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The Role of the Electronic Structure of the Porphyrin as Viewed by EPR/ENDOR Methods in the Efficiency of Biomimetic Model Compounds for Photosynthesis
Author(s) -
Huber Martina
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200112)2001:23<4379::aid-ejoc4379>3.0.co;2-6
Subject(s) - porphyrin , chemistry , electron paramagnetic resonance , molecular orbital , photochemistry , electronic structure , homo/lumo , resonance (particle physics) , acceptor , photosynthetic reaction centre , atomic orbital , covalent bond , electron acceptor , computational chemistry , electron transfer , molecule , electron , organic chemistry , atomic physics , nuclear magnetic resonance , condensed matter physics , physics , quantum mechanics
A project to investigate the parameters that determine the efficiency of biomimetic porphyrin quinone model compounds for photosynthesis is described. It should aid in understanding the origin of the high efficiency of energy conversion in the light‐induced charge separation in photosynthesis. Specifically, the contribution of the electronic matrix element to the electron transfer (ET) rates is addressed. Targeting the electronic structure of the ET components, EPR (electron paramagnetic resonance) and ENDOR (electron nuclear double resonance) spectroscopy were performed on the appropriate radical derivatives of the porphyrins and quinones to determine MO coefficients. Together with semiempirical MO methods, these coefficients were used to obtain information on the frontier orbitals of the donor and acceptor moieties, the porphyrins, and quinones of the model compounds. The porphyrin frontier orbitals (HOMO and LUMO) have been implicated in the balance of ET rates for charge separation and charge recombination, which is related to the efficiency. A model is presented to correlate the substitution pattern of the porphyrin with the efficiency of the model compound. In particular, the effect of porphyrin substitution, including the covalent link to an acceptor, and the comparison of porphyrins with chlorins and chlorophylls are addressed. The model allows for a quantification of these effects in some cases. Differences in the frontier orbital structure suggest that the [5,10,15,20 tetraaryl(alkyl)porphyrinato]metal complexes are sufficiently different from chlorophylls to be nonideal model compounds. These results are related to recent publications, and possible improvements of experimental and theory approaches are addressed.