Strong upconverted circularly polarized emission from a chiral tetrahedral Yb/Eu cage

Upconverted circularly polarized luminescence (UC-CPL) materials have attracted significant attention due to their potential in optical sensing and bioimaging. However, achieving UC-CPL in lanthanide supramolecular systems remains a challenge due to the extended distances between lanthanide ions. Here, enantiopure tetrahedral cages, (Yb/Eu)₄L₄(R/S-BINAPO)₄, are assembled using achiral C₃-symmetric ligands, Ln(III) ions and chiral ancillary ligands. Upon 980 nm excitation, the heterometallic tetrahedral cages exhibit strong UC-CPL (g_lum = 0.22) and high Φ_UC of 3.50 × 10^-6. Moreover, through femtosecond transient absorption spectroscopy, we reveal that the ligand's triplet state (T₁) serves as a critical mediator in the upconversion energy transfer process within the lanthanide complex, facilitating the efficient transfer of energy from the excited state of Yb³⁺ to the Eu³⁺ center via the mechanistic pathway Yb** → T₁ → Eu*.