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)(5)Ru}(mu-pz){Ru(NH3)(5)}](4+), giving rise to two closely lying excited states with mixed-valence character, one with a shorter lifetime (tau=136 ps) and weakly-coupled (Robin-Day Class II) character, the other a longer-lived (tau=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.

Automated summary

In a recent study by Cadranel and co-workers, time-resolved pump-probe spectroscopy was used to observe photo-induced metal-to-bridge charge transfer in the homovalent analogue of the Creutz-Taube ion. This resulted in two closely lying excited states with mixed-valence character, one with a shorter lifetime (136 ps) and weakly-coupled (Robin-Day Class II) character, and the other a longer-lived (2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer-lived species showed similarities with the photoinduced reactions of the photosynthetic special pair, suggesting its potential as a reference system for excited state mixed-valency and photocatalytic reactions.