1.2- Dioxetanes quantum yields

Other speculative mechanisms [26] may be proposed based on the presence of singlet oxygen and C = C in oxidized polymer. The reaction of the latter may lead to the transient formation of dioxetanes, the decomposition of which has an even higher quantum yield of luminescence than CIEEL mechanism [27],... 

Energy transfer to photoreactive acceptors has also been widely utilized for excitation quantum yield determination (chemical titration), mainly in the decomposition of dioxetanes ° . The quantum yields are calculated from the photoproduct yield obtained at infinite energy acceptor concentrations ( = 1.0) by extrapolation of the double-reciprocal relationship between the photochemically active energy acceptor concentration and the photoproduct yield ( Lp ). H the quantum yield of the photochemical reaction (excitation quantum yield (< > ) can be calculated (equation 8) . ... 

We have recently described a calibration procedure for the determination of excitation quantum yields on commercial fluorimeters, utilizing the luminol standard , and have thereby determined singlet excitation quantum yields for the peroxyoxalate reaction with bis(2,4,6-trichlorophenyl) oxalate (TCPO), hydrogen peroxide and imidazole, using various activators . The same calibration method has been utilized to determine the singlet quantum yields obtained in the induced decomposition of protected phenoxyl-substituted 1,2-dioxetanes 6 and and compared them to the well-investigated... 

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... 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

This qualitative interpretation of structural and electronic similarity has also been employed to rationalize the fact that the quantum yield for the dioxetane derivative 6, in which the phenoxy substituent is directly linked to the peroxidic ring, is two orders of magnitude higher than for the dioxetane 7, in which the trigger function is separated by a methylene bridge. Furthermore, the different quantum yields were rationalized in terms of a competition between the intramolecular (pathway A) and intermolecular back-electron transfer (pathway B) in the decomposition of 7, whereas the intramolecular back-electron transfer was believed to occur exclusively in the decomposition of 6, due to the higher stability of the radical anion of the benzaldehyde derivative, as compared with the radical anion of acetone (Scheme 14). 

Scheme 28) Quantum yields of lucigenin oxidation by hydrogen peroxide in alkaline media are comparable with the values obtained in luminol oxidation (1.24 x 10 E mol ) ° . However, the use of other peroxides, such as tcrt-butyl hydroperoxide, results in a decrease of chemiluminescence quantum yields of two orders of magnitude, confuming the hypothesis that a 1,2-dioxetane is the HEI, since its formation would be impossible with alkyl peroxides . 

The synthesis of A-methylacridan dioxetane derivatives would be very promising for analytical application as chemiluminescence probes. Despite the high quantum yields... 

Several other examples of 1,2-dioxetane derivatives containing easily oxidizable groups have been reported and the high singlet quantum yield observed in their decomposition was attributed to the occurrence of the intramolecular CIEEL sequence Based on this concept, Schaap and coworkers have introduced the concept of induced chemiluminescence, which is very relevant for investigations into firefly luciferin bioluminescence and has led to the development of chemiluminescent probes widely used in immunoassays (Section N. 

Tetramethylammonium ozonide, 736 Tetramethyl-l,2-dioxetane (TMD) chemical titration, 1224 chemiluminescence, 1221, 1234 quantum yield standard, 1224, 1226 N,N, N, A -Tetramethyl-p-phenylenediamine hydrogen peroxide determination, 735, 631, 633... 

This biradical-like concerted mechanism, in which the kinetic features reflect the biradical character and the formation of excited-state products can best be rationalized by the concerted nature of the complex reaction coordinate, was proposed to optimally reconcile the experimentally determined activation and excitation parameters of most 1,2-dioxetanes studied and has been called the merged mechanism51-87. Specifically, both thermal stability and singlet and triplet quantum yields in the series of methyl-substituted 1,2-dioxetanes, including the parent 1,2-dioxetane11 50 51, could be readily rationalized on the basis of the merged mechanism and qualitative quantum mechanics considerations86. 

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