Publication date: 15th May 2025
Integrating optical properties into porous materials offers exciting opportunities for multifunctional applications, including molecular sensing, energy harvesting, and photocatalysis. Metal–organic frameworks (MOFs), known for their versatile chemical environments and tunable porosity, are ideal candidates for such photonic functionalities. Previous demonstrations of upconversion in MOFs have relied on either ligand-based triplet–triplet annihilation or the incorporation of lanthanide ions via the antenna effect. Here we report on a fundamentally different mechanism involving energy transfer from the lanthanide ions to the bridging ligands. Furthermore, here, Yb(III)/Er(III)-trimesate MOF nano and micro crystals exhibit a highly non-linear, spectrally broadband emission, originating from a photon avalanche (PA) process within the organic ligand electronic states manifold. The lanthanide ions serve as the sensitizers, coupling through their excited states to the electronic manifold of the trimesate ligand. The long-lived ligand triplet states serve as a reservoir allowing for a population buildup. PA emission then proceeds through what seems to be a collective phenomenon involving multiple ligands. PA efficiency is comparable to that of recently emergent PA inorganic nanoparticles. Crucially, we find that PA activity is strongly dependent on MOF crystallinity: only fully coordinated frameworks support the cooperative energy transfer required for avalanche emission, while weakly coordinated MOF exhibit only Er³⁺ related upconversion emission. These findings extend the photon avalanche paradigm, offering a new node for spectral control beyond what is possible with the handful of rare-earth elements available. Our work highlights MOFs as a promising platform for engineering optical properties via straightforward synthetic strategies and crystal structure design.
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