Quantum yields luminol oxidation

In chemiluminescence immunoassay the antigen is tagged with a molecule such as luminol or an acridinium ester which emits light with a high quantum yield on oxidation. Alternatively, the antigen may be labelled with a bio-luminescent molecule such as luciferin, which emits light when oxidized by the enzyme luciferase. 

The overall reaction scheme of the luminol chemiluminescence in an aqueous medium is shown in Figure 1. The luminol oxidation leads to the formation of an aminophthalate ion in an excited state, which then emits light on return to the ground state. The quantum yield of the reaction is low ( 0.01) compared with bioluminescence reactions and the emission spectrum shows a maximum1 at 425 nm. 

As can be seen from Table 2, certain phenanthrene derivatives produce more chemiluminescence than luminol. The diphenyl derivative 33 diluminyl evidently has its benzene rings in a non-coplanar position this is apparent from the fact that this compound does not behave as a benzidine derivative under the oxidative conditions of chemiluminescence reactions. The chemiluminescence quantum yield is only about one third that of luminol 100>. 

Table 5. Luminol chemiluminescence quantum yields in different solvents with some oxidative systems (after J. Lee and H. H. Seliger 115>)...

The photosensitized results are from I.B.C. Matheson and J. Lee 118h It is seen that the quantum yields in photosensitized oxidation depend on the concentrations of luminol and base, and on temperature. At higher temperature (50°) and low luminol concentrations, the quantum yields reached those of hemin-catalyzed hydrogen peroxide oxidation of luminol in aqueous-alkaline solution. Primary products of the photosensitized oxidation are singlet oxygen (1Ag02) or a photoperoxide derived from methylene blue, but neither of these is directly responsible for the luminol chemiluminescence. 

Luminol amidine 132, synthesized from luminol and the Vilsmeier reagent from DMF and thionyl chloride, has been proposed as a suitable luminol derivative for analytical purposes because, unlike luminol, it can be easily purified by recrystallization from water. 132 exhibits a chemiluminescence quantum yield of about 20% of luminol in ferricyanide-catalyzed oxidation by aqueous alkaline hydrogen peroxide Amax of the emission is 452 nm 196>. 

The oxidation of luminol in basic solution is one of the best known and most efficient chemiluminescent reactions, having a quantum yield of CL of about 0.01 in water and 0.05 in DMSO. 

Scheme 28) Quantum yields of lucigenin oxidation by hydrogen peroxide in alkaline media are comparable with the values obtained in luminol oxidation (1.24 x 10 E mol ) ° . However, the use of other peroxides, such as tcrt-butyl hydroperoxide, results in a decrease of chemiluminescence quantum yields of two orders of magnitude, confuming the hypothesis that a 1,2-dioxetane is the HEI, since its formation would be impossible with alkyl peroxides . 

TABLE 3. Luminol chemiluminescence quantum yields in different solvents with some oxidative... 

Some chemical reactions give rise to light emission. A well-known example is that of luminol, used for non-electric emergency lighting or the manufacture of luminous colour wands. The reaction that causes emission is an oxidation that takes place in the presence of a catalyst. Its quantum yield is close to 1. 

Chemiluminescence generally arises from chemical reactions in solution in which an oxidation occurs that involves either molecular oxygen or hydrogen peroxide (H2O2). The quantum yield in solution is generally very low because certain derivatives inhibit luminescence, notably oxygen. There are some exceptions such as luminol, acridinium salts, and some oxalic esters. Although the mechanism is poorly understood, the reaction of luminol (5-amino-2,3-dihydro-l,4-phthalazinedione) can be written schematically ... 

The value of electron donation in any oxidative chemiluminescence is seen in the chemiluminescence of (12) under autoxidative conditions. The quantum yield is higher than well known bright compounds such as luminol, 0 being 4.6% ... 

That the donor and the fluorescing part of the molecules in (19) and (22) actually are separated can be demonstrated. As mentioned above unsubstituted phthalic acid hydrazide does not chemiluminesce, neither does 4-methyl phthalic hydrazide [36]. A mixture of phthalic acid hydrazide and N-methyl acridone gives no light on oxidation. The conjugate (19) chemiluminesces in the aprotic system with 8% of the luminol quantum yield (ca. 1 x 10" ), the emission matching... 

In (25) there is a true fluorescer moiety in the cyclophane system, and it represents a donor-acceptor complex system, whereas (24) and (23) very probably are forming exciplexes on oxidation [41]. The higher efficiency of this paracyclophane energy transfer in comparison with the methylene-linked energizer and fluorescer as in (22) is seen from the fact that in (22) the DPA-residue, having a fluorescence quantum yield of nearly unity exhibits a chemiluminescence efficiency of 26% of that of luminol whereas in (25) with the 1,4-dimethyl anthracene fluorescer (0jn ca. 0.30) a light yield of 100% luminol [41] is obtained. 

In aqueous systems using other oxidants, such as sodium hypochlorite, potassium ferricyanide or potassium persulfate the chemiluminescence quantum yields are reduced to 0.003-0.007, probably the result of concurrent dark reactions. A distinct influence of temperature and luminol concentration is observed. It should be noted that the concentration range concerned is 1 x 10 M and below, i.e. the luminol concentration range where self-quenching does not occur [48 a]. 

A very high chemiluminescence quantum yield (about eight times that of luminol) has been reported for the oxidation of N-methyl-9-benzyl acridinium chloride with potassium persulfate as catalyst [17]. Although no mechanism has been determined the reaction is reminiscent of the oxidation of 9-methyl acridine [15] in which the carbanion reacts with oxygen to produce the dioxetan which presumably decomposes in the accepted fashion for such compounds ... 

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