Coherent Energy Transfer under Incoherent Light Conditions

Abstract

Recent two-dimensional electronic spectroscopy (2DES) experiments have reported evidence of coherent dynamics of electronic excitations in several light-harvesting antennae. However, 2DES uses ultrafast coherent laser pulses as an excitation source; therefore, there is a current debate on whether coherent excitation dynamics is present under natural sunlight - incoherent - illumination conditions. In this letter, we show that even if incoherent light excites an electronic state with no initial quantum superpositions among excitonic states, energy transfer can proceed quantum coherently if nonequilibrium dynamics of the phonon environment takes place. Such nonequilibrium behavior manifests itself in non-Markovian evolution of electronic excitations and is typical of many photosynthetic systems. We therefore argue that light-harvesting antennae have mechanisms that could support coherent evolution under incoherent illumination.

Photosynthetic light harvesting: excitons and coherence

“…Are coherent mechanisms at play or can energy transfer be fully modelled in terms of a classical rate equation where light incoherently populates electronic states that then incoherently transfer excitation to other states? Because of the incoherent nature of sunlight, we expect an incoherent population of electronic eigenstates [73,108]. Based on our discussion in §3, coherent exciton dynamics could be involved in energy transfer in this case under a regime of intermediate to strong electron -phonon coupling (i.e.…”

“…Otherwise, no oscillatory behaviour is present during energy transfer and dynamics are fully described by incoherent hopping of excitons. In a recent study, we have discussed how the time-dependent equilibration process of the low-frequency continuum of environmental modes can induce coherent energy transfer when the initial electronic state corresponds to an eigenstate of the electronic system [73]. Thus, coherent energy transfer can arise not only as a consequence of preparing the molecular aggregate in a coherent superposition of electronic eigenstates, but environment-induced coherent excitation dynamics are also possible.…”

How Markovian is exciton dynamics in purple bacteria?

“…An alternative approach is to consider typical initial states of the system. For the case of light harvesting, the true nature of the initial state is still debated; 26 however, for simplicity it is often assumed that the exciton is initially localised at a single chromophore. Thus, as well as looking at N BLP , we also investigate the typical amount of non-Markovianity, based on the fully localised initial states for a dimer system.…”

“…Previous research into this question has resulted in differing conclusions. [23][24][25][26] In particular, within a dimer extracted from the Fenna-Matthews-Olson (FMO) complex, small amounts of non-Markovianity were observed for an overdamped Brownian spectral density; 23 however, in calculations using the quasiadiabatic path-integral approach (QUAPI) on the full FMO complex with realistic spectral densities, no non-Markovianity was found. 24 Here we look at the degree of non-Markovianity in a dimer parameterised to reflect the B850 ring of light harvesting complex two.…”

Coherent multidimensional optical spectra measured using incoherent light

“…Several groups studied the interpretation of femtosecond 2D ES measurements of photosynthetic proteins and what those measurement reveal about the natural conditions for photosynthesis [53][54][55][56] by focusing on the distinction between spectrally coherent and spectrally incoherent excitation conditions. We speculate that the significant distinction between excitation conditions is not the coherence of light but rather the four-wave mixing signal itself.…”