Week 3: It's all about them electrons

I was sad to miss the last lecture on indigo dye.

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 Indigo dye


I love dye. As a chemist, I see the process behind what makes the dye have its color. The structures are pieces of art work through my eyes. The process of why a dye is a dye (the natural reason before human intervention of using the dye) is so incredible. Since conjugated pi-systems are something I enjoy, for my week 3 blog I decided to take a moment to discuss Para-red, another dye. I have worked with Para-red in the laboratory. Para-red is a synthetic dye under the classification of an Azo dye. After reading some of the blogs, I see that there was a discussion of synthetic versus natural dyes, and in fact, the harmful effects synthetic dyes have on the environment. This does make me sad because the process of making the synthetic dye is beautiful in its own respect. In fact, after reading other blogs, I did some investigation and found that Old El Paso dinner kits were removed from market shelves due to a Para Red contamination in 2005. As I chemist I do think about the implications on our work in terms of the environment. There are particular career paths that I have already decided certainly against due to the hazards caused on the planet.


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Below is the synthesis of Para -red: 

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Para-red is a simple but beautiful structure with a conjugated pi-system. Conjugated pi-systems enable compounds to have a color. The stability of the compound by way of the pi-system allow for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to be close enough that electromagnetic wavelength absorbed by the compounds are within the visible spectrum.

If no visible light is absorbed, a compound will appear white or colorless. In the case of Para-red, 500 nm light is absorbed, corresponding to green. As the green light is absorbed, all other colors are reflected, enabling the compound to appear red. The more extensive a conjugated pi-system, the longer the wavelength (the lower the energy) of visible light will be absorbed. Thus, the color of the compound will change accordingly.

When a few drops of sodium hydroxide (NaOH) are added to Para-red, the maximum wavelength absorbed by the compound shifts to a higher wavelength. This results from the donation of a proton from the Para-red compound to the hydroxide ion. The conjugate base of Para-red adds to the conjugation present in the molecule, thereby further stabilizing the molecule and lowering the energy. This lowering of the energy allows for the highest occupied molecular orbital and the lowest unoccupied molecular orbital to become even closer together, increasing the wavelength absorbed by the molecule. This increase in absorbed wavelength thereby changes the color of Para-red accordingly. 

As you can see, the atomic process of what makes a dye a dye is beautiful. I hope this is something you can take with you as you look at the colors of fabric around you. I walk through life thinking about these tiny atomic processes. It makes life pretty magical. For instance, look at a stream of light from the sun and imagine the journey those thousands of photons took to reach the blades of grass that are undergoing photosynthetic processes.