FLUORESCENCE DECAY KINETICS and CHARACTERISTICS OF BIMOLECULAR EXCITON ANNIHILATION IN CHLOROPLASTS

Abstract— The decay profiles of the fluorescence of dark‐adapted spinach chloroplasts (0^A°C) excited with single 30 ps 532 nm laser pulses of varying intensities were measured with a low‐jitter streak camera system. By comparing the decay profiles of the fluorescence at low and high laser fluences, i.e. in the absence and presence, respectively, of dynamic bimolecular exciton‐exciton annihilation effects, the duration of such dynamic annihilation events can be estimated. A simple model suggests that the influence of bimolecular annihilation events on the fluorescence decay kinetics should disappear within a time interval corresponding to the low intensity, unimolecular lifetime of the exciton population which is subject to exciton‐exciton annihilation. The low intensity fluorescence decay profiles are characterized by three to four lifetimes (Reviewed by A. R. Holzwarth, Photochem. Photobiol. 43,707–725, 1986); it is shown here that only the shortest fluorescence components are subject to exciton annihilation, since the kinetics of the fluorescence decay are influenced by annihilations only within the initial 150–200 ps time interval after the excitation pulse. The amplitudes (but not the decay kinetics) of the longer‐lived fluorescence components are decreased at high levels of laser pulse excitations, suggesting that these components are derived from the shorter‐lived fluorescence decay components. The implications of these results are*discussed within the contexts of current models of the fluorescence in chloroplasts.