Evidence for the existence of sulfur-doped fullerenes from elucidation of their photophysical properties

Evidence is presented that carbon atoms of the hollow fullerene cage can be replaced by sulfur atoms, as has been suggested theoretically. S-doped fullerenes are obtained by arc-vaporization of graphite in the presence of thiophene or 3-methylthiophene, Mass spectra indicate the dominant processes are the substitutions of pairs of carbon atoms by sulfur atoms with the predominant ratios of sulfur to replaced carbon pairs being 1, 1/3, Or 1/4. Fractions that contain a large group of such compounds with a substantial S enrichment can be collected by column chromatography, but no single pure compound can be isolated. Since the mass spectra can be interpreted in alternate ways involving S incorporation as an adduct, evidence was sought for their existence from a detailed study of the fluorescence of these species. The fluorescence emission spectra are red-shifted from and consistent with absorption spectra and can be attributed to the emission from symmetry-broken distorted S-doped derivatives of fullerenes. An optical bandgap of 2.5 eV is derived for the S-doped fullerenes resulting from the splitting of the h(u) degenerate state. The fluorescence lifetimes of these molecules are 2-7 ns, considerably larger than those of the undoped fullerenes.