Factors Controlling Cage Escape Yields of Closed and Open Shell Metal Complexes in Bimolecular Photoinduced Electron Transfer

The cage escape yield, i.e. the separation of the geminate radical pair formed immediately after bimolecular excited-state electron transfer was studied in eleven solvents using six Fe(III), Ru(II), Os(II) and Ir(III) photosensitizers and tri-p-tolylamine as the electron donor. Among all complexes, the largest cage escape yields (0.67-1) were recorded for the Ir(III) photosensitizer showing the highest potential as a photocatalyst in photoredox catalysis. These yields dropped to values around 0.65 for both Ru(II) photosensitizers and to values around 0.38 for the Os(II) photosensitizer. Interestingly, for both open shell Fe(III) complexes, the yields were small (<0.1) in solvents with dielectric constants greater than 20 but were shown to reach values up to 0.58 in solvents with low dielectric constant. The results presented herein on closed shell pho-tosensitizers suggest that the low rate of triplet-singlet intersystem crossing within the manifold of states of the geminate radical pair implies that charge recombination towards the ground state is a spin forbidden process, favoring large cage escape yields that are not influenced by dielectric effects. Geminate charge recombination in open-shell metal complexes, such as the two Fe(III) photosensitizers studied herein, is no longer a spin forbidden process and becomes highly sensitive to solvent effects. Altogether, this study provides general guidelines for factors influencing bimolecular excited-state reactivity using prototypical photosensitizers but also allows to foresee a great development of Fe(III) photosensitizers with 2LMCT excited state in photoredox catalysis, providing that solvent with low dielectric constants are used.