Energy transfer quantum yields chemiluminescence

Energy transfer quantum yields, chemiluminescence, 1223 Enhanced chemiluminescence, 1219-20, 1221 Enol esters, dioxirane asymmetric epoxidation, 1150... 

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. 

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. 

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>. 

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... 

By means of steady-state kinetics, the relationship in Eq. 37 for the DPA-enhanced chemiluminescence quantum yield (0dpa) is derived in terms of the singlet excitation yield (0 ), the efficiency of singlet-singlet energy transfer (0 x), and the DPA fluorescence quantum yield (0dpa)- The 0 parameter can be readily assessed once the remaining terms are known. 

The energy-transfer term 0 x is unity under conditions of infinite DPA concentration. What is typically done is that one measures the DPA-enhanced chemiluminescence intensity (/dpa) function of DPA concentrations and constructs a plot of 1//dpa vs. 1/[DPA]. The intercept of such a double reciprocal plot represents the DPA-enhanced chemiluminescence intensity at infinite DPA concentration, that is, /foPA] - The DPA-enhanced chemiluminescence quantum yield that is calculated from this emission intensity, that is, 0fDPA] > represents complete singlet-singlet energy transfer, that is, 0 x is unity. 

The fluorescer of choice for counting chemienergized triplet states via triplet-singlet energy-transfer chemiluminescence has been 9,10-dibromoanthracene (DBA). Like DP A, it is readily available and easily purified unlike DPA it has a relatively low fluorescence quantum yield, that is, 0dba about 0.10 and is temperature- and solvent-dependent. For reliable triplet yields, the fluorescence quantum yields of DBA should be measured under the conditions at which the chemienergized carbonyl product K is generated. 

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. 

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... 

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. 

The activation energy in this case varied from 87 to 93 kJ/mol in different solvents. From the temperature dependence, several competitive reaction paths for this dimethyl-dioxetanone decomposition were deduced, all having a biradical as first intermediate. Heavy-atom effects often play a role in dioxetan chemiluminescence. If DBA is used as fluorescer, the quantum yield is markedly greater than that observed when DPA is used - although the latter has a fluorescence efficiency of 0.89, compared with 0.1 for DBA. In both cases triplet-singlet energy transfer is the origin of the chemiluminescence. 

In (25) there is a true fluorescer moiety in the cyclophane system, and it represents a donor-acceptor complex system, whereas (24) and (23) very probably are forming exciplexes on oxidation [41]. The higher efficiency of this paracyclophane energy transfer in comparison with the methylene-linked energizer and fluorescer as in (22) is seen from the fact that in (22) the DPA-residue, having a fluorescence quantum yield of nearly unity exhibits a chemiluminescence efficiency of 26% of that of luminol whereas in (25) with the 1,4-dimethyl anthracene fluorescer (0jn ca. 0.30) a light yield of 100% luminol [41] is obtained. 

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