Excitation quantum yields chemiluminescence

DPA) in dimethylphthalate at about 70°, yields a relatively strong blue Umax =435 nm) chemiluminescence the quantum yield is about 7% that of luminol 64>. The emission spectrum matches that of DPA fluorescence so that the available excitation energy is more than 70 kcal/mole. Energy transfer was observed on other fluorescers, e.g. rubrene and fluorescein. The mechansim of the phthaloyl peroxide/fluorescer chemiluminescence reaction very probably involves radicals. Luminol also chemiluminesces when heated with phthaloyl peroxide but only in the presence of base, which suggests another mechanism. The products of phthaloyl peroxide thermolysis are carbon dioxide, benzoic acid, phthalic anhydride, o-phenyl benzoic acid and some other compounds 65>66>. It is not yet known which of them is the key intermediate which transfers its excitation energy to the fluorescer. 

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. 

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%. 

A substantial effort has been appHed to iacreaskig i by stmctural modification (114), eg, the phthalaziQe-l,4-diones (33) and (34) which have chemiluminescence quantum yields substantially higher than luminol (115,116). The fluorescence quantum yield of the dicarboxylate product from (34) is 14%, and the yield of singlet excited state is calculated to be 50% (116). Substitution of the 3-amino group of lumiaol reduces the CL efficiency > 10 — fold, whereas the opposite effect occurs with the 4-amino isomer (117). A series of pyridopyridaziae derivatives (35) have been synthesized and shown to be more efficient than luminol (118). 

In bioluminescence and chemiluminescence reactions, light is emitted when the energy level of light emitter molecules falls from the excited state to the ground state. The quantum yield Q of the substance A is given by ... 

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. 

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. 

The hypothesis of Kellogg 38> described above, that autoxidation reactions display low quantum yields in spite of high yields of excited products, due to oxygen quenching in the solvent cage, is criticized by J. Beutel 13) who very thoroughly investigated the chemiluminescent autoxidation of dimedone (1.1. dimethyl 3.5 cyclohexandione). Here the recombination of dimedone peroxy radicals should be the excitation step ... 

Very weak chemiluminescence (quantum yields of 6.5.. . 9.1 X 10-10) in the spectral ranges 400. 540 nm (benzaldehyde phosphorescence) and 600 nm (emission from excited singlet oxygen collision pairs)) was also observed on thermolysis of 5 with no fluorescer present. 

Intermolecvlax energy transfer is apparently involved in the anomalous chemiluminescence of phthalic hydrazide in aprotic solvent (DMSO/tert.BuOK/Og) 124) the energy of excited phthalated ianion is transferred to phthal-hydrazide monoanion which then emits at 525 nm with relatively low quantum yield. This phenomenon has not been observed in aqueous systems 2>. 

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. 

Lee and coworkers postulated the involvement of 50 and 51 (Ar = 2,4-dinitrophenyl) as two HEIs formed in parallel in the uncatalyzed reaction of DNPO and hydrogen peroxide in the presence of perylene. Due to the experimental observations of light emission from the reaction of DNPO and TCPO also in the absence of hydrogen peroxide, Lee and coworkers postulated the involvement of a nonperoxidic HEI (additionally to 51 and 3, 48 or 52) under these conditions. However, neither chemiluminescence quantum yields nor even relative emission intensities have been reported. Furthermore, it was shown " that the intensities and the chemiluminescence quantum yields in the absence of hydrogen peroxide are five orders of magnitude lower than in the presence of 10 M H2O2, indicating that the proposed additional pathway is of extremely low efficiency for excited-state... 

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... 

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). 

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