Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl-Bipyridine Complexes Andrea Cannizzo, † Ana Maria Blanco-Rodrı ´guez, ‡ Amal El Nahhas, † Jakub S ˇ ebera, § Stanislav Za ´ lis ˇ,* ,§ Antonı ´n Vlc ˇek, Jr.,* ,‡,§ and Majed Chergui* ,† Laboratoire de Spectroscopie Ultrarapide, ISIC, FSB-BSP, Ecole Polytechnique Fe ´de ´rale de Lausanne, CH-1015 Lausanne-Dorigny, Switzerland, School of Biological and Chemical Sciences, Queen Mary, UniVersity of London, Mile End Road, London E1 4NS, United Kingdom, and J. HeyroVsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkoVa 3, CZ-182 23 Prague, Czech Republic Received December 12, 2007; E-mail: Majed.Chergui@epfl.ch; a.vlcek@qmul.ac.uk; zalis@jh-inst.cas.cz Abstract: Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b 1 A’ state of [Re(L)(CO) 3 (bpy)] n (L ) Cl, Br, I, n ) 0; L ) 4-Et-pyridine, n ) 1+) in acetonitrile was investigated using the femtosecond fluorescence up-conversion technique with polychromatic detection. In addition, energies, characters, and molecular structures of the emitting states were calculated by TD-DFT. The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a 3 A” state at longer times. The analysis of the data allows us to identify three spectral components. The first two are characterized by decay times τ 1 ) 85-150 fs and τ 2 ) 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b 3 A”), respectively. The third component corresponds to the long-lived phosphorescence from the lowest a 3 A” state. In addition, it is found that the fluorescence decay time (τ 1 ) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. τ 2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, molecular orbitals, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling constant of the ligand L, but we find a clear correlation between the ISC times and the vibrational periods of the Re-L mode, suggesting that the latter may mediate the ISC in a strongly nonadiabatic regime. I. Introduction Controlling the behavior of singlet and triplet metal-to-ligand charge transfer ( 1 MLCT and 3 MLCT, respectively) excited states of transition metal complexes is key to their efficient use in photonic applications. For example, operation of Ir III lumino- phores in organic light-emitting diodes (OLED), 1,2 Ru II -based sensitizers of solar cells, 3,4 Re I probes of protein relaxation dynamics, 5 or various luminescence sensors are all based on the presence of 3 MLCT states. Optical excitation of metal- containing chromophores prepares 1 MLCT states, from which the strongly phosphorescent triplet states are populated by intersystem crossing (ISC). Apart from this role as an optical gateway, 1 MLCT states can be exploited in ultrafast chemical processes such as electron injection, energy transfer, or metal-ligand bond dissociation, which can compete with ISC. Understanding the character and dynamics of optically excited 1 MLCT states presents a considerable challenge to contemporary photophysical research, as they are often very short-lived. Singlet-triplet (and also doublet-quartet) ISC rates were determined only in few cases and found to range from tens of femtoseconds to a few picoseconds. 6–14 1 MLCT fluorescence lifetimes of the generic photosensitizers [Ru II (bpy) 3 ] 2+ and [Ru(4,4′-(COOH) 2 -bpy) 2 (NCS) 2 ] (N3) were recently measured * To whom correspondence should be addressed. † Laboratoire de Spectroscopie Ultrarapide, ISIC, FSB-BSP, Ecole Polytechnique Fe ´de ´rale de Lausanne, CH-1015 Lausanne-Dorigny, Switzerland. ‡ School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom. § J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkova 3, CZ-182 23 Prague, Czech Republic. (1) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. ReV. 2006, 250, 2093. (2) Yersin, H. Triplet Emitters for OLED Applications. 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