Dimethyl-dioxetanone

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 importance of a charge transfer complex between the peroxide and the metal and the value of a low ionisation potential for the metalloporphyrin has been demonstrated using dimethyl dioxetanone [6]. Typical CIEEL behaviour was observed, and evidence for the importance of a charge transfer complex between the peroxide and the metallo-porphyrin obtained from competitive complexation experiments using pyridine and diethyl ether. Considerable reductions in the rate of reaction were observed. 

The thermolysis of dimethyl-dioxetanone (21) revealed two different mechanisms of decomposition. In a unimolecular mechanism, the dioxetanone is cleaved into CO2 and acetone in its excited triplet state (19) ... 

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. 

Surprisingly, the yield of excited products (triplet and singlet acetone) from dimethyl-dioxetanone (21) is only 5% of that observed in tetramethyl dioxetane decomposition, although the thermolysis of (23) is ca 84 kJ/mol. more exothermic... 

Interestingly, the quantum yield obtained recently by our research group for the catalyzed decomposition of dimethyl-1,2-dioxetanone (9) is two orders of magnitude lower... 

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. ... 

Dimethyl-1,2-dioxetanone, chemiluminescence quantum yield, 1226-7 Dimethyldioxirane epoxidation atkenes, 37-44 deuteriation, 1143 O NMR spectroscopy, 184-5 preparation, 26, 1130-2 structure, 26... 

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