Dyes are the heartbeat of organic chemistry. Dyes are where organic chemistry all began, with the very earliest organic chemists struggling to provide materials to meet the huge demand for colored fabrics. Today, dyestuffs are taken for granted, and even the most vividly colored t-shirt shouldn’t cost you more than a few dollars. It’s therefore reassuring to know that despite the hundreds of years of research into dyes, that there are still a few discoveries to be made. One such example was recently published in the journal Physical Chemistry: Chemical Physics and is a fantastic example of how intelligent molecular design can overcome difficulties thought to be inherent to the field.
One of the largest problems facing a chemist designing a new dye is that dyes are inherently self destructive. Dyes are molecules which interact with certain wavelengths of light. As particular wavelength ranges are absorbed, the remaining colors mix to produce the vibrant color of the dye. However, this absorption of light creates molecules which have a higher amount of energy than they normally have: the energy of the light goes into the molecule, creating temporarily a molecule labeled as “excited”. These excited-states are similar to a car perfectly balanced on the top of a very steep mountain. They are inherently reactive (something is bound to happen) and often the route which the molecule takes down from its excited state can lead to destruction of the molecule. This leads to fading of the color, which considering that the only function of a dye is to add color, is a pretty serious problem. Dyes are destroyed by exposure to light, yet being exposed to light is the sole purpose of the dye.
A new development published in Chemical Physics outlines one method of getting around this obstacle, and allows the production of dyes which are resistant to destruction by light. It relies on a property called energy transfer. The two ends of the molecule are chromophores, meaning that they soak up a particular wavelength of light quite greedily. This absorption leads to the two ends of the molecule going into an excited-state. Before anything destructive can occur, this energy is transferred from the two ends of the molecule into the center. Lying there is a chemical structure which is fluorinated. Fluorine, an element in the halogen family from the Periodic Table, forms extremely strong and stable bonds to carbon atoms. As a result, a molecule that contains fluorine (such as Teflon) is very resistant to destruction by heat or exposure to light. The amount of energy it would take to destroy the carbon-fluorine bond is too high, and so the molecule remains in one piece.
Upon absorption of the transferred light energy, the center of the new dye molecule goes into an excited state. However, as the destructive routes from atop that high energy perch are not available (due to the stability of the carbon-fluorine bonds), the only way the molecule can exit from the excited state is to reemit the light. Some of the energy is dissipated as heat, and the remaining energy is given off in the form of a longer wavelength (lower energy) color of light. This is a very clever way to avoid destruction of the dye molecule by light. By themselves, the ends of the molecules are too easily destroyed by exposure to light, and would make a terrible dye. By itself, the center of the molecule is too weak at absorbing light, and would have a hard time gathering enough energy to become an efficient dye. By working together (the ends harvesting the light, and the center using that energy for a productive purpose), the whole molecule becomes a team and does an amazing job of resisting degradation from light exposure.
Researchers hope to use this new dye for cases where the dye is continuously exposed to light and must last a long time, such as in ink for currency printing.
The source of this article can be found at:
“A near-IR emitting Bodipy-based dye fitted with ancillary light harvesting units”.
Anthony Harriman, Laura J. Mallon, Sébastien Goeb and Raymond Ziessel.
Physical Chemistry: Chemical Physics, a publication of the Royal Society of Chemistry.