acs c o m m e n t
Color, the senses, and chemical outreach
T
he five senses—sight, smell, hearing, taste, and touch—are irreplaceable tools for learning about chemistry: what it is, why it is important, and the myriad roles it plays in everyday life. Throughout its history, chemistry has been closely linked with the senses. Before science explained chemical composition and change, people judged the quality and character of substances by direct sensory experience: the color of pigments, the aroma of perfumes (see page 38), the sound of explosives, the taste of wines, the feel of leathers and oils. In recent decades, research has documented the potentially harmful effects of long-term exposure to even low concentrations of some chemicals. As a result, scientists have recognized that it is unwise to taste, smell, or touch most of the products and materials they study in the laboratory. Good practice requires many reactions to be run in dry boxes or under hoods. Experimenters wear gloves. The air in the laboratory is reassuringly bland. These steps are prudent; they protect us from potentially harmful interactions. At the same time, we pay a price for safety. Yet sensory experience can still enrich our understanding of chemistry. Volatile benzene derivatives have diverse and intense aromas yet are often similar in reactivity—the reactions that define "aromatic chemistry. " The smoothness of graphite and other layered compounds illuminates their structure at the atomic level, complementing two-dimensional pictures and verbal descriptions. Without these connections to sensory experience, chemistry may come across as an abstract intellectual activity. Our computerized instruments, for all their sophistication, provide an impersonal interface with nature. Complex theories of atoms and bonding may convey that chemistry is a distant, self-contained world rather than what it truly is: the science of many of the phenomena we experience each day. Yet this need not be the case. We
Ed Wasserman ACS President
can safely experience the aromas and tastes of food additives. Here the mechanisms by which molecular structures produce sensation are highly complex; they form a dynamic area of research with subtle biochemistry. Color provides a more direct connection between the senses and chemistry. Nearly everyone appreciates and responds to color. Ask someone to focus on a patch of color and understand that it arises from individual molecules. Suddenly, the concept of a single molecule becomes tangible. An explanation of the associated color is highly satisfying and can be a path for introducing other chemical concepts. Color can be explained as a consequence of molecular structure, and we can observe it without harm, provided the light is of only moderate intensity. Our understanding of color is a triumph of chemistry, one that visibly enhances daily life. It has been more than 100 years since chemists developed correlations between structure and color within families of organic molecules. Knowledge of these relationships guided development of synthetic dyes, the rainbow of hues we now take for granted. Point that out, and most people will immediately grasp how much grayer—
literally—-life would be without modern chemistry. This was made vivid for me when I was an undergraduate. One of our organic laboratory assignments was to make seven dyes, each a unique compound and color. This clearly demonstrated that color depends on structure. To acquire a deeper understanding, undergraduates could apply quantum mechanics, which was developed in the 1920s and '30s and explains molecular phenomena. The electronic levels of molecules and the frequencies of light-inducing transitions between them are fundamental to this understanding. Simple models—such as molecular orbital theory and the particle in a box—provide semiquantitative estimates for specific molecules, particularly conjugated organics. These electronic transitions absorb light. The light that is not absorbed— for example, that which passes through a solution—determines the color we see. With upper-level secondary school students, we may use analogies with the pitch of vibrating strings and wind instruments. We can emphasize the everpresent role of molecules and chemistry in daily life by having students look around, note the colors before their eyes, and use a simple model to estimate the size of the molecules responsible for their observations. For the general public, we may point to changes in color as molecules increase in size. Not only is color part of our history and scientific foundation, it is also at the center of some of the most productive areas of current research. When fabricated into devices, the new conducting organic polymers are able to emit a variety of colors. The recently commercialized blue gallium nitride laser is a triumph of materials science, one created by a lone researcher in a Japanese chemical company. The colors of nanoparticles alter with size, a visible expression of changes in the electronic levels as we move from small molecules to bulk metals and semiconductors. Literally before our eyes, color surrounds us in almost everything we do—a tangible example of how personally important chemistry is to each of us. In ACS and outside, color illuminates the nature and importance of our molecular science as onlyfirsthandexperience can.
Views expressed on this page are those of the author and not necessarily those ofACS OCTOBER 25,1999 C&EN
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