Chemiluminescence light emission kinetics

Chemiluminescence is light emission from the relaxation of electrons populating excited states in an elementary step of a chemical reaction. Since, the process of population of excited states is related kinetically to the kinetics of the given chemical reaction, the emission of chemiluminescence over time should thus be related to the rate of the chemical reaction. 

Equation (13) appears to be a good approximation for describing isothermal chemiluminescence kinetics for homogeneous systems where oxidation takes place uniformly. However, as has been shown by several authors [53-58], the different sections of a polymer sample may oxidize with its autonomous kinetics determined by different rates of primary initiation. A chemiluminescence imaging technique revealed that the light emission may be spread from some sites of the polymer film and the isothermal chemiluminescence vs. time runs are then modified, particularly in the stage of an advanced oxidation reaction [59]. 

Seitz, Suydam, and Hercules 186> recently developed on the basis of luminol chemiluminescence a method for chromium-III ion determination which has a detection limit of about 0.025 ppb. The method is specific for free chromium-III ions as chromium-VI compounds have no catalytic effect and other metal ions can be converted to a non-catalytic form by complexing with EDTA, since the chromium-III complex of EDTA, which is in any case not catalytically active, is formed kinetically slowly 186>. To detect extremely small light emissions, and hence very small metal concentrations, a flow system was used which allows the reactants to be mixed directly in front of a multiplier. (For a detailed description of the apparatus, see 186>). 

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 ... 

M12. Mattingly, G., Chemiluminescent 10-methyl-acridinium-9-(N-sulphonylcarboxamide) salts. Synthesis and kinetics of light emission. 7. Biolumin. Chemilumin. 6, 107-114 (1991). 

Oxygen quenching is also responsible for the remarkable kinetic behaviour of the chemiluminescent oxidation of hydrocarbons in closed vessels. Instead of the expected gradual decrease of intensity as oxygen is consumed, the intensity actually rises (Fig. 4) Depletion of O2 results in such an enhancement of emission that at the very point at which light emission should be negligible, it is maximal. The cessation of oxidation which inevitably results from consumption is all the more dramatic - the oxygen drop phenomenon [11]. 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

The figure also illustrates that a thexi state can be obtained in several ways (a) spectroscopically by direct light absorption, (b) by thermal activation or the reverse of radiationless deactivation, (c) by chemical reaction (as must be true in chemiluminescent processes), and (d) by photochemical reaction (as in chemical laser systems). Viewed in this light, both photochemical and photophysical processes (other than emission and light absorption) leave their usual spectroscopic milieu to join the broad field of reaction kinetics. 

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