Direct identification of energy transfer mechanism in CeIII-MnII system by constructing molecular heteronuclear complexes

Sensitization of metal-centered forbidden transitions is of great significance. Solid Mn^II-based phosphors with d-d forbidden transition sensitized by Ce^III with d-f allowed transition are promising light conversion materials, but the energy transfer mechanism in Ce^III-Mn^II is still in dispute for the uncertainty of distances between metal centers. Herein, for the first time, we explored the energy transfer mechanism in two well-designed luminescent heteronuclear complexes with clear crystal structures, i.e. Ce-N8-Mn and Ce-N2O6-Mn (N8 = 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane; N2O6 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane). Short distances between metal centers facilitate efficient energy transfer from Ce^III to Mn^II in both complexes, resulting in high photoluminescence quantum yield up to unity. After systematic study of the two heteronuclear complexes as well as two reference complexes Ce(N8)Br₃ and Ce(N2O6)Br₃, we concluded that dipole-quadrupole interaction is the dominant energy transfer mechanism in the heteronuclear complexes.