Excitation-Decomposition Mechanisms

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. 

The mercury-photosensitized decomposition of 1,1-difluoroethene, and of ( )- and (Z)-1.2-difluoroethene, proceeds by elimination of hydrogen fluoride via an excited molecule mechanism to produce fluoroacetylene (12).55... 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

For operation at telecommunication wavelengths, one must be concerned with two-photon excitation processes leading to photochemical decomposition mechanisms. Femtosecond pulse techniques, such as those described by Stro-hkendl et al. [185], prove useful in evaluating two-photon excitation cross sections and excited state energy transfer processes relevant to photochemical decay pathways. 

The rate of decomposition was directly proportional to the rate of light absorption and was directly proportional to the pressure. They found no effect of nitrogen on the reaction rate and they proposed a primary split to N and O atoms rather than the excited state mechanism,... 

In a pair of complementary studies, Archer et al. (9,10) have made observations that agree with the previous studies but differ somewhat in interpretation. Their work interprets the decomposition mechanism in terms of several species of electronically excited singlet states. These studies agree with the previously discussed studies, if the molecular products formed via process 47 are assumed to be due to "singlet" decomposition only and if 4>isc = 1 - I CO The yield of intersystem... 

The effects of solvent on the thermal chemiluminescent decomposition of 1 described above are summarized as follows. First, SPD proceeded in an aprotic polar solvent to afford bright light with AniaxSPD = 493-498 nm similar to BID by a CT-induced decomposition mechanism. Second, 1 underwent uncatalyzed TD to give excited 5 which emitted yellow light with A ,axTD = 536 nm due to ESIPT in nonpolarp-xylene. 

The CHsCHF F Unimolecular Decompositioii Mechanism. The large range of excitation energies for the nascent [CHgCHF Flt from Reaction 24 leads to a complex decomposition mechanism in which secondary and tertiary imimolecular processes compete with coUisional deactivation. As shown in Reactions 28-38, this cascading sequence continues until the excess energy is insufficient to induce further decomposition (70). 

Recent work on cyolobutane (Lee and Rowland, 1963) supports and extends these arguments. The excitation-decomposition reaction has also received support from recent work on the efiFect of various additives and of phase on the radiochemical yields of benzene, cyclohexane and a variety of cyolohexenes (Avdonina, 1962 Pozdeev et al., 1962a, b). As we shall see later, the implications of this residual energy in an excited intermediate has given rise to a controversy on the details of the mechanisms of the reactions involved in product formation. 

Urch and Wolfgang (1961a) have considered excitation-decomposition as an explanation for the observed labeled-radical formation but prefer a mechanism involving direct displacement of two hydrogen atoms, or an alkyl group and a hydrogen atom by the recoiling tritium atom, namely... 

CH2 FCH2l ratio in excess CF4 further showed that the CH2 F-CH2 radicals were almost uniformly monoenergetic indicating that the radicals received negligible additional excitation from extra translational enei of the F atom (see below). A similar F atom addition-plus-decomposition mechanism has been invoked to e q>lain the presence of C2H3F as a product from the photolysis of ONF in the presence of C2H4 (16). 

Bond cleavages that occur as a result of thermal excitation of all vibrational modes of the polymer lead to the formation of macroradicals that undergo secondary reactions via intra- or intermolecular mechanisms. Investigations of polymer decomposition mechanisms using Py-MS have been widely used to characterize the polymer microstructure and the accumulated knowledge applied to unknown polymers and to investigations of polymer decomposition products. 

Nonetheless, a dioxetan decomposition mechanism for lucigenin chemiluminescence, based on the exergonic processes described in Chap. V, seems well established [3]. A direct demonstration of the intermediacy of this dioxetane was first made [4] in 1969 by treating 10,10 -dimethyl-9,9 -biacrylidene (4) with singlet oxygen from several sources. Emission from N-methyl acridone was unequivocally shown. The lifetime of the intermediate was characteristic of the supposed dioxetane. Intramolecular electron transfer has been suggested as the excitation mechanism in the decomposition of this and other electron-rich dioxetans. 

In principle, emission spectroscopy can be applied to both atoms and molecules. Molecular infrared emission, or blackbody radiation played an important role in the early development of quantum mechanics and has been used for the analysis of hot gases generated by flames and rocket exhausts. Although the availability of FT-IR instrumentation extended the application of IR emission spectroscopy to a wider array of samples, its applications remain limited. For this reason IR emission is not considered further in this text. Molecular UV/Vis emission spectroscopy is of little importance since the thermal energies needed for excitation generally result in the sample s 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). 

Semiempirical (PM3) and ab initio (6-3IG basis set) calculations are in agreement with the hypothesis described in Section I (99MI233 OOOJOC2494). In the case of the sensitized reaction, when the excited triplet state is populated, only the formation of the radical intermediate is allowed. This intermediate can evolve to the corresponding cyclopropenyl derivative or to the decomposition products. In a previously reported mechanism the decomposition products resulted from the excited cyclopropenyl derivative. In our hypothesis the formation of both the decomposition products and the cyclopropenyl derivatives can be considered as competitive reactions. 

Fig. 1.12 Mechanism of the bioluminescence reaction of firefly luciferin catalyzed by firefly luciferase. Luciferin is probably in the dianion form when bound to luciferase. Luciferase-bound luciferin is converted into an adenylate in the presence of ATP and Mg2+, splitting off pyrophosphate (PP). The adenylate is oxygenated in the presence of oxygen (air) forming a peroxide intermediate A, which forms a dioxetanone intermediate B by splitting off AMP. The decomposition of intermediate B produces the excited state of oxyluciferin monoanion (Cl) or dianion (C2). When the energy levels of the excited states fall to the ground states, Cl and C2 emit red light (Amax 615 nm) and yellow-green light (Amax 560 nm), respectively.

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