Covert MOF-Based Photoluminescent Tags via Tunable Linker Energetics

Optical anticounterfeiting tags utilize
the photoluminescent properties
of materials to encode unique patterns, enabling identification and
validation of important items and assets. These tags must combine
optical complexity with ease of production and authentication to both
prevent counterfeiting and to remain practical for widespread use.
Metal–organic frameworks (MOFs) based on polynuclear, rare
earth clusters are ideal materials platforms for this purpose, combining
fine control over structure and composition, with tunable, complex
energy transfer mechanisms via both linker and metal components. Here
we report the design and synthesis of a set of heterometallic MOFs
based on combinations of Eu, Nd, and Yb with the tetratopic linker
1,3,6,8-tetrakis­(4-carboxyphenyl)­pyrene. The energetics of this linker
facilitate the intentional concealment of the visible emissions from
Eu while retaining the infrared emissions of Nd and Yb, creating an
optical tag with multiple covert elements. Unique to the materials
system reported herein, we document the occurrence of a previously
not observed 11-metal cluster correlated with the presence of Yb in
the MOFs, coexisting with a commonly encountered 9-metal cluster.
We demonstrate the utility of these materials as intricate optical
tags with both rapid and in-depth screening techniques, utilizing
orthogonal identifiers across composition, emission spectra, and emission
decay dynamics. This work highlights the important effect of linker
selection in controlling the resulting photoluminescent properties
in MOFs and opens an avenue for the targeted design of highly complex,
multifunctional optical tags.