Photoinduced electron-transfer reactions in a chlorophyllide-pheophorbide cyclophane. A model for photosynthetic reaction centers

Journal Article · Wed Sep 07 00:00:00 EDT 1983 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00356a007· OSTI ID:5580511

Overfield, R E ^[1]; Scherz, A; Kaufmann, K J; Wasielewski, M R

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1. Univ. of Illinois, Urbana

The fluorescence and the lowest excited triplet state yields for a chlorophyllide-pheophorbide cyclophane are strongly quenched as the dielectric constant of the solvent in which it is dissolved increases. The decrease in fluorescence yield is accompanied by a decrease in fluorescence lifetime. The radiative rate for fluorescence remains approximately constant. Fluorescence decays of thecyclophane are best fit with two exponentials. The short component is 250 +- 50 ps, while the long component is about 2 to 4 ns. Time resolved absorbance changes at 655 and 445 nm are used to follow the decay of the excited states back to the ground state. A residual absorbance having a lifetime in excess of 30 ns is found in all solvents and is ascribed to the lowest excited triplet state of the cyclophane. After correction for this contribution the decay of the absorbance changes is best fit with a single exponential. In dichloromethane and butyronitile the absorbance decay times, 1.6 ns and 435 ps, respectively, are longer than the 250-ps fluorescence emission component of the cyclophane in these solvents. This indicates that a nonemissive excited state is formed. The observed dependence of both the fluorescence and the absorption change decays on the solvent dielectric constant suggests that quenching of the lowest excited singlet state involves an electron-transfer reaction. The electron-transfer rate is 2 x 10/sup 9/ S/sup -1/, while the rate of the back electron transfer yielding the cyclophane in its ground electronic state is 10/sup 9/ S/sup -1/. The forward electron-transfer rate is about 100 times slower than that of photosynthetic reaction centers despite the close proximity of the donor and the acceptor (<6 A). The difference in electron-transfer rates between the cyclophane and photosynthetic reaction centers is rationalized in terms of the importance of donor-acceptor orientation.

• OSTI ID: 5580511
• Journal Information: J. Am. Chem. Soc.; (United States), Vol. 105:18
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
14 SOLAR ENERGY
MAGNESIUM COMPLEXES
EXCITATION
FLUORESCENCE
CHEMICAL REACTION KINETICS
CHLORINS
CHLOROPHYLL
COMPARATIVE EVALUATIONS
DATA ANALYSIS
DIELECTRIC PROPERTIES
ELECTRON TRANSFER
EXPERIMENTAL DATA
LASER RADIATION
LOW TEMPERATURE
MATHEMATICAL MODELS
ORGANIC SOLVENTS
PHOTOIONIZATION
PHOTOSYNTHESIS
PROGRAMMING
PULSE TECHNIQUES
RADIATIONLESS DECAY
REACTION INTERMEDIATES
STREAK PHOTOGRAPHY
STRUCTURAL CHEMICAL ANALYSIS
TEMPERATURE DEPENDENCE
TIME DEPENDENCE
ALKALINE EARTH METAL COMPLEXES
AROMATICS
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
COMPLEXES
DATA
DE-EXCITATION
ELECTRICAL PROPERTIES
ELECTROMAGNETIC RADIATION
ENERGY TRANSFER
ENERGY-LEVEL TRANSITIONS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROXY COMPOUNDS
INFORMATION
IONIZATION
KINETICS
LUMINESCENCE
NITROSO COMPOUNDS
NUMERICAL DATA
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHENOLS
PHOTOCHEMICAL REACTIONS
PHOTOGRAPHY
PHYSICAL PROPERTIES
PHYTOCHROMES
PIGMENTS
POLYPHENOLS
PORPHYRINS
RADIATIONS
REACTION KINETICS
SOLVENTS
SYNTHESIS
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