Evidence for the Existence of Sulfur-Doped Fullerenes from Elucidation of Their Photophysical Properties

Abstract

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.