Bandgap Engineering of Erbium-Metallofullerenes toward Switchable Photoluminescence

Encapsulating photoluminescent lanthanide ions like erbium (Er) into fullerene cages affords photoluminescent endohedral metallofullerenes (EMFs). Few reported photoluminescent Er-EMFs are all based on encapsulation of multiple (two to three) metal atoms, whereas mono-Er-EMFs exemplified by Er@C₈₂ are not photoluminescent due to its narrow optical bandgap. Herein, by entrapping an Er-cyanide cluster into various C₈₂ cages to form novel Er-monometallic cyanide clusterfullerenes (CYCFs), ErCN@C₈₂ (C₂ (5), C_s (6), and C_{2 v} (9)), the photoluminescent properties of CYCFs are investigated, and obvious near-infrared (NIR) photoluminescence only is observed for ErCN@C₂ (5)-C₈₂ . Combined with a comparative photoluminescence study of three medium-bandgap di-Er-EMFs, including Er₂ @C_s (6)-C₈₂ , Er₂ O@C_s (6)-C₈₂ , and Er₂ C₂ @C_s (6)-C₈₂ , this study proposes that the optical bandgap can be used as a simple criterion for switching the photoluminescence of Er-EMFs, and the bandgap threshold is determined to be between 0.83 and 0.74 eV. Furthermore, the photoluminescent patterns of these three di-Er-EMFs differ dramatically. It is found that the location of the Er atom within the same C_s (6)-C₈₂ cage is almost fixed and independent on the endo-unit; thus the previous statement on the key role of metal position in photoluminescence of di-Er-EMFs seems erroneous, and the geometric configuration of the endo-unit, especially the bridging mode of two Er ions, is decisive instead.