As photocatalysts tackle ever more demanding and complex reactions, accurate knowledge of redox potentials (E○) is increasingly important for mechanistic understanding and molecular design. Excited state redox potentials for photocatalysts are typically estimated with ground state E○ values and excited state energies using the Rehm-Weller equation. Cyclic voltammetry (CV) is a powerful and widely-used technique to find these E○ values but many photocatalysts and substrates can show irreversible redox behavior making it impossible to determine E○. Transient photoinduced absorption (PIA) is also very useful to help determine mechanisms but there can sometimes be ambiguity in assigning spectral features to species. Pulse radiolysis (PR) is a complementary technique to photoexcitation and voltammetry. In PR at the Laser Electron Accelerator Facility (LEAF), a 9 MeV electron pulse (~15 ps) generates a known concentration of non-equilibrium free ions in the solvent. These species are the most potent oxidants and reductants that can be generated in a solvent and can be used to rapidly create and study oxidized or reduced states of catalysts or substrates. Transient species can be probed in UV/vis/nearIR/IR/microwave. Spectral assignment is easier because we have created a known concentration of a specific oxidation state since it is the radiation chemistry of the solvent that determines this. Redox potentials for molecules or interest can be determined by electron transfer experiments to and from solutes with known redox potentials. An added advantage of PR is that relative potentials can be determined in any solvent and either with or without the 0.1 M electrolyte required in CV. As such, potentials can be found in the exact environment in which we need to know them. By studying bimolecular electron transfer rates, electron transfer equilibria and decay kinetics, we have investigated a number of systems that have demonstrated irreversible CVs including Ni(II) photocatalysts as well as various alkene and chloroamides substrates. We will summarize these findings and highlight remaining opportunities and challenges for determining redox potentials for short-lived intermediate species that are important in photocatalysis.

Redox potentials with pulse radiolysis for photocatalysis

ACS Fall 2020 Virtual Meeting

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