Beatings in electronic 2D spectroscopy suggest another role of vibrations in photosynthetic light harvesting.

Light harvesting by photosynthetic organisms is nature’s way to use solar energy for biomass growth. The process starts with light absorption in so-called antenna pigments, and is followed by transfer of the excited-state energy to reaction center proteins, where the energy is converted to an electrochemical gradient across the photosynthetic membrane (1). This potential is used to drive all energy-consuming processes in the photosynthetic organisms. Energy transfer in light harvesting occurs via various transport regimes. The limiting cases are the Förster-type incoherent excitation hopping from pigment to pigment and the exciton relaxation between energy levels, which are coherently delocalized over several antenna molecules. In both transfer regimes, vibrations play an important role in fulfilling the resonance condition of the rate equations. However, this is not the only way vibrations are used in light harvesting. The article in PNAS by Tiwari et al. (2) discusses the role of anticorrelated nuclear motions in driving energy transfer via nonadiabatic coupling (Fig. 1). The authors argue that the beatings observed in electronic 2D spectroscopy experiments of various antenna complexes are mainly of vibrational origin and provide evidence for this transport mechanism.