Dioxetanes, chemiexcitation

Besides the impressive difference in the chemiexcitation efficiency, also the fluorescence yield of the meta-pattemed emitter m-17 is by more than an order of magnitude ( ) higher than that of the para regioisomer p-17 . Evidently, crossed-conjugated emitters are advantageous for the design of efficient intramolecular CIEEL systems. In Sections V.A-V.C we shall consider additional internal (substrate structural effects) and external (medium influence) factors, which play an essential role in the development of efficient dioxetane-based analytical probes. 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

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). 

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. 

Like other peroxides, also dioxetanes are sensitive to the presence of metal ions and their complexes, which catalyze the decomposition of the dioxetane molecule. In most cases, this decomposition is dark, i.e. no chemiluminesce is generated in such a catalytic cleavage42. An informative exception, for instance, constitutes the chemiluminescent decomposition of the dioxetane 19 in Scheme 13, initiated by the ruthenium complex Ru(bipy)3Cl243. It has been shown that this chemiexcitation derives from the valence change of the ruthenium ion in the process Ru3+ I e — Ru2+, for which the efficiency of the excited-state generation may be as much as 40%44. Hence, when the radical anion of the carbonyl cleavage fragment from the dioxetane and the Ru3+ ion are formed in... 

The 1,2-dioxetane postulated as intermediate was never isolated219. However, indirect evidence of a 1,2-dioxetane as a reaction intermediate was obtained by chemiluminescence resulting from the reaction of 10,10 -d i methyl-9,9 -biacridene (40) with singlet oxygen219-220 (equation 11). Several sources of singlet oxygen were used and, in each case, the reaction resulted in chemiexcitation of /V-methylacridane. 

For example, 3-pentyl- and 3-neopentyl-l,2-dioxetane undergo thermolysis in xylene at 60 °C with first-order rate constants (kx) of 4.6 and 9.2 x 10 4 s, respectively (Scheme 4) <2001HAC459>. Chemiexcitation yields were in the order of 0.02 (0T) and <0.0005 (0s) for both derivatives. 

The first area of achievement was the discovery and subsequent intensive investigation of the chemiluminescent reaction of 1,2-dioxetanes. The discovery of this reaction, a simple unimolecular rearrangement, has allowed experimentalists to focus on the study of the key step of chemiexcitation. Previously studied chemiluminescent reactions often involved complicated reaction systems and sequences, required several reagents, and often afforded multiple products via transient intermediates. While some such systems have yielded to intensive investigation, the most revealing probes of chemiexcitation have been studies of simple unimolecular rearrangements such as that of the 1,2-dioxetanes. 

The recent studies of 1,2-dioxetane chemiluminescence, ECL, and CIEEL have brought significant advances to the field of chemiluminescence. The relatively simple nature of these processes has allowed attention to be focused on the nature of chemiexcitation and much has been learned. In addition, these relatively simple systems subsequently have been proposed as key intermediates, key steps or key sequences in many more complicated chemi- and bioluminescent systems. 

Dioxetanes have been the sole subject of several specialized reviews in recent years (Bartlett and Landis, 1979 Horn et al., 1978-79 Adam, 1977 T. Wilson, 1976 Turro et al., 1974a Mumford, 1975). These articles cover with depth which is not possible here such topics as (1) preparation, (2) physical and spectroscopic characterization, (3) experimental techniques, especially for the study of chemiluminescence, (4) mechanisms of decomposition and chemiexcitation, (5) ground state transformations, and (6) reactions involving dioxetanes as postulated intermediates. The interested reader is referred to these articles for details on these specialized topics, and for some interesting historical perspectives. 

In this treatment we shall limit ourselves to a discussion of work which deals directly with the mechanism of the decomposition of 1,2-dioxetanes and with the mechanism of chemiexcitation, paying particular attention to the efficiency and selectivity of this process. Emphasis will be placed on the most recent results and developments in this area. 

The data displayed in Fig. 1 support the yh/Z-electron-transfer mechanism. To understand the process of the excited-state generation, the nature of the CIEEL emitter and the chemiexcitation mechanism need to be established. Fig. 2a displays the CIEEL spectra of the monocyclic (/n-la) and bicyclic (w-lb) dioxetanes and the fluorescence spectra of the meta-oxybenzoate ions derived from the electron-... 

Adam W, Trofimov AV. The effect of meta versus para substitution on the efficiency of chemiexcitation in the chemically triggered electron-transfer-initiated decomposition of spiroadamantyl dioxetanes. J Org Chem 2000 65 6474-8. 

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