Ligand-Centred Fluorescence and Electronic Relaxation Cascade at Vibrational Time Scales in Transition-Metal Complexes

Using femtosecond-resolved photoluminescence up-conversion, we report the observation of the fluorescence of the high-lying ligand-centered (LC) electronic state upon 266 nm excitation of an iridium complex, Ir(ppy)(3), with a lifetime of 70 +/- 10 fs. It is accompanied by a simultaneous emission of all lower-lying electronic states, except the lowest triplet metal-to-ligand charge-transfer ((MLCT)-M-3) state that shows a rise on the same time scale. Thus, we observe the departure, the intermediate steps, and the arrival of the relaxation cascade spanning similar to 1.6 eV from the (LC)-L-1 state to the lowest 3MLCT state, which then yields the long-lived luminescence of the molecule. This represents the first measurement of the total relaxation time over an entire cascade of electronic states in a polyatomic molecule. We find that the relaxation cascade proceeds in