Picosecond spectroscopic study of chlorophyll-based models for the primary photochemistry of photosynthesis. [Dimers and trimers of chlorophyllide derivatives]
- Stanford Univ., CA
A series of covalently linked dimers and trimers of chlorophyllide derivatives was investigated by time-resolved absorption and fluorescence spectroscopy (3 to 10/sup 4/ ps). For these compounds, the free energy difference between the singlet excited state of the electron donor and the anticipated cation-anion photoproduct (..delta..G/sub ET/) is estimated to range from +200 to -400 MeV. For the dimers studied, the singlet-excited-state lifetimes range from 1 to 7 ns and depend inversely on the solvent's static dielectric constant. Since no decrease in lifetime or fluorescence quantum yield was found as ..delta..G/sub ET/ became more negative, this effect is unlikely to be due to slow electron transfer. It may be a result of fluctuating intramolecular association of the nonpolar macrocycles in solvents with a high dielectric constant. We also studied two trimers, each having the same chlorophyllide a dimer as the electron donor, but with pyropheophorbide a or pheophorbide a as the electron acceptor (the latter is 90 MeV easier to reduce than the former). For the trimer with pheophorbide a as the acceptor, there is evidence for a new path of radiationless decay which may involve an electron-transfer product. However, the rate of formation of this product is slow (less than or equal to 10/sup 10/ s/sup -1/), and its yield is low (less than or equal to 50%). Taken together, these results suggest that chlorophyll-based, donor-acceptor pairs connected by flexible chains longer than five atoms are not likely to duplicate the highly efficient excited-singlet-state electron-transfer reactions characteristic of the primary photochemistry of photosynthetic organisms.
- OSTI ID:
- 6938081
- Journal Information:
- J. Phys. Chem.; (United States), Vol. 86:11
- Country of Publication:
- United States
- Language:
- English
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ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
09 BIOMASS FUELS
CHLOROPHYLL
EXCITED STATES
PHOTOSYNTHESIS
PHOTOCHEMISTRY
ABSORPTION SPECTROSCOPY
DIMERS
ELECTRON TRANSFER
FLUORESCENCE SPECTROSCOPY
FREE ENERGY
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
CHEMISTRY
EMISSION SPECTROSCOPY
ENERGY
ENERGY LEVELS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHYSICAL PROPERTIES
PHYTOCHROMES
PIGMENTS
PORPHYRINS
SPECTROSCOPY
SYNTHESIS
THERMODYNAMIC PROPERTIES
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