Fluorescence Enhancement in the Solid State by Isolating Perylene Fluorophores in Metal–Organic Frameworks

Polycyclic
aromatic hydrocarbons such as perylene and pyrene and
their derivatives are highly emissive fluorophores in solution. However,
the practical applications of these materials in the field of molecular
electronic and light-emitting devices are often hindered by self-quenching
effects because of the formation of nonfluorescent aggregates in concentrated
solutions or in the solid state. Herein, we demonstrate that aggregation-caused
quenching of perylenes can be minimalized by molecular incorporation
into metal–organic frameworks (MOFs). This study utilized a
stable Zr6 cluster-based MOF, UiO-67, as a matrix. Linear
linkers containing photoresponsive moieties were designed and incorporated
into the parent UiO-67 scaffold through the partial replacement of
the nonfluorescent linkers of a similar length, forming mixed-linker
MOFs. The average distance between perylene moieties was tuned by
changing the linker ratios, thus controlling the fluorescence intensity,
emission wavelength, and quantum yield. Molecular modeling was further
adopted to correlate the number of isolated perylene linkers within
the framework with the ratio between the two linkers, thereby rationalizing
the change in the observed fluorescent properties. Taking advantage
of the tunable fluorescence, inherent porosity, and high chemical
stability of this MOF platform, it was applied as a fluorescent sensor
for oxygen detection in the gas phase, a model reaction, showing fast
response and good recyclability.