Breathing-Ignited Long Persistent Luminescence in a Resilient Metal–Organic Framework

Long
persistent luminescence (LPL) materials have been fascinating
from ancient times till today, whereas the generation and modulation
of LPL are still uncontrollable in most cases. In this paper, a smart
responsive metal–organic framework (MOF, LIFM-WZ-1) was reported
to combine resilient structural dynamics with reversible LPL modulation.
At room temperature in air, the as-prepared LIFM-WZ-1 has a staggered
packing structure with inhaled water molecules between cavities, exhibiting
fluorescence (F)-dominated blue emission. Under vacuum or by mild
heating, the water molecules can be exhaled by the lattice, resulting
in a crystal structure (LIFM-WZ-1a) with a reduced a axis and intensified intermolecular interactions. This brings transition
to green excimer (E) emission related with lowered energy states.
Meanwhile, orange-colored room temperature phosphorescence (RTP) is
also enhanced, giving an overall balanced F/E/RTP emission. More fascinatingly,
red LPL is ignited in the dehydrated sample, which is dumb in the
original LIFM-WZ-1 because of intensified singlet to triplet intersystem
crossing, damped high-energy oscillation, and compatible heat dissipation.
The above breathing-induced LPL turn-on process is reversible by facile
cooling, exposing to wet air, or purging in water vapor. This constitutes
a successful exemplification in modulating LPL by the dynamic MOF,
which provides potential applications in sensing, anti-counterfeiting,
display, and so on.