Excited State Mixed‐Valence Complexes: From the Special Pair to the Creutz–Taube Ion and Beyond

Compounds and complexes with mixed‐valence electronic ground states, such as the Creutz–Taube ion, have proven to be excellent vehicles through which to study intramolecular electron‐transfer processes. In a recent contribution by Cadranel and co‐workers, time‐resolved pump‐probe spectroscopy reveals photo‐induced metal‐to‐bridge charge transfer within the homovalent analogue of the Creutz–Taube ion, [{(NH3)5Ru}(μ‐pz){Ru(NH3)5}]4+, giving rise to two closely lying excited states with mixed‐valence character, one with a shorter lifetime (τ=136 ps) and weakly‐coupled (Robin‐Day Class II) character, the other a longer‐lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer‐lived species demonstrate analogies with the photo‐induced reactions of the photosynthetic special pair, suggesting this state as a reference system for excited state mixed‐valency, and a framework from which to explore photocatalytic reactions. Photoexcitation of the low‐lying metal‐bridge charge transfer (MBCT) transition in the homovalent analogue of the Creutz–Taube ion generates an excited state mixed‐valence species {Ru(III−δ)}(μ‐pz.−){Ru(II+δ)} with a delocalized electronic structure and remarkably long lifetime (2.8 ns), permitting intermolecular electron transfer and hinting at potential for designs of artificial photosynthetic systems.