Absorption of Light, Excitation Energy Transfer and Electron Transfer Reactions
The present chapter gives an overview of photophysical principles underlying the primary photosynthetic reactions in pigment–protein complexes, i.e., the absorption of light, excitation energy transfer and electron transfer reactions. From a common viewpoint the description of the above reactions requires theories that can include both the pigment–pigment (electronic) as well as the pigment–protein (electron–vibrational) coupling in an appropriate manner. Different theories are classified according to the relative strength of the two types of couplings. In the weak electronic coupling limit, a semiclassical and a quantum Fermi's Golden Rule type rate constant and their relation are discussed and applied to absorption and fluorescence, excitation energy transfer, and electron transfer, yielding the standard results by Lax/Kubo, Förster, and Marcus, respectively. Whereas for electron transfer reactions the weak electronic coupling limit is appropriate, in the case of excitation energy transfer and optical spectra, usually, the pigment–pigment coupling is of the same strength or even stronger than the pigment–protein coupling, a situation that is treated by theories that take into account delocalized excited states of the pigments. Multilevel Redfield theory describes excitation energy relaxation between different delocalized states and relates the relaxation times to optical line widths. In non-Markovian density matrix theories, besides a life time broadening due to exciton relaxation, vibrational sidebands of the optical lines are included. Another extension of Redfield theory, the modified Redfield theory, takes into account the reorganization of nuclei during exciton relaxation between different delocalized states. The generalized Förster theory allows a description of transfer between aggregates with strong intra- and weak inter-aggregate couplings. How the parameters (couplings, energies and spectral densities) of the dynamical theories can be extracted from experimental data and independent calculations is outlined also.