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.

Vibrational Relaxation and Redistribution Dynamics in Ruthenium(II) Polypyridyl-Based Charge-Transfer Excited States: A Combined Ultrafast Electronic and Infrared Absorption Study

“…To the extent that IVR affects the spatial localization of the excitation energy, this process can lead to differences in excited-state reactivity based on the geometric and compositional properties of the system. 1,[6][7][8][9][10][11][12][13] Despite its important role in excited state dynamics, detailed investigations of vibrational relaxation dynamics in transition metal-based compounds are relatively limited in number. 3,[6][7][8][9]12,[14][15][16][17][18][19][20][21] In considering the general problem of understanding the mechanism of excited-state evolution in transition metal complexes, it occurred to us that an examination of this issue must confront two critical problems.…”

“…2,[22][23][24] One can make a reasonable argument based on anticipated changes in Franck-Condon factors in support of this assertion, but it is still an indirect means of probing vibrational dynamics in electronic excited states. The development of ultrafast vibrational spectroscopies over the past several years presents tremendous opportunities for the direct measurement of vibrational dynamics in charge-transfer excited states, [6][7][8][9][12][13][14][15][16]20,21 but these methodologies tend to be more technically challenging and therefore not as commonly employed in the physical-inorganic community as electronic absorption spectroscopy.…”

Final Report: Vibrational Dynamics in Photoinduced Electron Transfer

“…13 With transient infrared absorption spectroscopy we observe two electron-transfer components of ~ 700 fs and ~ 5 ps by transient IR with 75 fs resolution. The two ET lifetimes are attributed to the presence of two ion-pair contact geometries with absorption origins different by about 1250 ± 350 cm −1 .…”

“…2,3,6 These measurements required innovative ultrafast laser measurements with transient infrared wavelengths, and during the period of the grant we improved from 2-3 picosecond resolution to 50 femtosecond resolution. 13 Thus far, these are the only measurements in the literature of vibrationally dependent electron transfer in solutions. We also developed electron transfer models 8,13 and computational structural models 7,10,14 to understand the experiments.…”

“…13 Thus far, these are the only measurements in the literature of vibrationally dependent electron transfer in solutions. We also developed electron transfer models 8,13 and computational structural models 7,10,14 to understand the experiments. We then generalized the treatment of vibrational properties of molecules for electron transfer or any exothermic kinetic process 10,11 ; where we discovered that proper treating of the vibrational distortion details can provide several orders of magnitude different rates than standard electron transfer models for exothermic cases.…”