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Abstract
In light harvesting, solar energy is captured by arrays of coupled chromophores and then funneled towards the site of energy conversion. A major roadblock in mapping the pathways of energy transfer had been that most previous experiments measured only a narrow portion of the broad solar spectrum captured by light-harvesting systems. I will present the development of ultrabroadband 2D electronic spectroscopy, an ultrafast spectroscopic tool that enables mapping of energy flow across the visible and the near-infrared range, and how this technique enabled the discovery of previously inaccessible photophysics in natural and synthetic light-harvesting systems. First, I will present the elucidation of pathways responsible for photoprotection of green plants against excess sunlight. In the major light-harvesting protein of green plants, LHCII, I directly resolved a sub-picosecond photoprotective chlorophyll-to-carotenoid energy transfer process, which had only been theoretically proposed. By introducing a systematically controllable near-native membrane platform that mimics the plant membrane, I also determined the previously inaccessible impact of physiological parameters such as protein crowding and protein-lipid interaction. I will then discuss another application where a new type of light-matter interaction, known as polaritons, is used to manipulate the dynamics of energy flow in semiconducting carbon nanotubes. Formed by strong coupling between photons and molecules, polaritons enable a plethora of new photophysics, such as long-range energy transfer, the presence and mechanism of which, however, had not been elucidated. A combination of ultrabroadband 2D measurements and quantum calculations revealed the spectroscopic signatures of the hypothesized long-range energy transfer as well as its underlying mechanism, which involves an intricate interplay between light-matter coupling and molecular parameters.