Trivalent lanthanide ions (Ln3+) hold an exceptional position in the field of optical science due to their f-f transitions which give rise to their sharp atomic-like emission spectra and long luminescence lifetimes. However, their low absorption coefficients make luminescence by direct excitation an inefficient process. This issue can be tackled by the indirect population of Ln3+ emitting levels by energy transfer from strongly absorbing chromophores in a process called sensitization. Metal-organic frameworks (MOFs), with their well-defined open network structures, which can incorporate a wide variety of chromophores, both in the form of bridging ligands and metal ions, offer an excellent platform for the sensitization and study of lanthanide luminescence. In this contribution, we present a series of mixed Ln3+/Eu3+ (Ln = La, Gd) and mixed ligand MOFs belonging to three different structural types with emissions spanning most of the visible region thereby constituting favorable materials for color tuning and white-light emission. We investigate the effect of the structure of the parent framework and the different metal and ligand doping levels on the luminescence properties of the materials. We observe emission quantum yields of up to 65% for the Eu3+-doped materials, while their ligand-and-Eu3+-doped counterparts show decreased quantum yields between 2.8% and 10%. A photophysical study, involving both steady-state and time-resolved emission measurements, allows us to discuss the possible energy migration and Eu3+ sensitization pathways that take place within these materials following selective ligand excitation. Furthermore, we attempt a fine tuning of the luminescence profiles of our materials by carefully changing the doping levels to achieve yellow and white light emission. Lastly, we establish that the materials show good chemical and photostability and can be predictably and repeatably synthesized up to certain doping percentages.

Luminescent lanthanide MOFs: A structural and photophysical study towards white light emission in mixed metal and mixed ligand frameworks

ACS Fall 2021

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