VIS-NIR, Raman and EPR Spectroscopy on Medium Cage Sized Endohedral Fullerenes

摘要

Eu@C_(74) and both isomers of Eu@C_(76) have been isolated from endohedral metal fullerenes containing raw soots produced by means of the radio-frequency method. M@C_(74) (M = Ca, Sr, Ba, Eu) and M@C_(76) (M = Sr, Eu) have been characterized by means of VIS and Raman spectroscopy. The VIS-NIR and Raman spectra are dominated by the spectral features of the fullerene cage. Small shifts of the band positions in the VIS spectra are mostly due to the variations of the ionization potentials of the respective metal. The analysis of the metal vs. cage vibration observed in the low frequency range of the Raman spectra depends on the atomic mass and the charge state of the endohedrally enclosed metal, irrespective of the type and molecular structure of the fullerene cage. Thus, the frequency of the metal vs. cage vibration is a first measure of the valence state of the endohedral metal atom. Preliminary W- and X-band ESR spectroscopic investigations on Eu@C_(74) clearly show the presence of a S = 2/7 spin state of the endohedral Eu atom pointing to an inner salt Eu~(2+)@C_(74)~(2–). The total fading of the ESR signal at temperatures above 15 K is a consequence of the thermal excitation of low lying spin states of the Eu~(2+) cation inside the C_(74) fullerene cage.