Plants absorb sunlight across the visible region of the solar spectrum and collect the energy required for photochemical reactions in lower-lying states. Previous 2D experiments to study plants were limited to these low-energy states. We describe the development of ultrabroadband 2D electronic spectroscopy and its application to the major antenna complex of plants, LHCII. We find that two chemically identical carotenoids bound within different protein pockets perform distinct roles. We identify a debated optically dark electronic state, Sx, on one carotenoid, lutein 2, that mediates relaxation from the higher-energy to the lower-energy excited states. We also measure chlorophyll-to-carotenoid energy transfer likely into the short-lived S<sub>1</sub> state of another carotenoid, lutein 1, which is a hypothesized but previously unobserved pathway to safely dissipate excess energy.

Exploring carotenoid-mediated photophysics in plants with ultrabroadband 2D electronic spectroscopy

ACS Fall 2021

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