Quantum yield photodecomposition

The quantum yield of the initiation process (<, ) is quite low 8 x 10, indicating the great stability of the chelate ring toward photolysis. However, the quantum yield of photodecomposition 4>d) under similar condition is 2 X 10, which is higher than It is clear, therefore, that not every molecule of Mn(acac)3 that is decomposed initiates polymerization apparently, ex-... 

Arylamine Photodecomposition. A number of researchers have alluded to the fact that the products produced from photolysis of aromatic carbamates (i.e., la) also degrade upon irradiation (10), 17). Indeed, we found that the aryl amine 2b and the photo-Fries products 2c and 2d (resulting from photolysis of 2a) decomposed with respective disappearance quantum yields of 0.035, 0.004, and 0.003 when irradiated at 280 nm. These latter results agree with those of Schwetlick et al. (17), who found the rates of disappearance of lc and Id to be quite small. 

The quantum yield of the selected azoalkanes photodecomposition are given in Table 3.18. The extinction coefficient s depends on the wavelength of light, and for AIBN in benzene solution (room temperature) has the values as given below [205] ... 

The inefficiency of photodecomposition of S02 in this spectral region is then ascribed to the small quantum yield of 3S02. 

Because of the paucity of rate data for the reactions of F and FO one can only speculate on the course of the reaction subsequent to the initial dissociation. The results of photochemical studies398,399 give some guidance. The quantum yield F2o of photodecomposition is 1.0 at 3650 A, independent of temperature in the range 15-45 °C, pressure of F20 and pressure of oxygen398 the primary step is almost certainly as in (2)398,3". Thus, at room temperature at least, any contribution from... 

While reactions (16)—(19) and (9) are adequate to explain the small quantum yields for the photodecomposition in solution ( Cio2 2 in CC14 for X < 4100... 

All the 1-pyrenylbismuthonium salts photochemically decompose (2ex > 320 nm / > 150 mW cm 2) to generate their respective protic acids, accompanied by the formation of bismuth(III) compounds and pyrene (Scheme 25). The quantum yields of the photodecomposition (<7>dcc) in acetonitrile were determined by chemical actinometry to be 0.20-0.22. These values are comparable to the values reported for the triarylsulfonium and diaryliodonium salts (dec = 0.17-0.22) [98, 99]. 

G(X) and quantum yield of the product used for the calculation, respectively. Two possible sources of errors should be mentioned. One is the energy dependence of the photodecomposition. In order to obtain intrinsic yield should be measured at photon energies as close as possible to the absorption onset, or appropriate corrections should be used [154,155]. Both photon emission and photodecomposition studies show that the energy dependence is more severe for the smaller molecules than for the larger ones. Therefore, G(Si) for the larger molecules can be estimated with higher accuracy. The second source of error is that X may not solely form in Si molecule transformation. 

Benkelberg, H.-J., and P. Warneck, Photodecomposition of Iron(III) Hydroxo and Sulfato Complexes in Aqueous Solution Wavelength Dependence of OH and SO/ Quantum Yields, J. Phys. Chem., 99, 5215-5221 (1995). 

The photochemical stabihty of the molecules is characterized by the quantum yield of photodecomposition, (P = N/Q [69], where N and Q are the numbers of decomposed molecifles and absorbed photons, respectively. The photochemical properties of the fluorene derivatives were investigated in different organic solvents (hexane, CH2CI2, ACN, and polyTHF) at room temperature by the absorption and fluorescence methods and comprehensively described [70-72]. These methods are based on measurements of the temporal changes in the steady-state absorption and fluorescence spectra during irradiation. For the absorption method, the quantum yield of the photodecomposition under one-photon excitation, c >ipa, can be obtained by the equation [73] ... 

Table 4 Quantum yields 2PA of photodecomposition under two-photon excitation into the main absorption band (Aexc = 720 nm and 840 nm ) in air saturated solutions with different concentrations C ...

In the absence of photodecomposition reactions, the quantum yield for electron injection from the excited sensitizer to the semiconductor is given by Equation 17.11. 

Although the net quantum yield of photodecomposition is found to be very small in the gas phase (typically 0.05), it has been found that in the presence of silver,16 mercury,16 oxygen,17 or nitric oxide,18 the quantum yield increases markedly. The two metals scavenge both atomic and molecular iodine, and the two gases scavenge alkyl radicals, thus suppressing both of the two possible inhibiting secondary steps. 

The photolysis of isopropyl iodide has been investigated by McMillan and Noyes.16 Addition of excess NO increased the quantum yield of iodine formation at 3130 A. to — 0.5, and in the presence of metallic silver the quantum yield of photodecomposition rose to unity. Hence the primary process at 3130 A. is exclusively... 

The photodecomposition of perfluoro diethyl-62 and perfluoro-di-w-propyl63 ketones has been shown to follow a course similar to that of hexa-fluoroacetone. The fluorescence is weaker in the case of the perfluoro-diethyl ketone and almost absent in the case of the perfluorodi-n-propyl ketone, and the pressure dependence of the quantum yields of carbon monoxide is consistent with this behavior. It is proposed that in the more complex structures, the energy may more easily be accommodated and lost by internal conversion, thereby reducing the contribution made by fluorescence and by Collisional quenching of the excited molecule. 

To explain the high chemical yield in the transformation A - D, coupled with the fact that the quantum yield for the photodecomposition of nitrites is less than unity, it was suggested that the Barton reaction might take place through a radical "cage mechanism.2 However, recent studies at the Institute on the mechanism of nitrite photolysis have shown this not to be so. Photolysis of an equimolecular mixture of 3/3-acetoxy-androstan-6/8-yl nitrite (102) and 3/8-acetoxy-cholestan-6/8-yf nitrite containing 98% of nitrogen as N16 (103) in iso-octane or toluene... 

H20 system. Quantum yields of 03 photodecomposition at 2537 A in the presence of ITjO are much larger than those of pure 03. They increase linearly with N/rP,lQ (640, 749). The results are interpreted on the basis of the following chain mechanism... 

Upon photolysis, there are two reactive species formed side by side, the vinylic uracilyl radical and Br The latter has oxidizing properties (E7 = 1.66 V Ward-man 1989), and in DNA it can oxidize neighboring G and A moieties (Wojcik et al. 2003). The quantum yield of photodecomposition of 5BrUra is only 1.8 x 10 3, but increases dramatically upon deprotonation (Campbell et al. 1974). In the presence of MeOH as an H-donor, it also increases substantially above 2 mol/1 MeOH. It has been speculated that above this concentration cage reactions come into play. Reported rate constants for the reaction of Br- with MeOH are 5 x 104 dm3 mol-1 s 1 in water and 1 x 106 dm3 mol-1 s 1 in acetonitrile (Neta et al. 1988). 

The transient spectrum of the phenoxy radical can be readily generated by reaction of phenol with tert-butoxy radicals, formed by the photodecomposition of di-jm-butyl peroxide. This is a very efficient reaction which takes place with a quantum yield of approximately 0.7-0.9 (13, 14). The transient spectra of foe phenoxy and guaiacoxy radicals are shown in Figure 4. 

Choudhary, G.G., Webster, G.R.B. (1985b) Quantum yields for the photodecomposition of polychlorinated dibenzo-p-dioxins (PCDDs) in water-acetonitrile solution. Chemosphere 14, 893-896. 

Similar cyclization reactions to polychlorodibenzodioxins have been reported under irradiation of the sodium salt of the conjugate base of perchloro-u-phenoxyphenol in methanol in the presence of a sensitizer and excess triethylamine. Photodecomposition of the conjugate base of perchloro-0-phenoxyphenol in methanol reveals a small amount of cyclization, while irradiation in methanol in the presence of a 10-fold excess of triethylamine increase the quantum yield of cyclization by 17-fold. These results were interpreted in terms of an ET to the substrate to from a radical dianion, which reacts through an intramolecular S l mechanism347. 

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