Metal-organic frameworks: platforms for photonic upconversion

Metal-Organic Frameworks (MOFs) are crystals forming from the self-assembly of organic molecules and metal ions through coordination bonds. Known for about 25 years, these structures have largely dominated the scene of modern materials chemistry due to their high porosity and the resulting applications (e.g. gas storage, catalysis, etc.). The combination of erbium and ytterbium ions is known in the literature to give up-conversion phenomena when inserted into inorganic crystalline structures. However, the use of MOFs as supporting platforms for this phenomenon is largely unexplored; the very first studies appeared only in very recent years [1, 2]. Here, we report the synthesis of new MOFs using as building blocks 1,2,3,4-butanetetracarboxylic acid, erbium and ytterbium salts. Thanks to the presence of lanthanides nodes, these MOFs are able to emit higher energy photons in the visible upon irradiation with an IR laser (λ = 980 nm). Different synthesis parameters of MOFs have been explored to obtain different structures and tune the emissions thereof (Figure). Furthermore, it was observed that the synthesized MOFs show a strong tendency to evolve over time if left in the reaction solution. Through various microscopy and spectroscopy techniques, we attempted to obtain information on the crystal growth mechanism, highlighting non-classical crystallization mechanisms (e.g. fusion processes).