Time-Resolved Spectroscopy of the Metal-to-Metal Charge Transfer Excited State in Dinuclear Cyano-Bridged Mixed-Valence Complexes

Visible pump-probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyanobridged mixed-valence complexes of the formula [LCoIIINCMII(CN)5]-, where L is a pentadentate macrocyclic pentaamine (L14) or triamine-dithiaether (L14S) and M is Fe or Ru. Nanosecond pump-probe spectroscopy on frozen solutions of [L14CoIIINCFeII(CN)5]- and [L14SCoIIINCFeII(CN)5]- at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350-400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L14CoIIINCRuII(CN)5]- was too short-lived to allow its detection. Femtosecond pump-probe spectroscopy on aqueous solutions of [L14CoIIINCFeII(CN)5]- and [L14SCoIIINCFeII(CN)5]- at room temperature enabled the lifetimes of their CoII-FeIII MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.