Nature harnesses sunlight by capturing and transporting photoenergy. Our experiments seek to understand light harvesting in natural systems and control it in synthetic ones.
Energy transfer in photosynthetic light harvesting. Photosynthesis begins with absorption of photoenergy followed by excitation energy transfer through a network of chlorophyll-containing proteins known as light-harvesting complexes. Energy transfers through the protein network to reach the reaction center, where electricity is generated, with a remarkable near unity quantum efficiency. The organization of the protein network within the biological membrane is dynamic; the network reorganizes under different environmental conditions with unpredictable effects on the efficiency of light harvesting. We investigate the role of membrane organization and composition using model membrane platforms and map out the energy transfer pathways using ultrafast transient absorption spectroscopy and 2D electronic spectroscopy.
Free energy landscapes (left) of the pigment-protein complex LHCSR3 (right) from green algae reveal two types of quenched and unquenched conformations. Likely switch between quenched and unquenched conformations indicated on the structural model.
Photoprotection in photosynthetic light harvesting. Under high light (i.e., sunny days), excess absorbed energy can cause damage. Thus, light-harvesting complexes have evolved a feedback loop that triggers photoprotective energy dissipation. It is this feedback loop that solves the “intermittency problem” in solar energy. However, under natural conditions, inefficiencies in photoprotection limit biomass yields by up to 30%.
While photoprotection is thought to involve conformational changes of the light-harvesting complexes, the nature of these changes as well as the associated dissipative pathways have not been identified. We probe the conformational dynamics with single-molecule spectroscopy and the dissipative pathways on a femtosecond timescale with ultrafast spectroscopy. Our mechanistic insights could guide efforts to rewire photosynthesis for increased biomass.
DNA-scaffolded molecular aggregates.
Bio-inspired DNA excitonic systems. Control over excitons and their dynamics enables energy to be directed for light harvesting and molecular electronics, yet requires nanoscale precision over the architecture of the constituent molecules. A remarkable example of this precision is photosynthetic light harvesting, where the protein network transports energy with near unity quantum efficiency. While photosynthesis exemplifies the potential of excitonic systems, synthetic platforms have lacked the structural precision to leverage this potential, or even test its limits.
We use DNA origami with attached dyes to generate arbitrary nanoscale 3D architectures for programmable exciton dynamics. In these structures, we control the intermolecular interactions including the electronic coupling between dyes and the system-bath coupling between the dyes and their environment, i.e., bath engineering. Using single-molecule and ultrafast spectroscopy, we have demonstrated that we can select the mechanism and efficiency of exciton transport by tuning these interactions.
Photocatalysis. Photoredox catalysis, which converts light energy into reactivity, has emerged as a powerful new synthetic tool with applications including pharmaceuticals, feedstocks, and upcycling. Typically, photoredox catalysts are light limited due to their low molar absorptivities and competing absorptions. Photosynthesis solved this problem through the evolution of the LHCs - separate architectures for light harvesting. We are mimicking this design by conjugating LHCs to the photocatalysts, improving product yields and absorption bandwidths. We are also gaining a deeper mechanistic understanding of emerging transition metal photocatalysts with ultrafast spectroscopy, which may help overcome challenges associated with their performance and lead to faster adoption of these more abundant molecules.