Journal of Photochemistry最新文献

The fluorescence and phosphorescence spectra of the dimethylphenols were studied at 77 K. Changes in the pK_a values in the excited singlet (S₁) and triplet (T₁) states for 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dimethylphenol in ethanol and ethanol—NaOH solutions were estimated by means of the Förster cycle.

We found that the acidity of the hydroxyl group in the first excited singlet is greater than that in the ground state. This is also true for the first triplet state, although the acidity of the first triplet state of 2,6-dimethylphenol is an exception, owing to the influence of the methyl groups on the hydroxyl group.

Photo-oxidation of leuco crystal violet (DH) to crystal violet dye (D⁺) was studied in the benzophenone (BP) derivatives—DH—acetonitrile systems. The BP derivatives with a tetraalkylammonium substituent (R₃N⁺; I, II and III exhibited much higher photo-oxidizing efficiencies than BP itself. Under deaerated conditions, nearly quantitative photo-oxidation of DH by the absorbed photons was attained by I whereas BP itself showed a poor ability to photo-oxidize DH. Addition of tetraalkylammonium salts (R₄NX), such as cetyltrimethylammonium chloride or tetra-n-butylammonium chloride to the BPDH system also enhanced the quantum yield by a factor of about 10. The enhancement of quantum yield by the use of I, II or III, or by the addition of R₄NX to BP was interpreted as due to stabilization of the BP anion radical or the DH cation radical by ion pairing with the tetraalkylammonium substituent on BP or with the added R₄NX. In the case of I, II and III, the electrostatic repulsion between the DH cation radical and the R₃N⁺ substituent on BP, which enchanced the charge separation efficiency, was the main cause of the high photo-oxidation yields.

The effect of hydrogen sulphide, ethanethiol, diethyl sulphide and diethylhydroxylamine on the gas phase photolysis of acetone has been studied. Diethylhydroxylamine was found to be much more reactive than thiols and thioethers towards the nπ^* triplet state of acetone. However, H₂S and C₂H₅SH proved to be more effective methyl-radical trapping agents than diethylhydroxylamine. The following kinetic data were determined for hydrogen-atom abstractions by CH₃ from C₂H₅SH and (C₂H₅)₂NOH respectively:

The exclusive quenching of a potentially fluorescent species in solution by its nearest quenching neighbor imposes a finite upper integration limit to the flux equation and provides a modified version, i.e.

Various aspects of the photochemical behaviour of linear p-dimethylaminobenzoylated polystyrene, 4-dimethylamino-4′-isopropylbenzophenone and of Michler's ketone have been studied in cyclohexane, benzene and chloroform. At low concentrations, the model compound is efficiently photoreduced in cyclohexane (φ = 0.58), parallel to the behaviour of Michler's ketone (φ = 0.37). Lower values were found in chloroform (φ = 0.12) and in benzene (φ = 0.055). The reduction quantum yield φ decreases as the model compound concentration increases and at concentrations higher than 10⁻⁴ M the quantum yield was found to be dependent on the extent of reaction, whereas it is independent in dilute solutions. A general kinetic scheme consistent with previous contributions to the subject and with our results has been proposed. In this scheme, a bimolecular hydrogen abstraction from the solvent with a second-order rate constant (k_r) competes with the triplet excimer formation (k_e) which is depopulated by two routes: physical deactivation to give the ground state ketones (k_ed) and hydrogen transfer (k_er) to give a pair of ketyl and aminomethyl radicals. The rate constants of hydrogen abstraction (k_r) in the three solvents studied were determined, as well as the ratio k_er/(k_er + k_ed) (approximately equal to 0.02) which measures the importance of hydrogen transfer through excimer formation. It is concluded that the excimer formation process leads principally to the generation of the starting ketone by physical deactivation, and only, to a small extent, to a pair of radicals via hydrogen transfer. Both ketones (the model compound and Michler's ketone) have similar lifetimes at room temperature and transfer their triplet excitaion energy to naphthalene and trans-stilbene with high rates. In contrast with this behaviour, the polymeric sensitizer shows very low reactivity and its energy donor ability is quite limited. These results can be explained by the efficient intramolecular self-quenching which takes place in the functionalized macromolecules owing to the large local concentration of chromophore.

Photolysis of alkyl cinnamates in water gave δ-truxinates in near quantitative yields. Dimerization of alkyl cinnamates at low concentration (about 10⁻⁴ M) in water is attributed to hydrophobic association of these solutes. At such low concentrations, photolysis in benzene and methanol yields only geometric isomers. This study illustrates the potential of water as a solvent for photoreactions and stresses the need for increased appreciation of its utility.

In the presence of a photosensitizer (anthraquinone) a surface film of pristane on synthetic sea water was photo-oxidized by sunlight. The photochemical processes are affected not only by the secondary carbons (producing different 2,6,10,14-tetramethylpentadecanones), but also, and more strongly, by the tertiary carbons. This photo-oxidation leads on the one hand to the formation of tertiary alcohols and on the other hand, after α-cleavage reactions, to different fragmentations of the pristane molecule into alkanes and ketones. These very important results provide new information that must be taken into account when considering the degradation of branched alkanes in a marine environment.

Flash photolytic studies on benzils are reported. Two different geometrical conformers, one puckered and the other planar, have been characterized for triplet benzils and benzil anions. They are produced under different conditions. The puckered benzil triplet formed in ethanol galss at 77 K shows an absorption peak at 490 nm accompanied by vibrational bands at 1600 and 1000 cm⁻¹. The planar benzil triplet, which appears at room temperature, absorbs at 470 nm.

Two different conformers of the benzil anion are also found to be present in two different solvents, ethanol and acetonitrile. The planar form absorbs at 540 nm in ethanol and the puckered form absorbs at 620 nm in acetonitrile. Mesitil and α-furil do not undergo a change in conformation under similar conditions.