Aggregation-Induced Emission-Responsive Metal–Organic Frameworks

Although
many studies on luminescent metal–organic frameworks
(MOFs) have been reported for chemical sensing applications, it has
yet to be realized in MOFs the precise linearity control over photophysical
characteristics and sensing sensitivity at the molecular level for
a fundamental understanding of the structure–property relationships.
Here we demonstrate the first example of aggregation-induced emission
(AIE)-responsive MOFs with precise linearity control of photophysics
and chemical sensing. We employ a multivariate strategy to tune the
number of AIE molecular rotors (dynamic phenyl rings) in a MOF system
by varying the ratio of tetraphenylethylene (TPE)-based organic linker,
leading to highly tunable photophysical characteristics (e.g., maximum
emission peak, quantum yield, and optical band gap) featuring linear
correlations with linker content. Importantly, the sensing sensitivity
of these dynamic MOFs can be enhanced by increasing the number of
AIE molecular rotors with perfect linearity control, as systematically
investigated by fluorescence responsive to temperature, viscosity,
guest molecular size, as well as theoretical calculations. Our study
shows that the sensing sensitivity of the AIE-responsive MOF in this
study (termed as NUS-13-100%) is better than those of our previously
reported materials. Significantly, the observed linear relationship
between emission intensity and molecular weight of polystyrene as
the analyte suggests that such AIE-responsive MOFs could be used as
molecular sensors for fluorescence-based determination of polymer
molecular weight. Eventually, the optical sensing device containing
NUS-13-100% shows a perfect linearity response with high sensitivity
for the detection of trace toxic benzene vapor. In short, our work
paves the way toward porous MOFs containing AIE molecular rotors with
a versatile responsive emission mechanism and suitable pore size/geometry
for broad applications in chemical sensing and environmental monitoring.