Molecule of the Month: November 1998:

Although discovered some twenty years ago, amid much hype, conducting organic polymers have hardly set the world alight. Not that there have not been successes - a number of research groups are developing light emitting diodes (LEDs) and Contex made by Milliken since 1990 is used to make antistatic fabric and camouflage netting. In some ways its surprising that so much effort has been made on their behalf. The first major group of researchers, electrochemists were more known for obsessive polishing of their electrodes rather than coating them with what had hitherto been a waste product of porphyrin synthesis. However when it was discovered that polypyrrole could be easily synthesised on an electrode surface then the process of development and analysis began. As is usual when something with great potential is discovered, making and testing of analogs and derivatives began. The most obvious analog was polythiophene and although harder to oxidise resulted in more stable polymer films.

This stability is ably demonstrated by an example published recently in Journal of the American Society for Mass spectrometry. Looking for a stable matrix for laser desorption mass spectrometry Robb and Blades found that using a polythiophene film in the ion trap produces peaks only for the counter ions - typically tetrabutylammonium salts . The base peak was due to the molecular TBA ion and the lower peaks accounted for by the loss of alkyl groups. What is good for the mass spectroscopists is that the peaks of the spectrum were stable for tens of thousands of laser shots compared to exponential degradation when a crystalline film was used as the matrix. Conducting organic polymers are materials of contrasts - they are conducting but only when oxidised or doped. They are easy to synthesis in situ but only because they become insoluble at fairly low molecular weight. Being made in an electrochemical cell makes studying their electrochemical properties straight forward, but makes conventional analysis difficult. AS the example of their use in mass spectrometers shows, their intractability makes them stable materials but makes for difficult structural analysis.

On the other hand their properties open up many possibilities - a variable conductivity makes them ideal as sensors. The stable matrix can be used to trap enzymes close to the electrode surface and even facilitate electron transfer between the biological system and a metal - a biotransducer. They can mimic transistors but be flexible. They can store charge - the possibility of light weight polymer batteries and solar energy cells is a real possibility and polymer LEDs are already here. When oxidised the polymers swell as they absorb the counter ions and a team at the University of Pennsylvania and the Kyushu Institute of Technology in Japan have made an actuator by affixing two pieces of polyaniline to cellophane film. Applying a potential difference causes the tape to bend more than 360 degrees.

As for the structure, the example given here is just for illustration (created in Chem3D). An all cis arrangement with a S-C-C-S torsional bond angle of 5 degrees suggests this helical structure - which is at least pretty to look at. Most publications, however, assume that the trans arrangement is most likely leading to linear polymers.

Take a quick tour of the web and it is easy to see that research in conducting polymers is continuing and growing. After all, as a discipline it is still only twenty years old.

A few web links for polythiophenes

(Last updated 20 October 1998)

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