TRANSFER OF EXCITATION-ENERGY IN A 3-DIMENSIONAL-DOPED MOLECULAR-CRYSTAL .5. SELF-CONSISTENCY OF THE TEMPORAL PROCESSES INVOLVED IN ENERGY-TRANSFER IN PHOTOSYNTHETIC UNITS

Numerical experiments were carried out to determine the timewise self- consistency of different physical processes involved in the energy tra nsfer in green plant photosynthetic units. A 6 x 6 x 6 array of chloro phyll-a with cubic lattice constants a = b = c = 20 Angstrom was chose n as a model of the thylakoid disc. Another model aggregate was obtain ed by substituting chlorophyll-b molecules for some of the chlorophyll -a molecules. In both models, a reaction center occupied a central sit e in the last xy plane. Two extreme arrangements were considered for t he orientation of molecules. In one, the transition moments of all mol ecules were directed along the y-axis. The other had chlorophyll molec ules randomly oriented. The four resulting model systems were used in our investigation on exciton generation, transport, decay by fluoresce nce, and trapping. All excitons were assumed to be generated by a 20 m s exposure to sunlight at high altitudes. The general trends noticed f rom these computations are as follows: The number of excitons generate d is influenced by lattice disorders. Disorders also increase the time for the establishment of an equilibrium distribution The decay of exc itons by fluorescence is always a monotonic function of time. The ener gy transfer is adversely affected by a lower degree of orientation in the crystal: The trapping time increases with disorder. The number of trappings decreases with the onset of fluorescence of the host molecul es and the trap. From these investigations, we also made three specifi c observations: (1) The efficiency of exciton utilization varies from 12% for a completely random arrangement of transition dipoles to 46% f or a perfectly ordered arrangement. This agrees with the experimental efficiency, about 20%. (2) The number of excitons trapped varies from one to six. This tallies with the time scale of electron transfer alon g the Z-scheme that requires at least two excitons trapped in about 20 ms. Thus, the photon density and the exciton transfer rate are consis tent with the rates of electron transfers. (3) The trapping rate also indicates that the thylakoid disc must resemble a considerably ordered system. (C) 1996 John Wiley & Sons, Inc.