Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO₂ reduction catalyst

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Br & oslash;nsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA(center dot+) and TEOA(center dot+) radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA(center dot) and TEOA(center dot), respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA(center dot) and TEOA(center dot) radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA(center dot) and TEOA(center dot) radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)(3) (bpy = 2,2 '-bipyridine), by these radicals [k(TEA center dot) = (4.4 +/- 0.3) x 10(9) M-1 s(-1) and k(TEOA center dot) = (9.3 +/- 0.6) x 10(7) M-1 s(-1)]. The similar to 50x smaller rate constant for TEOA(center dot) indicates, that in contrast to a previous assumption, TEA(center dot) is a more potent reductant than TEOA(center dot) (by similar to 0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA(center dot) radicals.