Journal of Photochemistry最新文献

The photochemical ring cleavage and ring contraction reactions of cyclic olefins are reviewed. The retro Diels-Alder processes cannot be unambiguously linked to a particular electronic state. Although there seem to be some differences between the bicycloalkane products observed in the photosensitization (triplet) and the direct photolysis (singlet) of cyclo-olefins, these differences do not warrant assignment of the formation of a given bicycloalkane to a particular electronic state. Ring contraction to the corresponding vinylcycloalkane can occur from either the triplet or the “hot” ground state. The formation of methylenecycloalkanes appears to be a reaction specific to a Rydberg singlet state of the cycloalkenes.

The pulse d.c. photoconductivity (PC) of the vacuum-degassed ethers tetrahydrofuran (THF), ethyl ether (Et₂O) and 1,4-dioxane was investigated. It was found that after a light pulse of duration 1 ms in degassed THF or Et₂O the photocurrent rises. A monophotonic ionic photodissociation is responsible for the charge generation since the photosignals were proportional to the amount of absorbed light. The photocurrent signals decreased exponentially with first-order rate constants which decreased linearly with the decreasing voltage applied between the platinum electrodes. This might indicate that the fairly stable photoproduced ions disappear mainly in the electrode discharge reactions. The bulk recombination, which is probably preceded by a monomolecular or pseudomonomolecular rearrangement (since this is a process also well described by an exponential equation) did not exceed about 10% of the total ion consumption under the conditions used. After examining different methods of purification of the ethers studied it was found that the phenomenon was caused by electron acceptor impurities, which were mainly the products of the oxidation of ethers. In the case of 1,4-dioxane the PC signals were hardly detectable, since the lower polarity of this solvent did not favour ionic dissociation of the ion pairs which are probably also produced. An improved method of purification of these ethers is proposed.

The paramagnetism loss of nitroxide, photosensitized by eosin, was analysed in basic aqueous and micellar media using electron paramagnetic resonance (EPR) spectroscopy. In cetyltrimethylammonium bromide micelles and sodium bis(2-ethylhexyl)sulphosuccinate—n-heptane microemulsions, the paramagnetism decay curves are monoexponential. This is not the case in neat water and sodium dodecylsulphate micelles, for which more complex decays were recorded. It is concluded from flash photolysis measurements that the eosin triplet state is physically quenched by nitroxide and that the latter can be reduced by electron transfer either from the triplet state or from the semireduced form of eosin. This sequence of reactions was confirmed by experiments in micellar media and leads to an understanding of the EPR results.

The gas phase photochlorination of 1,2-dichloro-1,2-difluoroethylene was studied in a static system at temperatures between 30 and 90°C. The rate was measured under both continuous and intermittent light of wavelength 436 nm. The only final product, CFCl₂,CFCl₂, is formed according to the equation + $\text{d[C}{}_2\text{F}{}_2\text{Cl}{}_4\text{]}\text{d}\text{t}$=k[Cl₂]J_abs^{$\text{1}\text{2}$} where J is the intensity of the absorbed light.

The rate constants determined under continuous illumination are as follows: k_30°C=5.63 ± 0.09 1^{$\text{1}\text{2}$} mol^{−$\text{1}\text{2}$} s^{−$\text{1}\text{2}$}, k_60°C = 10.43 ± 0.02 1^{$\text{1}\text{2}$} mol^{−$\text{1}\text{2}$} s^{−$\text{1}\text{2}$} and k_90°C = 21.26 ± 1.4 l^{$\text{1}\text{2}$} mol^{−$\text{1}\text{2}$} s^{−$\text{1}\text{2}$}.

The rate constants for the elementary reactions CFCl₂ĊFCl+Cl₂→C₂F₂Cl₄+Cl and 2CFCl₂ĊFCl→products are log k₃=8.68-$\text{4810±300 4.57}\text{T}$ and log k₄=8.91±0.21 respectively, where k₃ and k₄ are in litres per mole per second.

The conjugation of methyl 9,12-octadecadienoate (DH) is a reaction of industrial interest. It has been photochemically performed with iodine as photosensitizer (λ=520nm). The reaction is a radical chain process which is inhibited by galvinoxyl, the overall quantum yield decreasing from 1.9 to 1.3 with the irradiation time. The rate is given by the empirical expression k [i]^0.56 [ DH]^1.22 [I₂]^0.40 where i is the light intensity. The reaction leads to a mixture of methyl 9,11-octadecadienoate and methyl 10,12-octadecadienoate with E,E:Z,E:Z,Z in the molar ratio of 0.72:0.26:0.02 in 80% yield.

This study illustrates the possibility of applying solar radiation to an industrial process.

The fluorescence lifetime-temperature dependences for naphthalene and pyrene in 3-methylpentane were investigated in the temperature range 92 - 298 K using Čerenkov light as the source of photochemical excitation. The results are explained in terms of temperature-dependent intersystem crossing from S₁ to the adjacent triplet levels.