Evidence supporting a 1,2-dioxetanone as an intermediate in the benzofuran-2(3H)-one chemiluminescence.

The mechanism of the chemiluminescent reaction of ethyl (5-fluoro-2-oxo-2,3-dihydrobenzofuran-3-yl) carbamate (a 2-coumaranone derivative) with a base and molecular oxygen was investigated. New evidence from the reaction kinetics and absorption/emission profiles was obtained, supporting the existence of a 1,2-dioxetanone as an intermediate: (i) its characteristic activation parameters (ΔH(≠) = 7.2 ± 0.1 kcal mol(-1); ΔS(≠) = -45 ± 5 cal K(-1) mol(-1)) indicating a high degree of thermal instability and (ii) its bimolecular decomposition rate constant for the reaction with perylene. The newly developed methodology has been shown to be suitable for determining the reactivity of such thermally unstable peroxides, which are very difficult to prepare and isolate, using this alternative approach of in situ generation of a 1,2-dioxetanone.

Studies on the chemiexcitation step in peroxyoxalate chemiluminescence using steroid-substituted activators.

The peroxyoxalate reaction is utilized in a wide variety of analytical applications; however, its mechanism is still not very well understood, especially with respect to the excitation step, where the 'chemical energy' is transformed into 'excitation energy'. This base-catalysed reaction of activated oxalic phenyl esters with hydrogen peroxide in the presence of highly fluorescent aromatic hydrocarbons with low oxidation potentials is the only known chemiluminescence system for which exists experimental evidence for the occurrence of the intermolecular chemically initiated electron exchange luminescence (CIEEL) mechanism of proven high efficiency for excited state formation. We report here the singlet quantum yields and relative rate constants of the excitation step (k(CAT)/k(D)), obtained in the peroxyoxalate reaction, utilizing steroid-substituted oxazolinylidenes as activators. In agreement with the CIEEL mechanism, a linear correlation of ln(k(CAT)/k(D)) with the oxidation potential of the activators is obtained, and the singlet quantum yields can be rationalized in terms of the free energy balance of the back electron transfer, leading to the formation of the activator's excited state. Thus, these results contribute to the experimental validation of the widely employed, thus still controversial, CIEEL mechanism.