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 = CI, Br, 1, 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.