Er3+ Photoluminescence in Er2@C82 and Er2C2@C82 Metallofullerenes Elucidated by Density Functional Theory

Metallofullerenes with two erbium atoms encapsulated in IPR C82 cage isomers Cs-6 (I), C2v-9 (II), and C3v-8 (III) were investigated using density functional theory. The calculations suggest that erbium atoms assume a trivalent state with Er (4f11) valence electronic configuration in Er2@C82 and Er2C2@C82, where two electrons (6s2) per Er atom are transferred to the cage carrying four negative charges (C824–), while the third electron is promoted from the 4f to the 5d shell, becoming involved in covalent bonding to near atoms. Experimentally, Er3+-like emission from 4I13/2 to 4I15/2 was observed, and our calculations indicate that the Er–Er covalent metal bond in Er2@C82, and the Er–C/C2 covalent bonds in Er2C2@C82, can account for the observed photoluminescence despite the cage with C824–. Such existence is the reason that the C2 unit was found to be neutral on the basis of MEM-Rietveld X-ray measurements, although formally it should be described as C22–. Our prediction for isomer photoluminescence intensity agrees with the experimentally determined order (III > I > II), where the most pronounced activity of isomer III in Er2C2@C82 stems from its higher charge of formal Er3+ and its largest HOMO–LUMO gap.