Turning On the Near-Infrared Photoluminescence of Erbium Metallofullerenes by Covalent Modification.

The photoluminescence of lanthanide ions inside fullerenes is usually very weak due to the quenching effect of the fullerene cage. In the case of Er@C, the near-infrared emission from the Er ion is completely quenched by the C fullerene cage. It remains challenging to turn on the photoluminescence of Er@C and other monometallofullerenes. In this work, we adopt a covalent modification strategy to alter the electronic structure of the fullerene cage for sensitizing the near-infrared emission of Er ions in metallofullerenes Er@C (2 = 72, 76, and 82). After covalent modification with trifluoromethyl, phenyl, or dichlorophenyl groups, the erbium metallofullerenes exhibit photoluminescence at 1.5 μm, which is the characteristic emission of the Er ion. Particularly, the otherwise nonfluorescent metallofullerene Er@C is transformed into fluorescent derivatives by using this strategy. The photoluminescence from the Er ion is ascribed to energy transfer from the fullerene cage to the Er ion. According to theoretical calculations, the sensitization of the Er ion by the fullerene cage is associated with the large HOMO-LUMO gap and the closed-shell electronic structure of the metallofullerene derivatives. This work provides useful guidance for the design and synthesis of new fluorescent metallofullerenes.