Electronic coherence lifetimes of the Fenna-Matthews-Olson complex and light harvesting complex II.

Shawn Irgen-Gioro,Elad Harel,Karthik Gururangan,Robert E Blankenship,Rafael G Saer

doi:10.1039/c9sc03501j

Shawn Irgen-Gioro, Elad Harel + Show 3 more

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https://doi.org/10.1039/c9sc03501j

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Abstract

The study of coherence between excitonic states in naturally occurring photosynthetic systems offers tantalizing prospects of uncovering mechanisms of efficient energy transport. However, experimental evidence of functionally relevant coherences in wild-type proteins has been tentative, leading to uncertainty in their importance at physiological conditions. Here, we extract the electronic coherence lifetime and frequency using a signal subtraction procedure in two model pigment-protein-complexes (PPCs), light harvesting complex II (LH2) and the Fenna-Matthews-Olson complex (FMO), and find that the coherence lifetimes occur at the same timescale (<100 fs) as energy transport between states at the energy level difference equal to the coherence energy. The pigment monomer bacteriochlorophyll a (BChla) shows no electronic coherences, supporting our methodology of removing long-lived vibrational coherences that have obfuscated previous assignments. This correlation of timescales and energy between coherences and energy transport reestablishes the time and energy scales that quantum processes may play a role in energy transport.

Highlights

The timescale in which a system maintains phase correlation, or coherence lifetime, dictates the observation of quantum interference at a macroscopic level.[1]
We extract the electronic coherence lifetime and frequency using a signal subtraction procedure in two model pigment-protein-complexes (PPCs), light harvesting complex II (LH2) and the Fenna–Matthews–Olson complex (FMO), and find that the coherence lifetimes occur at the same timescale (
Results nDES spectra are taken for bacteriochlorophyll a (BChla), light harvesting complex 2 (LH2), and FMO, and in the case of LH2 and FMO a bimodal distribution of coherence lifetimes is observed

Summary

Introduction

The timescale in which a system maintains phase correlation, or coherence lifetime, dictates the observation of quantum interference at a macroscopic level.[1]. In the late 2000's, the assumption that coherences between two excited electronic states should be on a comparable timescale was challenged when a series of experiments observed coherences for hundreds of femtoseconds, with possible functional implications for energy transport in photosynthetic systems.[2,3,4,5,6,7,8] These initial experiments disputed the notions that the excited states' independent interaction with the thermal bath would rapidly destroy coherences between excitonic states (electronic coherences) and the conventional description of energy transfer using standard and modi ed Forster theory.[9,10] more recent work has suggested that the spectroscopic signatures that began this debate, The nDES signal is generated with a series of three compressed broadband laser pulses, measured as a function of delay between the pulses, generating a 3D dataset. On a longer timescale, delocalized vibrations coupled electronic states form “vibronic” coherences, which eventually turn into purely vibrational coherences a er a critical time sD y 2p/s, where s is the

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