Long-Lived Electronic Coherence in Dissipative Exciton Dynamics of Light-Harvesting Complexes

Recent experiments on light-harvesting complexes at ambient temperatures display oscillatory signals in two-dimensional spectroscopy. They suggest mechanisms supporting coherent exciton transport through molecular networks in noisy environments. We demonstrate a mechanism that relies on the continuum properties of the vibronic spectral density. We employ the spectral density of the Fenna-Matthews-Olson (FMO) complex and perform a nonperturbative calculation of two-dimensional spectra. They display long-lasting electronic coherence up to 0.3 Ps at a temperature of 277 K Two important properties of the spectral density found in the FMO complex emerge; (i) the coupling to higher-frequency vibrations is large, as required for efficient transport toward the reaction center, and (ii) the slope of the spectral density at zero frequency approaches zero. We demonstrate that electronic coherence and fast thermalization depend sensitively on the continuum part of the spectral density but can be simultaneously realized in molecular networks.