IR Irradiation Triggers Photon Avalanche and Crystal Dynamics in Metal-Organic Frameworks

Upconversion (UC) converts low-energy photons (IR) into a higher energy photon (visible). Photon avalanche (PA) is the most nonlinear UC: a burst of emission steeply arises above a critical threshold of the laser excitation power. PA was observed only in lanthanide-doped inorganic matrix, relying on electron buildup in the f levels of lanthanide ions. Due to the f-f selection rules, PA is efficient only in a few inorganic matrices. Here, we expand the PA material design. By irradiating lanthanide-trimesate metal-organic frameworks (MOFs) at 980 nm, we show a new mechanism of PA with electron buildup in the triplet of the organic ligand. PA efficiency is comparable with top inorganic matrices. We also observed that the IR irradiation triggers a phase change accompanied by microscopic MOF contraction, likely due to non-radiative pathways. This is the first example of PA in MOFs and IR triggered dynamic crystals and we aim to unravel the structural features supporting both processes.