Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy

Significance The remarkable near-unity quantum efficiency of photosynthetic charge separation has motivated decades of research to uncover the underlying design principles. Much of our current understanding of photosynthetic charge separation is rooted in studies of the bacterial reaction center (BRC). We present two-dimensional electronic spectroscopy of the BRC as it undergoes charge separation, resolving the energy-transfer and charge-separation processes with time and excitation frequency resolution. These measurements reveal the excitonic structure of the BRC, including a previously hidden exciton state. We present a multiexcitation 2D global analysis method that supports two-step sequential charge separation in the BRC without evidence for secondary charge separation pathways. We extract the spectral signatures of the charge-separation intermediates.