That article was a short review that described our understanding of the processes underlying electroluminescence in these materials. It was the result of a collaboration with Richard Friend's group at Cambridge as well as Bill Salaneck’s group in Linköping, Sweden and Carlo Taliani’s group in Bologna, Italy. It was written to provide a description of the state of the art in these conjugated polymers and also to give insight into the material requirements and into what the remaining issues were. I think it is a very good starting point for people who want an overview of what the field of polymer electroluminescence is all about. This field is attracting major interest because there is a big potential market for electronic polymers (plastics). Among other things, you can use them to build flexible displays. The review came at the right time, in the sense that at that point the field had matured for almost 10 years. As you know, when a new field develops, especially in materials chemistry or physics, there are so many observations that are actually dependent on the quality of the sample that the researcher is studying. So you often get contradictory results in the beginning, simply because people think they are looking at the same materials, but actually they are looking at polymers that are prepared in slightly different ways, and even small defects in the sample can lead to very different results. By 1999, I think the field had matured enough so that people could get a very clear understanding of the basic concepts. So the paper was timely, and being a review of just eight pages, it is very readable.

  The highest-impact paper out of your group alone was in Chemical Reviews in 1994: "3rd-Order Nonlinear-Optical Response in Organic Materials—Theoretical and Experimental Aspects." Here, you're going to have to explain what nonlinear-optical responses are and what makes third-order responses so interesting.

Here's a good way to look at it: imagine the interaction between light that is an electromagnetic radiation and matter. In most instances, what's really important is the interaction of the electric field of light with matter. When light strikes a molecule in a material, it induces a dipole moment in that molecule, and under the usual circumstances the magnitude of that induced dipole moment is a linear function of the electric field of the light. Nonlinear optics starts when there occur deviations from these linear relationships. Then, one has to add second-order (quadratic) or third-order (cubic) terms to describe the induced dipole moment in the molecule or, more generally speaking, the induced polarization in the material.

The "proportionality" coefficients at various orders are known as molecular polarizabilities and they are usually denoted as alpha, for the first-order (linear) polarizability, beta for the second, gamma for the third, and so on. Generally speaking, beta is about six orders of magnitude smaller than alpha, and gamma is about six orders of magnitude smaller than beta. The fact that alpha is so much bigger than beta and gamma, explains why only the linear effect is seen in normal circumstances. However, when you have laser light, which can be of very high intensity, you can have extremely large electric fields associated with it; the second- or third-order effects can then become significant and can be observed. Thus, nonlinear optics is a field that really started with the advent of lasers in the 1960s.

Well, second-order effects, for instance, can be used to double the frequency of light. You can have an input beam interact with a material in which the molecules have a very large beta value and you can get an output signal that will have twice the input frequency. It means you can have an input beam with the frequency of red light and the output can be blue light. So you can "up-convert" the frequency of light. Some 15 years ago, people were interested in the high-beta organic materials to double the frequency, for instance, of the lasers that are used to read compact discs. When the frequency becomes twice as large, the wavelength is twice as small, and it means one can read four times as many bits of information in the same area. A currently much sought-after application is known as the electro-optic effect, in which one can modulate the characteristics of the input beam of light through interaction with a static electric field. That's one of the applications in telecommunications for which people are trying to use conjugated organic materials. One advantage of these materials is that the modulation of the light can be done at very high frequencies, meaning a faster processing of telecommunications signals.

With third-order effects, there are many problems to overcome, but it has been demonstrated that, at least in some prototypical cases, one can use them for all optical computing or optical manipulation of light. So instead of manipulating a ray of light with an external electric field, as in the electro-optic effect, one manipulates light with light. This is the field people refer to as photonics, to be contrasted with electronics or optoelectronics. That means all the signals that are generated, manipulated, and stored are optical signals. And you don't deal anymore with electronic signals.

A third-order effect of high current interest is two-photon absorption, which corresponds to the simultaneous absorption of two photons by a molecule. The applications are numerous and range from biomedical imaging to nanofabrication and optical storage of information.

What we were doing in that paper was first trying to describe the basic principles that make organic materials so interesting for possible third-order nonlinear optical effects. I believe it is one of the reasons why the paper is so widely cited. Then, there is a second part of the paper in which we described the techniques that are used to probe the third-order nonlinear optical effects; this was written in collaboration with the group of my excellent colleague and friend, André Persoons at KU Leuven, Belgium. In the third part of the paper, we looked at families of organic materials and described their third-order, nonlinear optical properties. It was a pretty big paper and it took me a long time to get it to the point where I was happy with how it looked.

Yes. Often when you write for Chemical Reviews, the papers are expected to have good impact, but the magnitude of this impact still surprised me. It was good, though, because we worked pretty hard to put it together. At one point, I took two full months and did virtually nothing but teach and work on the paper and make sure everything was right. So the fact that it has been so highly cited makes it worth the effort.

I must that say that in my field, in the 20 years since I got my Ph.D., it's just been one exciting discovery after another. I am continually fascinated by these conjugated organic materials. For instance, we are now involved in a totally new type of application, in which the conjugated polymers are used for their transport properties; for instance, they can be exploited as the active elements in new generations of (flexible) transistors.

In fact, what we've been doing for the past two to three years is trying to understand what is the effect on transport of the electronic interactions among neighboring chains or neighboring molecules. And we have been trying to understand that from a microscopic point of view, which is a rather original approach. Indeed, until fairly recently, when people were considering the transport processes in organic materials, they did it from a mostly macroscopic standpoint. It was a very phenomenological type of approach. We are now trying to bridge that approach with one that was developed originally in the 1950s in chemistry, and which relates to electron transfer reactions. Rudolph Marcus from Caltech got the Nobel Prize in Chemistry in 1992 for what has become known as Marcus theory, which is the theory of how charges can hop (transfer) from one atom or molecule to another to make a chemical reaction. We are trying to adapt those techniques to take a very microscopic, chemical point of view to study how a charge might move along a polymer chain and how it might hop from chain to chain. That requires a new level of sophistication in the calculations: Whereas in the past, we only looked at isolated molecules or polymer chains, now we look at what happens between neighboring chains or neighboring molecules. From the point of a view of a theorist, it's a much more complicated task, but it's very rewarding. It defines kind of a new era and that is a very positive feeling.

Let me describe it this way. It is performing electronic-structure calculations and applying them to materials chemistry and physics, in particular to uncover the fascinating electronic and optical properties of conjugated organic materials. A main characteristic of our research is that, even though it is theoretical in nature, it has always been conducted in very close collaboration with experimentalists; it’s been a wonderful ride because we have been fortunate to collaborate with the leading experimentalists in the field.

Over the years, each new major development has been based on new applications of conjugated polymers that, in turn, required yet a new level of theoretical analysis. For instance, when I finished my Ph.D. in 1979, it was a glorious time right after the discovery of conducting polyacetylene, which eventually led to the 2000 Nobel Prize in Chemistry awarded to Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa. At that time, the most important issue was to characterize the nature of the ground state of the polymers from an electronic structure standpoint. A major question was: How easy is it to try to take an electron away from these polymers or to add an excess electron, so as to modify their electrical conductivity? To answer such a question required mostly the evaluation of the polymer ground-state properties. The next generation of polymers or oligomers that caught people's attention came along in the mid-1980s; these were conjugated materials with good nonlinear optical properties. To describe those properties, theorists needed to describe accurately the excited states. They needed to know the nature of the excited states that couple strongly with the ground state, and which other excited states couple strongly with these excited states. So being able to characterize the excited states of polymers or long oligomers was the next step in theory.

Then, in 1990, Richard Friend discovered that polyphenylenevinylene, or PPV, is electroluminescent; that is, when you drive an electric current through PPV it emits light, which is what you need to build a light-emitting diode (these are the processes described in our Nature paper discussed above). In that case, it required theorists to go beyond a static picture of the excited states themselves, and to look at the dynamics in the excited states, for instance to consider all the relaxations that can take place and study in detail the fate of the excited states and how you can optimize the material to get as much light out of it as you can.

So it has been a steady progression toward new understanding. What is remarkable is how much the field of conjugated polymers mixes basic and applied research, how new fundamental understanding helps in designing more efficient devices. Electronic and photonic plastics have a bright future!

Dr. Jean-Luc Brédas
University of Arizona
Department of Chemistry
Tucson, AZ, USA