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 C_s-6 (I), C_2v-9 (II), and C_3v-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 (C₈₂^4-), 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 ⁴I_13/2 to ⁴I_15/2 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 C₈₂^4-. 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 C₂^2-. 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.