Journal of Photochemistry最新文献

The tunnel effect theory is applied to study the photocycloaddition of ketones to olefins. This theory can quantitatively explain the reactivity, regioselectivity, stereospecificity and efficiency of the photocycloadditions studied. The study covers a large diversity of olefins (electron rich and electron poor) and ketones (alkyl, alkylphenyl and phenyl), in both their ¹(n,π*) and ³(n,π*) states. The theory does not need to postulate the questionable intermediacy of an exciplex to explain the characteristics of these reactions. Evidence for the dominancy of tunnelling over thermal activation is presented. The results of this study are in line with previous applications of tunnel effect theory and complete the explanation of ketone photoreactivity in solution under the framework of the general intersecting-state model.

Quantum yields for the formation of hydrated electrons (Φ_e−) and single-strand breaks (Φ_ssb) were measured for polyadenylic acid (poly A), polycytidylic acid (poly C), polyuridylic acid (poly U) and single-stranded (ss) DNA in deoxygenated aqueous solution at room temperature upon nanosecond laser excitation at 248 nm. The values Φ_e⁻ = (1.7 − 3) × 10⁻² and Φ_ssb = (0.5 − 5) × 10⁻⁴ at intensities of (0.5 − 1.5) × 10⁷ W cm⁻² were obtained. The results show that base radical cations of these polynucleotides and of ssDNA lead to ssb formation. The large difference between Φ_e⁻ and Φ_ssb (a factor of 60 – 380) is explained as being the result of back reactions of geminate pairs of radicals and/or radical ions. Conductivity changes after the laser pulse, representing ssb formation, reveal a fast increase and a consecutive show increase for all nucleic acids studied. The activation energies and pre-exponential factors, calculated from the rate constant k_obs of the slow conductivity increase at pH 7.3, are reported. Dithiothreitol (DTT) inhibits ssb formation of poly C (rate constant k_DTT = 1.7 × 10⁶ M⁻¹ s⁻¹) and poly U whereas for poly A and ssDNA a smaller effect was found (k_DTT < 0.7 × 10⁶ M⁻¹ s⁻¹. The pH dependence of k_obs is very similar for poly A and ssDNA, but different from that of poly C and poly U. This indicates that the slow strand break formation in ssDNA upon laser excitation is related to the purine moiety.

The triplet—triplet absorption spectra of three diaminopyridines have been assigned from variable-temperature flash photolysis studies. In all cases the lowest triplet state is ππ* with a phosphorescence lifetime of about 2 s at 77 K in ethyl ether—pentane—ethyl alcohol. Complete neglect of differential overlap—configuration interaction calculations were performed to supplement the interpretation of the fluorescence and phosphorescence data and to provide a satisfactory correlation of the observed and predicted behavior of these molecules.

Measurementds of photoconductivity on thin polymer films are described using an electrostatic recordimg device built in our laboratory from components actually utilized in a commercial photocopier. The voltage applied to the surface of thin films may be measured using an electrostatic voltmeter and photodecays may be easily monitored by a digital-type storage oscilloscope which when coupled to a BBC microcomputer, via the RS423 input, gives an accurately stored decay profile of the charge with time on the millisecond scale. In this paper, the construction of the equipment and its operation are described and some measurements are presented which support its practical viability under various conditions

Decarboxylation and oxidation reactions of carboxylic acids in the presence of various light receivers are reviewed from the experimental and mechanistic standpoints. The homogeneous photoreceivers studied are the ions UO₂²⁺, Fe²⁺ and Cu²⁺. They differ in activity and in selectivity as far as the nature of the hydrocarbons formed by decarboxylation is concerned. The activity of the heterogeneous photocatalyst TiO₂ (anatase) in oxidation is shown to be greatly increased in the presence of dissolved substances, e.g. Fe³⁺ and Cu²⁺.

Photolyses of p-, m- and o-chlorotoluene in methanol gave formation quantum yields of methylanisoles of the order of 10⁻², which are substantially smaller than that for chlorobenzene. The values for the para and ortho isomers are smaller than that for the metal isomer. This result can be explained in terms of the effect of the methyl substituent, which induces negative charges at the ortho and para positions. The effect of biacetyl addition was also examined and the results indicated that the triplet route is significant, as in the case of chlorobenzene. However, about half the methylanisole was formed without any dependence on the biacetyl concentration, indicating the presence of an intermediate state with a very short lifetime.

It is clearly demonstrated that the photodimerization of 5-acyl dibenz[b,f]azepine (DBA) derivatives occurs via excited triplet intermediates. The excited triplet state of 5-acyl DBA derivatives is barely formed on direct excitation, but is formed efficiently by triplet—triplet (T—T) energy transfer from a triplet sensitizer. Benzophenone-sensitized photoexcitation gave a unique reaction product, the anti-cyclobutane dimer of DBA. Under the sensitized conditions, the quantum yield for the photodimerization was estimated to be 0.15 at 25°C ([DBA]=2.0×10⁻³ mol l⁻¹). The triplet intermediate of this reaction was detected by nanosecond laser photolysis and the kinetic parameters of the triplet species were determined.