Silvio Pietra

The photochemistry of azo-dyes. The wavelength-dependent photo-reduction of 4-diethylamino-4′-nitroazobenzene

4-Diethylamino-4′-nitroazobenzene (I) is photoreduced upon irradiation at 254 nm, and in alcohols, also at 313 nm, while visible light is inactive. The products are the corresponding hydrazo-derivative (II) or the amines (III) and (IV). Triplet sensitizers of sufficiently high energy are effective, while the lowest triplet is not reduced. The reaction is inhibited by nitroxyl radicals. A mechanism implying hydrogen-abstraction on the part of some high lying triplet states of trans-(I) is proposed. Other than by energy transfer, these states can be reached by intersystem crossing from high lying singlet states.

The photocyclization of N-acyl-2-nitrodiphenylamines to phenazine N-oxides : scope and mechanism

The photocyclization of N-acyl-2-nitrodiphenylamines to phenazine N-oxides is extended to several dinitro derivatives and a pyridine analogue obtaining N-oxides of otherwise difficult access. In the presence of some additives, the reaction takes a different course. Thus, with acids deacylation occurs, with triphenylphosphine a N-phosphoranylidene amine is formed and with 2,6-di-ter-butylphenol the corresponding nitrosodiphenylammine is obtained. A mechanism starting from the nitroamide triplet and involving several discrete intermediates is proposed in order to account for such observations

The photochemistry of azo dyes. Photoisomerisation versus photoreduction from 4-diethylaminoazobenzene and 4-diethylamino-4′-methoxyazobenzene

4-Diethylaminoazobenzene (2) and 4-diethylamino-4′-methoxyazobenzene (3) undergo geometrical isomerization with a quantum yield of almost unity by irradiation in the visible region, and, with lower efficiency, by irradiation at a shorter wavelength or sensitization both with high and low energy hydrocarbons. These dyes are also photoreduced, this time however only by irradiation at λ⩽ 313 nm and with low quantum yield (10–4–4 × 10–2). Photoreduction also takes place by energy transfer from high energy sensitizers as well as by radical initiation by ketones. It is concluded that the lowest singlet and triplet states of these azobenzene derivatives, which are reasonable models for commercially useful monoazo-dyes, are only capable of geometrical isomerization, whereas hydrogen abstraction takes place from the high-lying triplet state of both the trans- and cis-configuration of these dyes.

Singlet oxygen photo-oxidation of some triazapentalenes

The self-sensitized photo-oxidation of 1,3-dimethyl-5H-pyrazolo[1′,2′:1,2]-1,2,3-triazolo[3,2-a]phenazin-4-ium inner salt (1) leads to the cleavage of the triazapentalene moiety, yielding an epoxy-ketone. If the two methyl groups are absent, as in compound (2), the photo-oxidation takes place much better in the presence of dyes and yields an αβ unsaturated aldehyde. The first reaction is shown to involve singlet oxygen, the formation of which requires oxygen-promoted intersystem crossing from singlet to triplet (1), followed by energy transfer from the latter state. The rate of the singlet oxygen addition onto (1) was measured in some solvents, and showed no dependence on the polarity of the medium. The mechanism of the addition is discussed, taking into account also the results from the photo-oxidation of the 6,7-dihydro-derivatives of compounds (1) and (2).

Photochemical decomposition of 2,4-dinitro-4′-dimethylaminoazobenzene in aerated solution

The irradiation of various ketones in the presence of 2,4-dinitro-4′-dimethylaminoazobenzene (1) causes decomposition of the latter. The chemical process actually observed depends on the solvent and on the ketone structure. In benzene the dye (1) undergoes demethylation in the presence of 2,2-dimethoxy-2-phenylacetophenone (DMPA) and benzil. This is due to the reactions of benzoylperoxyl radicals formed from the ketone sensitizers in the presence of oxygen. Benzophenone and anthraquinone cause the same reaction, in this case through direct interaction of the nπ* triplet with (1). In alcohols the situation is more varied, as with DMPA, benzil, and benzophenone in methanol, demethylation of (1)via the same mechanisms as above is observed, while with anthraquinone and acetone in methanol and with benzophenone in propan-2-ol, 2,4-dinitro-4′-dimethylamino-NNO-azoxybenzene, (4), is formed. The latter reaction is initiated by attack of ketyl radicals, formed in the primary photoreaction of the ketone with the solvent, onto (1). The relevance of this model for the fading of azo dyes under applicative conditions is discussed.

Is self-sensitized photo-oxidation a significant pathway in the photofading of azo dyes

The photofading in aerated solution of 4-diethylaminoazobenzene (1) and of the corresponding 4′-methoxy (2) and 4′-nitro derivatives (3) under various conditions has been examined and compared with naphthol derivatives which are subject to azo–hydrazone tautomerism. Dyes (1)–(3) are efficient physical quenchers of singlet oxygen but show very little chemical reactivity with oxygen. Their ability to sensitize oxygen is low. More efficient photo-oxidative fading is observed upon short-wavelenght irradiation, which results from the reaction of oxygen with hydrazyl radicals, formed by initial hydrogen abstraction from the solvent.

Photoreaction of 2-nitrophenazine 10-oxide with amines

The photodeoxygenation and the photosubstitution of the nitro-group of 2-nitrophenazine 10-oxide has been investigated by kinetics methods supplemented by quenching measurements. Photodeoxygenation by triethylamine occurs quite efficiently (Φrca. 0, 2) in acetonitrile and methanol and is little affected by triethylamine concentration. The other amines employed are much less efficient; their efficiency strongly depends on the amine concentrations. Examination of the fluorescence quenching data with the quantum yields of reduction allows the conclusion that in the case of triethylamine, the main pathway of reduction involves an exciplex derived from the singlet state. The same pathway is also proposed for the reduction by concentrated solutions of other amines in acetonitrile. In different conditions a pathway from a triplet state seems to be more important. Both the pathways lead to the same intermediate. Quenching of the reduction by oxygen and by tetramethyldiazetine dioxide supports this point. The mechanism of photosubstitution of the nitro group is also discussed.

Solvent effects in the photoisomerization of phenazine 5-oxide

The photoisomerization of phenazine 5-oxide in acetonitrile–water mixtures has been investigated. The quantum yields of 10H-azepino[1,2-a]benzimidazol-10-one (3) and 2-(4-cyanobuta-1,3-dienyl)benzoxazole (4) are closely related to solvent composition in that they decrease strongly with increasing content of water. In acetonitrile ϕ3 and ϕ4 are temperature independent. The quantum yield of oxepino[2,3-b]quinoxaline (5) is almost independent of the composition of the solvent mixtures. A mechanism is proposed which relates the quantum yield variations to the properties of the excited state.

Photochemical decomposition of 4-arylazo- and 4-arylazoxy-N,N-dialkylaniline N-oxides

The 4-aryl-N,N-dialkylaniline N-oxides (1a–c) decompose on u.v. irradiation in aprotic solvents, undergoing deoxygenation as well as intramolecular hydrogen abstraction, to give amides and dealkylated products. The latter process is more important for the diethyl derivative (1b)(photo-Cope elimination). Visible irradiation is ineffective. The 4-arylazoxy-N,N-dialkylaniline N-oxides (2a–d) undergo photochemical deoxygenation from the amino group (major process) as well as from the azo group. The mechanism of these photoprocesses is discussed in general and in relation to the possible involvement of these N-oxide derivatives in photofading of the related azo dyes.

PHOTOCHEMICAL DECOMPOSITION OF 2‐AZIDOPHENAZINE IN THE PRESENCE OF AMINES

Die Photozersetzung der Titelverbindung (I) in 0,1 M-Losungen der Amine (II) in Acetonitril liefert neben dem Aminophenazin (III) die Alkylamino-aminophenazine (IV).