Photoinduced charge separation involving an unusual double electron transfer mechanism in a donor-bridge-acceptor molecule

Scott E. Miller, Aaron S. Lukas, Emily Marsh, Patrick Bushard, Michael R. Wasielewski*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

67 Scopus citations

Abstract

A series of rodlike donor-bridge-acceptor (D-B-A) molecules was synthesized to study the role of bridge energy levels on electron transfer (ET) rates. In these compounds, a 4-aminonaphthalene-1,8-imide (ANI) electron donor is linked to a 1,8:4,5-naphthalenediimide acceptor (NI) via the 1,4 positions on a phenyl bridge. The phenyl bridge is substituted at the 2 and 5 positions with methyl or methoxy groups to yield ANI-diMe-NI and ANI-diMeO-NI. These molecules differ only in the energy levels of the bridge molecular orbitals. Other parameters affecting ET rates such as donor-acceptor distance, orientation, and driving force are constant between the two systems. The rate constants for charge separation (CS) and charge recombination (CR) within ANI-diMeO-NI in toluene are 32 and 1400 times larger, respectively, than the corresponding rate constants for ANI-diMe-NI. Solvents of higher polarity diminish these differences in rate constants, making them comparable to those observed for ANI-diMe-NI. The relative energies of the ion pair states suggest that it is possible for the reaction 1*D-B-A → D-B+-A- to occur via a double electron-transfer process that is somewhat analogous to Dexter energy transfer. The lowest excited singlet state of the donor, 1*ANI, possesses about 70% charge-transfer character, so that significant positive charge is localized on its amine nitrogen, whereas significant negative charge is localized on its naphthalene-1,8-imide ring. Electron transfer from the naphthalene-1,8-imide ring of 1*ANI to NI is concomitant with electron transfer from the p-dimethoxybenzene bridge to the electron-deficient amine nitrogen atom in 1*ANI. A series of reference molecules in which the p-dimethoxybenzene bridge moiety is attached only to ANI or NI alone is used to establish the structural and electronic requirements for this unusual charge separation mechanism.

Original languageEnglish (US)
Pages (from-to)7802-7810
Number of pages9
JournalJournal of the American Chemical Society
Volume122
Issue number32
DOIs
StatePublished - Aug 16 2000

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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