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Investigation of polar and stereoelectronic effects on pure excited-state hydrogen atom abstractions from phenols and alkylbenzenes

Photochemistry and Photobiology, ISSN: 0031-8655, Vol: 82, Issue: 1, Page: 310-317
2006
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  • Citations
    21
    • Citation Indexes
      21
  • Captures
    11

Conference Paper Description

The fluorescence quenching of singlet-excited 2,3-diazabicyclo[2.2.2]oct-2- ene (DBO) by 22 phenols and 12 alkylbenzenes has been investigated. Quenching rate constants in acetonitrile are in the range of 10-10 Ms for phenols and 10-10 Ms for alkylbenzenes. In contrast to the quenching of triplet-excited benzophenone, no exciplexes are involved, so that a pure hydrogen atom transfer is proposed as quenching mechanism. This is supported by (1) pronounced deuterium isotope effects (k/k ca 4-6), which were observed for phenols and alkylbenzenes, and (2) a strongly endergonic thermodynamics for charge transfer processes (electron transfer, exciplex formation). In the case of phenols, linear free energy relationships applied, which led to a reaction constant of ρ = -0.40, suggesting a lower electrophilicity of singlet-excited DBO than that of triplet-excited ketones and alkoxyl radicals. The reactivity of singlet-excited DBO exposes statistical, steric, polar and stereoelectronic effects on the hydrogen atom abstraction process in the absence of complications because of competitive exciplex formation. © 2006 American Society for Photobiology.

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