Schematic representation of the mechanism of PL excitation, energy transfer, and PL emission in Mn2+-doped material. The orange lines represent the energy level diagram of the Mn2+ ion in a free-ion state (right) and tetrahedrally coordinated environment (left) in a supertetrahedral nanocluster. The dotted arrows indicate nonradiative transitions. PL, photoluminescence. Reprinted with permission from Lin et al. (J Am Chem Soc 2014;136:4769-4779) . Copyright 2014 American Chemical Society.