Excitation quantum yields

Energy transfer to photoreactive acceptors has also been widely utilized for excitation quantum yield determination (chemical titration), mainly in the decomposition of dioxetanes ° . The quantum yields are calculated from the photoproduct yield obtained at infinite energy acceptor concentrations ( = 1.0) by extrapolation of the double-reciprocal relationship between the photochemically active energy acceptor concentration and the photoproduct yield ( Lp ). H the quantum yield of the photochemical reaction ( ) can be calculated (equation 8) . ... 

We have recently described a calibration procedure for the determination of excitation quantum yields on commercial fluorimeters, utilizing the luminol standard , and have thereby determined singlet excitation quantum yields for the peroxyoxalate reaction with bis(2,4,6-trichlorophenyl) oxalate (TCPO), hydrogen peroxide and imidazole, using various activators . The same calibration method has been utilized to determine the singlet quantum yields obtained in the induced decomposition of protected phenoxyl-substituted 1,2-dioxetanes 6 and and compared them to the well-investigated... 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

Figure 3.79. The viscosity dependence of the singlet excitation quantum yield given by Eq. (3.646) at different rate constants of recombination to the triplet state kt — 1011, 1010,10 ,M 1s 1 (from top to bottom). The points are the same as in Figure 3.78 and the excitation rate constant k = 8.8 x 108 M-1s-1. (From Ref. 231.)...

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 slope of the logarithmic plot of absorbance change versus the logarithm of the pump intensity is 1.84, indicating the occurrence of a nonlinear absorption process. The TPA cross section of isomer A is about 0.76 GM and an increased 8 was observed for isomer B (8 = 6 GM) due to the closed ring form and higher planarity. One-photon and two-photon induced isomerization of 143 shown in Eq. (62) results in similar spectra and in the same isosbestic points observed at 329 nm, 377 nm, and 429 nm. These results clearly indicate participation of the same excited state applying either OP or TP excitation. Quantum yields are close to unity. 

Photoproducts and quantum yields for representative Co(III) complexes are summarized " in Table 2. As noted above, charge-transfer excitation of the Co(lll) amines is accompanied by photoreduction to Co(II) plus oxidations of ligands. For LF excitation, quantum yields of photosubstitution are wavelength dependent. Even the nature of the predominant substitution is dependent, as illustrated for [Co(NHj)jCl], for which NHj aquation is favored for excitation of the lowest-energy singlet LF absorption (488 nm), whereas Cl labilization is predominant for excitation of the triplet LF absorption (647 nm). In contrast, the Co(CN)jX photoaquations are independent of... 

Excited state formed in initial excitation. Quantum yield for photoequations in mol/E. 

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... 

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