Tb³⁺ as an Energy-Mediating Node: Boosting Ho³⁺ Upconversion in Metal-Organic Frameworks via Cascaded Energy Transfer

Upconverting luminescence in lanthanide-based metal-organic frameworks (Ln-MOFs) plays a pivotal role in emerging photonic applications, including information encryption, ion sensing, and bioimaging. Nevertheless, optimizing intrinsic energy transfer pathways to enhance the intensity and efficiency of upconverting luminescence in conventional systems remains a significant challenge. In this study, we report the successful observation of upconverting luminescence (under 980 nm excitation) from Tb^{3 +} and Ho^{3 +} ions in Yb/Tb and Yb/Ho co-doped MOFs, respectively, constructed from oxalic acid and formic acid. Most notably, the tri-doped Yb/Tb/Ho MOFs reveal a previously undocumented Tb³⁺→Ho³⁺ energy transfer mechanism that significantly enhances Ho³⁺ upconversion emission. To the best of our knowledge, this is the first observation of such energy transfer in MOFs under upconversion conditions. Tb³⁺ serves as an efficient energy-mediating node, establishing a cascading energy transfer pathway from Yb³⁺→Tb³⁺→Ho³⁺ via its long-lived ⁵D₄ excited state. This pathway effectively suppresses detrimental back energy transfer from Ho³⁺ to Yb³⁺, thereby promoting population of Ho^{3+ 5}F₅ level and consequently enhancing its upconverting emission at 657 nm. This finding broadens the scope of energy transfer mechanisms in Ln-MOFs. More importantly, it lays a foundation for rational design of upconverting luminescence in lanthanide-based materials, opening new possibilities for advanced photonic applications.