Photosubstitution yields

The work reports wavelength dependent quantum yields for the photoreaction and photosubstitution yields for the Os(III) complex are much lower than those of the iron or ruthenium analogs. The authors also report that the complexes exhibit weak luminescence in the green the Os(III) complex has a reported emission yield of 0.008 [35]. 

The reactions outlined above and in Table 1 can be generalized to other lone-pair (n-donor) ligands. ir-Donor ligands such as olefins, also behave in an analogous fashion, but Mo(CO) CjHj-h , produced by normal photosubstitution, yields an intermediate hydride upon irradiation ... 

N-Alkylacridinium ions and other cationic heteroarenes likewise underwent photoalkylation, but these processes were reductive additions, rather than photosubstitution, yielding acridanes [46]. In these cases, both tetra-aUcylstarmanes and the more environmentally benign 4-aIkyl-l-benzyl-l,4-dihydronicotinamides have recently been used as a source of alkyl radicals [47]. 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

Figure 1. Spectrum of Ru3(CO) 2 in octane. Quantum yields for photosubstitution and photofragmentation in 25°C argon flushed octane in the presence in 0.012 M P(OCH3>3 represented as a function of irradiation wavelength (from reference 5).

Complex 7 is stable in water, does not react with 5 -GMP in the dark, and only 5% of Pt(IV) was reduced by GSH after 21 days. Irradiation of the complex alone in water yielded little reduction to Pt(II) and photosubstitution of one or both of the azides was observed. In the presence of 5 -GMP, however, rapid photoreduction resulted in the formation of the mono-adduct (SP-4-4)-[Ptn (5 -GMP-Ar7)(N3)(NH3)(py)]+ (7rG) and bis-adduct trans-... 

On irradiation, the cluster molecule Os3(CO)12 is resistant to Os-Os bond cleavage and instead undergoes photosubstitution with a low quantum yield ... 

Generally, clusters of nuclearity >3 undergo photosubstitution with low quantum yield ... 

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

Mechanistic studies also indicate that 4-nitroveratrole (equation 20) and 4,5-dinitroveratrole (equation 21) undergo both singlet and triplet nucleophilic aromatic substitution with ethyl glycinate23. An electron transfer process competes against the nucleophilic aromatic photosubstitution for singlet excited 4-nitroveratrole, causing a decreased product yield in equation 20. 

A competition between substitution and two kinds of quenching processes, either dependent or independent on the added nucleophile, is envisaged for the photosubstitutions of p-nitroanisole (pNA) ni.ns). With cyanide ions in aqueous aerated solution pNA is transformed on irradiation into 2-cyano-4-nitroanisole in high yield ns,ii8) probably via addition of cyanide to excited pNA. 

Nitropyrrole does not seem to undergo any photosubstitution with either methoxide, cyanate, cyanide or water 123> 2-Nitrothiophene 1 a) and 5-bromo-2-nitrothiophene (1b) undergo photocyanation smoothly and efficiently. The disappearance quantum yield for 7 a,b equals the product quantum yield. 

Already, with one of the first nucleophilic aromatic photosubstitutions encountered, curious behaviour was found when studying the rate of reaction as a function of pH. m-Nitrophenyl sulphate shows no increase in the quantum yield of photohydrolysis with increase of hydroxide ion concentration up to values as high as 0-1 M. This behaviour, also found with one other compound (5-chloro-3-nitroanisole), is in clear contradistinction to what is... 

With the reactions discussed so far, variation of the wavelength of irradiation has only a minor influence on the quantum yield of nucleophilic aromatic photosubstitution (Section 1). Nitroazulenes are found to be interesting exceptions (Lok et al., 1973). 

Chlorine and bromine react under favourable conditions (activation by other substituent, appropriate nucleophile). Iodine may also be photosubstituted by nucleophiles but is easily induced to enter into homolytic reaction pathways. One has to bear in mind that the heavier substituents (iodine, bromine and even chlorine) increase the rate of intersystem crossing which, depending on the conditions, increases or decreases the quantum yield. 

To test the first hypothesis, solutions of 3,5-dinitroanisole and hydroxide ions were flashed and the absorption spectra at different time intervals after excitation were compared. The absorption ( max 400-410 nm) that remains after all time-dependent absorptions have decayed can be shown to be due to 3,5-dinitrophenolate anion, the photosubstitution product of 3,5-dinitroanisole with hydroxide ion. When the absorption band of the 550-570 nm species is subtracted from the spectrum of the solution immediately after the flash, there remains an absorption at 400-410 nm, which can also be ascribed to 3,5-dinitrophenolate anion. The quantity of this photoproduct does not increase during the decay of the 550-570 nm species. Therefore the 550-570 nm species cannot be intermediate in the aromatic photosubstitution reaction of 3,5-dinitroanisole with hydroxide ion to yield 3,5-dinitrophenolate. Repetition of the experiment with a variety of nucleophiles on this and other aromatic compounds yielded invariably the same result nucleophilic aromatic photosubstitution is, in all cases studied, completed within the flash duration (about 20jLts) of our classical flash apparatus. 

Thiophene on irradiation in presence of dihalogenomaleimides leads to low yields of the 2-substituted products (Scheme 89) (78TL125). When benzo[6]thiophene is irradiated in the presence of dibromomaleic anhydride, both photosubstitution and photocycloaddition are observed (Scheme 90) (77BCJ1797). The ratio of photosubstitution relative to cycloaddition increases with the temperature and polarity of the solvent. A common triplet exciplex has been suggested for both processes. 

Table 3 Photosubstitution Quantum Yields for some d6 Complexes...

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

The complexation of coordination compounds may make it possible to control their photochemical behaviour via the structure of the supramolecular species formed. For instance, the binding of cobalt(m) hexacyanide by macrocyclic polyammonium receptors markedly affects their photoaquation quantum yield in a structure-dependent manner [8.73-8.77]. It thus appears possible to orient the photosubstitution reactions of transition-metal complexes by using appropriate receptor molecules. Such effects may be general, applying to complex cations as well as to complex anions [2.114]. 

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