Chemiluminescence excitation yields determination

Most of these excitation yields have been determined by energy-transfer chemiluminescence using 9,10-diphenylanthracene (JJPA) and 9,10-dibromo-anthracene DBA) as fluorescers. 

The direct chemiluminescence quantum yield is given by Eq. 35, where is the singlet excitation quantum yield and 0 is the fluorescence quantum yield of the singlet excited carbonyl product. The latter is directly responsible for the observed chemiluminescence. If 0 is known from photoluminescence work, determination of 0° allows us to calculate the desired 0 -parameter. Frequently 0 is not known and it is necessary to measure it, using routine fluorescence techniques. 

Once the standardized and calibrated direct chemiluminescence quantum yield (0 ) has been acquired experimentally, the singlet excitation yield (0 ) can be calculated for the chemienergized process from Eq. 35. However, as already stated, this requires that the fluorescence quantum yield (0 ) be known under the same experimental conditions at which 0 was determined. This is not always the case... 

Thus, the greater the numbers or rates of processes competing with fluorescence for deactivation of the lowest excited singlet state, the lower the value of <()/. The quantum yield of fluorescence is important in determining how intense chemiluminescence can be for a particular reaction. 

The main features of the chemiluminescence mechanism are exemplarily illustrated in Scheme 11 for the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) as base catalyst and the chemiluminescent activators (ACT) anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene and rubrene. In this mechanism, the replacement of the phenolic substituents in TCPO by IMI-H constitutes the slow step, whereas the nucleophilic attack of hydrogen peroxide on the intermediary l,l -oxalyl diimidazole (ODI) is fast. This rate difference is manifested by a two-exponential behavior of the chemiluminescence kinetics. The observed dependence of the chemiexcitation yield on the electrochemical characteristics of the activator has been rationalized in terms of the intermolecular CIEEL mechanism (Scheme 12), in which the free-energy balance for the electron back-transfer (BET) determines whether the singlet-excited activator, the species responsible for the light emission, is formed ... 

An indirect method has been used to determine relative rate constants for the excitation step in peroxyoxalate CL from the imidazole (IM-H)-catalyzed reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide in the presence of various ACTs18. In this case, the HEI is formed in slow reaction steps and its interaction with the ACT is not observed kinetically. However, application of the steady-state approximation to the reduced kinetic scheme for this transformation (Scheme 6) leads to a linear relationship of 1/S vs. 1/[ACT] (equation 5) and to the ratio of the chemiluminescence parameters /ic vrAi), which is a direct measure of the rate constant of the excitation step. Therefore, this method allows for the determination of relative rate constants for the excitation step in a complex reaction system, where this step cannot be observed directly by kinetic measurements18. The singlet quantum yield at infinite activator concentrations ( °), where all high-energy intermediates formed interact with the activator, is also obtained from this relationship (equation 5). 

Several other chromophores have been used in the development of sensors based upon ECL. For example, the luminol reaction is a conventional chemi-luminence reaction that has been studied in detail and it is believed that the mechanism of the ECL reaction is similar, if not identical, to that of the chemiluminescence. As shown in Fig. 2, the luminol ion undergoes a one-electron oxidation to yield a diazaquinone, which then reacts with peroxide or superoxide ( OOH) to give the excited 3-aminophthalate which has an emission maximum of 425 nm. This reaction is particularly versatile and has been utilized in a variety of ECL assays, many of which have been previously summarized by Knight [1], The luminol ECL reaction can be used for the determination of any species labeled with luminol derivatives, hydrogen peroxide, and other peroxides or enzymatic reactions that produce peroxides. A couple of examples are described later. 

The most recent threshold determination for the radical process (I) utilizes the.Meinel band chemiluminescence of the O(- P) + HCO - OH + CO reaction (20). The basic photochemical questions have been and remain the determination of the true precursors for the two product channels and the energy (or excitation wavelength)-dependent quantum yields for each channel. 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

Considerable interest is being shown in the chemiluminescent reactions of barium. Much of this work is directed towards measurements of photon yields and determination of rotational—vibrational excitation in the various states. The reaction between barium and nitrous oxide has evoked most interest [374—378]. In the chemiluminescent reactions between barium and nitrosyl chloride [377, 379], nitrogen dioxide [374], nitric oxide[374], oxygen [374] and ozone [374], electronically excited BaO is produced, except with nitrosyl chloride which produces BaCl. 

D24.5 Infrared chemiluminescence. Chemical reactions may yield products in excited states. The emission of radiation as the molecules decay to lower energy states is called chemiluminescence. If the emission is from vibrationally excited states, then it is infrared chemiluminescence. The vibrationally excited product molecule in the example of Figure 24.13 in the text is CO. By studying the intensities of the infrared emission spectrum, the populations of the vibrational states in the product CO may be determined and this information allows us to determine the relative rates of formation of CO in these excited states. 

Not all of the factors responsible for chemiluminescence will respond in the same way to changes in substitution. However even when separately determined for alkyl substituted isoluminol derivatives (Table 9), the relatively small differences observed are difficult to interpret. The fluorescence quantum yield stays reasonably constant but the efficiency of excited state population (0es) shows no clear trend. 

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