I recently read an article in the journal Chemical Communications that described a clever use of donor-acceptor chemistry. I thought that I’d talk about this topic a little as it’s something that most people can understand (even if they have no chemistry training) and it gives some insight into the principles of molecular design that are at the core of organic chemistry. I’ll then finish with a quick look at how this method was applied in the journal article.
Molecules are made up of atoms, which are connected to other atoms by chemical bonds. So for a typical organic molecule, you might have anywhere from 20 to 40 atoms, all interconnected and forming one final structure that doesn’t have any atoms floating free – it’s one big molecule made up of tiner parts. The bonds between the atoms are made up of electrons, two electrons per bond, and the electrons are not necessarily equally shared between the two atoms that they connect. It depends on the atoms placements in the Periodic Table. Those atoms which are closer to the upper right of the Periodic Table pull in more than their fair share of the electon density, and those atoms which are in the lower left of the Periodic Table pull in less than their fair share of electron density. This principle is called electronegativity.
If one section of a molecule is comprised mainly of those atoms which are closer to the upper right, then that section becomes a “magnet” of sorts for electron density within the surrounding bonds. None of the bonds are broken, but a partial negative charge begins to build up on the “electron withdrawing” portion of the molecule. This part of the molecule is also called an acceptor, as it accepts more electric charge than its neighbors. Parts of the molecule which have extra electrons to give, such as oxygens and long chains of carbon atoms, are called “donors”, as they give up some of their electron density. So, instead of having a perfectly equal 50:50 share of the electrons amongst all of the bonds in a molecule, you have sort of a permanent imbalance. One section of the molecule (the acceptor) has more electrons, and the other section (the donor) has less. There’s a constant electronic “push”, starting from the donor and ending up at the acceptor. This molecular component is called a dipole, and is a foundation of organic chemistry.
It turns out that the amount of light that a molecule can absorb, and the frequency of light that it can absorb (the lights color), depend heavily on what type of dipole is present and how strong it is. Molecules with a strong charge separation (strong donor, strong acceptor) absorb a wider frequency of light than a molecule such as hexane, which is a nonpolar (non-dipole containing) organic solvent which is perfectly clear and colorless. It only absorbs ultraviolet light, and therefore has no distinctive color to the human eye.
Ok, so why is this important? It is extremely important for the area of solar cells. A solar cell – the active component in a solar panel – absorbs incoming sunlight and produces useful electricity in the process. Light is absorbed at the donor site of the molecule and transported to the acceptor section, at which point the electrons jump from the acceptor down to the titanium dioxide surface of the solar panel and are transported away by circuitry to be used for useful purposes. The amount of electricity produced is directly tied to the amount of light that can be absorbed, as it is the light energy which is being transformed into the electrical energy. No light absorbtion means no electricity production.
Dye-sensitized solar cells contain an organic dye (a molecule with both a donor and an acceptor). Light energy comes in and “excites” the molecule (an actual scientific term), which then releases electrons which flow to the semiconductor surface. Most chemists use a molecule named cyanoacrylic acid to anchor the dye to the semiconductor. However, this choice of molecular tether makes it difficult to modify the dye, which is sometimes necessary to precisely finetune the dyes behavior. It is through modification of the donor and acceptor groups that changes to the dyes light absorbtion can be made. Chemists in China published in this particular journal article that by switching to a different tether, one named propanoic acid, it became much easier to make modification to the structures of the dye. Their new dyes were therefore much better at absorbing the full spectrum of incoming sunlight, including some of the infrared wavelengths, which meant that much more energy was available for transformation into electricity. This is a fantastic approach as it means that none of the incoming sunlight is wasted, which is a problem with current models of solar panels – a lot of the light is either reflected off the surface or it simply passes straight through without being absorbed. This new discovery should help boost the efficiency of solar panels, and maybe finally we will have a product available which makes commercial sense to implement in our homes and businesses.