Electrogenerated Chemiluminescence of Bithiophenes with Methylthio Functionalities

Abstract

The photoluminescence and electrogenerated chemiluminescence (ECL) of 5,5′-bis(methylthio)-2,2′-bithiophene (BMTbT) have been studied and compared to those of 2,5-bis(methylthio)thieno[3,2-b]thiophene (f-BMTbT) and 5,5′-diethyl-2,2′-bithiophene (DEbT), the fused-ring and alkyl analogues of BMTbT, respectively. All of these compounds emitted in the blue/violet region of the visible spectrum, with a fluorescence quantum yield of ca. 0.03. In acetonitrile, BMTbT and f-BMTbT each exhibited an irreversible reduction wave in addition to the well-documented two-electron oxidation processes. ECL was observed from solutions of BMTbT and of f-BMTbT via annihilation of the electrogenerated anion and dication, with emission following only the reduction step, likely due to an unstable anion species. Lack of solvent stability at potentials required to reduce or oxidize DEbT in acetonitrile or tetrahydrofuran, respectively, precluded annihilation ECL in these solvents. For all three compounds, sweeping to negative potentials in the presence of the coreactant benzoyl peroxide produced ECL. Emission from BMTbT was detected for over 1000 cycles using benzoyl peroxide. Herein, it is demonstrated that thioether functionalities may provide a stable alternative to alkyl side chains in oligothiophenes in light-emitting applications.