Using Synthetic Model Systems to Understand Charge Separation and Spin Dynamics in Photosynthetic Reaction Centers

Abstract

Over the past 20 years many biomimetic systems have been synthesized to model the photosynthetic reaction center (RC).(1) The purpose of these modeling studies is to use relatively simple electron donor-acceptor molecules with restricted donor-acceptor distances and orientations to probe the electronic interactions between photosynthetic pigments that lead to efficient, long-lived charge separation in photosynthetic reaction centers. These assemblies are designed to mimic several key properties of the RC protein: 1) multi-step ET to increase the lifetime of the radical pair (RP) product, 2) high quantum yield, fast formation and slow recombination of the singlet-initiated charge separation, 3) temperature independent ET rates, and 4) spin polarization of the RP states. Most RC models fulfill only a subset of these criteria. Moreover, there is one key property of the RC primary photochemistry that has proven very difficult to mimic. It is the unique ability of the RP intermediate within the photosynthetic RC to yield, upon charge recombination, a triplet state that retains a memory of the precursor RP spin state, and exhibits the time-resolved EPR (TREPR) signal characteristic of this triplet state.