| Summary |
Excitons are the molecular scale currency of electronic energy. Control over excitons and their dynamics enables energy to be directed and harnessed for light harvesting and molecular electronics, yet these dynamics strongly depend on the surrounding environment. Describing – and controlling – this dependence is challenging in condensed phase systems owing to the large number of degrees of freedom. We disentangled the effects of the environment for two systems. First, we performed ultrabroadband 2D electronic spectroscopy on the primary antenna protein from green plants, LHCII. We found that two chemically identical carotenoids bound within different protein pockets perform distinct roles. On one carotenoid, we identified a debated dark state that mediates exciton relaxation to collect absorbed energy from the higher lying electronic states, serving as a nexus of light harvesting [Son, et al., Chem, 2019]. On the other, we directly measured exciton transfer into its short-lived S1 state, a hypothesized but previously unobserved pathway to safely dissipate excess excitons, regulating light harvesting [Son, et al., Nat Communs, 2020]. Second, we developed a synthetic system consisting of chromophores held in a DNA scaffold in which we can select for desired photophysical processes via the properties of the scaffold alone. In a cy3-DNA assembly, we increased the efficiency of exciton transfer by 30% through a flexible scaffold that fluctuates out of inefficient configurations [Hart, et al., Chem, 2021]. In a squaraine DNA assembly, we precisely controlled the chromophore geometry to activate symmetry breaking charge transfer, which had not been previously demonstrated for molecules despite decades of study [Hart et al., Chem Sci, 2022]. Collectively, these two systems provide an experimental demonstration of how the environment can dictate exciton dynamics.
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