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

Citation

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. https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366

Abstract

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 ac-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 109 sec-1 in N,N-dimethylformamide to 1.15 x 1010 sec-1 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 κET on reaction driving force. The classical trend of increasing rate versus increasing exothermicity occurs from 0.7 eV ≤ |ΔG0'(R)| ≤ 1.0 eV until a maximum is reached (κET = 3 x 108 sec-1 rising to 1.15 x 1010 sec-1 in acetonitrile). The rate remains insensitive to ΔG0 for ~ 300 mV from 1.0 eV ≤ |ΔG0’(R)| ≤ 1.3 eV, and then slightly decreases in the most exothermic case studied (cyanoquinone, κET = 5 x 109 sec-1).

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Dervan, Peter B. (advisor)
  • Hopfield, John J. (co-advisor)
Thesis Committee:
  • Dervan, Peter B. (chair)
  • Hopfield, John J.
Defense Date:24 March 1986
Funders:
Funding AgencyGrant Number
NSFUNSPECIFIED
Record Number:CaltechTHESIS:11052015-135421366
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:11052015-135421366
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9271
Collection:CaltechTHESIS
Deposited By: Bianca Rios
Deposited On:05 Nov 2015 23:43
Last Modified:02 Dec 2020 01:39

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