Water modulates the chemiexcitation efficiency of the peroxyoxalate reaction: Enhancement at low and decrease at higher concentrations

Abstract

Chemiluminescence transformations are widely utilized in analytical and bioanalytical applications. Specifically, the peroxyoxalate system can be employed for sensitive quantification of a wide variety of analytes. Under anhydrous conditions, the peroxyoxalate reaction shows extremely high emission quantum yields, comparable to highly efficient bioluminescence systems. However, in aqueous media, important for bioanalytical applications, the emission efficiency suffers a significant decrease of several orders of magnitudes, although the reason for this loss in efficiency is unknown up to now.In the present work, we show the results of a detailed photophysical study on the effect of water on the chemiexcitation efficiency of the peroxyoxalate reaction using bis(2,4-dinitrophenyl) oxalate as reagent and imidazole as base catalyst in the presence of different chemiluminescence activators. It is shown that in 1,2-dimethoxyethane/water mixtures (DME:H2O), singlet quantum yields increase with increasing the water concentrations up to 0.7 mol/L, followed by a steady decrease at higher water concentrations. The decrease in quantum yields is not predominantly due to concurrent ester hydrolysis, as quantum yields could be corrected for this side reaction. This indicates that the quantum yield decrease is due to the diminished chemiexcitation efficiency in the presence of water. Using different activators, it is shown that chemiexcitation occurs via electron transfer and electron back-transfer steps between a high-energy intermediate and the activator. Additionally, its efficiency is governed by the energetics of the electron back-transfer step, in agreement with the Chemically Initiated Electron Exchange Luminescence (CIEEL) mechanism, also in partially aqueous environment. Lower excitation efficiency in aqueous media may be due to formation of solvent-separated radical ion pairs, inhibiting chemiexcitation.