Ultrafast photoinduced electron transfer in rigid donor-spacer-acceptor molecules

modification of spacer energetics as a probe for superexchange

Michael R Wasielewski, Mark P. Niemczyk, Douglas G. Johnson, Walter A. Svec, David W. Minsek

Research output: Contribution to journalArticle

127 Citations (Scopus)

Abstract

Four fixed-distance porphyrin-quinone molecules, 1-syn, 1-anti, 2-syn, and 2-anti, were synthesized. These molecules possess a zinc 5-phenyl-10,15,20-tripentylporphyrin electron donor attached to a naphthoquinone via a rigid pentiptycene spacer. The central benzene ring of the spacer is unsubstituted in 1 and possesses p-dimethoxy substituents in 2. The naphthoquinone is oriented either syn or anti to the porphyrin across the spacer. These molecules provide information concerning the orientation dependence of electron transfer between the porphyrin and the quinone, and the dependence of this transfer on low-lying ionic states of the spacer. The rate constants for the oxidation of the porphyrin lowest excited singlet state by the naphthoquinone are 1-syn: 8.2 x 109 s-1; 1-anti: 1.7 x 1010 s-1; 2-syn: 8.5 x 109 s-1; 2-anti: 1.9 x 1010 S-1. The corresponding rate constants for the porphyrin cation - naphthoquinone anion recombination reaction are 1-syn: 1.4 x 1010 s-1; 1-anti: 2.5 x 1010 s-1; 2-syn: 5.0 x 1010 s-1; 2-anti: 8.2 x 1010 s-1. The rate constants for the syn isomers are uniformly a factor of about 2 slower than those of the anti isomers. The charge separation reaction rates for 1 and 2 are similar, while the ion pair recombination reactions are about 3-4 x faster in 2 than in 1. The conformational effect is attributed to better overlap of the spacer wave functions in the anti vs the syn conformation, while the increase in recombination rate for 2 over 1 is attributed to a superexchange interaction involving an electronic configuration of the spacer in which the dimethoxybenzene cation contributes.

Original languageEnglish
Pages (from-to)4785-4806
Number of pages22
JournalTetrahedron
Volume45
Issue number15
DOIs
Publication statusPublished - 1989

Fingerprint

Porphyrins
Naphthoquinones
Electrons
Molecules
Genetic Recombination
Rate constants
Isomers
Cations
Wave functions
Benzene
Excited states
Reaction rates
Anions
Conformations
Zinc
Ions
Oxidation
benzoquinone

ASJC Scopus subject areas

  • Biochemistry
  • Organic Chemistry
  • Drug Discovery

Cite this

Ultrafast photoinduced electron transfer in rigid donor-spacer-acceptor molecules : modification of spacer energetics as a probe for superexchange. / Wasielewski, Michael R; Niemczyk, Mark P.; Johnson, Douglas G.; Svec, Walter A.; Minsek, David W.

In: Tetrahedron, Vol. 45, No. 15, 1989, p. 4785-4806.

Research output: Contribution to journalArticle

Wasielewski, Michael R ; Niemczyk, Mark P. ; Johnson, Douglas G. ; Svec, Walter A. ; Minsek, David W. / Ultrafast photoinduced electron transfer in rigid donor-spacer-acceptor molecules : modification of spacer energetics as a probe for superexchange. In: Tetrahedron. 1989 ; Vol. 45, No. 15. pp. 4785-4806.
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abstract = "Four fixed-distance porphyrin-quinone molecules, 1-syn, 1-anti, 2-syn, and 2-anti, were synthesized. These molecules possess a zinc 5-phenyl-10,15,20-tripentylporphyrin electron donor attached to a naphthoquinone via a rigid pentiptycene spacer. The central benzene ring of the spacer is unsubstituted in 1 and possesses p-dimethoxy substituents in 2. The naphthoquinone is oriented either syn or anti to the porphyrin across the spacer. These molecules provide information concerning the orientation dependence of electron transfer between the porphyrin and the quinone, and the dependence of this transfer on low-lying ionic states of the spacer. The rate constants for the oxidation of the porphyrin lowest excited singlet state by the naphthoquinone are 1-syn: 8.2 x 109 s-1; 1-anti: 1.7 x 1010 s-1; 2-syn: 8.5 x 109 s-1; 2-anti: 1.9 x 1010 S-1. The corresponding rate constants for the porphyrin cation - naphthoquinone anion recombination reaction are 1-syn: 1.4 x 1010 s-1; 1-anti: 2.5 x 1010 s-1; 2-syn: 5.0 x 1010 s-1; 2-anti: 8.2 x 1010 s-1. The rate constants for the syn isomers are uniformly a factor of about 2 slower than those of the anti isomers. The charge separation reaction rates for 1 and 2 are similar, while the ion pair recombination reactions are about 3-4 x faster in 2 than in 1. The conformational effect is attributed to better overlap of the spacer wave functions in the anti vs the syn conformation, while the increase in recombination rate for 2 over 1 is attributed to a superexchange interaction involving an electronic configuration of the spacer in which the dimethoxybenzene cation contributes.",
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