Dioxetanones activated decomposition

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

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. 

Finally, in activated chemiluminescence, an added compound also leads to an enhancement of the emission intensity however, in contrast with the indirect CL, this compound, now called activator (ACT), is directly involved in the excitation process and not just excited by an energy transfer process from a formerly generated excited product (Scheme 5). Activated CL should be considered in two distinct cases. In the first case, it involves the reaction of an isolated HEI, such as 1,2-dioxetanone (2), and the occurrence of a direct interaction of the ACT with this peroxide can be deduced from the kinetics of the transformation. The observed rate constant (kobs) in peroxide decomposition is expected to increase in the presence of the ACT and a hnear dependence of kobs on the ACT concentration is observed experimentally. The rate constant for the interaction of ACT with peroxide ( 2) is obtained from the inclination of the linear correlation between obs and the ACT concentration and the intercept gives the rate constant for the unimolec-ular decomposition ( 1) of this peroxide (Scheme 5). The emission observed in every case is the fluorescence of the singlet excited ACT" ° . ... 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

Intensive studies in the field of mechanistic CL by several research groups have resulted in the description of a large variety of peroxides which, in the presence of appropriate activators, show decomposition in an activated CL process and might involve the CIEEL mechanism . Even before the formulation of the CIEEL mechanism, Rauhut s research group obtained evidence of the involvement of electron-transfer processes in the excitation step of the peroxyoxalate CL. Results obtained in the activated CL of diphe-noyl peroxide (4) led to the formulation of this chemiexcitation mechanism , and several 1,2-dioxetanones (a-peroxylactones), such as 3,3-dimethyl-l,2-dioxetanone (9) and the first a-peroxylactone synthesized, 3-ierr-butyl-l,2-dioxetanone (14), have been shown to possess similar CL properties, compatible with the CIEEL mechanism Furthermore, the CL properties of secondary peroxyesters, such as 1-phenethylperoxy acetate (15) , peroxylates (16) , o-xylylene peroxide (17) , malonyl peroxides... 

However, the most severe criticism of the CIEEL hypothesis relates to the chemiexcita-tion efficiency experimentally obtained for the standard CIEEL systems, diphenoyl peroxide (4) and 1,2-dioxetanone (2) . In a study on the electron transfer catalyzed decomposition of l,4-dimethoxy-9,10-diphenylanthracence peroxide (21), Catalan and Wilson obtained very low chemiexcitation quantum yields with various commonly utilized activators (4>s =2 10 EmoH ) and reinvestigated the CL of diphenoyl peroxide (4), determining quantum yields in the same order of magnitude (4>s = (2 1)10 Emol ) as those obtained by 21 (Table 1). We have more recently determined the quantum yields in the rubrene-catalyzed decomposition of dimethyl-1,2-dioxetanone (9) and also found a much lower value than the one initially reported (Table 1) °. Since the diphenoyl peroxide and the 1,2-dioxetanone systems are the two prototype CIEEL systems, the validity of this hypothesis itself might be questioned due to its low efficiency in excited-state formation. ... 

Recent work by Schmidt and Schuster (1978a, 1980a) has shown that the addition of any of several easily oxidized, fluorescent aromatic hydrocarbons or amines to solutions of [21] results in greatly enhanced chemiluminescence. Moreover, addition of these molecules accelerates the rate of reaction of [21]. The catalyzed reaction is first order in both [21] and aromatic hydrocarbon or amine (which is termed the activator, act). Acetone is still produced quantitatively, and the activator is not consumed in the reaction, but rather serves as a catalyst for the decomposition of the dioxetanone. The kinetic behavior is thus described by the simple rate law (27), where kt is the rate... 

The activation energy in this case varied from 87 to 93 kJ/mol in different solvents. From the temperature dependence, several competitive reaction paths for this dimethyl-dioxetanone decomposition were deduced, all having a biradical as first intermediate. Heavy-atom effects often play a role in dioxetan chemiluminescence. If DBA is used as fluorescer, the quantum yield is markedly greater than that observed when DPA is used - although the latter has a fluorescence efficiency of 0.89, compared with 0.1 for DBA. In both cases triplet-singlet energy transfer is the origin of the chemiluminescence. 

H02 " as nucleophile [73]. The intramolecular electron transfer is in fact a combination of the hypothesis for the firefly dioxetanone decomposition (p. 152) and the (intramolecular) chemiluminescent activation of secondary peresters (p. 35). However it is only one hypothesis among many which lack confirmatory evidence in this exceedingly complicated reaction. 

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