  A series of fluorescein-encapsulated zeolitic imidazolate framework-8
(fluorescein@ZIF-8) luminescent nanoparticles with a scalable guest loading has
been fabricated and characterized. The successful encapsulation of the organic
dye (fluorescein) is supported by both experimental evidence and theoretical
simulations. The measured optical band gap is found to be comparable with the
computed values of a hypothetical guest-host system. Isolated monomers and
aggregates species of fluorescein confined in ZIF-8 nanocrystals have been
systematically investigated through fluorescence lifetime spectroscopy. The
quantum yield (QY) of the obtained solid-state materials is particularly high
(QY~98%), especially when the concentration of the fluorescein guest is low.
Combining a blue LED chip and a thin photoactive film of fluorescein@ZIF-8, we
demonstrate a device with good optical tunability for multicolor and white
light emissions. Additionally, we show that the fluorescein@ZIF-8 nanoparticles
exhibit an improved photostability due to the shielding effect conferred by the
nanoconfinement of host framework, making them promising candidates for
practical applications such as solid-state lighting, photonics, and optical
communications.
