In my second year of graduate school, I had to give a departmental seminar that discussed azobenzenes. These molecules consist of benzene rings (six carbon atoms set into a hexagon pattern) that are connected by two nitrogen atoms. The nitrogen atoms have a trigonal arrangement, which means that the two benzene rings can exist in one of two different orientations. The rings can lie on the same wide of the nitrogens (which is called a cis-isomer) or they can lie on opposite sides of the nitrogens, which would be the trans-isomer. These two compounds have the same atomic composition (six carbons, five hydrogens, and two nitrogens) but the orientations of the atoms are different. Organic chemists such as myself call them stereoisomers. Each of the two different isomers has entirely different electronic properties, and they’re highly colored. Probably the most exciting property of azobenzenes is that either isomer can absorb light and then use that energy to flip orientation into the other form. So, a cis-azobenzene can be irradiated and then flip over into the trans form, and vice versa. This type of molecular switch is not very common in organic chemistry, and my seminar that day in 1999 concerned the use of these materials in “molecular rotors” / machines.
I’ve done what I can over the years to keep track of developments in this subject area, as I found the azobenzenes interesting and I even used them in my own research for a number of projects. I was happy to read a recent article published in the American Chemical Society journal Nano Letters that discussed how azobenzenes are being coupled with another molecule, carbon nanotubes, to act as a type of energy storage. The American Chemical Society publishes most of the top quality chemistry journals in the world, and Nano Letters is one of the best publications for breaking news in nanoscience. This particular article was published by a group of MIT chemists and used carbon nanotubes, which are hollow rolled-up tubes of carbon atoms that are in the form of a thin sheet. The researchers coupled these nanotubes to an azobenzene. The central point of this article was that the cis and trans forms of the azobenzenes are not at different energy levels. The trans form is ever so slightly more stable, and is usually the predominant form. The final molecule was then tested as a method of storing solar energy.
The sequence of events goes something like this: first, the molecules (which are in their lowest energy state, the trans isomer ground state) absorb sunlight. This bumps the nanotubes:azobenzene hybrid into it’s elevated energy state (the excited state). When this happens, the nitrogen-nitrogen bond has the energy it needs to isomerizes into the cis form. The molecule falls down into the ground state, but it’s no longer the trans form, and so it ends up slightly higher in energy than the starting cis isomer. The solar energy has been transformed, in a sense, into chemical energy; the energy difference in stability between the cis and trans form is now locked into the chemical bonds. This molecule can be stored in the dark for an indefinite period of time, and is perfectly stable. However, if it is then exposed to a small stimulus – a tiny amount of heat, or some sort of mechanical stress – the extra energy comes rushing out at once as the molecule twists back into the trans form. This cycle can be repeated, if desired, and the heat that is reemitted can be used to power a variety of useful processes.
This article was very satisfying to read. It really drove home the notion that sunlight is no different from any other type of energy; the different isomers of azobenzene have stability difference that is more than just theoretical. It brings the undergraduate organic chem. “energy level diagrams” out of the textbook and firmly into the real world. As an organic chemist, I thoroughly enjoyed reading about this success.
The source of this article can be found at:
Kolpak, A., et al. “Azobenzene-functionalized carbon nanotubes as high energy density solar thermal fuels”. Nano Letters 2011, 11, 3156-3162.