Journal of Photochemistry最新文献

The rate constants for the quenching of the fluorenone fluorescence by substituted benzaldehydes and benzoic acids (substitutents: p-nitro, p-cyano, p-chloro, p-bromo, p-H, p-methyl, p-methoxy, p-methoxy-m-methyl, and p-dimethylamino) are reported in ethanolic and micellar media. These are compared with the diffusion rate constant and evidence is presented that, under chemiluminescence mimetic conditions, the quenching leaving moiety in close proximity to the emitter is responsible for most of the quenching observed. The binding constants and partition coefficients of the above compounds as well as those of the corresponding substituted 9-benzylidene fluorenes in cetyltrimethylammonium chloride are also reported together with quencher molecules per micelle. The influence of the micellar medium on the quenching rate constants is discussed and the position of the above compounds inside the micelle is reported.

The separation of the products of the photodegradation of 7-amino-4-methylcoumarin (C120), a laser dye, has been achieved by a reverse phase high performance liquid chromatography method. Two reverse phase columns (C6 and ODS) were used to optimize the separation conditions and various mixtures of methanol and water served as the mobile phase. The chromatograms representing the separation of the photolyte from the photoproducts under various experimental conditions are also shown.

The fluorescent photodegradation product in an oxygenated methanolic solution of the dye has been separated by column chromatography assisted by thin layer chromatography and high performance liquid chromatography. The structure of the compound has been ascertained by mass spectrometric and IR spectroscopic analysis and found to be 7-amino-3-hydroxy-4-methyl-coumarin. A mechanism is suggested for the formation of this product by the interaction of singlet oxygen and the dye. On the basis of the behaviour of the dye in degassed solutions, a general scheme is postulated to represent the photobleaching of C120.

A series of 9-(p-substituted) benzylidene fluorenes (p-nitro, p-cyano, p-chloro, p-bromo, p-H, p-methyl, p-methoxy, p-methoxy-m-methyl and p-dimethylamino) was synthesized and the rates of fluorenone production on oxidation with ozone were followed fluorimetrically (apparent rates) and spectrophotometrically (true rates). The rates were plotted against Hammett's σ constants and any quenching effect of the leaving moiety on fluorenone at the moment of their production was illustrated as deviation from the plot of the apparent rates. The above effect was intensified in micellar media as a result of a more rigid confinement of the two species inside the micelle, supporting an earlier suggestion that, in chemiluminescence, even a small degree of quenching of the primary emitter by the leaving moiety can be very effective as a result of their inherent proximity at the moment of their production.

Three pseudoazulenes derived from 7-azaindole reveal photophysical properties characteristic of this class of compounds: strongly temperature-dependent S₁ → S₀ fluorescence, weak phosphorescence, very small S₁—T₁ splitting, decrease in dipole moment on excitation to S₁ (ππ*) and S₂(ππ*) states and dimerization at low temperatures in non-polar solvents. Photoacoustic studies show that internal conversion is the dominant channel for the deactivation of the lowest excited singlet state.

The binding constants, mean occupation numbers per micelle, partition coefficients and per cent micellization of nine 9-(p-substituted) benzylidene fluorenes (p-nitro (i); p-cyano (ii); p-chloro (iii); p-bromo (iv); p-H (v); p-methyl (vi); p-methoxy (vii); p-methoxy-m-methyl (viii); p-dimethylamino (ix)) in cetyltrimethylammonium chloride and sodium dodecylsulphate were determined from the corresponding Stern—Volmer plots, with pyrene as the fluorescent probe. The ozonolysis reaction rates of the nine benzylidene fluorenes were measured in the micellar systems and, from the above partition coefficients together with the reaction rates in the bulk phase, the reaction rates in the micellar pseudophase, independent of the reaction rate in the bulk phase and the partition coefficient, were determined; the plots of these reaction rates (log k_tr or log k_app) against Hammett's σ constants were linear.

The excited state behavoir of 2-phenylbenzthiazole and 3-phenyl-1,2- benzisothiazole was examined by laser flash photolysis techniques in order to determine which reaction intermediates preceded their subsequent photochemistry. Both compounds, together with related derivatives, form longlived readily detectable triplet states. Quenching constant with representatives olefins have been determined and the energy transfer equilibrium between 3-phenyl-1,2-benzisothiazole and 1-methylnaphthalene has been examined. The results suggest that olefins and alkynes react with excited states of 2-phenylbenzthiazole and 3-phenyl-1,2-benzisothiazole by 1,2 addition, and no evidence for prior ring opening was obtained.