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-centred (LC) electronic state upon 266 nm excita-tion of an iridium complex, Ir(ppy)3, with a lifetime of 70±10 fs. It is accompanied by a sim-ultaneous emission of all lower lying electronic states, except the lowest triplet metal-to-ligand charge transfer (3MLCT) state that shows a rise on the same time scale. Thus, we ob-serve the departure, the intermediate steps and the arrival of the relaxation cascade span-ning ~1.6 eV from the 1LC state to the lowest 3MLCT state, which then yields the long-lived luminescence of the molecule. This represents the first measurements of the total relaxation time over an entire cascade of electronic states in a polyatomic molecule. We find that the relaxation cascade proceeds in ≤ 10 fs, which is faster than some of the highest frequency modes of the system. We invoke the participation of the latter modes in conical intersec-tions and their overdamping to low frequency intramolecular modes. Based on literature, we also conclude that this behaviour is not specific to transition metal complexes, but also applies to organic molecules.