Theoretical aspects of Dexter-type excitation energy transfer for understanding optical phenomena on photosynthetic systems

Dexter-type excitation energy transfer (EET) has a deep relationship in various physical phenomena on photosynthetic systems. For example, green plants have a system to efficiently dissipate excess excitation energy absorbed by chlorophylls. This is due to the Dexter-type excitation energy transfer between chlorophylls and carotenoids in their vicinity. In addition, in the light-harvesting antennas LH1 and LH2 of purple bacteria, aggregates of regularly arranged pigments have an important role to absorb light energy for charge separation reaction. The excited state of the pigment aggregate is delocalized throughout the aggregate to form excitons. Accurate prediction of exciton energy requires the Dexter-type excitation energy transfer caused by the overlap of wavefunctions between adjacent pigments. Since the origin of Dexter-type excitation energy transfer is exchange coupling, theoretical estimation of the magnitude of exchange coupling provides important clues for understanding the optical phenomena occurring in photosynthetic systems. This review first outlines theoretical methods for evaluating the exchange coupling by the Dexter mechanism. The exchange coupling is a matrix element of the Hamiltonian about charge transfer (CT) excited states. Various methods have been developed to obtain charge transfer excited states by transforming adiabatic energy states obtained by ordinary quantum chemical calculations. Next, from the standpoint of theoretical analysis, the quenching process of excess excitation energy in photosynthetic systems and the optical properties of excitons in light-harvesting antennas are introduced. These results demonstrate the importance of theoretical analysis of the Dexter mechanism in photosynthetic systems.