Quantum yield recombination

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

Quantum yield of luciferin. Various values of quantum yield have been reported for coelenterazine in the luminescence reaction catalyzed by Renilla luciferase 0.055 (Matthews et al., 1977a), 0.07 (Hart, et al., 1979), and 0.10-0.11 (with a recombinant form Inouye and Shimomura, 1997). The quantum yield is significantly increased in the presence of Renilla green fluorescent protein (GFP) see below. 

Em., luminescence emission Q, quantum yield of coelenterazine (unpublished data included). Recombinant protein. 

Although the electrostatic field on the polyelectrolyte surface effectively impedes back ET, it is unable to retard very fast back ET or charge recombination of the primary ion pair within the photochemical cage. The overall quantum yield of photoinduced ET is actually controlled in most cases by the charge recombination. Hence, its retardation is the key problem for attaining high quantum yields in the photoinduced ET. 

Core/shell-type nanoparticles ovm ated with higher band inorganic materials exhibit high PL quantum yield compared with uncoated dots d K to elimination of surface non-radiative recombination defects. Such core/shell structures as CdSe/CdS [6] and CdSe ZnS [7] have been prepared from organometaHic precursors. 

Neither the scavenger for electrons nor the scavenger for positive holes are adsorbed at the colloidal particles and both are rather unreactive. Neither of the two can compete efficiently with the recombination of the charge carriers, the result being that the quantum yields of the products are very low. 

Nosaka and Fox determined the quantum yield for the reduction of methyl viologen adsorbed on colloidal CdS particles as a function of incident light intensity. Electron transfer from CdS to MV " competes with electron-hole recombination. They derived a bimolecular rate constant of 9 10 cm s for the latter process. 

In order to minimize the fast electron-hole recombination, Calzolari et al. [50] attached a donor-acceptor boronic acid spacer between the anthocyanins and a gold electrode, and the same molecule between the dye and a P-cydodextrin which is able to bind I3 dissolved in the electrolyte near the chromophore. In that configuration, the device shows an increase in the quantum yield of the photocurrent from a negligible value to 1.75%. Such studies were not repeated on Ti02 layers. 

Electrons and holes generated by energetic illumination in a small semiconductor particle can easily reach the surface and can carry out reduction and oxidation processes of adsorbed species, respectively, by providing and accepting electrons before they have a chance to recombine. Therefore, the quantum yield can be high. 

The quantum yield is defined here as the ratio of photons emitted to the number of initiator molecules one molecule of DCPD yields 2 radicals which in turn produce one molecule of ketone in the recombination reaction. 

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 ... 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

Somewhat surprisingly, the pressure dependence of quantum yields from the photolysis of NO at 1236 A, where the primary process is almost entirely photoionization199, is very similar to that observed for 1470-1650 A198. Clearly, recombination of the ionic fragments must lead to an excited state of NO which predissociates upon collision. However, the neutralization reaction... 

Using light of 3660 A the quantum yield of reaction (1), zero time. At any other time some light is absorbed by M(CO)5NEt3 giving an apparent value of < 1. In the absence of Et3N quantitative recombination of M(CO)5 and CO occur when the irradiation is stopped9. 

This case is shown in Fig. 10.6c and d where through absorption of light a photohole in the vb and a photoelectron in the cb are formed. The probability that interfacial electron transfer takes place, i.e. that a thermodynamically suitable electron donor is oxidized by the photohole of the vb depends (i) on the rate constant of the interfacial electron transfer, kET, (ii) on the concentration of the adsorbed electron donor, [Rads]. and (iii) on the rate constants of recombination of the electron-hole pair via radiative and radiationless transitions,Ykj. At steady-state of the electronically excited state, the quantum yield, Ox, ofinterfacial electron-transfer can be expressed in terms of rate constants ... 

Hole trapping by electron donors bound to the surfaces of the semiconductor particles competes with the e - h+b recombination, allowing e b to react with molecular 02 via Eq. (10.23). Fig. 10.10 shows that the quantum yield, peroxide formation increases with increasing concentration of the electron donor. 

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