Chemiluminescence peroxide decomposition

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

A number of chemiluminescent reactions may proceed through unstable dioxetane intermediates (12,43). For example, the classical chemiluminescent reactions of lophine [484-47-9] (18), lucigenin [2315-97-7] (20), and transannular peroxide decomposition. Classical chemiluminescence from lophine (18), where R = CgH, is derived from its reaction with oxygen in aqueous alkaline dimethyl sulfoxide or by reaction with hydrogen peroxide and a cooxidant such as sodium hypochlorite or potassium ferricyanide (44). The hydroperoxide (19) has been isolated and independentiy emits light in basic ethanol (45). 

Decomposition of diphenoylperoxide [6109-04-2] (40) in the presence of a fluorescer such as perylene in methylene chloride at 24°C produces chemiluminescence matching the fluorescence spectmm of the fluorescer with perylene was reported to be 10 5% (135). The reaction follows pseudo-first-order kinetics with the observed rate constant increasing with fluorescer concentration according to = k [flr]. Thus the fluorescer acts as a catalyst for peroxide decomposition, with catalytic decomposition competing with spontaneous thermal decomposition. An electron-transfer mechanism has been proposed (135). 

Special review articles published since 1968 on these topics are one by E. H. White and D. F. Roswell 2> on hydrazide chemiluminescence M. M. Rauhut 3) on the chemiluminescence of concerted peroxide-decomposition reactions and D. M. Hercules 4 5> on chemiluminescence from electron-transfer reactions. The rapid development in these special fields justifies a further attempt to depict the current status. Results of bioluminescence research will not be included in this article except for a few special cases, e.g. enzyme-catalyzed chemiluminescence of luminol, and firefly bioluminescence 6>. 

Intramolecular electron transfer initiated peroxide decomposition. . 1236 HIGH-EFFICIENCY ORGANIC CHEMILUMINESCENT REACTIONS INVOLVING PEROXIDE INTERMEDIATES. 1238... 

Finally, in activated chemiluminescence, an added compound also leads to an enhancement of the emission intensity however, in contrast with the indirect CL, this compound, now called activator (ACT), is directly involved in the excitation process and not just excited by an energy transfer process from a formerly generated excited product (Scheme 5). Activated CL should be considered in two distinct cases. In the first case, it involves the reaction of an isolated HEI, such as 1,2-dioxetanone (2), and the occurrence of a direct interaction of the ACT with this peroxide can be deduced from the kinetics of the transformation. The observed rate constant (kobs) in peroxide decomposition is expected to increase in the presence of the ACT and a hnear dependence of kobs on the ACT concentration is observed experimentally. The rate constant for the interaction of ACT with peroxide ( 2) is obtained from the inclination of the linear correlation between obs and the ACT concentration and the intercept gives the rate constant for the unimolec-ular decomposition ( 1) of this peroxide (Scheme 5). The emission observed in every case is the fluorescence of the singlet excited ACT" ° . ... 

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

Singlet-oxygen studies have being reported to such diverse areas as chemiluminescence [14], photocarcinogenity [15], ozonolysis [16], photodynamic action [17], peroxide decomposition [7], photosynthesis [14], air pollution [18], metallocatalyzed oxygenation reactions [19,20], synthetic applications [21], and polymer degradation [10]. 

Rauhut, M. M., Chemiluminescence from Concerted Peroxide Decomposition Reactions, Acc. Chem. Res. 1969, 2, 80 87. 

The strongly exothermic transfer of electrons between fluorescent organic molecules represents one of the most general mechanisms in chemiluminescence [50, 51]. It can be found in electroluminescence, radical ion annihilation and peroxide decomposition. The basic concept was introduced by Hercules [39] following the general theory of Marcus [52]. The reaction co-ordinate can be roughly indicated by the potential energy curves shown. 

The mechanism of the chemiluminescent decomposition of secondary peroxy-esters shows similar features [7], in that the loss of the secondary proton allows electron transfer to the activator radical cation. Activators are fluorescent compounds of low ionisation potential. In the example shown below the activator is not consumed, and the products, acetophenone and acetic acid, are formed quantitatively. The unactivated decomposition is very closely related to peroxide decomposition generally, and it is not surprising to find that the quantum yield is very low. 

In the search for highly exergonic reactions, which might be chemiluminescent, the decomposition of cis-1,2-dihydro phthaloyl peroxide (24) was considered. 

Interestingly, the most efficient chemiluminescent reaction so far observed during acene endo-peroxide decomposition has been exhibited by the 1,4-endo-peroxide (41) [38]. However, this very efficient chemiluminescence evidently requires treatment of (41) with acids, and it has been shown [45] that it is not the direct decomposition of the endo-peroxide to yield (43) that produces most of the excitation energy, but the acid catalyzed cleavage of (41) to yield the compounds (44) and (45). The reaction itself had been previously described [46-47]. 

Later, fireflv oxyluciferin was successfully synthesi2ed (403. 408) and has been isolated and identified in firefly lanterns (luciola cruaciata) after the lanterns were treated with pyridine and acetic anhydride to prevent decomposition (409). In 1972, Suzuki and Goto firmly established that oxyluciferin is involved in the bioluminescence of firefly lanterns and in the chemiluminescence of firefly luciferin (403. 410).. A. mechanism involving a four-membered ring cyclic peroxide has been proposed for the reaction (406. 411). However, it was not confirmed by 0 -labelinE experiments (412). 

In many chemiluminescent reactions of peroxides, two carbonyl groups are formed simultaneously by decomposition of an intermediate such as compound (1) ... 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Most likely singlet oxygen is also responsible for the red chemiluminescence observed in the reaction of pyrogaHol with formaldehyde and hydrogen peroxide in aqueous alkaU (152). It is also involved in chemiluminescence from the decomposition of secondary dialkyl peroxides and hydroperoxides (153), although triplet carbonyl products appear to be the emitting species (132). 

---