 |
 |
| Back Issues |
|
|
|
|
ACS Members can sign up to receive C&EN e-mail newsletter.  |
|
|
 Join ACS |
|
|||||||||
|
SUPRAMOLECULAR CHEMISTRY
Polymer wires sheathed by cyclodextrin rings have been used to prepare blue- and green-light-emitting diodes (LEDs) by a team of scientists in England and Germany.
Anderson's group carried out the work with University College London physics lecturer Franco Cacialli and scientists at Humboldt University Berlin and the University of Cambridge [Nat. Mater., 1, 160 (2002)]. The team prepared the materials by threading conjugated macromolecules, such as poly(para-phenylene), poly(4,49-diphenylenevinylene), and polyfluorene through "These are the first examples of conjugated polyrotaxanes--that is, conjugated polymers threaded through macrocyclic units with bulky stoppers," Anderson tells C&EN. "And it is the first time that the semiconductivity and electroluminescence of any type of insulated molecular wire have been investigated." The aims of the work were to reduce molecular interactions between the polymer strands and to minimize packing effects that partially quench luminescence and shift light emission to a broader and longer wavelength (the red shift). "Our approach preserves the fundamental semiconducting properties of the conjugated wires and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge transport," the authors note. Blue shift is a shift toward a shorter wavelength. "Blue is the hardest color to achieve because, although many molecules emit in the blue in dilute solution, aggregation in the solid state often results in a red shift, so that the color is green or white rather than blue," Anderson explains. The group prepared the LEDs by spin-coating 80- to 100-nm-thick films of the polyrotaxanes onto indium tin oxide anodes and using thermally evaporated calcium or aluminum thin films as cathodes. "That LEDs can be fabricated with the materials shows that charge transport across the sample is possible," comments Guillermo C. Bazan, a chemistry professor at the University of California, Santa Barbara. "The authors have nicely demonstrated that the cyclodextrin insulation diminishes the emission quenching by interchain contacts in conjugated organic structures and that it substantially changes the morphology of the bulk sample," he adds. "However, quantitative measurements will be required to fully understand to what extent charge mobility is affected." François Diederich, professor of organic chemistry at the Swiss Federal Institute of Technology (ETH), Zurich, thinks the work is elegant. "The benefits of this approach do not stop with the optimization of the optoelectronic properties such as the enhanced electroluminescence of the encapsulated polymers and the significant blue shift of the emission," he says. "Other rewards of the approach are substantially enhanced stability of the emitting polymers by protection from outside impurities and solubilization of the wires in water, thereby allowing spin-coating without using toxic solvents. "The use of supramolecular encapsulation concepts for the protection of nanomaterials for optoelectronic applications is quite a general one, and this paper provides the first clear proof of concept for this approach, which should have a bright future," he suggests. Anderson points out that the efficiencies of their LEDs are low. "We are currently making LEDs from these materials blended with other polymers to get better charge transport and higher electroluminescence efficiencies," he says. "Another limitation with our current insulated molecular wires is that they are polyelectrolytes. The lithium countercations tend to migrate in the LEDs, so we are currently making nonionic versions of these structures."
|
|||||||||
|
Chemical & Engineering News |
|||||||||
- or 
-cyclodextrin rings.
|
||||||||||
|
||||||||||
|
||||||||||
|
||||||||||
|
||||||||||
|
||||||||||
