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1 January 2006 Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes
Uwe Pischel, Digambara Patra, Apurba L. Koner, Werner M. Nau
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Abstract

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 108–109 M−1s−1 for phenols and 105–106 M−1s−1 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 (kH/kD 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.

Uwe Pischel, Digambara Patra, Apurba L. Koner, and Werner M. Nau "Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes," Photochemistry and Photobiology 82(1), 310-317, (1 January 2006). https://doi.org/10.1562/2005-02-07-RA-434
Received: 7 February 2005; Accepted: 6 May 2005; Published: 1 January 2006
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