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), Cs(6), and C2v(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@Cs(6)‐C82, Er2O@Cs(6)‐C82, and Er2C2@Cs(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 Cs(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. The photoluminescent property of monometallic cyanide clusterfullerenes is investigated. By encapsulating a triangular erbium (Er)‐cyanide cluster into a C2(5)‐C82 cage, photoluminescence of mono‐Er‐metallofullerene is switched on. Combined with three medium‐bandgap di‐erbium‐metallofullerenes, the bandgap threshold for judging whether an Er‐metallofullerenes is photoluminescent is determined to be between 0.83 and 0.74 eV.