Quantum yield mechanisms

A careful study of the photoinitiated addition of HBr to 1-hexene established the following facts. (1) The quantum yield is 400, (2) The products are 1-bromohexane, 2-bromohexane, and 3-bromohexane. The amounts of 2- and 3-bromohexane formed are always nearly identical and increase from about 8% each at 4°C to about 22% at 63°C, (3) During the course of the reaction, small amounts of 2-hexene can be detected. Write a mechanism that could accommodate all these facts. 

McCapra, F., and Hysert, D. W. (1973). Bacterial bioluminescence — identification of fatty acid as product, its quantum yield and a suggested mechanism. Biochem. Biophys. Res. Commun. 52 298-304. 

Shimomura, O., and Johnson, F. H. (1970a). Mechanisms in the quantum yield of Cypridina bioluminescence. Photochem. Photobiol. 12 291-295. 

Oplopborus luminescence, 82-87 effects of pH and temperature, 83-86 luminescence spectrum, 84 mechanism, 85-87 quantum yield of coelenterazine, 85 Orfelia, 2, 27, 337... 

Terminally deuterium-labeled phenylacetylene was also used to elucidate the possible mechanism of this reaction. In view of all these results, a rationalization for the loss of the trimethylsilyl and the migration of the ethoxy group from its original position in the complex 96 has been put forward. Due to the contribution of the conjugated diarylcyclopentadiene moiety in 98 and 99, these molecules showed intense fluorescence with a relatively high quantum yield of 46%. 

Between 1923 and 1927, the concepts of quantum efficiency (number of photons emitted divided by number of photons absorbed by a sample) and quantum yield (fraction of excited molecules that emit) had been defined and values determined for many compounds by Vavilov (34). The quantum yield indicates the extent that other energy loss mechanisms compete with emission in an excited molecule. Although the quantum yield is influenced by the molecular environment of the emitter, for a given environment it depends on the nature of the emitting compound and is independent of concentration and excitation wavelength, at least at low concentrations (35). Tlius, it serves as another measurable parameter that can be used to identify the compounds in a sample and also, because of its sensitivity to the surroundings of the luminophore, to probe the environment of the emitter. 

The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

DSP crystal, a detailed picture of the lattice motion and related displacements was constructed and related to the topochemical postulate and the mechanism of phonon assistance. Holm and Zienty (1972) have measured the quantum yield for the overall polymerization process of a,a -bis(4-acetoxy-3-methoxybenzylidene)-p-benzenediacetonitrile (AMBBA) crystals in slurries and reported it to be 0.7 on the basis of the disappearance of two double bonds ( = 1.4 if assigned on the basis of the number of double bonds consumed). 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

It is interesting to note that some reactions proceed with large quantum yield, e.g.

transfer mechanisms so that little interference between anodic and cathodic processes occurs . ... 

As a rule, high quantum yields for two-electron transfer reactions are expected when the mechanism is one-electron/two-hole or two-electron/one-hole. In the cases of twQ-electron/two-hole or one-electron/one-hole efficient back reactions of the intermediates on the colloidal particles or in solution, respectively, will lead to a low yield of the final products. 

Instead of postulating Zn," as intermediate, as it has a highly negative potential and is possibly unstable in ZnO, one may write the above mechanism with Zn e pairs. The blue-shift in the absorption upon illumination was explained by the decrease in particle size. The Hauffe mechanism was abandoned after it was recognized that an excess electron on a colloidal particle causes a blue-shift of the absorption threshold (see Fig. 19). In fact, in a more recent study it was shown that the blue-shift is also produced in the electron transfer from CH2OH radicals to colloidal ZnO particles When deaerated propanol-2 solutions of colloidal ZnO were irradiated for longer times, a black precipitate of Zn metal was formed. In the presence of 10 M methyl viologen in the alcohol solution, MV was produced with a quantum yield of 80 %... 

These quantum yield values appear to be much higher than unity and therefore demonstrate that carbonization occurs by a chain reaction process. The mechanism of the laser-induced dehydrochlorination of photodegraded C-PVC can be schematically represented by the follo-... 

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