Peroxyoxalate reaction, chemiluminescence

In this part of the chapter, we will focus essentially on mechanistic aspects of the peroxyoxalate reaction. For the discussion of the most important advances in mechanistic aspects of this chemiluminescent system, covering mainly literature reports published in the last two decades, we will divide the sequence operationally into three main parts (i) the kinetics of chemical reactions that take place before chemiexcitation, which ultimately produce the high-energy intermediate (HEI) (ii) the efforts to elucidate the structure of the proposed HEIs, either attempting to trap and synthesize them, or by indirect spectroscopic studies and lastly, (iii) the mechanism involved in chemiexcitation, whereby the interaction of the HEI with the activator leads to the formation of the electronically excited state of the latter, followed by fluorescence emission and decay to the ground state. 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

A variant of postcapillary derivatization is chemiluminescence (CL) detection [8]. In this case, the chemical reaction replaces the light source for excitation. The detector is a PMT run at high voltage. Solutes can be tagged with CL reagents such as luminol or directly excited via the peroxyoxalate reaction. The latter works best for aminoaromatic hydrocarbons such as dansylated amines. The LCDs using CL detection approach laser levels because of the low background. However, the need for specialized apparatus has limited the applicability of CL detection. 

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

Most peroxyoxalate chemiluminescent reactions are catalyzed by bases and the reaction rate, chemiluminescent intensity, and chemiluminescent lifetime can be varied by selection of the base and its concentration. Weak bases such as sodium saUcylate or imidazole are generally preferred (94). 

A number of other chemiluminescent reactions appear to be related to peroxyoxalate chemiluminescence although thek mechanistic details may vary. For example, various chlotinated esters and ethers react with and a fluorescer to emit light (98—101). Other examples have been given... 

In order to optimize the chemiluminescence response, we have investigated the mechanism of the complex reactions leading to chemical generation of chemiluminescence. A new peroxyoxalate-hydrogen peroxide reaction mechanism has emerged from our preliminary studies on the five contributing factors listed above. Two kinetic models are discussed, one for the... 

Though we and others (27-29) have demonstrated the utility and the improved sensitivity of the peroxyoxalate chemiluminescence method for analyte detection in RP-HPLC separations for appropriate substrates, a substantial area for Improvement and refinement of the technique remains. We have shown that the reactions of hydrogen peroxide and oxalate esters yield a very complex array of reactive intermediates, some of which activate the fluorophor to its fluorescent state. The mechanism for the ester reaction as well as the process for conversion of the chemical potential energy into electronic (excited state) energy remain to be detailed. Finally, the refinement of the technique for routine application of this sensitive method, including the optimization of the effi-ciencies for each of the contributing factors, is currently a major effort in the Center for Bioanalytical Research. 

The brilliant emissions resulting from the oxidation of certain oxalic acid derivatives, especially in the presence of a variety of fluorophores, are the bases of the most active area of current interest in CL. This group of chemiluminescent reactions has been classified as peroxyoxalate chemistry because it derives from the excited states formed by the decomposition of cyclic peroxides of oxalic acid derivatives called dioxetanes, dioxetanones, and dioxetanediones. 

The leaving group of the oxalic ester has a strong effect on the efficiency of the peroxyoxalate chemiluminescent system. The electron-attracting power of the substituents on the phenyl rings of the substituted diphenyl oxalates is important to the overall efficiency of the chemiluminescent reactions. Steric effects... 

A wide variety of different classes of fluorescent molecules has been investigated in the peroxyoxalate chemiluminescent systems. Among those screened were fluorescent dyes such as rhodamines and fluoresceins, heterocyclic compounds such as benzoxazoles and benzothiazoles, and a number of polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and perylenes. The polycyclic aromatic hydrocarbons and some of their amino derivatives appear to be the best acceptors as they combine high fluorescence efficiency with high excitation efficiency in the chemiluminescent reaction [28],... 

Tsunoda, M., Nagayama, M., Funatsu, T., Hosoda, S., and Imai, K., Catecholamine analysis with microcolumn LC-peroxyoxalate chemiluminescence reaction detection, Clinica ChimicaActa 366(1-2), 168-173, 2006. 

An indirect method has been used to determine relative rate constants for the excitation step in peroxyoxalate CL from the imidazole (IM-H)-catalyzed reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide in the presence of various ACTs . In this case, the HEI is formed in slow reaction steps and its interaction with the ACT is not observed kinetically. However, application of the steady-state approximation to the reduced kinetic scheme for this transformation (Scheme 6) leads to a linear relationship of l/direct measure of the rate constant of the excitation step. 

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