2.5: Dioxetanes and oxalates

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Peroxy-oxalate chemiluminescence (PO-CL) was first reported in 1963 as a very weak bluish-white emission from oxalyl chloride, Cl-CO.CO-Cl, on oxidation by hydrogen peroxide; a similar blue emission occurs from related oxalyl peroxides. Much more intense emission is obtained in the reaction between aryl oxalates and hydrogen peroxide in the presence of a fluorophore; it is this version of the reaction that is analytically useful^[1]^[2]. Liquid chromatography is a major area of application^[3].

Because in PO-CL analysis, the analyte is an added fluorophore to which energy is transferred, the various applications have much in common. The rate of PO-CL depends especially on pH and on the presence of a nucleophilic base catalyst for ester hydrolysis. Aryl oxalates differ in the effect of pH on the intensity and decay of the chemiluminescence. They also differ in their solubilities, which affects their usefulness as detection reagents for HPLC. There are wide variations in their stabilities in the presence of hydrogen peroxide, so some are more suitable than others for premixing with the oxidant. Taking all these things into account, Honda et al. proposed that the preferred oxalate varied with pH as follows:

<2: bis(pentafluorophenyl)
2 to 4: bis(2-nitrophenyl)
4 to 6: bis(2,4-dinitrophenyl)
6 to 8: bis(2,4,6-trichlorophenyl)
>8: bis(2,4,5-trichloro-6-pentyloxycarbonylphenyl)

PO-CL is thought to follow a chemically initiated electron exchange luminescence (CIEEL) mechanism as proposed by Koo and Schuster^[4]. An electron is transferred from the fluorophore to an intermediate, which, as it decomposes, transfers it back again; as a result the fluorophore is raised to an exited state and subsequently radiates. In support of this it has been demonstrated that the relative excitation yields of different fluorescers have a significant negative correlation with their oxidation potentials – in other words, the more difficult it is to oxidize the fluorescer, the lower its probability of excitation. High chemiluminescence intensity can be predicted if a fluorescer has a low singlet excitation energy ; a low oxidation potential is at least as important. The formation of a linear peroxide intermediate, ArO-CO.CO-OOH, which decomposes to radical ion-pairs comprising the fluorophore and a carbon dioxide molecule, has also been proposed as the mechanism of energy transfer. Background emission in the absence of a fluorophore occurs at 450 nm (which could be carbon dioxide) and at about 550 nm (which varies with the aryl group and could be due to an excited carbonyl intermediate containing the aryl group). Dioxetanes luminesce on warming, producing excited carbonyl compounds and the may have a role in PO-CL. However, decomposition of 1,2-dioxetanedione into carbon dioxide, though possible, is unlikely to be the sole source of the emission as the chemiluminescence depends on the electronegativity of the aryl group, so is unlikely to arise from an intermediate that would be the same whatever the aryl group.