Journal article

Ultrafast Nonadiabatic Dynamics of [FeII(bpy)3]2+ in Solution

The ultrafast relaxation of aq. Fe(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) was characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence expt. shows a very short-lived broad 1MLCT emission band at .apprx.600 nm, which decays in ?20 fs, and a weak emission at .apprx.660 nm, which the authors attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) <400 nm, and a longer-lived 1 >550 nm, along with the ground-state bleach (GSB). The authors identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decompn. and a global anal. of the TA data, based on a sequential relaxation model, reveal 3 characteristic time scales:120 fs, 960 fs, and 665 ps. The 1st is the decay of the 3MLCT, the 2nd is identified as the population time of the 5T2 state, while the 3rd is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit const. of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, the authors propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate .apprx.2000 cm-1/100 fs. This rate is discussed, and it nevertheless can be described by the Fermi golden rule, despite its high value.


Related material