Photolysis mechanism photoproducts

Indoles were high (0.02-1.2), evidence that auto-catalysls occurred In distilled water. In aqueous CRM-1, Indoles underwent sensitized photolysis, possibly through a superoxide or singlet-oxygen Intermediate. The same photoproduct of 3-MI, o-(N-formyl)amlnoacetophenone, was found In both distilled water and In aqueous CRH-1. Carbazole underwent direct photolysis In both distilled water and aqueous CRM-1, Indicating that It has a different photolysis mechanism from that of Indoles In the two systems studied. 

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

Pliotolytic. Bussacchini et al. (1985) studied the photolysis (7, = 254 nm) of phenmedipham in ethanol, ethanol/water, and hexane as solvents. In their proposed free radical mechanism, homolysis of the carbon-oxygen bond of the carbamate linkage gave the following photoproducts 3-(hydroxylphenyl)carbamic acid methyl ester, m-toluidine, 2-hydroxy-4-aminomethyl benzoate, 3-hydroxy-5-aminomethyl benzoate, 2-amino-4-hydroxymethyl benzoate, and 2-amino-6-hydroxymethyl benzoate. 

Considerable detail about the photolysis of cytidylic acid has been reported by Johns et al.62 The photoproducts, Cp f and Up, were separated from Cp by rapid electrophoresis. Since Cp labeled with 32P was used, the separated substances could be located on the dried paper by autoradiography, the spots cut out, and the eluted products assayed by 32P counting. The formation of products was described in terms of the mechanism in Chart 2. 

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. 

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... 

In this context, it has been reported that (Mes)3Mo=N=N=Mo (Mes)3 with Mes = 2,4,6-Me3C6H2 is light sensitive and the photoproducts imply the intermediate formation of (Mes)3Mo=N (55). This photolysis was carried out rmder conditions that are usually applied for synthetic pmposes. It may be thus difficult to elucidate the photochemical mechanism. For example, the photolysis mentioned above was performed for 18 h. In such cases, the photoproducts could be the result of subsequent reactions that may not be easily related to the primary photochemical steps. 

It was shown during this period that photolysis of the octacyano d species of Mo(V) and W(V) produces the reduced octacyanometalate(IV) complexes as main photoproducts, and much research has concentrated on the first mechanistic step of action. On the other hand, photolysis of the primary photoproduct, [MfCNlg] of Mo(IV) and W(IV), forming [M02(CN)4] (and the mono- and the diprotonated [M0(0H)(CN)4] and [M0(H20)(CN)4] , depending on the solution pH) as the final product during exhaustive photolysis, was also extensively studied. Photolysis of both these metals d and d systems has also been studied in organic media as an aid in the elucidation of the photolytic mechanisms. The discussion below concentrates on aspects of the research that has been performed with regard to the photochemistry of these M(V) and M(IV) complexes of Mo and W. 

Examples of the vinylcyclopropene photorearrangement are known where the chromophore is external to the three-membered ring. The results suggest that the carbene mechanism operates and that it is not contingent upon the chromophore being a part of the carbocycle. For example, direct photolysis of 245d leads to three photoproducts (equation 79) and the presence of an allene requires a [1,2]H shift in the carbene intermediate. 

The results of early flash studies were irreproducible and led to various conclusions concerning the nature of the primary photoproduct. For example, flash photolysis of Cr(CO)g in room temperature degassed cyclohexane solutions and in polystyrene film was reported to produce a transient having a maximum absorbance in the visible at 450 nm and which decayed with second-order (equal concentration) kinetics (83). The following scheme was proposed for the overall mechanism ... 

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. 

---