Analyzing spectral distributions of charge transfer character in ensembles: a case study on the reaction center of photosystem II

Understanding the primary charge separation events in Nature's photosynthetic reaction centers is a key step toward harnessing the microscopic processes of light conversion into chemical energy. Despite intense research efforts employing state-of-the-art spectroscopic and theoretical techniques, the precise nature of energy transfer and charge separation events in these systems are still insufficiently understood. Herein, we present a computational approach that enables analysis of the charge transfer character in excited electronic states with inclusion of thermal effects in ensembles. We showcase an application of this approach to the reaction center of photosystem II, focusing on the Chl_D1Pheo_D1 and P_D1P_D2 pairs of pigments. We find that the Chl_D1Pheo_D1 pair is a more likely candidate for the primary charge separation than the P_D1P_D2 pair. Our computational approach is transferable to other biological and man-made charge separation and charge transfer systems.