Sugar-coated polymer strings

by David Bradley

Like so many sweet little beads, doughnut-shaped cyclodextrin molecules can be threaded on to a strand of polymer. But, this is no rhinestone necklace on the molecular scale. If the polymer threads are of the conjugated variety, then, according to Franco Cacialli, and his colleagues at University College London, Oxford and Berlin Universities, they can provide the backbone of a new class of materials for organic light-emitting diodes (LEDs). The new materials could find use in LED devices, displays, compact TVs, and as backlighting for liquid crystal displays as well as many other applications, as well as in the biomedical area.

During the early 1990s scientists at the University of Cambridge and elsewhere began working on conjugated polymers that had some rather interesting properties. For one, these materials were electrical conductors, or better semiconductors, depending on their specific chemical structure. Not only that, some of them, like poly(p-phenylene vinylene), PPV, and later, poly(para-phenylene) were found to go with the glow when an electric current was passed through them, producing an eerie fluorescence.

Since then, much effort has been applied to fine-tuning the properties of these materials. Extending their life to make them viable for commercial electronics applications has been one avenue the research has taken while other efforts have wound the polymers up to cover the full visible spectrum of colours.

Always hampering the research was the problem of efficiency. Intermolecular interactions and solid-state packing effects almost inevitably caused a red-shift of the luminescence wavelength and partially quench the light. In an attempt to get around this problem once and for all, Oxford's Harry Anderson, Cacialli and their teams began working on ways of insulating these glowing polymers from the bulk environment and to reduce quenching, maintain wavelength integrity and protect the polymer threads as a positive side-effect.

According to Cacialli: “Achieving a good degree of control of intermolecular interactions is vital to the use and exploitation of these molecular semiconductors both for LEDs and other organic electronics applications, as well as for nano-engineered devices based on single-molecules,” he told us. Anderson and his colleagues reasoned that one of the most popular macrocycles in supramolecular chemistry - cyclodextrin -might provide their polymers with the insulation they need.

The researchers have used the well-established building blocks of the polyarylene and polyarylenevinylene families for their luminescent wires. Threading the polymer strands through either alpha- or beta-cyclodextrin is possible under the driving force of the hydrophobic effect. In a polar environment, the water-repellent, or hydrophobic, exterior of the polymer chains finds the protective hydrophobic cavity of the cyclodextrin rings more attractive than the polar solvent and so once the end of a polymer chain chance to collide with a cyclodextrin ring, the ring is inexorably threaded on to the polymer, bulky end groups are then attached to prevent the rings from slipping off the chain.

The result is a supramolecular structures that are known generically as polyrotaxanes. Polyrotaxanes themselves are not new and have been used by teams such as that of Fraser Stoddart at the University of California at Los Angeles and others for many years to create prototype nanoscale devices and computational sub-units. Cacialli and his colleagues used nuclear magnetic resonance spectroscopy to count the number of cyclodextrins that become threaded on to each polymer per unit length.

The researchers also used tapping-mode scanning force microscopy to reveal how the individual sugar-coated wires would aggregate once formed into thin films. The technique revealed, for instance, that beta-cyclodextrin-poly(para-phenylene) (ß-CD-PPP) polyrotaxanes are densely, but not closely, packed. They could see individual molecular rods and suggest that this provides unambiguous evidence of how interactions between conducting polymer strands are greatly reduced by the presence of the cyclodextrin coating. Some of the threads were found to be several hundred nanometres long. In contrast, the researchers found that uninsulated PPP when deposited on mica it aggregates to form a mono-layer of stacked polymer threads.

Their approach allows them to preserve the underlying semiconducting properties of the conjugated polymer wires. Indeed, it effectively increases the photoluminescence efficiency and blue-shifts their emission in the solid state. The team has prepared single-layer light-emitting diodes using calcium and aluminium cathodes and were able to produce blue and green light.

A final advantage that Cacialli and his colleagues highlight, is that their materials are water soluble. Processibility has always been a bugbear of OLED science meaning that spin-coating has to rely on toxic solvents. Cacialli believes the creation of the cyclodextrin insulating layer will allow environmentally friendly processing to be done in water. Interestingly, this could also pave the way for bio-compatible processes and structures.

SOURCE: Nature Materials

Fifty years after - Rosalind Franklin

by David Bradley

Fifty years ago next year, the story of DNA was unravelled famously by Watson and Crick. But, in Brenda Maddox's Rosalind Franklin (out October 2002), we read the "untold" story of the scientist without whom that famous base pair may never have understood the double helix.
  Franklin, of course, was never eNobled perhaps because she had died of ovarian cancer at the age of 37, four years prior to the Nobel award. That award was ultimately to lock the names of Watson and Crick in the popular mind but exclude hers. Franklin has on occasion been labelled as the "the Sylvia Plath of science". Now, Maddox offers a clearer picture of the woman without whom WC would never have come to mean what they do in biomedical circles.

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Elemental Discoveries - April 2003, Issue 64