Lessons Learnt in Photocatalysis - the Influence of Solvent Polarity and the Photostability of the Photocatalyst

The yields obtained in a photocatalysis reaction are frequently dependent on the choice of solvent. Yet, the intrinsic optoelectronic properties of photocatalysts (PCs) that form the basis for the thermodynamic driving force of the photocatalysis are often acquired in a solvent different to that used in the photocatalytic reaction, despite these crucial parameters being solvent dependent. Herein, we report the detailed computational modelling and optoelectronic characterization of eight popular PCs, encompassing transition metal complexes and organic compounds, in four commonly used solvents of varying polarity. Significant variation of up to 270 mV in the experimental ground-state and excited-state redox potentials is noted as a function of solvent polarity, while experimental triplet energies are found to be dependent on solvent (up to 110 meV) when the excited state is charge transfer, rather than locally excited, in nature. A range of photocatalytic electron and energy transfer reactions were investigated using a subset of the PCs and solvents to verify the impact of the changes in optoelectronic properties on the yields of the reactions. For the photoredox reactions, the yields are not correlated with solvent polarity. Instead, when the PC could promote the formation of the target product, we observed photodegradation for all PCs across all solvents, something that is rarely investigated in the literature. This, therefore, makes it difficult to ascertain whether the parent PC and/or the photodegraded product is responsible for the photochemistry, or indeed, whether photodegradation is actually detrimental to the reaction yield. Conversely, the PCs were found to be photostable for energy transfer reactions; however, yields were not correlated to the triplet energies of the PCs, highlighting that triplet energies alone are not a suitable descriptor to discriminate the performance between PCs in photoinduced energy transfer processes.