Vilnius University (VU) Faculty of Physics Institute of Chemical Physics PhD student Arunjyoti Baidya and Prof. Darius Abramavičius are working on a “PhotoCAM” (Photosynthetic Antennas in a Computational Microscope) project to better understand photosynthesis. Using theoretical and computational methods, the researchers aim to identify the most efficient ways to capture light energy and control energy production.
Arunjyoti Baidya. Photo from personal archive.
How does nature harvest light?
If you need the most efficient example of a sunlight-capturing system, look to nature. For example, in plants. Firstly, photosynthetic cells absorb sunlight, generating excitons that travel through a series of light-harvesting proteins. Finally, they reach the “photosynthetic reaction centre”, where the exciton is converted into electron-hole pairs. Those charges provide the power to run chemical reactions and produce an essential energy source – sugar-like molecules.
“Since these processes happen very fast and at a very small scale, despite decades of experimental and theoretical work, it remains extremely challenging to accurately determine how the excitons move through these systems and why some of it is lost along the way, as well as how the proteins self-protect from burnout. In my research, I develop advanced computational models to accurately describe processes occurring throughout the lifetime of the exciton and how it eventually contributes to energy conversion,” says Baidya.
Into the funnel
Excitons are almost impossible to study individually due to their short lifetimes, small size, and sensitivity to their environment. They are quasiparticles, which exist only inside the material as specific collective excitations of chlorophyll molecules.
“All quasiparticles are defined relative to a state of equilibrium. Take, for example, plant cells. At night, there are no sun photons – the material is at equilibrium – there are no drivers to “trigger” any kind of activity, such a state can be denoted as an effective “vacuum” of excitations. In semiconductors, an incoming photon initiates the creation of free electrons and holes, again as quasiparticles, both of which carry charge and can be inspected individually. In photosynthesis, there is an intermediate step because a photon in molecular materials cannot directly create a free electron and its hole. Instead, it creates a molecular excitation that can be shared between molecules. This collective excitation is what physicists describe as an exciton.
They have properties other than an electron or a hole – for example, they have no charge. However, they can diffuse through the materials. We believe that photosynthetic molecular complexes operate according to a funnel principle. The exciton “senses” this funnel and rolls down it, eventually reaching the region where all additional electrical processes occur. When they reach a specific type of molecular arrangement, an electron and a hole can be generated from that exciton. Electrons and holes are “simple” because they have a charge – by applying an electric field, we can push them to the left or right, but we cannot manipulate an exciton in this way,” explains Badya.
What models show?
To understand energy flow through the system, scientists must track the energies of excitons, how pigments interact with each other, how molecular vibrations affect energy flow, and dipole moments, which determine how pigments interact with light. All of these determine how an exciton is created, shared, and transferred.
These physical properties are combined into a specific mathematical model that describes excitons, vibrational motion, and their mutual interactions. This way, researchers simulate energy transfer within a single pigment–protein complex or between different complexes.
In reality, things aren’t simple. “Various quantum relaxation, and spectroscopic modelling methods have already been developed, and AI is now being incorporated into these processes. Using computational tools, we can model the process and show that everything works exactly as we set it up. However, experiments show one picture, and while our models resemble it, they aren’t identical. Therefore, our goal is to identify the point where theory and experiments diverge, and what this tells us about the real mechanism of energy transfer in nature,” says Baidya.
Prof. Darius Abramavičius. Photo from personal archive.
“We model the processes occurring during the lifetime of this exciton. Since it is born from light, we can observe it via various optical and spectroscopic measurements. It is important not to forget that our modelling results must also match those observed in experiments. Of course, we can’t lock ourselves into our theory; we collaborate and co-author papers together with colleagues internationally. There are cases where theory doesn’t match practice, so we investigate whether the discrepancies lie in the calculations or the experimental part,” Prof. Abramavičius adds.
An interdisciplinary network of researchers
This research is part of the Marie Skłodowska-Curie Actions doctoral network, funded by the European Union. The “PhotoCAM” project is primarily an interdisciplinary fundamental research effort aimed at studying photosynthesis by modelling it from the perspectives of physics, chemistry, biology, and industry. The network brings together 10 different research groups across Europe.
“This is the first European Research Council grant of its kind at the VU Faculty of Physics. The main goal is not to invent or improve something from a practical standpoint, but to understand how it works and whether our current theoretical computer modelling methods are sufficient to understand this highly complex process. Each university has one doctoral student working in a research group within the network, thereby forming a consortium to address this specific problem concerning photosynthesis.
One of the industry representatives develops computer modelling software. This way, they can incorporate the latest knowledge and determine what is needed to solve such problems from a software perspective. The knowledge is directly applied in the agricultural sector for plant engineering, so that, for example, even when plants grow in the shade, they can yield the same harvest as when growing in the sun. In other words, they can use available sunlight more efficiently.
Fundamental research conducted in this network is the first step toward practical knowledge, since we cannot speak of application until we fully understand the process itself,” concludes Prof. Abramavičius.