Simple Dioxetans

The singlet oxygen + olefin route is particularly effective when the double bond has electron releasing substituents. 

Very many dioxetans, particularly simple ones like tetramethyl-dioxetan are light yellow in colour owing to a long absorption tail which extends into the visible region of the spectrum. The properties of dioxetans have been reviewed, such reviews [2, 6, 11] being particularly useful for those interested in the physical aspects of the decomposition reaction. 

---

Dloxetanes. Simple dioxetanes (3) decompose thermally near or below room temperature to generate excited states of carbonyl products... 

As mentioned in Section II. C., the concerted bond cleavage of 1.2-dioxetane derivatives has been proposed to be of general importance in respect of the excitation step of a large number of chemiluminescence reactions. The first experimental results concerning simple dioxetanes were obtained by M. M. Rauhut and coworkers in their work on activated oxalic ester chemiluminescence 24>. From experimental data on the reaction of e.g. bis (2.4-dinitrophenyl)oxalate with hydrogen peroxide in the presence of rubrene, they concluded that 1.2-dioxetanedione... 

In sum, then, a good deal of experimental evidence has been gathered which supports, although indirectly, the intermediacy of a 1,4-biradical in the chemiluminescent reaction of simple dioxetanes. Yet there is no direct evidence that such biradicals exist with finite lifetimes. An attempted independent generation of a 1,4-biradical by decomposition of a dinitrite proved inconclusive (Suzuki, 1979). The influence of quenchers, radical scavengers, and external heavy atoms on the chemiluminescent reaction of trimethyldioxe-tane (Simo and Stauff, 1975) and adamantylideneadamantane-l,2-dioxetane [8] (Neidl and Stauff, 1978) was studied. While the authors interpret their results in terms of a relatively long-lived precursor to the excited-state product, namely the 1,4-biradical, the results are open to alternative explanations (Horn etal., 1978-79). 

Dioxetane substrates for alkaline phosphatase and p-galac-tosidase The four-membered ring peroxides, 1,2-dioxetanes, are too unstable to be used as chemiluminescent substrates due to their low energy of activation (Adam and Cilento, 1983). Substitution of the carbonyls in the dioxetane ring has substantial effects on the rate of decomposition (Wieringa et al., 1972), i.e., half-lives from less than a second for simple dioxetanes to over 21 years for some of the... 

High yields of excitation without strong visible emission are possible if 0p is very low, as is the case for simple dioxetans. Phosphorescence is difficult to observe under the conditions of most of the reactions, but in principle 0phos can replace 0p. 

Although the idea of a concerted decomposition initiated work in the area, as the number of simple dioxetans increased and methods for titrating singlets and triplets appeared, it became obvious that triplet states predominated overwhelmingly in the products. A concerted route with no opportunity for spin inversion should of course produce only singlets as the primary product. It is still not clear. 

These seemingly easily distinguished mechanisms are considered together as an alternative to the diradical mechanism for reasons which will become apparent. Electron transfer is particularly important in the more electron deficient, cyclic per-ester dioxetanones. Dioxetans in general do not respond to activators -fluorescent compounds of low ionisation potential (see p. 40) almost certainly because they are poorer oxidants than dioxetanones. There is however a hint that simple dioxetans, if sufficiently strained may accept electrons from an activator [36]. The light emitted from the cyclic compound (S) shows a linear dependence on DP A concentration, and the ionisation potential of several fluorescent compounds. However, true electron exchange luminescence cannot be... 

The evidence presented for almost total 0-0 cleavage before the C-C bond breaks seems incontrovertible. If a concerted mechanism is to be considered, it can probably only be done in the context of a very unsymmetrical transition state. The lifetime of the diradical is certainly extremely short, and whether it has a discrete existence in all cases of simple dioxetan cleavage may never be established. Experiments in the gas phase intensify this difficulty. Absorption of infra-... 

It has already been mentioned that alkyl substituted dioxetans do not normally show exhanced rates of decomposition on intermolecular collision with low ionisation potential fluorescers. The geometric arrangement (particularly orbital overlap) of dioxetan and would-be electron donor in the compounds just discussed is less than ideal. They are in fact simple dioxetans with an ill-disposed electron donor. It is not clear then why they should show electron transfer from these donors (which are sometimes, e.g. in (12), considered unlikely activators in CIEEL) when they would certainly not react that way m ermolecularly. 

The rare earth chelate Eu(fod)3, does not catalyse the decomposition of simple dioxetans but acts as an excellent acceptor of the energy from the triplet carbonyl product. However it is an effective catalyst for the electron rich dioxetan (11), and the complex formed emits the characteristic bright red light of the europium chelate [48]. 

Although activated luminescence has not yet been observed with simple dioxetans, amines and olefins with low ionisation potentials are powerful catalysts for the dark decomposition of certain dioxetans [49] (e. g. (16) and (17)) but tetramethyldioxetan (perhaps because it has no electronegative substituents) is apparently unaffected by amines [6]. 

---