Metal–organic frameworks (MOFs) have been widely
used for
separation, but amplifying subtle differences between similar molecules
to achieve effective separation remains a great challenge. In this
study, we utilize the fluorescent molecule uranine (Ura) to modulate
the pores of zeolitic-imidazolate framework 8 (ZIF8), creating an
unusual throttle effect. By monitoring fluorescence intensity changes
in Ura, the transport diffusion process could be quantified to reveal the diffusion constant of solvents.
When we pushed the Ura occupancy to its limit (from 59% to 76% and
98%), the diffusion rate decreases by 2 orders of magnitude. Most
importantly, there is a significant dissymmetry between the two-way
exchange rates of solvents, and the rates of H2O and D2O became distinguishable. Such unusual throttle effects disappear
at low Ura occupancy of 59% and 76%. We believe that the throttle
effect with small-molecule loading could provide a universal design
principle for MOF-based applications, especially for isotope separation.