Tb and Eu in MOF‐76: Elucidating the Mechanisms Responsible for the Divergent Excellent and Poor Photoluminescence Quantum Yields

Metal–organic frameworks (MOFs) have emerged as promising hosts for
lanthanide ions owing to their ability to sensitize the lanthanides and provide
a rigid topology while still permitting interactions with guest species. Among
these lanthanide MOFs, MOF-76 is a noteworthy example composed of
1,3,5-benzenetricarboxylic acid (H$_3$BTC) and lanthanide ions. In this study, a
high photoluminescence quantum yield (𝚽$_{PL}$ ) of 91% for Tb-based MOF-76
(Tb-MOF) is reported, while the efficiency of Eu-based MOF-76 (Eu-MOF) is
over five times lower. A potential circumvention of the poor performance of
Eu$^{3+}$ is energy transfer from Tb$^{3+}$ to Eu$^{3+}$, but it is found that although energy
transfer is efficient the 𝚽$^{PL}$ of the Eu$^{3+}$ remains low. The study investigates
the cause of the lower quantum yield in Eu-MOF by ruling out possible factors
such as crystal size, guest solvent molecules, and energy transfer from or
back to the linker. None of these are significant loss mechanisms, leading to
conclude that the MOF-76 ligand unfortunately leads to fast non-radiative
deactivation of the Eu$^{3+}$ excited state. The results suggest that MOF-76 is an
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excellent host for Tb$^{3+}$ ions, but exploration is required for MOF linkers to
simply access the potentially excellent emission efficiency of Eu$^{3+}$ ions.