Isolated π‑Interaction Sites in Mesoporous MOF Backbone for Repetitive and Reversible Dynamics in Water

We
report the introduction of π-interaction sites into a
series of chemically robust metal–organic frameworks (MOFs),
MOF-526, -527, and -528, with progressively increased pore size, 1.9–3.7
nm, and the inclusion and release of large organic molecules in water.
The mesopores in these MOFs lead to fast adsorption kinetics, whereas
the π-interaction between isolated porphyrin units in the MOF
backbone and polycyclic structure of the organic guests provides excellent
reversibility. Specifically, seven large organic dyes were quantitatively
captured by the porphyrin units of these MOFs in a 2:1 molar ratio,
exhibiting unprecedented kinetics for MOFs [e.g., 4.54 × 105 L/mol for rhodamine B] at an extremely low concentration
(10 ppm) in water. Rotational-echo double-resonance NMR experiments
revealed that the distance between the guest molecules and porphyrin
units in MOFs was in the range from 3.24 to 3.37 Å, confirming
the specific π-interaction. Repetitive and reversible dynamics
was achieved in these MOFs for 10 complete inclusion–release
cycles without any decay in performance, which is ideally suited for
the removal and recycle of large polycyclic organic molecules from
water. The performance of MOF-526 rivals that of state-of-the-art
carbon and polymers.