High-energy intermediate chemiluminescence

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

In the first proposal of a mechanism for chemiluminescent luminol oxidation, Albrecht postulates a bicyclic endoperoxide as the high-energy intermediate. The endoperoxide is presumably formed by nucleophilic attack of hydrogen peroxide monoanion on one of the diazaquinone 27 carbonylic groups to form 28, followed, after deprotonation to 29, by ring closure to 30 (Scheme 21) . 

Analogously to the firefly luciferin/luciferase system, the general chemiluminescence mechanism postulated for 9-carboxyacridinium derivatives proposes the 1,2-dioxetanone 45 as high-energy intermediate However, this 1,2-dioxetanone is the only intermediate that has not yet been isolated . The cleavage of the peroxidic ring presumably results in the release of CO2 and the formation of an acridan residue in its electronically excited state (Scheme 32). 

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. 

Hexanal, hpid oxidation assessment, 669 1-Hexene, primary ozonide, 720 High-density lipoprotein (HDL) oxidation, 612 TEARS assay, 667 High-energy intermediate (HEI) chemiluminescence, 1215 activators, 1220, 1222 peroxyoxalates, 1188-9, 1257, 1261-6, 1267, 1269 stmcture, 1262, 1263... 

Peroxyoxalate chemiluminescence, continued) high-energy intermediate, 1188-9, 1257, 1261-6, 1267, 1269 mechanism, 1257-61... 

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 ACTs18. 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 1/S vs. 1/[ACT] (equation 5) and to the ratio of the chemiluminescence parameters /ic vrAi), which is a direct measure of the rate constant of the excitation step. Therefore, this method allows for the determination of relative rate constants for the excitation step in a complex reaction system, where this step cannot be observed directly by kinetic measurements18. The singlet quantum yield at infinite activator concentrations ( °), where all high-energy intermediates formed interact with the activator, is also obtained from this relationship (equation 5). 

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

Lee JH, Rock JC, Park SB, Schlautman MA, Carraway ER. Study of the characteristics of three high-energy intermediates generated in peroxyoxalate chemiluminescence (PO-CL) reactions. J Chem Soc Perkin Trans 2 2002 802-9. 

CHEMILUMINESCENCE INVOLVING THE PHOSPHORUS CHEMISTRY. PHOSPHA-l,2-DIOXETANES AS THE MOST LIKELY HIGH-ENERGY INTERMEDIATES IN AUTOXIDATION OF PHOSPHONATE CARBANION... 

Motoyoshiya J, Ikeda T, Tsuboi S, Kusaura T, Takeuchi Y, Hayashi S, Yoshioka S, Takaguchi Y, Aoyama H. Chemiluminescence in autoxidation of phosphonate carbanions. Phospha-l,2-dioxetanes as the most likely high-energy intermediates. J Org Chem 2003 68 5950-5. 

Studies with Cypridina (154), Oplophorus 150) and Renilla 155) luciferases have uncovered the following steps (Scheme 26) 156). First, the appropriate luciferase deprotonates the luciferin (62) to its tetrahedral carbanion (64). Reaction of the latter with oxygen yields the hydroperoxide (65). Cyclization affords the so-called high energy intermediate, the dioxetanone (66), which on cleavage loses carbon dioxide to give the amide (67) accompanied by chemiluminescence. 

In these systems, a high-energy intermediate excites a suitable fluorophore, which then emits its characteristic fluorescence spectrum consequently, they are termed indirect or sensitized chemiluminescence. The most common analytical application has been as a postcolumn reaction detector for liquid chromatography. Various fluorescent analytes (polycyclic aromatic hydrocarbons and polycyclic aromatic amines) and compounds derivatized using dansyl chloride, fluorescamine, or o-phthalaldehyde have been determined with sub-femtomole detection limits. 

Since its discovery by Chandross and to this day, peroxy-oxalate chemiluminescence has been controversial because of its enormous complexity in view of the many alternative steps involved in this process. The principal mechanistic feature of the peroxy-oxalate chemiluminescence pertains to the base-catalyzed (commonly imidazole) reaction of an activated aryl oxalate with hydrogen peroxide in the presence of a chemiluminescent activator, usually a highly fluorescent aromatic hydrocarbon with a low oxidation potential . A variety of putative high-energy peroxide intermediates have been proposed for the generation of the excited states . In the context of the present chapter, it is of import to mention that recent work provides experimental evidence for the intervention of the 1,2-dioxetanedione 18 (Scheme 11) as the high-energy species responsible for the chemiexcitation. Furthermore, clear-cut experimental data favor the CIEEL mechanism as a rationalization of the peroxy-oxalate chemiluminescence . 

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. 

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