Rare-earth metal-organic frameworks (REMOFs) have found many applications based on their photoluminescent properties, including in lighting, sensing, and bioimaging. These photoluminescent properties also make them excellent candidates for optical anticounterfeiting tags. The tunability of MOFs allows for the creation of complex tags with combined overt and covert methods of encoding in a single material, a property lacking in many currently available tag fluorophores.<br/><br/>We report the synthesis of a family of heterometallic REMOFs based on a predetermined nonanuclear metal cluster linked by tetratopic organic ligands. The resulting materials contain up to three different metal centers and are permanently porous with tunable, multi-emissive spectra and luminescence lifetimes in both the visible and NIR ranges. Importantly, this study demonstrates that the fine tunability afforded by the control over the composition, has direct implications in the ligand-to-metal and metal-to-metal charge transfer mechanisms and the resulting photoluminescent properties.<br/><br/>The materials were characterized through a suite of complementary structural probes, including X-ray single crystal diffraction, X-ray powder diffraction, and Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM-EDS). Tag authentication was validated via a comprehensive evaluation of the photophysical properties, specifically emission properties and time domain photoluminescence lifetimes. The rational materials design principles shown here can be leveraged to create a library of optical tags with sophisticated, multicomponent encoding.

Optical encoding via rational design of heterometallic rare-earth MOFs

ACS Fall 2021

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