| Résumé | Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S1) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S1 state of ππ∗ character for fluorinated benzenes with four or less F atoms, and S1 state of πσ∗ character for PFB and HFB. The πσ∗ character of the S1 state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the π1π∗ (Lb)←S0 transition, which can be identified with the predicted low-energy π1σ∗←S0absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the π1σ∗ state and efficient S1 (πσ∗)→S0internal conversion between two electronic states of very different geometries. |
|---|
