Ultrafast nonadiabatic dynamics of [FeII(bpy)3]2+ in solution

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band ((MLCT)-M-1) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad (MLCT)-M-1 emission band at similar to 600 nm, which decays in <= 20 fs, and a weak emission at similar to 660 nm, which we attribute to the (MLCT)-M-3, populated by intersystem crossing (ISC) from the (MLCT)-M-1 state. The TA studies show a short-lived (< 150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the (MLCT)-M-3 state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state T-5(2) lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the (MLCT)-M-3, the second is identified as the population time of the T-5(2) state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [Ru-II(bpy)(3)](2+) and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the (MLCT)-M-1 absorption, we 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 T-5(2) state dissipate similar to 2000 cm(-1)/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.