Biomimetic modeling of photosynthetic reaction center function: Long-lived, spin-polarized radical ion pair formation in chlorophyll-porphyrin-quinone triads

Journal Article · Wed Nov 03 04:00:00 EST 1993 · Journal of the American Chemical Society; (United States)

DOI:https://doi.org/10.1021/ja00075a103· OSTI ID:5164287

Wasielewski, M R; Gaines, III, G L; Wiederrecht, G P; Svec, W A; Niemczyk, M P ^[1]

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1. Argonne National Lab., IL (United States)

Anisotropic spin-spin interactions within photogenerated radical ion pairs that possess restricted distances and orientations between the radical ions can be used to probe the structures of these pairs with EPR spectroscopy. A radical ion pair with an optimized free energy of recombination and a lifetime on the order of milliseconds should possess an electronic coupling matrix element V < 0.001 cm[sup [minus]1] ([approximately] 10 G). Under these conditions, the electron-electron exchange interaction between the radicals, 2J, which is on the same order of magnitude as V, is sufficiently weak that differences in local magnetic fields surrounding each radical result in singlet-triplet mixing of the radical pair spin sublevels. This mixing produces a non-Boltzmann population of the spin sublevels of the radical pair and often results in spin-polarized EPR spectra. We report here on 1-3, the first molecules based on a chlorophyll electron donor possessing sufficiently weak spin-spin interactions in the radical pair product state to exhibit both long lifetimes and spin-polarized EPR spectra analogous to those observed in photosynthetic reaction centers. The radical ion pairs produced within 1-3 directly probe how V changes with molecular structure when the orientation of the quinone relative to the chlorophyll is changed. Our results strongly suggest that we can mimic the weak radical pair interactions found in natural photosynthesis by using 1-3. 27 refs., 1 fig.

• DOE Contract Number: W-31109-ENG-38
• OSTI ID: 5164287
• Journal Information: Journal of the American Chemical Society; (United States), Journal Name: Journal of the American Chemical Society; (United States) Vol. 115:22; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102 -- Chemical &amp; Spectral Procedures
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AROMATICS
CARBOXYLIC ACIDS
CHARGED PARTICLES
CHEMICAL REACTIONS
CHLOROPHYLL
ELECTRON SPIN RESONANCE
EVALUATION
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
IONS
MAGNETIC RESONANCE
MOLECULAR STRUCTURE
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOSYNTHESIS
PHYTOCHROMES
PIGMENTS
PORPHYRINS
PROTEINS
QUINONES
RADICALS
RESONANCE
SYNTHESIS