Periodical Variation of Electronic Properties in Polyhydroxylated Metallofullerene Materials

Endohedral metallofullerenes (fullerenes with metal atom(s) encapsulated), as a novel form of carbon-related materials, have attracted special attention for their potential applications in electronic devices, biomedical fields such as therapeutic medicine, and a new generation of magnetic resonance imaging (MRI) contrast agents, etc. Recently, the polyhydroxylated Gd@C82 material was found to possess a very high efficiency of inhibiting cancer growth in vivo, and to have a strong capacity to improve immunity and interfere with tumor invasion in normal muscle cells. Unlike conventional anticancer chemicals that use a high toxicity to kill cells, this material shows non-toxicity in vivo and in vitro and does not kill normal cells directly. This finding indicates that this type of material with proper surface modifications may realize the human dream of cancer chemotherapeutics of high efficacy but low toxicity. Although the structural and electronic properties of endohedral metallofullerenes have been well investigated, the chemical functionalization of the metallofullerene and its properties are not yet well studied and understood. Recently, a unique electronic structure of Gd@C82 caused by the encapsulation of the Gd atom was theoretically predicted. How do the electronic properties of the material vary if the C82 nanostructure is further modified with chemical groups? This is an intriguing topic for exploring the functions of a new nanomaterial because the modulation of the electronic properties of metallic atoms restricted to a nanospace is of significant and wide interest, though it is especially difficult. Recently, it was found that electronic configurations of atoms inside the fullerene cage can be tuned via chemical modifications to the cage surface. But, on the chemical modifications of metallofullerenes it was found theoretically that i) the highest occupied molecular orbital (HOMO) in M@C82 tends to distribute locally, ii) the addition locations prefer to initiate at the cage surface opposite to the metallic position, and iii) the addition pattern on the hollow fullerene cage tends to array as a cluster which shifts with the increasing number of added groups. These raise many intriguing questions, for example, how do the electronic properties of the modified material vary with the changing number of added groups to the outer surface? To this end, we synthesized and purified the Gd@C82 and Gd@C82(OH)x materials with a changing number of hydroxyls. The electronic properties of the Gd@C82(OH)x film were then studied using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray absorption spectroscopy (SRXAS). Surprisingly, when the OH groups reach a certain number in Gd@C82(OH)x, the electron emission of the innermost Gd shows a periodical emergence or disappearance, depending on how many hydroxyls are added to the outer surface of the fullerene cages. Such a unique phenomenon is observed for the first time. The results suggest that polyhydroxylation of metallofullerene may be a new way for designing materials with novel electronic, optical, or magnetic functions. Figure 1(A1–D1) shows results from the Gd valence band photoemission spectroscopy (PES) of Gd@C82 and Gd@C82(OH)x films obtained with an incident photon energy of 140.0 eV. In Gd@C82 the 31.4 eV energy level is not observed but it emerges in Gd@C82(OH)12. Surprisingly, this photoemission property vanishes in Gd@C82(OH)20 and appears again in Gd@C82(OH)26. The energy level periodically appears or disappears with the changing number of OH groups added to the fullerene-cage surface. To identify the 31.4 eV peak, we investigated the energy dependence of the valence band PES for Gd@C82 and Gd@C82(OH)x, the results are given in Figure 2. The PES spectra (measured and normalized by the beam flux of the synchrotron radiation) of Gd@C82(OH)20 are similar to those of Gd@C82, while Gd@C82(OH)26 and Gd@C82(OH)12 show similar spectra. So only the spectra for Gd@C82 and Gd@C82(OH)12 are shown in Figure 2 as representative results. The spectra have less dependence on the incident photon energy, excluding the possibility that the observed C O M M U N IC A TI O N S

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