Efficiency of chemiluminescence

Recent investigations (see e.g. 12>13>) therefore make a special point of differentiating as far as possible between the chemical and physical efficiencies of chemiluminescence reactions. 

R. Bezman and L. R. Faulkner 189> developed methods for defining a concise set of parameters which quantitatively describe the efficiencies of chemiluminescent electron-transfer reactions (see Section VIII. A.) by means of analysis of chemiluminescence decay curves. 

RELATIONSHIP BETWEEN HEAT OF REACTION AND CHEMILUMINESCENCE EFFICIENCY OF CHEMILUMINESCENT REACTIONS... 

Meta-para selection rules for chemiluminescence. Accumulated experimental results for efficiency of chemiluminescence have clearly demonstrated that meta-substituted aromatic rings emit efficiently like luciferin, but para-substituted isomers do not exhibit such a remarkable property. Meta-para selection rules for efficiency of chemiluminescence reactions have been explained by the orbital interaction modes LUMO-LUMO interaction for meta isomers and the LUMO-HOMO interaction for para isomers.5 6 The selection rules were also confirmed by natural orbital analysis of the BS solutions. Thus it is concluded that the BS and SA MO theoretical calculations are very useful for theoretical illustration of the electronic mechanisms of chemi- and bio-luminescence reactions. 

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

Several new oxalates have been developed for use ia analytical appHcations. Bis(2,6-difluorophenyl) oxalate (72) and bis(4-nitro-2-(3,6,9-trioxadecylcarbonyl)phenyl) oxalate (97) have been used ia flow iajection and high performance Hquid chromatography (hplc) as activators for chemiluminescence detectors. These oxalates are generally more stable and show better water solubiUty ia mixed aqueous solvents yet retain the higher efficiencies ( ) of the traditional oxalates employed for chemiluminescence. 

To our surprise and satisfaction, the general approach worked the CBI derivatives did chemiluminescence, and the sensitivity enhancement was 30- to 50-fold over fluorescence With this success, we embarked on a more thorough study of chemiluminescence with the goal of optimizing the method. Identifiable parameters that affected the efficiency of light emission from a chemically generated fluorescent molecule included ... 

To understand how these parameters affected the efficiency of the chemiluminescent reaction, we examined the mechanism originally proposed by Rauhut (26). As shown in Scheme 2, hydrogen peroxide reacts with an oxalate ester, such as 2,4,6-trichlorophenyl oxalate (TCPO), in a two-step process to form a reactive intermediate for which Rauhut suggested structure 1, the 1,2-dioxetanedione. The dioxetanedione then interacts with an acceptor (ACC) to produce two molecules of COj and the excited state of the acceptor. The last stage of the sequence is fluorescence emission from the acceptor. 

We have also investigated other oxalate esters as a potential means to improve the efficiency. The most commonly used oxalates are the 2,4,6-trichlorophenyl (TCPO) and 2,4-dinitrophenyl (DNPO) oxalates. Both have severe drawbacks namely, their low solubility in aqueous and mixed aqueous solvents and quenching of the acceptor fluorescence. To achieve better solubility and avoid the quenching features of the esters and their phenolic products, we turned to difluorophenyl oxalate (DFPO) derivatives 5 and 6 (Figure 14). Both the 2,4- and the 2,6-difluoro esters were readily synthesized and were shown to be active precursors to DPA chemiluminescence. In fact, the overall efficiency of the 2,6-difluorophenyl oxalate 5 is higher than for TCPO in the chemical excitation of DPA under the conditions outlined earlier. Several other symmetrical and unsymmet-rical esters were also synthesized, but all were less efficient than either TCPO or 2,6-DFPO (Figure 14). 

Finally, we have also attempted to improve on the fluorescence and chemiluminescence efficiencies of the labeling reagent, the naphthalene dialdehyde. Derivatives 7, 8, and 9 (Table IX) have been synthesized and studied. As shown in Table IX, none of these new derivatives presents an advantage over the parent unsubstituted CBI-Ala. 

Cx electronically excited product) depends on the efficiency es of the production of excited product molecules, and on the efficiency of the excited product molecules (or other molecules present in the reaction mixture) in transforming excitation energy into light. In most of the chemiluminescence reactions investigated so far this efficiency is identical with the fluorescence efficiency of the molecules concerned, so that... 

Brundrett, Roswell, and White 12> subdivide the efficiency es, the chemical efficiency of a chemiluminescent reaction, into the efficiency r (fraction of molecules following the correct chemistry) and the efficiency es (fraction of molecules crossing over to the excited state after having taken the correct chemical path). 

Therefore a low chemiluminescence quantum yield can be due to the fact that the fluorescence efficiency of the product molecules is high but the chemical efficiency of the reaction producing excited molecules is low, or the reverse, or that ifies and n are both low. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

Hydrazide chemiluminescence has been investigated very intensively during recent years (for reviews, see 1>, p. 63, 2>, 90>). Main topics in this field are synthesis of highly chemiluminescent cyclic diacyl hydrazides derived from aromatic hydrocarbons, relations between chemiluminescence quantum yield and fluorescence efficiency of the dicarboxylates produced in the reaction, studies concerning the mechanism of luminol type chemiluminescence, and energy-transfer problems. 

Another rather striking example demonstrates that the fluorescence efficiency of the respective dicarboxylates is not the most important factor in determining the chemiluminescence efficiencies of the hydrazides 9.10-diphenylanthracene-2.3-dicarbonic acid 25 has a fluorescence efficiency of about 0.9 (as has the parent compound 9.10-di-phenylanthracene) 94>. The corresponding hydrazide 26, however, gives a quantum yield of 48% that of luminol only (in DMSO/t-BuOK/ O2) 95) although 3-aminophtalic acid has a fluorescence efficiency of about 0.3 only. 

Table 1. Chemiluminescence and fluorescence efficiencies of some substituted phthalic hydrazides (after Brundrett, Roswell, and White 12>)...

This is not due to the relatively extended aromatic system in 25, for C. C. Wei and E. H. White 96> recently succeeded in synthesizing the benzoperylene compound 27 which is the most efficient hydrazide yet known with a chemiluminescence quantum yield of 7.3 % (in DMSO/ t-BuOK/02). The corresponding dicarboxylate has a fluorescence efficiency of 14% and emits at 420 and 450 nm which matches the chemiluminescence emission of 27 9 ). 

Hetero-excimer chemiluminescence yields were measured by A. Weller and K. Zachariasse 214) the system dimethylanthracene anion radical/tri-p-tolylaminium perchlorate in tetrahydrofurane exhibits particularly strong chemiluminescence with quantum yields of about 7.5 x 10-2 215>. A. J. Bard and coworkers 216> very thoroughly investigated the influence of several parameters, e.g. supporting electrolyte concentration, on the efficiency of electrogenerated chemiluminescence. 

If cl is the efficiency of the chemiluminescent reaction, which is the ratio of the number of photons emitted to the number of molecules of reactant reacting in toto, it can be defined for a type I reaction as... 

Lophine emits yellow CL upon oxidation by molecular oxygen in alkaline solution. The oxidation is believed to produce a free radical [3], which is further oxidized to a hydroperoxide, which is the light-emitting species [4-6], A number of chemiluminescent derivatives of lophine have been synthesized and have been shown to exhibit varying efficiencies of CL. Lophine has been used in the analysis of metal ions such as Co2+ that catalyze the chemiluminescent reaction between it and hydrogen peroxide [7], It has also been used as a chemiluminescent indicator in titrimetry [8],... 

The leaving group of the oxalic ester has a strong effect on the efficiency of the peroxyoxalate chemiluminescent system. The electron-attracting power of the substituents on the phenyl rings of the substituted diphenyl oxalates is important to the overall efficiency of the chemiluminescent reactions. Steric effects... 

The structure of the fluorescent molecule can also contribute substantially to the overall efficiency of a chemiluminescent process. Excitation and fluorescence can be strongly influenced by the structure of the fluorescer. 

HTAC cationic micelles also markedly enhance the CL intensity of fluorescein (FL) in the oxidation of hydrogen peroxide catalyzed by horseradish peroxidase (HRP) [39], However, no CL enhancement was observed when anionic micelles of sodium dodecyl sulphate (SDS) or nonionic micelles of polyoxyethylene (23) dodecanol (Brij-35) were used (Fig. 9). CL enhancement is attributed to the electrostatic interaction of the anionic fluorescein with the HTAC micelles. The local concentration of fluorescein on the surface of the micelle increases the efficiency of the energy transferred from the singlet oxygen (which is produced in the peroxidation catalyzed by the HRP) to fluorescein. This chemiluminescent enhancement was applied to the determination of traces of hydrogen peroxide. The detection limit was three times smaller than that obtained in aqueous solution. 

The analytical detectability applying a CL method should, in principle, be comparable to that obtained using radioactive labels, without all the disadvantages related to the use of isotopic labeling. In fact, assuming reasonable values for the quantum efficiency of the chemiluminescent reaction (Cl 0.01), for the overall photon collection efficiency of the optical system-CCD camera assembly (T) 0.01%), and for the intensity of the lowest detectable CL signal (about... 

Luciferin is one of the simplest examples of chemiluminescence, and it is remarkably efficient. The overall yield is one photon per molecule of luciferin. 

The high efficiency of the Mg/NaNOs system at producing light has been ascribed to a chemiluminescence interaction between excited Mg atoms and ground state Na atoms. It is also possible that the surplus Mg burns in air increasing the total available energy. 

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