1.2- Dioxetanes excitation parameters

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 namre 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 mechanism . Specifically, bofh fhermal sfabil-ity and singlel and friplef quanfum yields in fhe series of mefhyl-subsliluled 1,2-dioxelanes, including fhe parenf 1,2-dioxefane" , could be readily rationalized on the basis of the merged mechanism and qualitative quanmm mechanics considerations . 

TABLE 4. ACTIVATION PARAMETERS AND EXCITATION PARAMETERS OF SOME SELECTED DIOXETANES... 

As already pointed out on several occasions, the unique property of dioxetanes is to generate electronically excited states on thermolysis, which then manifest themselves by light emission (Eq. 28). The total yield of excited states (Eq. 33), that is, the sum of the singlet excitation yield (0 ), triplet excitation yield (0 ), and the spin-state selectivity (Eq. 34), that is, the ratio of the triplet and singlet excitation yields, are excitation parameters that characterize a particular dioxetane. 

V. HEAVY ATOM SUBSTITUTION. The series of bromine-substituted 1,2-dioxetanes, shown in Eq. 65, were studied to assess whether such a heavy atom influences the excitation parameters through its spin-orbit coupling property. Qearly, the effect is dramatic on both the total efficiency and the spin-... 

For isolated HEI such as dioxetanes and other cyclic and linear peroxides that act directly as reagents in the excitation step, kinetic studies lead to rate constants and activation parameters for this excitation step and conclusions with respect to the mechanism of chemiexcitation can be obtained from the structural and conditional dependence of these parameters. For complex CL systems, in which the HEI is formed in rate-limiting steps prior to the excitation step, the kinetic parameters of this essential reaction step can only be obtained indirectly (see below). 

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... 

Although the activation parameters obtained from the thermal decomposition of a great number of diverse dioxetane derivatives have been interpreted on the basis of the biradical mechanism, no general interpretation of the excitation efficiencies has been given on fhe basis of fhis mechanism Furfhermore, most theoretical... 

Adam, W, Griesbeck, A.G., GoUnick, K., and Knutzen-Mies, K., 1,2-Dioxetanes derived from 4,5-dimethyl-2,3-dihydrofuran synthesis via photooxygenation, activation parameters and excitation properties, /. Org. Chem., 53,1492, 1988. 

More general orbital symmetry requirements might be expected to affect the probability of entering an excited state. A good example [45] is that of the formation of excited SO2 and NO2 discussed below. The relative ease with which the kinetic parameters may be obtained for such simple reactions allows quantitative confirmation of the importance of this effect. The other reaction for which such factors have been discussed is that of the dioxetans. It is more difficult to produce convincing evidence of the operation of symmetry rules in this case, although many calculations have been performed in pursuit of the idea [37-41]. A more complete discussion appears in the section dealing with the dioxetans (Chap. V). 

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