Makenzie Provorse is a graduate student in Professor Jiali Gao’s (MSI Fellow) group in the Department of Chemistry. She entered the University of Minnesota in the fall of 2009 and joined the Gao group in January 2010. She’s been using MSI since then.
Ms. Provorse was a finalist at the 2013 MSI Research Exhibition, which was held on April 11, 2013. She submitted a poster entitled "Quantum Coherence in Singlet Fission From Multistate Density Functional Theory (MSDFT)." Recently, MSI talked with Ms. Provorse to discuss her poster and the work she does using MSI.
MSI: What do you use MSI for?
Makenzie Provorse: I mainly run molecular simulation and electronic structure programs on Calhoun and Itasca. They’re the two machines I use the most. For this specific project, we used a locally modified version of GAMESS, a molecular electronic structure theory program, and CHARMM, a molecular simulation package. We do what is called combined QM/MM - quantum mechanics and molecular mechanics - simulations, so we use CHARMM to run molecular dynamics and have it call GAMESS as a subroutine whenever a quantum calculation is needed.
MSI: Can you explain what your poster describes?
MP: Pentacene is a planar [flat] molecule with five hexagonal benzene rings fused together. In the crystal or thin film form, pentacene displays some very unusual properties. Say you have two pentacene molecules side by side in a crystalline lattice. When one molecule absorbs energy from, for example, the sun, this absorbed energy - called a photon - excites an electron from one molecular orbital to another, higher-energy, molecular orbital. This forms an excited electronic state, but - what’s interesting about pentacene - is that through a process called singlet fission, you actually end up with two excited electrons, each on one of the neighboring pentacene molecules. So you’re getting two excited-state molecules from the absorption of just one photon. It’s kind of a two-for-one energy conversion process. The potential application is that we can use pentacene to make much more efficient solar cells as a source for renewable energy.
MSI: And that’s not a violation of thermodynamics laws?
MP: Right, that’s the interesting part! Quantum mechanically, when a molecule is excited from its ground state, which is typically a singlet state - all electrons are paired with one spin up and one spin down - its lowest-energy excited state is also a singlet. That is, after excitation, one electron has moved to a higher-energy molecular orbital, which makes two electrons unpaired, but they retain their respective spin states - one up and one down. The two unpaired electrons can also have the same spin - both spin up or both spin down - called a triplet state, but transitions from a singlet ground state to an excited triplet state are not allowed quantum mechanically. Regardless, the two excited pentacene molecules produced by singlet fission are in fact in the triplet state. So, after a photon is absorbed by a pentacene molecule and it’s excited to the lowest-energy singlet state, then, through some quantum mechanical process, the singlet state transitions to two excited triplet states.
Thermodynamics come into play when looking at the relative energy of the singlet and triplet excited states. Typically, the singlet excited state is lower in energy, but in pentacene, the triplet state is slightly less than half of the singlet state energy, which means that twice the energy of a triplet state is less than the singlet state energy. This quantum phenomenon makes it thermodynamically favorable for pentacene to undergo singlet fission.
The fact that pentacene undergoes singlet fission has been known for a long time. What we’re interested in is how this transition occurs. It’s been hypothesized that singlet fission involves an intermediate state, called a multiexciton (ME), which consists of two excited triplet states coupled together to form an overall singlet excited state. This state is optically-forbidden, meaning that it cannot be populated directly from the ground state, but it’s thought that the excited state pentacene shares its energy with a neighboring pentacene molecule to form a pair of correlated triplet states, each on one pentacene molecule. We’re investigating the way this ME state is populated and how it is coupled with the singlet excited state of a pentacene molecule.
MSI: So, you use the computer programs on the supercomputers to model this process?
MP: Yes, we use the computer programs to model each state, with the electrons excited in various ways, and then we calculate the electronic coupling between the states. There are three states important to the singlet fission process: the initially excited singlet state (S1); a charge-transfer (CT) state; and a pair of correlated triplet states, or a multiexciton (ME). If you go directly from the S1 to ME states, there’s a much smaller coupling than if you couple with the charge-transfer state, S1-to-CT then CT-to-ME. So these calculations show that including the intermediate charge-transfer state is necessary to get significant coupling to produce two excited, triplet state pentacene molecules.
MSI: Is this research something that can be applied immediately, or is it more basic research?
MP: This is basic research, but yes, there have been efforts to fabricate solar cell devices that make use of singlet fission. Here, we focus on understanding the underlying mechanism of this process.
MSI: Do you know if there’s anything else this could be used for?
MP: Mostly, the current interest in singlet fission comes from its potential to improve the efficiency of solar cells. Right now, the theoretical efficiency of single-junction solar cells to convert solar energy into electricity is limited to about 33%, the so-called Shokley-Queisser limit. But, if materials that undergo singlet fission are used, we could exploit this two-for-the-price-of-one energy conversion process and potentially break this limit.
MSI: Would that reduce the cost, if they could be made more efficient?
MP: I don’t know if will reduce the cost. But looking at where solar cells are now, with the materials that are currently used, efficiency is around 11%. One of the key motivations for this project is this quantum-mechanical limit that even if the solar cell is completely 100% efficient in all these other ways, due to the quantum mechanics, 33% efficiency is all you’ll ever get. That’s if everything else is perfect. This project is a way to increase that limit, because we’re getting two excited electrons that can both, potentially, be harvested to generate an electrical current. So, that’s the major advantage of using pentacene, because it undergoes singlet fission, we can break that theoretical barrier. If that’s improved, everything will be improved.
MSI: So, the long-term goals for this research are to prove your theory?
MP: The goal is to investigate the mechanism of how singlet fission happens in these kinds of organic monolayers. If we can understand what drives the reaction and what molecular properties affect this process, then down the road, we can use that knowledge to our advantage.
MSI: Is there anything else you’d like to mention about your work?
MP: Yes, the multistate density functional theory method I used for this poster was developed in our group. Density functional theory is a well-known electronic structure theory method. It’s very efficient computationally, it’s easy, it’s inexpensive, but it’s delocalized. With multistate density functional theory we can localize states on individual pentacene molecules. That’s why we can calculate these couplings. I’m also currently using this MSDFT method to study other processes related to solar energy conversion, such as proton-coupled electron transfer in photosynthesis.
MSI: This method was developed before you joined the Gao group?
MP: Yes, and it’s been ongoing. This project is just one of its many applications. This is a group effort, involving a number of people developing the theory, writing the computer code, and applying and validating the method in practical applications.
MSI: So, this is something that needs the supercomputers.
MP: Oh, definitely.
posted July 3, 2013