Photoproducts solutions, quantum yields

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

Haug75 has reported that the concentration of TpC reaches a photostationary state at 280 nm and pH 7 at about half its initial value. Three photoproducts, separable by electrophoresis and stable to heat, are formed. The quantum yield for decay of TpC is independent of wavelength of light, and is 0.006 at pH 7 and 0.002 at pH 2. Heating the solution at 80°C restores about 40% of the absorption loss at 270 nm for the solution irradiated at pH 2 and about 70% of the loss for solutions irradiated at pH 7. Irradiation at 240 nm does not increase the absorption at 270 nm. One would expect dimers and hydrates to be formed in the photolysis, but the observations summarized here are difficult to reconcile with such products exclusively and suggest deep-seated irreversible changes. 

DHAs undergo an efficient photoreaction to the corresponding VHFs the quantum yields at room temperature (4>dha vhf) range from very small values (< 0.0004) to a respectable 0.6 (Table 1). The VHFs are non-emitting and photoche-mically inactive. X-ray analytical investigations of crystallized photoproducts have revealed the exclusive formation in the crystalline phase of the s-trans VHF iso-mer.ns.i In solution, where a thermal equilibrium exists between the s-trans and s-cis isomers, we assume a high concentration of the thermodynamically favorable s-trans... 

As we have seen above, if the solution is sufficiently acidic (pH < 4), the Cc isomer is rapidly protonated, with conversion to the 4 -methoxyflavylium ion AH+, which is kinetically stable under such pH conditions and exhibits an intense absorption band with a maximum at 434 nm (Figure 24) and an emission band with maximum at 530 nm (Figure 6). At higher pH values, however, protonation does not occur and the Cc photoproduct is back-converted into Ct. As far as Co(CN) is concerned, excitation by 254 or 365 nm light in acidic or neutral aqueous solution causes the dissociation of a CN ligand from the metal coordination sphere (quantum yield = 0.31), with a consequent increase in pH [S71... 

Irradiation at 254 nm of an aqueous neutral solution of the reduction product of wave I led to stepwise appearance of a new band in the UV with 305 nm, with a quantum yield of 0.08 (Fig. 5), with on overall yield of 80%. Chromatography and polarographic analysis demonstrated that the photoproduct was the parent l-methyl-l,2-dihydro-2-iminopyrimidine... 

The effect of the halogen substituent (fluoro, chloro, bromo and iodo) on the yield and mechanism of 4-halophenol photolysis was investigated by Durand et al. [24], Transient spectroscopy in aerated aqueous solutions indicated the formation of p-benzoquinone O-oxide from each derivative except 4-iodophenol for which no transients were detected p-benzoquinone and hydroquinone were found as photoproducts for all four compounds. It was concluded that the carbene mechanism was valid for the whole series. Under continuous irradiation, the 4-halophenol degradation quantum yields were determined to be

fluorescence lifetimes decreased in the same order, from 2.1 ns for 4-fluorophenol to 0.4 ns for 4-chlorophenol and < 0.1 ns for 4-bromophenol. 

Dicamba is a dichlorinated 2-methoxybenzoic acid derivative. It was found to be photodegraded in aqueous solution with a quantum yield of 0 = 0.0215 at A, = 275nm [99]. The photoproducts were the dihydroxylated derivative and an unexpected dicyclic compound, only formed in the presence of O2 (Scheme 10). 

A photoinduced electron transfer mechanism has been proposed by de Mayo and coworkers to explain the products arising from the irradiation of 1,1-di-p-anisylethene in benzene-trifluoroacetic acid solutions [73]. Photolysis (>430 nm) of the (di-p-anisylmethyl)methyl cation 30 resulted in electron transfer from the neutral substrate 29 as shown in the Scheme 10. No reaction occurred under these conditions in the absence of acid. The quantum yields of the photoproducts are... 

The Matsuura method (57) compares the photolysis result of a solution actinometer with that of a thin crystalline film of equal surface area after exposure. Thus, evaporation of a solution containing the photoreactive substrate results in a thin crystalline film on the glass wall of a test tube, which is subsequently exposed to actinic radiation in a merry-go-round type photolysis apparatus. To test for complete absorption of the incident photons within the crystalline film, the evaporation process is carried out at various concentrations of the substrate, which leads to films of different thickness. If the yield of photoproduct after a certain exposure time is independent of the concentration of the original solution before evaporation, complete absorption of all actinic photons is established. The quantity of the photons absorbed by the crystalline film is then estimated by parallel photolysis of a 0.1 M solution of 2,4,6-triisopropylbenzophenone in methanol solution, which has a well-established quantum yield of 0 = 0.52 (58). The volume of this actinometer solution in the test tube is adjusted so that the crystalline film and the solution exhibit irradiated surfaces of identical size. In summary, this method provides approximate estimates of solid-state quantum yields however, differences in the reflection of the... 

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

The photochemistry of 4-chloroanilines in methanol, dioxane-water and diox-ane-methanol solvents has been investigated for more than thirty years by Latowski185,186. Large quantum yields of HC1 formation (hci) have been observed for the photolysis of 91a in protic solvents (e.g. Hci = 0.78 in methanol at 254 nm). However, the values of 4>hx are relatively small for 4-bromoaniline (HBt = 0.19), 4-iodoaniline (cbm = 0.29), 2-chloroaniline (hci < 0.02) and 3-chloroaniline (hci = 0.02) under the same condition. N-Acetylation of 91a to 4-chloroacetanilide also inhibits the photolytic process. In conjunction with the solvent- and concentration-dependent photolysis rates of 91a, these results indicate an electron-transfer mechanism for the photochemical reaction electron transfer occurred from an excited 91a to an unexcited 91a molecule, followed by ionization reactions. However, recent analysis of photoproducts from 91a in water/methanol mixtures has shown that benzidine (92) is a major product along with aniline (equation 29)187. As a result, a carbene mechanism that leads to the formation of aniline radicals was put forward in analogy to the photochemistry of 4-halophenols188,189. For example, the photolysis of 91a in aqueous solution first results in the transient species carbene 93 followed by the formation of the aniline radical 94 that was observed as the primary product (Scheme 13)190. In addition to la and 92, other identified secondary products include 4-aminodiphenylamine, 2-aminodiphenylamine, hydrazobenzene, 4-chloronitrosobenzene and 4-chloronitrobenzene, but they are all in low yields191. 

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