Journal of Photochemistry最新文献

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.

Cyanoaromatic-sensitized photo-oxidations of substrates (α-pinene, β-pinene, 3,4-dihydro-2H-pyran, 1,4-diphenyl-1,3-butadiene, limonene and cyclohexadiene) which react readily with singlet oxygen were studied. It was proved that a singlet-oxygen reaction takes place in each of the above photo-oxidations. The solvent polarity and solvent viscosity have the same influence on both the quantum yields of photo-oxidations and the fluorescence quenching rate constants. All the experiments support the suggestion that electron transfer occurs between the sensitizer excited singlet state and the substrate. The singlet oxygen formed then acts as a reactive intermediate in the subsequent process through back electron transfer. In carbon tetrachloride, the products of all the above substrates are the same as in the dyesensitized photo-oxidations and singlet oxygen may be generated through intersystem crossing which leads to the triplet-excited reaction.

The absorption and fluorescence spectra obtained in various solvents indicate that aminophenols and anisidines act as proton donors in ether and acetonitrile but as proton acceptors in methanol and water in the S₀ state and as proton donors to all the solvents in the S₁ state. Stretched sigmoid curves are observed for the equilibrium between the monocation and the neutral species for all the compounds, except p-aminophenol (pAMP) and p-anisidine, giving both ground state and excited state pK_a values. For pAMP and p-anisidine, proton-induced fluorescence quenching of the neutral species is observed at moderate hydrogen ion concentrations. The values of the quenching constant are 3.3×10⁹ dm³ mol⁻¹ s⁻¹ and 2.0×10⁹ dm³ mol⁻¹ s⁻¹ for pAMP and p-anisidine respectively. The proton transfer reaction studied in methanol-water systems has indicated that, in low dielectric constant solvents, the excited state equilibrium is not established and thus the ground state pK_a value is observed from fluorimetric titration curves.

All combinations of functionalization in the C-2′ and C-6 positions of rose bengal was reported. Derivatives obtained are the C-2′ ester, C-6 ether, the C-2′ ester, C-6 salt and the C-2′ salt, C-6 ether. Strategies for manipulating the reactive functional groups in the presence of one another are reported.

Fluorescence spectra, quantum yields and lifetimes are comparatively studied for aniline, N,N-dimethylaniline (DMA) and some further alkylsubstituted and rigid aniline derivatives in various solvents. A good correlation of Stokes shift with macroscopic solvent parameters is found for DMA but not for aniline. General and specific solvent effects are distinguished by studying the functional dependence of various spectroscopic and photophysical parameters on the polar co-solvent concentration in binary solvent mixtures. Excited state complexation of primary and secondary amines with alcohols is demonstrated by the double-exponential decay of the monomer fluorescence and the grow-in of exciplex emission found in aniline—ethanol—n-hexane ternary systems. Exciplex formation is attributed to solvent—donor hydrogen bonding and primarily causes enhanced intersystem crossing compared with that in the free molecule, an effect also evoked by N-methylation. Non-specific long-range interactions give rise to a general reduction in the non-radiative deactivation rate, with the exception of o,o′-dimethyl-substituted anilines, for which an increase in this rate was found as the bulk becomes polar.

The photoreaction of some polycyclic ketones, derived from natural sesquiterpenic hydrocarbons, involves the formation of the ketene intermediate. This ketene can either add to protic solvents such as alcohols to give the corresponding esters, or decarbonylate in aprotic solvents. This decarbonylation of the ketene occurs only if its vibrational energy reaches a sufficient level, and leads to a carbene with a good yield. In comparison with aprotic solvents, the quantum yield of photoreactivity is increased when alcohol is present, while the quantum yield of phosphorescence is lower.

The photoisomerization about the αβ and γδ double bonds and the electrocyclic ring-closure reaction are competing processes within the excited singlet state of the phenylfulgides. Both processes are induced by the torsion of bulky molecular parts. Because of the extreme steric hindrance the torsion processes occur on potential curves without any activation barrier and consequently the photoisomerization, the photocyclization and the radiationless deactivation of the phenylfulgides are ultrafast processes proceeding within a few picoseconds, or even less, as the absorption measurements performed on a picosecond excite-and-probe beam spectrometer reveal. The competition between the E—Z isomerization and the cyclization is expressed by means of the partial quantum yields of these processes. The experimental results are interpreted in terms of the πbond orders and the sums of free-valence indices for the bondforming atoms in the S₁ state calculated by the Pariser—Parr—People method.