Light Collection and Exciton Dynamics in Photosynthetic Membranes

Abstract

The first steps in photosynthesis involve light absorption by specialized antenna pigments followed by efficient energy migration to a reaction center complex whereupon the excitation is utilized to drive the photosynthetic process. Figure 1 illustrates how the harvesting is thought to occur. The dots represent chlorophyll a (Chl a) molecules and RC represents a reaction center. Mot shown are accessory pigment molecules which assist in absorbing light. There are a variety of accessory pigment species [1] with complimentary overlapping absorptions that together span the visible spectrum. Once excited, the accessory pigment molecules very quickly (~ 1 ps) transfer their energy via a nonradiative dipole-dipole interaction to a Chl a molecule. The Chl a’s first excited singlet state has a lower energy than the singlet states of the other antenna piqments and so the excitation cannot easily jump back. Possible, however, are further similar Förster transitions between neighboring Chl a molecules and consequently the excited state, or Frenkel exciton, begins a random walk through the Chl a antenna matrix until it wanders near the reaction center complex whereupon it is quickly captured. The entire process, from initial pigment absorption to reaction center capture, although involving many hundreds of separate energy transfer steps, occurs in less than one nanosecond and has quantum yields for photon utilization approaching 100%.

Work performed under the auspices of the U. S. DOE