0′d (−0.54 V versus Ag/Ag+) exhibited electrocatalytic activity toward both the oxidation and reduction processes of DMcT-2Li. On the other hand, PTh, poly(3-methylthiophene) (PMTh), and poly(3,4-dimethoxythiophene) (PDMTh), whose windows of conductivity did not overlap with E 0′d, did not exhibit electrocatalytic activity. The standard charge transfer rate constants for the dimerization process of DMcT-2Li at PEDOT, PProDOT, and PDMTh film-modified glassy carbon electrodes (GCEs) were estimated to be 7.4 × 10−4, 3.2 × 10−4, and 6.9 × 10−5 cm/s while the rate constant was 6.3 × 10−5 cm/s at an unmodified GCE. Moreover, EIS studies for PEDOT, PProDOT, and PDMTh film-modified GCEs indicated the smallest charge transfer resistance for a PEDOT film and highest for a PDMTh film at E 0′d, indicating that the higher the electrical conductivity of a film at E 0′d the higher the electrocatalytic activity toward the redox reactions of DMcT-2Li. These results clearly indicate that in order to accelerate the redox reactions of DMcT-2Li (and likely of other organosulfur compounds) the window of conductivity of a conducting polymer needs to overlap the formal potentials of the organosulfur compounds and, thus, support our previous observations that the electrocatalytic reactions proceed via electron exchange reactions between DMcT-2Li and conducting polymers such as PEDOT. Additional computational results for oligomers of PEDOT, ProDOT, and PDMTh showed that substituents at the 3,4-positions of the thiophene ring influence the window of conductivity via steric and electronic effects. This study provides imp