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

The excited state absorption and fluorescence characteristics 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 photophysical 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 8-ps flash at 528 nm varied dramatically with a change in solvent, CH₂Cl₂ vs. CCl₄. At 190 K the absorbance changes of the dimer in CCl₄ 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 CH₂Cl₂. Second, similar dramatic lifetime variations were observed for folded dimer in CH₂Cl₂ 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 290 200 K temperature range. As the sample of folded pyrochlorophyll a dimer in CH₂Cl₂ 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 CCl₄ 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.