Probing Coherence Dynamics in Model Systems to Understand Energy Transfer in Photosynthesis
Dr. Daniel Turner1, Principal Research Scholar
Co-PIs: Olga Mass1, Ryan Pensack1, Eric Bittner2
1: Boise State University, Boise, ID 83725
2: University of Houston, Houston, TX 77004
The multi-step process of photosynthesis begins with the absorption of light followed by transfer of electronic excitation energy to a reaction center, effecting the charge separation necessary to drive the biochemical reactions that feed most life on earth. The entire photosynthetic process in most organisms has about a 1% energetic efficiency. By contrast, the quantum efficiency—which is the ratio of photons absorbed to charge-separated states produced—of the electronic energy-transfer process in photosynthesis can be nearly 100%. This fact is surprising because the structure of a photosynthetic protein is not a perfect lattice like that of single-crystal silicon used in photovoltaic panels. Hence, an improved understanding of energy-transfer mechanisms in photosynthesis is both of fundamental research interest and may offer new design principles for improving solar-energy conversion technologies. One question is the effect of so-called quantum coherences on the overall energy-transfer process. A quantum coherence is the superposition of multiple energetic states, which can be conceptualized as wavepackets of energy that move with time. The question of coherence has persisted for decades, remaining unanswered in part because natural photosynthetic complexes have limited synthetic tunability. The objective of this research is to overcome the issue of limited synthetic tunability of natural photosynthetic complexes by using DNA-templated molecular aggregates as model systems. These tailored molecular aggregates have vast synthetic tunability, and this project will incorporate bilins and chlorins as chromophores. Some bilins and chlorins are natural photosynthetic pigments, but also have synthetic tunability in the laboratory using well-understood chemical reactions. The studies of DNA-templated aggregates will use time-resolved spectroscopic measurements, paired with quantum-chemical computations, to evaluate two key questions: (1) Do the coherences persist longer as the electronic coupling comes into resonance with a vibrational mode? and (2) Does the frequency of the coherence downshift to very long frequencies, as predicted by theoretical model, as the electronic coupling comes into resonance with a vibrational mode? Answers to these questions will improve the interpretation of the coherences that appear in measurements of natural photosynthetic light-harvesting proteins and thereby produce new knowledge into photosynthetic structure-function relationships, which can be used to build new bio-inspired energy-conversion technologies having improved efficiencies or characteristics.
This research was selected for funding by the Office of Science, Basic Energy Sciences
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