Effect of halogenated compounds on the photophysics of C70 and a monoadduct of C70: Some implications on optical limiting behaviour Original Research Article

The fluorescence spectra, quantum yields and lifetimes of C70 and a pseudo-dihydro derivative (C70R) have been measured in a wide range of solvents at room temperature. This information is important for the development of reverse saturable absorbers. Phosphorescence spectra and phosphorescence lifetimes were also measured at low temperature. The fluorescence is subject to quenching by halogenated compounds. The efficiency of quenching follows the order I>Br≫Cl. The nature of the quenching is shown to vary, with chlorinated compounds exhibiting static quenching of fullerene fluorescence, owing to nonfluorescent complex formation, whilst those compounds containing bromine and iodine exhibit dynamic quenching due to the external heavy-atom effect, that increases the intersystem crossing rate constant in the fluorophore–perturber complex. This constant is evaluated by an original method from the bimolecular quenching rate constants. The phosphorescence quantum yield of both fullerenes at 77 K slightly increases in the presence of iodobenzene, in spite of a strong decrease in phosphorescence lifetime. The marked increase of the intersystem crossing rate constant in concentrated solutions owing to the external heavy-atom effect is of interest for the application of fullerenes as fast-responding optical limiters (reverse saturable absorbers) of intense laser pulses, even in cases where the triplet quantum yield is of the order of unity.