Effect of halogenated compounds on the photophysics of C-70 and a monoadduct of C-70: Some implications on optical limiting behaviour

The fluorescence spectra, quantum yields and lifetimes of C-70 and a pseudo -dihydro derivative (C70R) have been measured in a wide range of solvents a t room temperature. This information is important for the development of re verse saturable absorbers. Phosphorescence spectra and phosphorescence life times were also measured at low temperature. The fluorescence is subject to quenching by halogenated compounds. The efficiency of quenching follows th e order I > Br >> Cl. The nature of the quenching is shown to vary, with ch lorinated compounds exhibiting static quenching of fullerene fluorescence, owing to nonfluorescent complex formation, whilst those compounds containin g bromine and iodine exhibit dynamic quenching due to the external heavy-at om effect, that increases the intersystem crossing rate constant in the flu orophore perturber complex. This constant is evaluated by an original metho d from the bimolecular quenching rate constants. The phosphorescence quantu m yield of both fullerenes at 77 K slightly increases in the presence of io dobenzene, in spite of a strong decrease in phosphorescence lifetime. The m arked increase of the intersystem crossing rate constant in concentrated so lutions owing to the external heavy-atom effect is of interest for the appl ication of fullerenes as fast-responding optical limiters (reverse saturabl e absorbers) of intense laser pulses, even in cases where the triplet quant um yield is of the order of unity. (C) 2001 Elsevier Science B.V. All right s reserved.