Picosecond photophysics of covalently linked pyrochlorophyllide a dimer. Unique kinetics within the singlet manifold

Abstract

The excited state absorption and fluorescence characteristcs of the folded configuration of bis(pyrochlorophyllide a) ethylene glycol diester were studied on a picosecond time scale. This model for the primary photochemical electron donor in photosystem I of green plants displayed photophyscial properties that were found to depend strongly on both solvent and temperature. First, at 290 K the fluorescence lifetime and the lifetime of the excited state absorbance changes following an 80-ps flash at 528 nm varied dramatically with a change in solvent, CH/sub 2/Cl/sub 2/ vs. CCl/sub 4/. At 290 K the absorbance changes of the dimer in CCl/sub 4/ showed a large positive optical density change at 660 nm and bleaching at 700 nm. The 660-nm positive optical density change was absent for the folded dimer dissolved in CH/sub 2/Cl/sub 2/. Second, similar dramatic lifetime variations were observed for folded dimer in CH/sub 2/Cl/sub 2/ as a function of temperature. Both fluorescence lifetimes and absorption change decay rates were very similar with a lifetime at 290 K of 110 ps increasing to 4.6 ns at 200 K. However, the quantum yield for fluorescence of the dimer remained relatively constant in the 290 to 200 K temperature range. As themore » sample of folded pyrochlorophyll a dimer in Ch/sub 2/Cl/sub 2/ was cooled from 290 to 270 K the bleaching centered at 700 nm broadened somewhat and a new positive optical density change appeared at 660 nm. This spectrum was completely analogous to that obtained for the folded dimer in CCl/sub 4/ at 290 K. These results were interpreted in terms of a dual excited singlet state model with one singlet state fluorescent while the other remains nonfluorescent. Initial excitation of the dimer into the nonfluorescent state followed by a kinetically controlled population distribution between both excited states was shown to account for the data. 8 figures, 1 table.« less