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 ((3)MLCT) 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 ∼1.6 eV from the (1)LC state to the lowest (3)MLCT 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 ≤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 intersections and their overdamping to low-frequency intramolecular modes. On the basis of literature, we also conclude that this behavior is not specific to transition-metal complexes but also applies to organic molecules.
Laboratoire de Spectroscopie Ultrarapide (LSU) and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, ISIC, FSB , CH-1015 Lausanne, Switzerland.|Dipartimento di Fisica e Chimica, Università degli Studi di Palermo , Via Archirafi 36, 90123 Palermo, Italy.
Fabrizio
F
Messina
Laboratoire de Spectroscopie Ultrarapide (LSU) and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, ISIC, FSB , CH-1015 Lausanne, Switzerland.
Enrico
E
Pomarico
Laboratoire de Spectroscopie Ultrarapide (LSU) and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, ISIC, FSB , CH-1015 Lausanne, Switzerland.
Mahsa
M
Silatani
School of Chemistry, University of Birmingham , Edgbaston, Birmingham B15 2TT, United Kingdom.
Etienne
E
Baranoff
Laboratoire de Spectroscopie Ultrarapide (LSU) and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, ISIC, FSB , CH-1015 Lausanne, Switzerland.
Majed
M
Chergui
electronic relaxation; fluorescence up-conversion; iridium complexes; ligand-centered fluorescence; ultrafast