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

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.