Zinc and copper give nanocapsules blue fluorescence
17 July 2012
Tokyo Institute of Technology (Tokyo Tech) researchers have
developed blue-fluorescent molecular nanocapsules by simply mixing metal
ions and bent organic blocks. The nanocapsules have potential
applications as sensors, displays, and drug delivery systems.
Michito Yoshizawa, Zhiou Li, and collaborators at Tokyo Tech
synthesized molecular capsules of about 1 nanometer size with an
isolated cavity using inexpensive and environmentally safe zinc and
copper ions. In sharp contrast to previous molecular capsules and
cages composed of precious metal ions such as palladium and platinum
that show only poor fluorescence, these nanocapsules emit blue
fluorescence with 80% efficiency.
The researchers expect to be able to prepare multicolour
fluorescence composites by the simple insertion of appropriate
fluorescent molecules into the isolated cavity of the nanocapsules.
Fluorescence has widespread applications, helping researchers to
understand issues in the fundamental sciences and develop practical
materials and devices. Among the useful fluorescent compounds in
development, capsule-shaped molecular architectures, which possess
both strong fluorescent properties and a nanometer-sized cavity, are
Molecular cages and capsules can be prepared through a simple
synthetic process called co-ordinative self-assembly. However, most
of them are composed of precious metal ions such as palladium and
platinum, and are non-emissive due to quenching by the heavy metals.
Now, Michito Yoshizawa, Zhiou Li, and co-workers from the
Chemical Resources Laboratory at Tokyo Institute of Technology
report novel molecular nanocapsules with the M2L4 composition (where
M represents zinc, copper, platinum, palladium, nickel, cobalt, and
manganese). Their zinc and copper capsules, in particular, display
unique fluorescent properties.
The M2L4 capsules self-assemble from two metal ions and four bent
ligands that include anthracene fluorophores (fluorescent parts).
X-ray crystallographic analysis verified the closed shell structures
where the large interior cavities of the capsules, around one
nanometer in diameter, are shielded by eight anthracene panels.
The structure of a molecular nanocapsule
The zinc capsule emitted strong blue fluorescence with a high
quantum yield (80%), in sharp contrast to the weakly emissive nickel
and manganese capsules and the non-emissive palladium, platinum, and
cobalt capsules. The fluorescence of the copper capsule, on the
other hand, depends on the solvent; for example, it shows blue
emission in dimethyl sulfoxide but no emission in acetonitrile.
This study is the first to show such emissive properties of
molecular capsules bearing an isolated large cavity. The researchers
believe their nanocapsules could have novel applications in devices
such as chemosensors, biological probes, and light-emitting diodes.
Zhiou Li, Norifumi Kishi, Kenji Yoza, Munetaka Akita, Michito
Yoshizawa, “Isostructural M2L4 Molecular Capsules with Anthracene
Shells: Synthesis, Crystal Structures, and Fluorescent Properties”,
Chemistry, 18, 8358 (2012). DOI: 10.1002/chem.201200155.