Photochemistry and Photolytic Reactions

Photochemistry and Photolytic Reactions Table 2.1 Physical constants... 

Jeveral aspects of the photolytic behavior of aqueous complex ions have been studied in this laboratory over the past few years. One continually interesting question has been the extent to which the photochemistry of a complex depends on the absorption band irradiated. In the case of Co(III) acidopentamines, such as Co(NH3)5Br+2, we found that irradiation of Ajg —> g) bands showing appreciable charge transfer led to redox and aquation reactions which were competitive. It was reasonable to suppose that the common precursor was the species formed by a prompt heterolytic bond fission (7). The ( Aig —> Tig) band was far less photoactive, and in model cases, irradiation led only to aquation. Each excited state or excited state manifold thus tended to show a distinct photochemistry, which meant that conversion from one excited state to another was not important. 

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

There is a multiplicity of pathways for thermal dediazoniations. An analogous situation is to be expected for photochemical dediazoniations. Based on the general experience that light-sensitive reactions often involve free radical intermediates, it was commonly assumed that all photolytic dediazoniations are free radical reactions. Horner and Stohr s results (1952), mentioned above, could lead to such a conclusion. More sophisticated methods of photochemistry also began to be applied to investigations on arenediazonium salts, e. g., the study of photolyses by irradiation at an absorption maximum of the diazonium ion using broad-band or monochromatic radiation. This technique was advocated by Sukigahara and Kikuchi (1967 a, 1967 b,... 

There are a number of reports in the literature on the photochemistry of amides, including the photodegradation of serum albumin,224 and of polypeptides.1 Photodegradation involves reactions of acyl and imine free radicals generated by photolytic scission of the amide group.23 33 The photooxidation of N-pentylhexanamide led to the formation of n-valeraldehyde and valeric acid from the amine part of the molecule... 

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. 

Over the past few years it has often been observed that the photochemical behaviour of adsorbed molecules is distinctly different to that of their gas phase counterparts. Even direct dissociations of molecules physisorbed on insulator substrates were found to have different dynamics to the analagous gas phase reaction, and exhibited a dependence on the coverage. This needs to be understood. For adsorbed molecules a new kind of "dissocation" is possible, namely desorption, Photolytic (non thermal) desorption has been reported from all kinds of substrate. On metal surfaces it is often found that the quantum yield for a direct photodissociation reaction is much lower than in the isolated molecule. This must be accounted for. Finally, the observation which has stimulated a great deal of research in surface photochemistry, photolysis is observable at energies where the gas phase molecules are transparent. It turns out that all of these interrelated effects can be interpreted by a delicate interplay of excitation mechanism and transient quenching. The fine details of course depend on particular adsorbate-substrate systems, which are described in section 4. 

The photochemistry of bridging borylene complexes differs significantly from that of their terminal counterparts. The aminoborylene complex 23a,b proved to be entirely unreactive under photolytic conditions, which is in sharp contrast to the terminal aminoborylene complexes 1 and 2. The chloro derivative 34, however, yielded upon irradiation in the presence of a CO donor (i.e. M(CO)6> M = Cr, Mo, W) the dimetalla-mdo-tetraborane [B2Cl2 ( 75-C5R5)Mn(CO)2 2] (43) as shown in Scheme 17 [64]. The reaction provides a direct synthetic link between electron-precise borylene complexes and electron-deficient metallaboranes. 

In many cases photophysical effects are much influenced by photochemical reaction. The fluorescence of naphthacene is affected by dimerization and oxidation. Interaction with anthracene and quinones also occurs. The adiabatic photolytic cycloreversion of substituted lipidopterenes into intramolecular exciplexes shows an example involving anthracene derivatives. A series of very detailed papers on conformational effects on the fluorescence and photochemistry of [2/j] 9,10-anthracenophanes have been published by Ferguson and coworkers.It is not possible in this review to summarize this very detailed work... 

S02,403 on the photoreduction of quinones,404 on acetyl and acetyl peroxide radicals in gas-phase reactions,405 on fluoroalkyl radicals in solution,408 on the photochemistry of 2,2-dimethoxy-2-phenylacetophenone,407 on pyridine-3,5-dicarboxylic acid,408 on photolytically reduced pyrazine,409 410 on the dimerization... 

An allylic intermediate has also been proposed for the photolytic isomerisation of the COD ligand in Tp Rh(l,5-COD) (90), affording Tp Rh(l,3-COD) (171, Scheme 10). This reaction is believed to occur by dechelation of the diene ligand to afford an f/ -complex that undergoes C—H activation to give, transiently, Tp RhH(f/ f -C8Hi2), intramolecular reductive elimination then yields 171. The photochemistry of 171 has also been explored, and is discussed later (Sections II-C.2 and IV-A.2). 

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