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. CCI4. At 290 K the absorbance changes of the dimer in CCI4 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 CH2CI2 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 CCI4 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.