Electronic excitation quantum yields chemiluminescence

The excitation quantum yield ( ex) is the product of the efficiencies of (1) the chemical reaction, (2) the conversion of chemical potential into electronic excitation energy and in the case of sensitized chemiluminescence, and (3) the energy transfer. As a consequence, most chemiluminescent reactions have relatively low quantum yields compared to those of photoluminescence the exception being the enzymatically mediated bioluminescent processes. In spite of this low quantum efficiency, chemiluminescence remains an attractive option for chemical analysis. This stems from three factors (1) improved... 

In principle, one molecule of a chemiluminescent reactant can react to form one electronically excited molecule, which in turn can emit one photon of light. Thus one mole of reactant can generate Avogadro s number of photons defined as one einstein (ein). Light yields can therefore be defined in the same terms as chemical product yields, in units of einsteins of light emitted per mole of chemiluminescent reactant. This is the chemiluminescence quantum yield which can be as high as 1 ein/mol or 100%. 

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. 

Rauhut and coworkers proposed the occurrence of a charge transfer complex between the HEI and the ACT in order to explain the electronically excited-state generation in the peroxyoxalate system. Chemiluminescence quantum yield (4>cl) measurements with different activators have shown that the lower the ACT half-wave oxidation potential (Ei/2° ) or singlet energy (Es), the higher the electronically excited-state formation rate and 4>cl- According to the mechanistic proposal of Schuster and coworkers for the CIEEL... 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

Another method of measuring the relative quantum yield of the radical decomposition process (eq. 22) was also devised recently (144). This involves HNO chemiluminescence photoexcitation spectroscopy. When an H atom recombines with an NO molecule, an electronically excited HN0 ( A A" ) is formed. Fluorescence emission from HNO occurs at 762 nm. The HNO chemiluminescence in a low-pressure 1 10 mixture of H2CO and NO is proportional to the H-atom quantum yield from the photolysis of H2CO. The photoexcited HNO (red) chemiluminescence excitation spectrum of a H2CO/NO mixture obtained with a tunable laser at high resolution is shown in Fig. 2 together with an absorption spectrum and a H2CO fluorescence (blue) excitation spectrum (237). The relevant reaction scheme is as follows ... 

Further studies have been conducted with (46), (49)-(51) in 1-phenyldecane at high temperatures (170-220°C) <94HCA1851>. On heating, the trioxanones decompose unimolecularly to give electronically excited singlet ketones with an efficiency of ca. 0.2%. The chemiluminescence quantum yields depend on the nature of the substituents at C(6) and increase linearly with temperature. The rate of decay of chemiluminescence affords the Arrhenius activation energies, which are found to be 35.6, 22.9, 30.4, and 34.2 kcal mol respectively. 

Chemiluminescence (CL) is a phenomenon whereby the electronically excited state product of a chemical reaction generates optical radiation during relaxation to its ground state. There are two general mechanisms of CL that are employed in the context of detection for CE. In direct CL, the photon(s) are emitted by the excited state reaction product as it relaxes to the ground state. In the second, the relaxation of the excited state takes place via energy transfer to a fluorophore, which subsequently fluoresces this is referred to as sensitized CL, because a fluorophore with a high quantum yield can be used. 

It can be seen from the above that it is necessary to account for coUisional effects on the emission quantum yields when interpreting chemiluminescence profiles to deduce mechanistic information. In this section, we summarize the results of a series of experiments on quenching and energy transfer in electronically excited OH and CH, which are pertinent to such flame studies. 

Dioxetane and 1,2-dioxetanone are key chemical structures in many chemiluminescent and bioluminescent systems. The molecular structure of these systems bears the chemiluminophore properties of the chemi/bioluminescent molecules based on them, providing a channel for a thermally activated chemical reaction that produces a compound in an electronically excited state.The efficiency of the chemiluminescent process in 1,2-dioxetane and dioxetanone is however low and requires an electron-donor group to increase the quantum yield of luminescence. In addition, it is observed experimentally that the triplet emission is significantly more favourable than singlet emission in these small systems. From a theoretical standpoint, the general aspects of the... 

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