Absolute calibration for cyclic voltammetry from the solution-phase ionisation of ferrocene

Accurate determination of the energy levels of materials is crucial to many fields of science and technology, including electronics, catalysis, and energy generation and storage. The frontier molecular orbital levels of molecules are commonly inferred from their oxidation and reduction potentials measured in solution using voltametric techniques, which are reported versus a standard, typically an internal one such as a ferrocenium/ferrocene (Fc⁺/Fc) redox couple. At present, however, multiple reference electrode scales are used across the literature, leading to discrepancies of up to 0.3 eV. Here, we report an absolute energy level measurement for (Fc⁺/Fc) in acetonitrile solution. Specifically, we determined the adiabatic ionisation energy of ferrocene in acetonitrile solution to be 4.94 ± 0.05 eV using ambient pressure photoemission spectroscopy. By comparing the energy-dependence of photoemission from different solution concentrations with a model for photoemission from solution, we confirm that we measure the adiabatic ionisation energy and that liquid surface barrier effects are minimal. This value is consistent with one of several conflicting reference values used in the literature. The result therefore provides a benchmark value for the Fc⁺/Fc internal reference, widely used for the conversion of voltammetry data to the absolute energy scale.