Ultrafast Electron Transfer in the [Co(Cp)2|V(CO)6] Radical Pair

Abstract

We have reexamined our earlier report of electron transfer in the [Co(Cp)2|V(CO)6] radical pair using ultrafast infrared transient absorption spectroscopy. The radical pair is created from the [Co(Cp)2+|V(CO)6-] ion pair by ultrafast visible charge-transfer excitation. Transient absorption experiments with <75 fs resolution reveal two major direct electron-transfer (ET) components with ∼700 fs and ∼5 ps time constants. A small ET component with a ∼75 ps time constant is due to some separation and re-formation of the radical pairs. Transient absorption experiments monitoring the recovery of the ion-pair state show that both fast components are due to ET rather than some other vibrational relaxation (VR) process in the radical state. By modeling the visible charge-transfer band, the two fast ET decay times are assigned to two ion-pair contact geometries with absorption origins different by about 1250 ± 350 cm-1. The ∼700 fs ET lifetime depends on the vibrational quantum state of the nontotally symmetric CO stretch in the V(CO)6 radical, where the lifetime decreases by ∼10% for the first vibrational quantum and ∼45% for the second quantum. There is no quantum effect for the second ion-pair geometry with a 5 ps ET lifetime. Standard ET rate models cannot explain the rate dependence upon vibrational quantum state for a nontotally symmetric vibration, and it may arise from a breakdown of the Condon approximation. We also find that the intramolecular vibrational redistribution (IVR) time to transfer vibrational energy from the totally symmetric CO stretch to the nontotally symmetric stretch is less than 75 fs for a 1-quantum IVR process. This is unusually fast for metal carbonyls and may be assisted by the Jahn−Teller geometry change of the radical. The 2-quantum IVR time is ∼200 fs for 800 and 700 nm charge-transfer excitation wavelengths. At excitation wavelengths of 620 and 555 nm all quantum levels show a 200 fs rise time, which is unexpected for the zero quantum level. We assign this effect to the onset of sufficient internal vibrational energy in low-frequency vibrations to cause geometric interconversion between energetically similar Jahn−Teller geometries in the V(CO)6 radical. The 200 fs rise time is the time for the V(CO)6 radical species to assume a stable geometry, which requires VR of low-frequency vibrations to the solvent. These results demonstrate that earlier measurements from our group on the same molecule had insufficient time resolution to observe the ultrafast ET component and thereby inferred a vibrational quantum effect in a single ET rate of longer duration.