Electronic Structures of Exciplexes and Excited Charge-Transfer Complexes

Ian R. Gould, Ralph H. Young, Leonard J. Mueller, Samir Farid, A. C. Albrecht

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Abstract

For the exciplexes and excited charge-transfer (CT) complexes formed between 9,10-dicyanoanthracene and 2,6,9,10-tetracyanoanthracene as electron acceptors and alkylbenzenes as donors, the radiative rate constants (kf) increase with increasing emission energy. The increase in kf is attributed to a corresponding decrease in the charge-transfer character of the emitting species. This is explained in terms of the relative contributions of pure ion-pair and locally excited states to the emitting state. With decreasing solvent polarity and with increasing redox energy of the acceptor/donor pair (EoxD - EredA), the energy of the pure ion-pair state is raised and mixing with the locally excited states increases. The dependence of kf on emission energy is analyzed quantitatively using a three-state model in which mixing among the first locally excited singlet state of the cyanoanthracenes, the pure ion-pair state, and the neutral state is taken into account. Simplified methods for data analysis are also discussed. From the analyses, the relationship between the electronic structures of the exciplex/excited CT complexes and the emission frequency is obtained. For these acceptor/donor systems, the emitting species can be considered to be essentially pure contact radication pairs (>90% CT character) when their emission maxima are lower in energy than the 0,0 transition of the acceptor excited singlet states by ca. 5000 cm-1. Values of ca. 1300-1350 cm-1 are obtained for the electronic matrix elements coupling the locally excited and ion-pair states. The corresponding matrix elements for coupling the ion-pair and the neutral states are ca. 750-900 cm-1, which are similar to those estimated previously from studies of the rates of nonradiative electron transfer in closely related species.

Original languageEnglish (US)
Pages (from-to)8188-8199
Number of pages12
JournalJournal of the American Chemical Society
Volume116
Issue number18
DOIs
StatePublished - Sep 1 1994
Externally publishedYes

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ASJC Scopus subject areas

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

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