Transfer reactions relative quantum yield

Electron transfer to the metal centre results in oxidation of the halogen and the complex then dissociates. The relative quantum yields are high (0.1 to 1) when irradiation is made in the GT band in complexes with Br or I this reaction is still observed with irradiation in the d-d bands, but the quantum yields are then lower. 

Photosubstitutions. Selective control of reactivity at different positions of the aromatic ring included in the CD cavity has been obtained in the nucleophilic photohydroxylation and photocyanation of fluoroanisoles (FA) by Liu and Weiss [308,309]. The reaction (see Scheme 27 for 4-FA) involves the FA" " radical cation intermediate formed by electron transfer between a triplet and a ground state molecule. Tables 19 and 20 list the relative quantum yields of the two photoreactions in mixtures of bound and free 4-FA and 2-FA. Both reactions appear to be almost totally inhibited in the... 

On the other hand. Type II process competes efficiently with the electron-transfer pathway in aerobic environments where the concentration of ground triplet state molecular oxygen is relatively high ( 0.27 mM), and singlet molecular oxygen (1O2) is the most abimdant ROS generated under these conditions, with a quantum yield 0.48 (Valle et al., 2011), eqn. 8. It is also possible an electron-transfer reaction from 3RF to 02 to form anion superoxide, but this reaction occurs with very low efficiency <0.1% (Lu et al., 2000). 

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 quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Consequently, the formation of a labile addition product, Sensrad. .. 02, was postulated. This should transfer its oxygen to a substrate A via a collision complex Sensrad... 02... A, which collapses into the ground-state sensitizer and an energy-rich AOa. The latter stabilizes to A02 by dissipating its excess energy as heat.26 Since photooxygenation reactions were shown to occur even at very low temperatures with relatively high quantum yields, the identity of Sensrad with a sensitizer in the excited triplet state was assumed.61,68-70... 

When the quantum yield is so low that the recombination reaction becomes relatively dominant, Eq. (5.14) is simplified to e = (kekjkrg)l/2, or [e ] = (khg/kIke)l/2. This relationship indicates that the steady amount of photoinduced electron is proportional to the square root of the light intensity. A similar relationship can be also obtained for the case of the photoinduced holes, and an equation similar to Eq. (5.14) is used to calculate the hole transfer yield,[Pg.44]

Quantum yields for adduct 63 and total product (63-65) formation from the reaction of - -t with several tertiary amines are summarized in Table 12. Quantum yields measured at 1.0 M amine concentration are lower than the values extrapolated to infinite amine concentration due to incomplete quenching of It. Extrapolated total quantum yields range from 0.07 to 0.33, providing a lower limit for the efficiency of the proton transfer step, kh> in Fig. 11. The other reaction products, 1,2-diphenylethane (64) and 1,2,3,4-tetraphenylbenzene (65), are formed mainly via in-cage radical pair disproportionation and out of cage combination, respectively. The relative importance of radical pair combination, disproportionation, and cage escape is dependent... 

In terms of photophysics, electron transfer reactions create an additional non-radiative pathway, so reducing the observed emission lifetimes and quantum yields in A-L-B dyads in comparison with a model compound. However, there are other processes, such as molecular rearrangements, proton transfer and heavy-atom effects, which may decrease the radiative ability of a compound. One of the most important experimental methods for studying photoinduced processes is emission spectroscopy. Emission is relatively easy to detect and emission intensities and lifetimes are sensitive to competing processes. Studying parameters such as emission quantum yields and lifetimes for a given supramolecular species and associated... 

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