Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process

<p>A series of meso-phenyloctamethylporphyrins covalently bonded at the 4'phenyl position to quinones via rigid bicyclo[2.2.2]octane spacers were synthesized for the study of the dependence of electron transfer reaction rate on solvent, distance, temperature, and energy gap. A general and...

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Main Author: Joran, Alvin David
Format: Others
Language:en
Published: 1986
Online Access:https://thesis.library.caltech.edu/9271/1/Joran_AD_1986.pdf
Joran, Alvin David (1986) Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/chrv-qx20. https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366 <https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366>
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spelling ndltd-CALTECH-oai-thesis.library.caltech.edu-92712021-04-20T05:01:41Z https://thesis.library.caltech.edu/9271/ Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process Joran, Alvin David <p>A series of meso-phenyloctamethylporphyrins covalently bonded at the 4'phenyl position to quinones via rigid bicyclo[2.2.2]octane spacers were synthesized for the study of the dependence of electron transfer reaction rate on solvent, distance, temperature, and energy gap. A general and convergent synthesis was developed based on the condensation of <i>ac</i>-biladienes with masked quinonespacer-benzaldehydes. From picosecond fluorescence spectroscopy emission lifetimes were measured in seven solvents of varying polarity. Rate constants were determined to vary from 5.0 x 10<sup>9</sup> sec<sup>-1</sup> in N,N-dimethylformamide to 1.15 x 10<sup>10</sup> sec<sup>-1</sup> in benzene, and were observed to rise at most by about a factor of three with decreasing solvent polarity. Experiments at low temperature in 2-MTHF glass (77K) revealed fast, nearly temperature-independent electron transfer characterized by non-exponential fluorescence decays, in contrast to monophasic behavior in fluid solution at 298K. This example evidently represents the first photosynthetic model system not based on proteins to display nearly temperature-independent electron transfer at high temperatures (nuclear tunneling). Low temperatures appear to freeze out the rotational motion of the chromophores, and the observed nonexponential fluorescence decays may be explained as a result of electron transfer from an ensemble of rotational conformations. The nonexponentiality demonstrates the sensitivity of the electron transfer rate to the precise magnitude of the electronic matrix element, which supports the expectation that electron transfer is nonadiabatic in this system. The addition of a second bicyclooctane moiety (15 Å vs. 18 Å edge-to-edge between porphyrin and quinone) reduces the transfer rate by at least a factor of 500-1500. Porphyrinquinones with variously substituted quinones allowed an examination of the dependence of the electron transfer rate constant κ<sub>ET</sub> on reaction driving force. The classical trend of increasing rate versus increasing exothermicity occurs from 0.7 eV ≤ |ΔG<sup>0'</sup>(R)| ≤ 1.0 eV until a maximum is reached (κ<sub>ET</sub> = 3 x 10<sup>8</sup> sec<sup>-1</sup> rising to 1.15 x 10<sup>10</sup> sec<sup>-1</sup> in acetonitrile). The rate remains insensitive to ΔG<sup>0</sup> for ~ 300 mV from 1.0 eV ≤ |ΔG<sup>0’</sup>(R)| ≤ 1.3 eV, and then slightly decreases in the most exothermic case studied (cyanoquinone, κ<sub>ET</sub> = 5 x 10<sup>9</sup> sec<sup>-1</sup>).</p> 1986 Thesis NonPeerReviewed application/pdf en other https://thesis.library.caltech.edu/9271/1/Joran_AD_1986.pdf Joran, Alvin David (1986) Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/chrv-qx20. https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366 <https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366> https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366 CaltechTHESIS:11052015-135421366 10.7907/chrv-qx20
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description <p>A series of meso-phenyloctamethylporphyrins covalently bonded at the 4'phenyl position to quinones via rigid bicyclo[2.2.2]octane spacers were synthesized for the study of the dependence of electron transfer reaction rate on solvent, distance, temperature, and energy gap. A general and convergent synthesis was developed based on the condensation of <i>ac</i>-biladienes with masked quinonespacer-benzaldehydes. From picosecond fluorescence spectroscopy emission lifetimes were measured in seven solvents of varying polarity. Rate constants were determined to vary from 5.0 x 10<sup>9</sup> sec<sup>-1</sup> in N,N-dimethylformamide to 1.15 x 10<sup>10</sup> sec<sup>-1</sup> in benzene, and were observed to rise at most by about a factor of three with decreasing solvent polarity. Experiments at low temperature in 2-MTHF glass (77K) revealed fast, nearly temperature-independent electron transfer characterized by non-exponential fluorescence decays, in contrast to monophasic behavior in fluid solution at 298K. This example evidently represents the first photosynthetic model system not based on proteins to display nearly temperature-independent electron transfer at high temperatures (nuclear tunneling). Low temperatures appear to freeze out the rotational motion of the chromophores, and the observed nonexponential fluorescence decays may be explained as a result of electron transfer from an ensemble of rotational conformations. The nonexponentiality demonstrates the sensitivity of the electron transfer rate to the precise magnitude of the electronic matrix element, which supports the expectation that electron transfer is nonadiabatic in this system. The addition of a second bicyclooctane moiety (15 Å vs. 18 Å edge-to-edge between porphyrin and quinone) reduces the transfer rate by at least a factor of 500-1500. Porphyrinquinones with variously substituted quinones allowed an examination of the dependence of the electron transfer rate constant κ<sub>ET</sub> on reaction driving force. The classical trend of increasing rate versus increasing exothermicity occurs from 0.7 eV ≤ |ΔG<sup>0'</sup>(R)| ≤ 1.0 eV until a maximum is reached (κ<sub>ET</sub> = 3 x 10<sup>8</sup> sec<sup>-1</sup> rising to 1.15 x 10<sup>10</sup> sec<sup>-1</sup> in acetonitrile). The rate remains insensitive to ΔG<sup>0</sup> for ~ 300 mV from 1.0 eV ≤ |ΔG<sup>0’</sup>(R)| ≤ 1.3 eV, and then slightly decreases in the most exothermic case studied (cyanoquinone, κ<sub>ET</sub> = 5 x 10<sup>9</sup> sec<sup>-1</sup>).</p>
author Joran, Alvin David
spellingShingle Joran, Alvin David
Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process
author_facet Joran, Alvin David
author_sort Joran, Alvin David
title Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process
title_short Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process
title_full Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process
title_fullStr Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process
title_full_unstemmed Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process
title_sort photochemical electron transfer at fixed distance: a synthetic model of the photosynthetic primary process
publishDate 1986
url https://thesis.library.caltech.edu/9271/1/Joran_AD_1986.pdf
Joran, Alvin David (1986) Photochemical Electron Transfer at Fixed Distance: A Synthetic Model of the Photosynthetic Primary Process. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/chrv-qx20. https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366 <https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366>
work_keys_str_mv AT joranalvindavid photochemicalelectrontransferatfixeddistanceasyntheticmodelofthephotosyntheticprimaryprocess
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