This primer will build on ideas presented in my previous post, so if you haven’t read that yet I encourage you to do so. You should be familiar with the idea of different states in molecules, specifically the ground state and excited states.
We’ve seen that when a molecule absorbs a photon it gains enough energy to move up to an excited state and then once in the excited state, the molecule can go on to use this stored energy to do something useful, like make a fuel or provide electricity. Unfortunately it is not as easy as I make it out to be, and one of the reasons is kinetics – which is the study of how fast certain reactions happen. Once in the excited state, the system can either go on to do the chemistry that you want it to do, or it can return to the ground state. If returning to the ground state occurs much faster than the reaction that you want to achieve, then there will be problems…
The Basics of Barriers and Kinetics
Let’s look at the state diagram again thinking about it as a boulder-in-the-valley picture. In figure 1 you’ll see the same energy states (ground and excited state) with the boulder sitting up in the excited state, but they are connected in different ways. In scenario A there is no barrier between the excited state and the ground state, the boulder can just roll right down. In B, there is a slight barrier, and in C there is a very large barrier. I now pose to you this question: In which scenario will it be easiest to push the boulder from the excited state back down to the ground state?
If you answered A give yourself a cookie! There’s no barrier so it would only take the slightest of breezes to push it back down. Whereas in C it’s going to take quite the effort to push the boulder up over the barrier before it will roll down to the ground state. In the molecular picture it is similar; there may be a barrier of varying height in the path connecting one state to another. If the molecule is going to change states, it first has to get over that barrier.
The astute reader will now ask “but how is the molecule going to get over the barrier? If the boulder were sitting in that excited state it would never spontaneously have enough energy to get over by itself.” That is a good point, reward yourself with another cookie. Certainly if the boulder were sitting up in the excited state in scenario B it would never go over that hump by itself, it would need some kind of push to get started. In the molecular picture, the molecule actually needs some sort of push as well. The only difference is that it is much easier for the molecule to get a push than it is for the boulder. If the molecule is in solution, it is constantly colliding with other molecules and bouncing around non-stop like a kid after eating all his Halloween candy. Every bump and collision with another molecule is like a little push up that barrier. If the molecule gets hit with enough force it can easily be pushed up an over the barrier and arrive back at the ground state.
In the molecular picture the height of the barrier is called the activation energy. The higher the activation energy, the slower it will be for the molecule to go from state to state. Conversely the lower the activation energy, the faster the molecule can move between states.
We all know that we shouldn’t waste energy in our homes but it is also important not to waste energy on the molecular scale as well! If we’ve absorbed all the energy from the photon and stored it by putting the molecule in the excited state, the last thing we want to have happen is for the molecule to go back to the ground state. It would waste the energy that was so carefully stored! Going back to the ground state from the excited state is generally called recombination because the electron is returning back to where it came from. This is the problem with designing solar materials that make use of stored energy – how can we make use of the stored energy before it has time to return to the ground state?
Of course if there was some way to control and slow down the recombination process… then there would be more time available for the excited state to do useful work! This is exactly the problem that we are trying to solve in the paper we had published in JACS this summer. I’ll explain how we achieve this control in the next post!
- There are energetic barriers that prevent direct movement between states
- The larger the barrier the slower the movement between states can occur
- If the molecule goes from the excited state back to the ground state, this is recombination and results in wasted energy