5,5′-bis(methylthio)-2,2′-bithiophene:: A potential cathode electroactive material for energy storage devices

We present a study of the electrochemical properties of a set of oligothiophenes including 2,5-bis(methylthio)-thiophene (BMTT), its derivatives, and 5,5'-bis(methylthio)-2,2'-bithiophene (BMTbT). BMTbT, which possesses an alpha-coupled bithiophene capped with methylthio groups, exhibited two one-electron reversible processes at 3.8 and 4.0 V vs Li/Li+, respectively, while BMTT exhibited only one reversible process. The chemically and electrochemically reversible processes of BMTbT derive from the stabilization of the monocation and dication by the methylthio groups at the 2,5-positions and by the extension of the conjugated resonance structure (i.e., from mono- to bithiophene). Compared to other substituents at the 2,5-positions such as methoxy, methyl, sulfinylmethyl, t-butylthio, and pyrrolidine groups, we found that methylthio groups uniquely stabilize the oxidized species more effectively, giving rise to the chemically reversible redox couples. Furthermore, the stabilizing effect of the methylthio groups was found to be effective only with a thiophene ring but not with other structures such as thiadiazole, N-methylpyrrole, or benzene. On the basis of the redox potentials and the theoretical gravimetric capacity (209 mAh/g), BMTbT is capable of providing an energy density of 836 mWh/g. Combined with its fast charge-transfer kinetics (k(0) = 2.3 x 10(-2) cm/s for the first redox process at 3.8 V vs Li/Li+), BMTbT is highly promising as an energy-storage chemical system for a redox capacitor, lithium-ion battery, and/or lithium-ion capacitor applications upon additional chemical modification to incorporate it (confine the. site) to a current collector (e.g., hybridization with a polymeric backbone).