Femtosecond fluorescence and intersystem crossing in rhenium(I) carbonyl-bipyridine complexes.
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 tau 1 = 85-150 fs and tau 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 (tau 1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. tau 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.
Laboratoire de Spectroscopie Ultrarapide, ISIC, FSB-BSP, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne-Dorigny, Switzerland.
Andrea
A
Cannizzo
Ana Maria
AM
Blanco-Rodríguez
Amal
A
El Nahhas
Jakub
J
Sebera
Stanislav
S
Zális
Antonín
A
Vlcek
Majed
M
Chergui
D015082:2,2'-Dipyridyl / Q000737:chemistry*; D008163:Luminescent Measurements / Q000379:methods; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D012211:Rhenium / Q000737:chemistry*; D013050:Spectrometry, Fluorescence / Q000379:methods
D009942:Organometallic Compounds; D015082:2,2'-Dipyridyl; D012211:Rhenium