A THEORY OF DISPERSIVE KINETICS IN THE ENERGY-TRANSFER OF ANTENNA COMPLEXES

The inherent glasslike structural heterogeneity of photosynthetic ante nna protein complexes is now known to result in a distribution of valu es for any given donor-acceptor energy gap. The width of the distribut ion is sufficiently broad to raise the possibility that the kinetics o f D-A energy transfer might be dispersive and strongly dependent on te mperature especially since the chlorophyll optical transitions are cha racterized by weak electron-phonon coupling. An approximate theory is presented that allows far computationally simple analysis of this prob lem. It is based on the familiar nonadiabatric energy-transfer theory and Condon approximation and, thus, is applicable to conventional Fors ter transfer involving localized phonons. The case of delocalized phon ons is also treated. Calculations are presented for a model D-A system which, based on hole-burning data, can be considered to be realistic and typical of many complexes. The results reveal pronounced and tempe rature-dependent dispersion in the kinetics. The effect of pure electr onic dephasing on the calculated results is considered. The theory is not applicable to the situation where excitonic level splittings are l arge relative to the pure dephasing frequencies of the levels. i.e., w here one has to go beyond the Condon approximation. Nevertheless. the theory can be easily extended to cover this strong coupling case which can be expected to be important for many photosynthetic complexes at sufficiently low temperatures. The possibility that strong coupling mi ght lead to a diminuation in the degree of dispersive kinetics is cons idered.