Physical principles of efficient excitation transfer in light harvesting

After light absorption the primary process in light harvesting is the transfer of excitation to a reaction center which facilitates a separation of charge across a cell membrane. The physical principles underlying excitation transfer are explained. Theoretical methods for the description of the excitation migration process, including an expansion for excitation lifetime in terms of repeated trapping and subsequent detrapping events, and the construction of representative pathways for excitation transfer based on mean first passage times, are presented. Measures for robustness and optimality of excitation transfer in terms of quantum yield are introduced. Photosystem I (PSI) is used as an example to illustrate the methods discussed. Some conclusions for the design of artificial light harvesting systems are also discussed.