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.