TY - BOOK

T1 - Shining Light on Charge Separation in Nature

AU - Northcote, Lea Kjærgaard

A2 - Hansen, Thorsten

PY - 2024

Y1 - 2024

N2 - Plants are extremely efficient at harnessing energy from the sun and could be the inspiration for improving renewable energy resources. In plant chloroplasts, solar energy travels from one chlorophyll molecule to another until it eventually reaches the reaction centre (RC). Here, an electron splits from a hole, and the two charges move in opposite directions. Both moving charges contribute energy to the chemical reactions of photosynthesis that produce sugar and oxygen. Although we know the atomic structure of the RC and the speed at which the energy splitting occurs, we do not have a detailed understanding of the dynamics of these moving charges. An in-depth knowledge of the mechanism can potentially enable us to create synthetic molecules that can harness solar energy as efficiently as plants.A detailed understanding of RC dynamics requires an interplay between experimental and theoretical quantum mechanical models. Modern coherent laser experiments can capture ultrafast processes and reveal new details. However, to extract all experimental information, we need quantum dynamics simulations of what we expect to see. Only when measurement and calculation match can we genuinely comprehend the process and be confident in our underlying theoretical model. We intend to perform the necessary calculations for the photosystem II (PSII) RC at the heart of the energy conversion process. We aim to uncover the complexities of the RC and gain a deeper understanding of photosynthesis using advanced quantum chemical computational techniques and open quantum system models.This PhD thesis provides a detailed study of the PSII RC, including multireference site energies and excitonic couplings. We developed a framework for simulating quantum dynamics and two-dimensional electronic spectroscopy, anchored by implementing the Redfield equation to evolve the open quantum system in time.To better understand the energetics of the system, we made a deliberate decision to omit the protein environment from our calculations. This approach allowed us to separate the effects of the environment from the intrinsic site energies. By doing so, we could employ a theoretical model for the environment in quantum dynamics simulations without the risk of double-counting environmental effects. Another motivation was the computational infeasibility of performing quantum mechanics/molecular mechanics geometry optimisations with our chosen method, underscoring the necessity for an alternative strategy. We are confident that this methodology will yield more accurate and reliable results in future studies.As an essential component of our research, we thoroughly analysed the Redfield equation to ensure its accurate application to our model system. Our analysis revealed that adhering to the Markov approximation and monitoring spectral function stability were essential to avoid non-positivity issues. Furthermore, we employed the eigenvalues of the Redfield matrix as a powerful diagnostic tool to validate our findings.Our ongoing research into the dynamics of the PSII RC has yielded significant progress by developing a flexible framework capable of accommodating various environmental models. In the future, we plan to explore the impact of protein surroundings on energy transfer to gain a deeper understanding of this vital process. This thesis represents a significant step forward in understanding the PSII RC dynamics, paving the way for invaluable insights in the field with future studies. We now have the framework for unlocking fundamental insights into nature’s most elegant energy conversion mechanism.

AB - Plants are extremely efficient at harnessing energy from the sun and could be the inspiration for improving renewable energy resources. In plant chloroplasts, solar energy travels from one chlorophyll molecule to another until it eventually reaches the reaction centre (RC). Here, an electron splits from a hole, and the two charges move in opposite directions. Both moving charges contribute energy to the chemical reactions of photosynthesis that produce sugar and oxygen. Although we know the atomic structure of the RC and the speed at which the energy splitting occurs, we do not have a detailed understanding of the dynamics of these moving charges. An in-depth knowledge of the mechanism can potentially enable us to create synthetic molecules that can harness solar energy as efficiently as plants.A detailed understanding of RC dynamics requires an interplay between experimental and theoretical quantum mechanical models. Modern coherent laser experiments can capture ultrafast processes and reveal new details. However, to extract all experimental information, we need quantum dynamics simulations of what we expect to see. Only when measurement and calculation match can we genuinely comprehend the process and be confident in our underlying theoretical model. We intend to perform the necessary calculations for the photosystem II (PSII) RC at the heart of the energy conversion process. We aim to uncover the complexities of the RC and gain a deeper understanding of photosynthesis using advanced quantum chemical computational techniques and open quantum system models.This PhD thesis provides a detailed study of the PSII RC, including multireference site energies and excitonic couplings. We developed a framework for simulating quantum dynamics and two-dimensional electronic spectroscopy, anchored by implementing the Redfield equation to evolve the open quantum system in time.To better understand the energetics of the system, we made a deliberate decision to omit the protein environment from our calculations. This approach allowed us to separate the effects of the environment from the intrinsic site energies. By doing so, we could employ a theoretical model for the environment in quantum dynamics simulations without the risk of double-counting environmental effects. Another motivation was the computational infeasibility of performing quantum mechanics/molecular mechanics geometry optimisations with our chosen method, underscoring the necessity for an alternative strategy. We are confident that this methodology will yield more accurate and reliable results in future studies.As an essential component of our research, we thoroughly analysed the Redfield equation to ensure its accurate application to our model system. Our analysis revealed that adhering to the Markov approximation and monitoring spectral function stability were essential to avoid non-positivity issues. Furthermore, we employed the eigenvalues of the Redfield matrix as a powerful diagnostic tool to validate our findings.Our ongoing research into the dynamics of the PSII RC has yielded significant progress by developing a flexible framework capable of accommodating various environmental models. In the future, we plan to explore the impact of protein surroundings on energy transfer to gain a deeper understanding of this vital process. This thesis represents a significant step forward in understanding the PSII RC dynamics, paving the way for invaluable insights in the field with future studies. We now have the framework for unlocking fundamental insights into nature’s most elegant energy conversion mechanism.

M3 - Ph.D. thesis

BT - Shining Light on Charge Separation in Nature

PB - Department of Chemistry, Faculty of Science, University of Copenhagen

ER -