Highly Fluorescent Metal–Organic-Framework Nanocomposites for Photonic Applications

Metal-organic frameworks (MOFs) are porous hybrid materials built up from organic ligands coordinated to metal ions or clusters by means of self-assembly strategies. The peculiarity of these materials is the possibility, according to specific synthetic routes, to manipulate both the composition and ligands arrangement in order to control their optical and energy-transport properties. Therefore, optimized MOFs nanocrystals (nano-MOFs) can potentially represent the next generation of luminescent materials with features similar to those of their inorganic predecessors, that is, the colloidal semiconductor quantum dots. The luminescence of fluorescent nano-MOFs is generated through the radiative recombination of ligand molecular excitons. The uniqueness of these nanocrystals is the possibility to pack the ligand chromophores close enough to allow a fast exciton diffusion but sufficiently far from each other preventing the aggregation-induced effects of the organic crystals. In particular, the formation of strongly coupled dimers or excimers is avoided, thus preserving the optical features of the isolated molecule. However, nano-MOFs have a very small fluorescence quantum yield (QY). In order to overcome this limitation and achieve highly emitting systems, we analyzed the fluorescence process in blue emitting nano-MOFs and modeled the diffusion and quenching mechanism of photogenerated singlet excitons. Our results demonstrate that the excitons quenching in nano-MOFs is mainly due to the presence of surface-located, nonradiative recombination centers. In analogy with their inorganic counterparts, we found that the passivation of the nano-MOF surfaces is a straightforward method to enhance the emission efficiency. By embedding the nanocrystals in an inert polymeric host, we observed a +200% increment of the fluorescence QY, thus recovering the emission properties of the isolated ligand in solution.