(Photo)redox Processes Involving the Eu(II)/Eu(III) Couple

Processes involving electron transfer driven by differences in chemical potential and/or light hold promise for chemical transformation. One system that has been proposed for these applications is the versatile Eu(II)/Eu(III) redox couple, where either species has diverse and tunable optical and electrochemical properties. Here, the details of the processes involved in electron transfer from and to europium centers are considered and probed using seven differing organic dyes in dimethyl sulfoxide (DSMO)─the only solvent where all species are soluble and can be oxidized and reduced. Most dyes were reduced by Eu(II), but one dye, triazatriangulenium (TATA), enabled an experimental determination of quenching by Eu(III) and Eu(II) via photoelectron transfer quenching. It was found that while Eu(III) does not interact strongly with the organic dyes, Eu(II) is a potent reducing agent for organic molecules, disregarding whether the substrate is in the ground state or the excited state. We concluded that ground state reduction of any substrate by Eu(II) must be considered in all cases, where photoredox chemistry involving the Eu(II)/Eu(III) redox couple is explored. Finally, we advocate that the redox potentials of all involved species be determined if a mechanism is proposed.