It’s been roughly three weeks since the start of this blog, but I am finally coming around to answering the question that started this off in the first place: what does this paper that I helped to author have to do with anything?
Here’s a summary of what I’ve covered in the previous few posts and the logic leading up to the point of this paper.
- Solar energy processes use the energy stored in the excited state. (Read about excited states here)
- Excited states can easily return to the ground state (recombination) wasting the stored energy (read about recombination here)
- If we can control the speed at which the recombination occurs, then there is more time available for the stored energy to do productive work (Topic of the paper and this post!)
That’s it in a nutshell. In fact, the topic of this paper is the meat and potatoes of what we do in our lab: focus on ways we can learn about and achieve control over excited states in various molecular systems that are used in solar energy conversion devices. In this paper we are using molecular design – how the molecule is put together and arranged – to control and slow down recombination.
All your questions are answered after the jump…
Deconstructing Overly Complex Titles
Here is the title again in all of it’s complex, science jargon filled goodness:
Breaking it down:
- “Exploiting Conformation Dynamics…” –> “Using the way the molecule moves and is put together…”
- “…to Facilitate Formation and Trapping…” –> “…to make and trap…”
- “…Electron-Transfer Photoproducts…” –> “…excited states…”
- “…in Metal Complexes” –> “…in complexes with metals in them.”
The title is just a fancy way of describing what we had mentioned above. We’re simply designing molecules such that they trap stored energy in the excited state and prevent recombination. I realize the explanation of metal complexes doesn’t really add anything there, so in the next section I will describe the type of molecules that are used in this report.
Background: Ruthenium Complexes and their Excited State
Metal complexes are a type of molecule that typically have some positively charged metal center surrounded by what are called ligands. In this case we have a ruthenium metal center (shown as Ru2+ in figure 1) surrounded by bipyridine ligands (the rings containing nitrogen, N, surrounding the Ru2+ center).
These types of metal complexes containing ruthenium and bipyridine ligands are commonly used in solar energy applications. Firstly, they absorb light very strongly right in the middle of the spectrum of visible light, which is perfect when using them for solar energy. The other useful property they have is that when the ruthenium complex absorbs a photon it promotes an electron from the ruthenium center and places it on one of the bipyridine ligands. This is important because it means that it physically separates the negatively charged electron from the positively charged ruthenium metal center. This is the excited state of the molecule and it is typically referred to as the Metal-to-Ligand-Charge-Transfer State commonly abbreviated as MLCT. This MLCT excited state is extremely useful. When the molecule is in this state and the electron is sitting out on the ligand, it has stored nearly all of the photon’s energy. Again though, we want to make sure that the electron does not go back to the ruthenium center, otherwise all of our stored energy is lost.
Preventing Recombination – The Natural Way
There are a few ways to prevent recombination and Mother Nature has figured out one of the ways to do so. In photosynthetic processes that go on in plants, once the excited state is formed the electron is quickly moved away by providing a number of states for it to go to that are each physically a little bit further away from where the electron started. By “hopping” down from state to state the electron can easily move far away and charge recombination is avoided. However as you can see in figure 2, each time the electron hops to a new state that is further away, it looses a little bit of the stored energy. By the time it is sufficiently far away it has used up most of the stored energy just moving the electron a large distance away from where it started. While good in preventing recombination, this greatly reduces the efficiency of the overall process.
A low efficiency process is fine for Mother Nature, because she can simply make up for this shortcoming by just making more leaves. We don’t really have that option because solar energy devices are expensive and we have a limited amount of places where they can feasibly be used. That is why we need another option…
Achieving Control – The Designed Way
Now onto the new and improved way! Looking at figure 2 you’ll see that there are three parts of the molecule highlighted. The part on the left is the electron donor (a ruthenium polypyridyl complex) whereas the part hanging off on the right is the electron acceptor (a methyl viologen). This naming means that once the molecule is excited by a photon, an electron comes from the donor and travels to the acceptor.
The critical part of this though is the third part – the connection between the donor and the acceptor which in this case we will call the bridging ring. This ring is connected to the other two parts such that it can can rotate freely in place. Imagine a coin spinning around as you held it between your thumb and index finger. The ring can lie flat, parallel to the other ring that it is connected to, or it can spin around its axis by 90° and be perpendicular to the neighboring ring. When it is parallel, the barrier for electron movement from donor to acceptor is low, and the pathway for the electron is open – it can freely move from donor to acceptor (or acceptor back to donor). When the ring is perpendicular to the neighboring ring, the barrier is high, the pathway is closed, and the electrong cannot move from one side of the molecule to the other. This is illustrated in figure 3.
What was done in the report is that the two R groups indicated in Figure 2 where modified. In molecule 1 both R groups are hydrogens and should not hinder the rotation of the bridge ring in any way. The ring preferably will lie flat when needed to transfer the electron back and forth. In 2 one of the R groups is replaced with a much larger methyl group (CH3). This means that the ring will preferentially be in the perpendicular position because the larger methyl group with start to ‘bump’ into the neighboring ring when they are in the parallel position. In 3 both R groups are replaced with the large methyl units meaning that the bridge ring is much more stable in the perpendicular position, making it much harder to transfer the electron back and forth.
So… Does It Work?
So – in examining molecules 1, 2, and 3, we expect to see that the amount of time the molecule spends in the excited state increases as we add more methyl groups and force the bridging ring to spend more time in the perpendicular state. Does it work? See for yourself in figure 4:
The data shown in Figure 4 compares how long each of the molecules, 1, 2, and 3 spend in the excited state with an electron on the acceptor. Very briefly, at time 0 a short pulse of light excites an electron on the donor. Within the first 100 picoseconds (abbreviated ps – 1 ps is 0.000000000001 seconds) you can see the signal rises, indicating that the electron is being transferred from the donor through the bridge to the acceptor. After a peak, the signal decays, indicating that the electron is leaving the acceptor, going back to the donor, and the system is returning to the ground state. However, look at the difference in the time it takes for this excited state to decay between 1 (green), 2 (purple), and 3 (black). At 400 ps after excitation, 1 is pretty much out of the excited state, 2 has a small amount still in the excited state, and 3 is almost entirely still in the excited state! Comparing the lifetime (sort of an average) of each molecule reveals in increase from 98 ps for 1 to 789 ps for 3. That’s nearly an increase of 8x in the lifetime of the excited state!
So – we demonstrated that by simply altering two small parts of the molecule, we can extend the lifetime of the excited state by nearly eight times! Aside from being pretty cool, this means we can attempt to implement these types of systems in actual molecules that are used for solar energy instead of just test systems, to see if this could actually improve their efficiencies.
I hope you enjoyed reading about this research. I find it fairly fascinating, and I hope I’ve been able to make it understandable! This is one of the more technical posts that I’ve done so I would love to hear your feedback on the post. Too wordy? Too confusing? Too many ancillary topics? Not enough science? Let me know in the comments!
Meylemans, H., Hewitt, J., Abdelhaq, M., Vallett, P., & Damrauer, N. (2010). Exploiting Conformational Dynamics To Facilitate Formation and Trapping of Electron-Transfer Photoproducts in Metal Complexes Journal of the American Chemical Society, 132 (33), 11464-11466 DOI: 10.1021/ja1055559