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.