Mixed-Lanthanide Metal–Organic Frameworks for Near-Infrared-I Bioimaging and Near-Infrared-III Four-Photon Upconversion

Lanthanide-based metal-organic frameworks (Ln-MOFs) provide a structurally precise platform for upconversion luminescence engineering, yet practical applications are limited by insufficient structural stability, restricted color tunability, and the lack of excitation in the NIR-III window. Here, we report a MOF incorporating mixed-lanthanide Yb³⁺, Tb³⁺, and Eu³⁺ ions that addresses these challenges. The optically transparent and periodic framework enforces a uniform spatial distribution of lanthanide centers and establishes stepwise interionic energy transfer pathways, while ether-containing organic linkers enhance photo and thermal stability. Under NIR-I (980 nm) excitation, the Ln-MOFs exhibit tunable multicolor upconversion emission with high hydrolytic stability and biocompatibility, enabling reliable in vivo imaging in zebrafish models. Remarkably, under NIR-III (1960 nm) excitation, a four-photon upconversion process is observed for the first time in Ln-MOFs. Together, these results expand the excitation landscape of Ln-MOFs from NIR-I to NIR-III and confirm the potential of structurally programmable Ln-MOFs for bioimaging and nonlinear photonic applications.