PHOTOINDUCED ELECTRON-TRANSFER REACTIONS IN A CHOROPHYLLIDE-PHEOPHORBIDE CYCLOPHANE. A MODEL FOR PHOTOSYNTHETIC REACTION CENTERS.

Robert E. Overfield, Avigdor Scherz, Kenneth J. Kaufmann, Michael R Wasielewski

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Abstract

The fluorescence and the lowest excited triplet state yields for a chrlorophyllide-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 fluoresence remains approximately constant. Fluorescence decays of the cyclophane are best fit with two exponentials. The short component is 250 plus or minus 50 ps, while the long component is about 2-4 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 butyronitrile 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 process.

Original languageEnglish
Pages (from-to)5747-5752
Number of pages6
JournalJournal of the American Chemical Society
Volume105
Issue number18
Publication statusPublished - Sep 7 1983

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ASJC Scopus subject areas

  • Chemistry(all)

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