Journal of Photochemistry最新文献

Excimer formation in 2,5-diphenyloxazole (PPO) in ethanol is shown not to obey the expected biexponential kinetics because of the presence of time-dependent quenching. Between 25 and 50°C the decay of monomer fluorescence can be described by a model based on the linear superposition of excimer reverse dissociation and transient quenching to give values for the diffusion coefficient and the interaction radius. However, at lower temperatures the model provides an incomplete description and the indications are that this is caused by the emergence of a heterogeneous distribution of PPO molecules.

The quantum yield for the photodestruction of N-phenylbenzenecarbohydroxamic acid was determined in a number of solvents. Protonation was found to photostabilize both the hydroxamic acid and its anion. Quenching studies showed that the reaction in cyclohexane occurs from both the singlet and the triplet state, giving benzanilide as the main photoproduct. The results are discussed in the light of other recent investigations in the field.

Fluorescence spectra were recorded as a function of temperature (10-60°C) for dilute dioxane solutions of four homopolymers containing the benzyl group in different positions with respect to the backbone, poly(α-benzylmethyl acrylate), poly(benzyl methacrylate), poly(monobenzyl itaconate) and poly(dibenzyl itaconate). In all cases the intensity decreases with increasing temperature. Arrhenius plots show two lines with different slopes intercepting at a point which we ascribe to a transition temperature between two states of the coil characterized by different activation energy parameters. The transition temperatures are around 29°C for all the systems except for poly(α-benzylmethyl acrylate) which gives simple linear behaviour without a transition. Activation energy parameters are compared for the different structures studied.

Forward and backward rate constants for the excited state deprotonation reaction of carbazole in ammoniacal aqueous media were determined from both steady state (Weller's fluorescence quenching technique) and direct kinetic studies. The results establish that the equilibrium pK^* may be significantly different from the “apparent” pK^* obtained by the fluorometric titration method.

Kinetic electron spin resonance spectroscopy has been applied in order to follow the variation in peroxy radical concentration in the cumene—anthraquinone system in the presence of oxygen and during UV irradiation. A periodic variation in concentration was observed, which can be caused either by hydrodynamic convections or by diffusion-controlled photochemical oscillations. For the latter case, a model was suggested in which the anthraquinone regeneration, controlled by the local oxygen concentration, can serve as a feedback mechanism, and the oscillations can be maintained by concentration gradients between the irradiated and the dark zones and by the different diffusion rates of oxygen and anthraquinone.

The solvent-dependent electronic absorption and fluorescence spectra indicate that quinoxaline-2,3(1H,4H)dione exists in its keto form, even in non-polar solvents, but the deprotonation reaction is observed from its enol form. The protonation reaction in comparison with those of the parent quinoxaline and 1,4-dimethylquinoxaline-2,3-dione suggests that the cation is formed by the gain of a proton on one of the carbonyl groups of the molecule.

The beam gas chemiluminescence reactions of samarium with SF₆ and samarium with CCl₄ have been studied. The emission is attributed to SmF₂^* and SmCl₂^* emitters. The reactions are first order with respect to both metal and oxidizer gas at low pressures. The total cross-section, chemiluminescence cross-section and photon yields for the reactions have been found to be 280 Ų, 0.44 Ų and 0.0015 respectively for the samarium plus SF₆ reaction and 315 Ų, 0.32 Ų and 0.001 for the samarium plus CCl₄ reaction.