Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes†

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 10(8)-10(9) M(-1)s(-1) for phenols and 10(5)-10(6) M(-1)s(-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 rho = -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.

In-vitro antioxidant activities of an ethanolic extract of the oyster mushroom, Pleurotus ostreatus

“…These results strongly suggest that the mushroom P. ostreatus can quench DBO fluorescence (free radical source) in a concentrationdependent manner, possibly due to its antioxidant components such as β-carotene, α-tocopherol, ascorbic acid, chrysin and rutin. The quenching activity of similar types of antioxidants, such as glutathione, ascorbic acid, uric acid and α-tocopherol, has been reported by Nau (1998), while Pischel et al (2006) described the fluorescence quenching of singlet-excited DBO by 22 phenols and 12 alkyl benzenes.…”

“…Its n,π⁎-excited states behave in a radical-like way. Since the reactivity resembles simple alkoxyl radicals (Valgimigli, Banks, Ingold, & Lusztyk, 1995), it has been extensively employed as a fluorescent probe for antioxidants (Gramlich, Zhang, & Nau, 2004;Pischel, Patra, Koner, & Nau, 2006). In the present study, DBO was used as a source of free radicals or antioxidant probe for the extract of the mushroom P. ostreatus.…”

In-vitro and in-vivo antioxidant effects of the oyster mushroom Pleurotus ostreatus

“…The n,π*-excited states of this compound mimic free radical behavior. Since this reactivity resembles that of simple alkoxyl radicals (Sies, 1998;Valgimigli, Banks, Ingold, & Lusztyk, 1995), it has been extensively employed as a fluorescent probe for antioxidants (Gramlich, Zhang, & Nau, 2004;Pischel, Patra, Koner, & Nau, 2006). Natural antioxidants, such as glutathione, ascorbic acid, uric acid and α-tocopherol, have been reported to show quenching activity (Nau, 1998).…”

“…Natural antioxidants, such as glutathione, ascorbic acid, uric acid and α-tocopherol, have been reported to show quenching activity (Nau, 1998). Twenty-two phenolic and 12 alkyl benzenes have also shown antioxidant-like properties by quenching the intensity of fluorescence of singlet-excited DBO (Pischel et al, 2006). In our study, a steady-state quenching plots for the quenching of DBO by the mushroom extract suggest that the extract of P. ostreatus is an excellent quencher of DBO in a concentrationdependent manner ( Fig.…”

Investigation of the fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by certain substituted uracils

“…Another piece of evidence is that the excited DBO emits at 420 nm at this wavelength, these two uracils possess absorption (overlap of emission spectrum of DBO and absorption spectrum of 5-nitrouracil and 5-fluorouracil). This reveals that the energy transfer from the excited DBO to 5-nitrouracil and 5-fluorouracil presents an efficient competitive quenching mechanism in this case [23]. Substitution at the 5-position of uracil can substantially alter the electronic nature of uracils.…”