Electron Transfer Theory Elucidates the Hidden Role Played by Triethylamine and Triethanolamine during Photocatalysis

Triethylamine (TEA) and triethanolamine (TEOA) are renowned, in part, for their ability to reductively quench excited states by outer-sphere electron transfer with vast and still growing applications as sacrificial electron donors for photocatalytic systems. Upon amine oxidation, the resulting TEA^•+ and TEOA^•+ radical cations undergo proton transfer (or hydrogen atom transfer), resulting in the formation of a chemical reductant that has an α-carbon centered radical adjacent to the nitrogen center (TEA^• and TEOA^•). In this contribution, we have electrochemically and spectroscopically characterized a set of electron acceptors which, upon accepting an electron, are a series of photocatalysts, [ReCl(R₁R₂-bpy)(CO)₃]^•-, where R₁ and R₂ are electron-donating and electron-withdrawing groups in the 4,4'- and 5,5'-positions on the bipyridyl ligand. We substantiated the formation of the electron donors, TE(O)A^•, by spin trapping electron paramagnetic resonance spectroscopy, where TE(O)A^• reacts with 2,4,6-tri-tert-butylnitrosobenzene to generate N-centered and O-centered radical adducts. Having established the chemical behaviors of the electron acceptors and donors individually, the electron transfer rate constants were determined across a 1.43 V range in driving force. The redox potential of TEA^• was benchmarked to within ±80 mV on an absolute scale in V vs Fc⁺/Fc in CH₃CN by using an empirical rate vs free-energy correlation, electron transfer theory, and density functional theory calculations. The equilibrium potentials for TEA^• and TEOA^• were determined to be -1.98 V and -1.76 V, respectively. Based on the kinetic and thermochemical analysis presented for TEA^• and TEOA^•, these transient radicals can be broadly considered strong homogeneous chemical reductants within the wider context of photoredox potentials. Thus, this work clarifies a frequently unnoticed secondary function for these sacrificial electron donors during photocatalysis and rationalizes the possibility of a one-photon/two-electron conversion process that is dependent on the free-energy exchange between TE(O)A^• and photocatalysts.