In this primer, I will cover some background needed to explain the results of our group’s JACS paper that I had mentioned previously. Here you’ll learn about how chemists and physicists describe a molecule’s state, and how that relates to the processes that occur in materials used in solar energy devices.
The Ground State
Imagine you are taking a walk through a narrow valley that has steep hills on either side of it. You come across a boulder sitting in bottom of the valley, blocking the path. The boulder is quite stable at the bottom of the valley and certainly isn’t going anywhere on it’s own. You decide that the boulder doesn’t look so big, maybe you could just roll it up one of the valley walls. You get on one side of the boulder and with great effort manage to roll the boulder a few feet off the valley floor. You need to take a break so you let go of the boulder, which promptly rolls back down into the bottom of the valley. Damn. Try again, push the boulder up the valley wall as far as you can, exerting more effort this time, but when you let it go it will inevitably roll back down to the bottom of the valley (figure 1). We all intuitively understand what is going on here. Once we let go of the boulder it rolls down because the force of gravity pulls it down as far as it can go, which in this case is the bottom of the valley. This may seem quite Sisyphian but there is a point!
The boulder-in-valley metaphor presented above is actually a pretty good way to think about molecules. The boulder represents what is called the state of the molecule. When the boulder is in the bottom of the valley, that is called the ground state of the molecule. The molecule is configured (see note 1), such that it is as low in energy as possible. Just as the boulder cannot go down any further once it is in the bottom of the valley, the molecule can’t be changed in any way to decrease its energy. If one were to put energy into changing the configuration of the molecule, by stretching it, squeezing it, rotating certain parts, or even moving the electrons around, the molecule would “roll” back to the ground state in the same way that the boulder would roll back to the bottom of the valley once pushed up the valley walls (see note 2).
The Excited State
Now I will modify the analogy a little bit. Picture the same valley with the same boulder, only this time, halfway up one of the walls of the valley you spy a small ledge with a lip, just large enough for the boulder to sit securely. Perfect! You can now push the boulder up to the ledge and it will sit there instead of in the valley. All of the work you put into pushing the boulder up is not instantly wasted because the boulder now has a place to sit without rolling back down to the bottom (Figure 2). This ledge applies to the molecular picture as well. When the boulder is sitting up in its ledge, it is in a higher energy state than when it was in the bottom of the valley, and similarly with the molecule there are stable states that lie higher in energy than the ground state. These are referred to as excited states.
In the boulder picture it’s pretty easy to imagine yourself pushing the boulder up onto the ledge. However, in the molecule there is no tiny person available to do the “work” invovled in moving the molecule into the excited state. What is available though is the energy that falls upon us every day from the sun: light! Light is a fascinating subject and I could write much more about it (topic for a future post I imagine) but for our purposes here, light is simply a packet of energy (commonly called a photon). When a molecule absorbs this piece of energy it can use that energy to move to an excited state. Picture the absorption of light as giant that can pick up the boulder and easily throw it onto the ledge.
To be more precise in showing how scientists think about states, just strip away the valley walls from our previous picture, and place lines where the stable states of the molecule (or boulder) are. Now we can label each state accordingly. The difference in height between the states is simply the difference in energy between the states, or to put it another way, it is the amount of energy stored in the excited state (figure 3).
Putting the Excited State to Work
One could think of the boulder sitting up in its “excited state” ledge as having stored all the work and energy you put into pushing it up there. You could push the boulder back into the bottom of the valley and change all that stored energy (called potential energy) into kinetic energy, or if you were clever you could hook the boulder up to some kind of pulley system and you could put the stored energy into any number of useful purposes: use the pulley to pull something else up as the boulder rolls to the valley floor, attach the other end of the rope to an axle and set something spinning, or even connect it to a generator and make a little bit of electricity. In molecular systems the idea is the same: once the molecule is in an excited state, it is possible to use the stored energy to do something useful like driving a chemical reaction that makes or breaks bonds, or transferring an electron to into a circuit to create electricity.
This is the whole point here. If the molecules are well designed, then the energy of the photon is stored in an excited state, and that stored energy can be used to do something useful! By making or breaking bonds, the energy in the excited state can be used to make fuels (ethanol or hydrogen) that could run your car or heat your home. By transferring electrons out of a molecule in the excited state efficiently and creating a circuit, the stored energy of the photon can be used to make electricity that can power a device or even your home (solar power). If these processes work well enough and the systems can be made cheaply enough then in the future we could begin to replace gasoline as a fuel source and reduce the amount of electricity that is produced from fossil fuels.
Unfortunately It isn’t as easy as I make it out to be above to use these materials to produce solar energy. In my next primer I’ll talk about kinetics – how fast these processes happen, and why it is so hard to make molecules that use solar energy efficiently.
- When I talk about the ‘configuration’ of the molecule I am referring to two things a) The physical geometry of the molecule, such as the placement of atoms in relation to one another, how they are bonded, and how the different parts of it are folded, flattened, or rotated. b) given a geometric arrangement of a molecule (part a) how the electrons are arranged around the molecule.
- When dealing with chemistry and physics, people tend to personify inanimate particles. One hears explanations such as “the molecule wants to be over here” or “the atom likes 8 electrons”. While useful as a tool to aid in understanding, remember that the electrons and atoms don’t like or want anymore than the boulder wants to be at the bottom of the valley. It is simply a way of saying that the atom or molecule is more stable or at a lower energy. So instead one could say “the molecule is lower in energy over here” or “the atom is more stable with 8 electrons.”