Coherences in photosynthetic energy and charge transfer have generated immense interest due to the possibility of a functional role for such fragile quantum superpositions. Fluorescence-detected Two-dimensional Electronic Spectroscopy (F-2DES) measurements on LH2 antenna protein isolated from Rps. Palustris, show room-temperature quantum coherences which dephase rapidly within ~100 fs. Isolation of this signal using a two-color pulse sequences unequivocally shows that the signatures arise on the excited state of the protein, and likely involve both, B850 as well as the B800 excitonic manifolds of the protein. Such signatures are also found to be prominent in vivo. Moreover, such signatures survive in the LH2 proteins isolated from bacteria grown under low-light conditions where the ‘light-stress’ is well known to strongly perturb the excitonic manifold. Preliminary control experiments on a pucD mutant bacteria, which lacks the B850 excitonic manifold, do not show such coherences. The above control experiments, and theoretical analysis with parameters based on the previous single molecule experiments on LH2, suggest that the observed quantum coherences are not purely vibrational superpositions. Figure 1. (Left) T = 0 fs F-2DES spectrum of LH2 isolated from Rps. Palustris grown under high-light. (Center) Absorption spectrum of LH2 showing the B800 and B850 bands. Overlaying curves in red (R) and blue (B) show the two-color pulse spectra. (Right) Rapidly dephased signal (grey dots) isolated by RB sequences, with excited state specific sequence highlighted in color.

Robust, rapidly dephased, excited state coherences in a photosynthetic bacteria isolated through fluorescence-detected two-dimensional electronic spectroscopy

2019 Fall National Meeting

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