Journal of Photochemistry最新文献

Molar absorption coefficients of NONH₂ have been measured for bands at 285 nm, 365 nm and 550 nm, and the bands assigned to the tautomeric species HNNOH, NH₂NO and HNNHO respectively. Solutions of NO in liquid ammonia have an absorption band at 425 nm which is assigned to a charge transfer complex of NO with NH₃. It is suggested that coloured solutions of NONH₂ and NO in liquid ammonia may contribute to the colours observed in the atmosphere of Jupiter.

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 absorption cross-sections σ_CF2 for the CF₂ radical in the UV for the transition $\text{A}$ ¹B₁←$\text{X}$ ¹A₁ at 0.5 nm intervals were obtained. CF₂ was produced by the flash photolysis of C₂F₄ and C₃F₆ between 185 and 209 nm. The σ_CF2 values determined with the quantum yield φ_CF2 of CF₂ as 1.0 from the photodissociation of CF₃CFCF₂ and φ_CF2=2.0 in the photolysis of C₂F₄ were identical within the experimental error of ±15%. HBr actinometry with the emission spectra of the lamps and the σ vs. wavelength characteristics of C₂F₄ and C₃F₆ were used to determine the absolute number of photons absorbed per flash. A maximum σ_CF2 of (2.91±0.44)×10⁻¹⁷ cm² molecule⁻¹ was obtained at 249.0 nm. This was compared with values reported in the literature. The oscillator strength of the CF₂ transition was determined to be 0.026.

The decay of CF₂ was studied in flashed C₂F₄, C₂F₄ plus added gas, C₃F₆ and C₃F₆ plus added gas mixtures. In all cases CF₂ was found to decay by the second-order reaction CF₂+CF₂→C₂F₄. The rate constant for this dimerization was found to be (4.26±0.43)×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. No evidence for the reaction of CF₂ with C₂F₄ and C₃F₆ to form the next higher perfluorocarbon was observed.