Elucidation of the timescales and origins of quantum electronic coherence in LHCII

Photosynthetic organisms harvest sunlight with near unity quantum efficiency. The complexity of the electronic structure and energy transfer pathways within networks of photosynthetic pigment–protein complexes often obscures the mechanisms behind the efficient light-absorption-to-charge conversion process. Recent experiments, particularly using two-dimensional spectroscopy, have detected long-lived quantum coherence, which theory suggests may contribute to the effectiveness of photosynthetic energy transfer. Here, we present a new, direct method to access coherence signals: a coherence-specific polarization sequence, which isolates the excitonic coherence features from the population signals that usually dominate two-dimensional spectra. With this polarization sequence, we elucidate coherent dynamics and determine the overall measurable lifetime of excitonic coherence in the major light-harvesting complex of photosystem II. Coherence decays on two distinct timescales of 47 fs and ~800 fs. We present theoretical calculations to show that these two timescales are from weakly and moderately strongly coupled pigments, respectively. Quantum coherence has been observed in the major light-harvesting complex of photosystem II (LHCII) from green plants. By controlling the laser pulse polarization in two-dimensional electronic spectroscopy, signals from quantum coherence have been separated from other molecular processes, offering insight into the role of quantum coherence in photosynthetic light-harvesting.