Source=Missourian; Date=15.02.2001; Section=Features; Page=6;

Light is difficult to understand because of its dualistic nature and we need to know just a little bit about light to develop a conceptual understanding of dyes.

A "photon" is the smallest amount of light. In a way, a photon is to a light beam what a raindrop is to rain and, sometimes, photons actually behave just like tiny particles. On the other hand, a photon also behaves like a "wave" and this behavior is much harder to explain. Fortunately, all we need to know right now about the wave nature of light, is that the color of the light is related to the frequency of the waves.

Let's get some idea about the magnitude of the frequency by looking at the rainbow spectrum. For example, photons of violet light have a frequency of 750 TeraHertz. One TeraHertz means 1012 Hertz, that is, whatever it is that vibrates in the photon, "it" vibrates up and down on the order of one hundred trillion times per second. At the low-energy side of the rainbow spectrum, red light has a lower frequency of 375 THz. The higher the frequency of the light, the higher its energy.

Violet light is on the high-energy side of the visible spectrum. The energy of ultraviolet light, just outside the visible range on the high-energy side, is responsible for sun burns. Infrared "light" cannot be seen, instead it can be felt as heat and has frequencies below 375 THz. IR photons contain less energy than photons of visible light.

One kind of interaction between light and a molecule consists in absorption. The photon is absorbed and its energy is used to promote the molecule into a higher energy state. Such an "excited" molecule can return to its ground state by the reverse process; the molecule falls back into its lower energy state while emitting a photon of light (fluorescence and phosphorescence). Normal dyes are materials that allow for such absorption-emission sequences with photons of visible light. If a dye absorbs one color, then it reflects light of all other colors and is the color complementary to the absorbed light. This is simple to understand based on the three basic colors: red, green and blue. The dye of a blue dress absorbs orange (red and green), a red tie absorbs blue-green, and green lawn absorbs purple light (red and blue).

Many of the big chemical companies started out in the "age of the dyestuffs," the period between 1865 and 1900, which includes the discovery of the azo dyes and indigo. Dye discovery, development and fabrication remain one of the pillars of the modern chemical industry. The art and science of dye making is very mature. Organic chemists have been able to design complex organic molecules that are intensely colored and the dye molecules can be fine-tuned to absorb or emit light of any desired color of the rainbow spectrum.

In my research group, we are interested in a different and novel kind of "dye" - we are making "nonlinear dyes." We create materials that interact with light of one color and emit light of a different color - and best of all-they emit light of a color that is higher in energy than the source light. We create materials that emit light with twice the energy of the source light. Shining infrared laser light (very intense heatwaves) on such a so-called second-order nonlinear optical material leads to emission of visible light with twice the frequency of the infrared light. The design of such "nonlinear dyes" is still in its pioneering stages and fundamental problems remain. Nonlinear dyes are at the heart of photonics applications. In photonics, technology light is used as the primary carrier of information. Photonics is thought to replace much of today's electronics in communications and computing applications in the near future.

Normal and nonlinear dye molecules share a few features in that they often are rod-like and have large electrical-dipole moments along the long molecular axis. For the novel kind of dye, the relative orientation of the individual dye molecules becomes a key issue. This is very different from normal dyes. A blue material will be blue no matter what the orientations of the neighboring molecules happen to be. The color of all the dye molecules simply adds up for normal dyes.

For the novel kind of dyes, however, the optical effects add up only if the nonlinear dye molecules are oriented in the same direction; otherwise the optical effects cancel each other out. This is where the problem lies. Most dye molecules have large electrical dipole moments and nature prefers to arrange polar molecules such that neighboring molecules are oriented in opposite directions. This orientation problem is indeed fundamental. It has long been considered impossible to have polar molecules form crystals in which all the molecules in the crystal are oriented in the same direction. Trying to make such highly dipole parallel-aligned organic materials seemed like a worthwhile goal to pursue in academia.

To tackle this problem, we employ a multi-disciplinary approach that includes mathematical modeling of crystals of dipolar molecules, theoretical and computational studies to arrive at and to test rational design concepts, and, most importantly, the experimental realization of prototypes.

In the mid-90s, we convinced ourselves it was not impossible to realize such dipole-parallel aligned molecular crystals. In 1995, we made the first near-perfect dipole parallel-aligned organic molecular crystal of a nonlinear dye. The second and third prototypes were realized in 1997 and 2000. An improved design resulted in prototypes four and five in 2000.

Aside from the novel idea, truly innovative academic research takes time, good faith, patience, a supportive environment, and bright and talented students. Several graduate students worked with me on this project. They are Grace Chen (Ph.D. Chemistry 1996, Humboldt post-doctoral fellow in Zrich, Switzerland, and Heidelberg, Germany), Don Steiger (Ph.D. Mathematics 1999, University of Illinois, Urbana-Champaign, and University of California San Diego post-doctorals), Michael Lewis (Ph.D. Chemistry 2001, Harvard University), Zhengyu Wu, and Nathan Knotts. Two undergraduate students also contributed to this effort: Jason Wilbur (BS, Chemistry 1995) and Mitchell Anthamatten (BS, Chemical Engineering 1996, Ph.D. Chemistry Engineering 2001). We patented these materials and the patent is being issued this month. We are only beginning to understand, and the best is yet to come.

Dr. Rainer Glaser

MU Department of Chemistry

Copyright 1999 The Missourian - University of Missouri

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