Thermolysis chemiluminescence

DPA) in dimethylphthalate at about 70°, yields a relatively strong blue Umax =435 nm) chemiluminescence the quantum yield is about 7% that of luminol 64>. The emission spectrum matches that of DPA fluorescence so that the available excitation energy is more than 70 kcal/mole. Energy transfer was observed on other fluorescers, e.g. rubrene and fluorescein. The mechansim of the phthaloyl peroxide/fluorescer chemiluminescence reaction very probably involves radicals. Luminol also chemiluminesces when heated with phthaloyl peroxide but only in the presence of base, which suggests another mechanism. The products of phthaloyl peroxide thermolysis are carbon dioxide, benzoic acid, phthalic anhydride, o-phenyl benzoic acid and some other compounds 65>66>. It is not yet known which of them is the key intermediate which transfers its excitation energy to the fluorescer. 

Very weak chemiluminescence (quantum yields of 6.5.. . 9.1 X 10-10) in the spectral ranges 400. 540 nm (benzaldehyde phosphorescence) and 600 nm (emission from excited singlet oxygen collision pairs)) was also observed on thermolysis of 5 with no fluorescer present. 

Alkoxy-dioxetanes undergo chemiluminescent thermolysis in the pres-... 

Lipid hydroperoxides are also generated in singlet molecular oxygen mediated oxidations and by the action of enzymes such as lipoxygenases and cyclooxygenases. Chemiluminescence (CL) arising from lipid peroxidation has been used as a sensitive detector of oxidative stress both in vitro and in vivo . Several authors have attributed ultra-weak CL associated with lipid peroxidation to the radiative deactivation of O2 and to triplet-excited carbonyls (63, 72) (equations 35 and 36) " . It has been proposed that the latter emitters arise from the thermolysis of dioxetane intermediates (61, 62) (equation 35), endoperoxide (73) (equation 37) and annihilation of aUtoxyl, as well as peroxyl radicals ... 

The involvement of the CIEEL process in the thermolysis of [21] immediately offers new insight into many previously perplexing proposals of dioxetane or dioxetanone intermediacy in various chemi- and bio-luminescent reactions. For example, the discovery of activated chemiluminescence for [21], and the finding that intramolecular electron transfer can generate a very high yield of electronically excited singlet (Horn et al., 1978-79), prompts speculation that an intramolecular version (34) of the CIEEL mechanism is... 

The rates of thermolysis (37) of the peroxyesters in argon-purged benzene can be followed conveniently by the direct, indirect, or activated chemiluminescence. In all of the cases reported peroxyesters in benzene solution show clean first-order reaction for low initial peroxide concentrations (10-5-10-3 M). The activation parameters for the peroxyester thermolyses reveal some important details of the reaction mechanism. The activation enthalpy obtained for peroxyester [28] is quite similar to that reported by Hiatt... 

The thermolysis of peroxyacetate [28] and substituted peroxybenzoates [29] gives both direct and indirect chemiluminescence. Thermolysis of peroxyacetate [28] in benzene solution at 100° gives very weak direct chemiluminescence. The emission is so weak that an emission spectrum could not be obtained. When biacetyl, which has a considerably higher quantum efficiency for phosphorescence than acetophenone (Backstrom and Sandros, 1958), is used... 

Thermolysis of peroxide [29c] in benzene solution generates a chemiluminescent emission whose spectrum is identical to the fluorescence spectrum of photoexcited p-dimethylaminobenzoic acid under similar conditions. Thus the direct chemiluminescence is attributed to the formation of the singlet excited acid. The yield of directly generated excited acid is reported to be 0.24% (Dixon and Schuster, 1981). Since none of the other peroxybenzoates generate detectable direct chemiluminescence it was not possible to compare this yield to the other peroxides. However, by extrapolation it was concluded that the dimethylamino-substituted peroxide generates excited singlet products at least one thousand times more efficiently than does the peroxyacetate or any of the other peroxybenzoates examined. 

Activated chemiluminescence is observed from these secondary peroxy-esters as well. When the thermolysis of peroxyacetate [281 in benzene solution is carried out in the presence of a small amount of an easily oxidized substance the course of the reaction is changed. For example, addition of N,N-dimethyldihydrodibenzol[ac]phenazine (DMAC) to peroxyester [28] in benzene accelerates the rate of reaction and causes the generation of a modest yield of singlet excited DMAC. This is evidenced by the chemiluminescence emission spectrum which is identical to the fluorescence spectrum of DMAC obtained under similar conditions. Spectroscopic measurements indicate that the DMAC is not consumed in its reaction with peroxyester 28 even when the peroxyester is present in thirty-fold excess. The products of the reaction in the presence of DMAC remain acetophenone and acetic acid. These observations indicate that DMAC is a true catalyst for the reaction of peroxyacetate 28. The results of these experiments with DMAC, plotted according to (27) give k2 = 9.73 x 10-2 M-1 s-1. 

Thermolysis of [150] produces chemiluminescence due to emission from [151] and [152]. In the case of [151] the emission is due to an exciplex (Becker et al., 1981). Pyrimidines linked by a polymethylene chain and dihydropyri-dines similarly linked undergo intramolecular cycloaddition (Koroniak and Golankiewicz, 1978 Potts et al., 1977). 

Many examples exist of the reaction of anthracenes with alkenes and dienes to give products resulting from addition across the 9,10-positions of the anthracene ring. Yang has used this reaction to generate the (4-I-41 adducts (129) which are then converted to the novel para, para linked species (130). These comjpounds are chemiluminescent on thermolysis and yield benzene and, in the case of R=H, singlet excited anthroic acid. The (4+21 photoaddition of... 

Oxidation of luminol (see Example 7.22) and thermolysis of endoperox-ides or 1,2-dioxetanes provide important examples of chemiluminescent reactions. Tetramethyl-1,2-dioxetane (181) has been studied in great detail the thermolysis is clearly first order and the activation enthalpy in butyl phthal-ate is A// 27 kcal/mol. The enthalpy difference between the reactants and ground-state products is A// = -63 kcal/mol. 

Thermolysis of trara-4,4-dimethyl-2,3,5-trioxabicyclo[4.4.0]decane (55) in octane or diphenyl ether at temperatures of 160-189°C gives adipaldehyde (56) and acetone (Equation (3)) <78JOC52i>. Chemiluminescence is not observed. The kinetics are consistent with a unimolecular process and yield 39.9 + 1.4 kcal mol for ACT at 175°C. Cleavage is undoubtedly triggered by homolytic rupture of the O—O bond. 

Thermolysis of 40 in methanol gives a mixture of products, including 29, 30, and 31 (and some 23 from deoxygenation).44,48 Products 30 and 31 appear to arise via different (chemiluminescent) pathways (Scheme 60),48 although 29 is probably derived from 31 by hydrolysis. 

Chemiluminescence.—The triplet potential-energy surface of dioxetan has been found to intersect the singlet surface between dioxetan and the transition state for dioxetan decomposition to formaldehyde. This provides support for the argument that the reason for efficient triplet ketone formation on thermolysis of dioxetans is because intersystem crossing is an integral part of the reaction.293 The decomposition mechanism of 1,2-dioxetans has been the subject of another theoretical investigation.294 ... 

Bicyclol, fi]butanes Unlike the thermolysis of Dewar-benzene, benzvalene is converted into benzene by an essentially non-chemiluminescent and concerted process. Rearrangement of (255) to (257) proceeds largely via the orthoquino-dimethane (256) with benzvalene - benzene rearrangement as the initial step of the reaction.The iron tricarbonyl-co-ordinated analogue of (256) is the major product from the corresponding bicyclobutane. 

Most chemiluminescent reactions involve cleavage of a strained 0-0 bond in the key step that generates an excited state. For this reason the thermolysis of tetramethyldioxetane has been extensively investigated. We will summarize the results of those studies here. 

Transient Four membered Rings.—Evidence has been presented for the intermediacy of Dewar-benzenes in the thermal rearrangement of bis-cyclopropenyl to benzene. When (572) was heated in acetonitrile, in the presence of 9,10-dibromoanthracene, fluorescence of the dibromoanthracene was observed. The nature of the excited state produced was identified as triplet xylene, which was shown to be produced from (572) via an intermediate. The activation energy for thermolysis of the intermediate, ca. 26kcalmol"S was less than that for the disappearance of (572). Of the three possible benzene isomers, only Dewar-benzene produces a significant amount of the indirect chemiluminescence, and the activation energies for the decomposition of the Dewar-benzenes (573) and (574) are close to that found for the intermediate in the thermolysis of (573). Comparison of the chemiluminescence yields from (572), (573), and (574) allowed the conclusion that the rearrangement of (572) could proceed almost totally via a dimethyl-Dewar-benzene. 

The chemiluminescence emission in polystyrene and in polycarbonate has maxima at 450 and 530 nm which reach their highest intensity ca. 60 s after the start of the thermolysis at 70-80°, with DCPD as initiator. 

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