Showing papers on "Excimer published in 1970"

Evidence for exciplex laser action in coumarin dyes by measurements of stimulated fluorescence

Abstract: Experiments have been performed that measure the time dependence of stimulated fluorescence from acidified solutions of 4‐methylumbelliferone and 7‐hydroxycoumarin. The results show that laser action takes place from excited state complexes. It was found that exciplex emission is delayed after the neutral molecule emission. The delay gives a measure of the exciplex formation time. The measured dependence of exciplex formation time on hydrogen ion concentration, temperature, and pumping power was found to agree qualitatively with a simple model. Evidence for multiple exciplex formation is also presented.

The Interaction of Oxygen with Organic Molecules. II. Oxygen Quenching of the Excited States of Adsorbed Aromatic Hydrocarbons

Abstract: Quenching of phosphorescence and fluorescence of aromatic hydrocarbons adsorbed on powdered porous glass or silica gel have been examined as a function of oxygen pressure. It has been found that the phosphorescence quenching at low temperature can be explained mostly by assuming that only those aromatic molecules having an oxygen molecule or molecules within an effective interaction radius are nonphosphorescent (static quenching model). The fluorescence quenching occurs only at much higher oxygen pressures and it has been proposed that contact pairs between aromatic and oxygen molecules, which have shorter intermolecular distances compared to the distances effective for the phosphorescence quenching and give rise to the charge-transfer absorption bands, are relevant. It has been observed that the excimer fluorescence of naphthalene appearing at a higher sample concentration is more strongly affected by oxygen than the monomer fluorescence. The emission spectra from naphthalene-porous glass adsorbate immer...

Absorption and fluorescence of a nonionic detergent in aqueous solution

Abstract: The absorption and fluorescence spectra of a detergent, polyoxyethylene octylphenyl ether, were measured in water at different concentrations. The absorption spectra had a peak at 275 mμ and a shoulder at 281 mμ. The spectra were independent of concentration below the critical micelle concentration, but the molar extinction coefficients of the peak and the shoulder increased with concentration above the critical micelle concentration. The critical micelle concentration value derived from the absorption data was in good agreement with those obtained by other methods. The fluorescence spectra of the detergent in water were independent of concentration in dilute solutions, if the intensities of the spectra were normalized at the peak at 302 mμ. At higher concentrations, a weak excimer band appeared at 345 mμ, whose intensity increased with concentration. The excimer band manifested itself at a concentration slightly lower than the critical micelle concentration. The main band decreased and the excimer band increased, as the temperature was raised. An excimer band was observed in the same region of the spectra for the pure detergent, either in the solid or liquid state. The equilibrium solid spectra exhibited a very strong excimer emission. It was concluded that the excimers were formed within micelles in the case of aqueous solutions.

The Luminescence of the Pyrene-Perylene System: Evidence for Mixed Excimers in the Solid State

Abstract: Absorption and fluorescence data are presented for the mixed crystal system—perylene in pyrene. It is shown that there is an anomalous luminescence from the mixed crystals above ca. 125°K, at which temperature we suggest there is a phase transition occurring in the host lattice. The anomalous emission is studied between 125°K and 450°K and the temperature dependence of its spectral shape and location is consistent with there being two superimposed emission spectra, one corresponding to a perylene weakly associated with pyrene, and the other corresponding to perylene more strongly associated to host molecules. The latter emission is spectrally diffuse, and could be the mixed excimer, or exciplex of perylene and host pyrene.

Polarized absorption and fluorescence spectra of crystalline pyrene

Abstract: The study of polarized absorption spectra on the ab face of crystalline pyrene at 90K has been reported. Measurements on the a:b polarization ratio and vibrational analysis for the different absorption systems are presented. It has been found that the second absorption system could be analysed in terms of frequencies of totally symmetric vibrations, whereas many of the vibrations in the first absorption system could not be unambiguously assigned. The excimer fluorescence of crystalline pyrene has also been studied on the ab and bc' faces. From the analysis of the data it has been found that, while the major contribution to the excimer fluorescence is derived from the transition directed along the long axis of the monomer molecule, the contribution due to an out-of-plane polarized transition is not negligible.

Some Aspects of the Quantum Theory of Photochemical Reactivity of Organic Molecules

Abstract: Publisher Summary This chapter highlights differences and the analogies between thermo and photoreaction in order to indicate the methods that can be applied in both cases, and special procedures that can be used in studying photochemical processes. It also discusses base strength of conjugated molecules in their electronic excited states and rate of photochemical reaction. It is difficult to obtain good wave functions for molecules in their ground state. Very often for conjugated molecules only π orbitals are considered. Recently, an all-electron self-consistent field (SCF) wave function for the ground state of pyridine was calculated, that led to the conclusion that the π orbitals have negligible contributions to the electronic charges on the various carbon atoms in this molecule. For the excited states the situation is worse. Conjugated molecules are usually assumed to have well-separated bands of π and σ orbitals. The representation of the first excited states is assumed to retain the same σ orbitals as the ground state. In fact this separation is known not to be true, even for ethylene. The chapter also provides summary concerning the calculation of rate constants for molecules in their ground state.

Fluorescence spectra of polystyrene excited by beta-rays and uv-light

Abstract: Monomer free polystyrene samples, produced by evaporation from a solvent excluding oxygen, show by excitation with UV-light (λ=270 nm) or with β-rays besides the excimer peak at 330 nm (P-peak) an additional fluorescence peak at 390 nm (G-peak). This peak disappears by melting of the sample or admission of air. The influence of additives on the fluorescence spectra was investigated. 1,2-di-(2-pyridyl)-ethylene, an efficient quencner for the first excited singlet state of polystyrene, reduces the P-peak much more than the G-peak. Therefore it is inprobable that the states responsible for the P-and G-peak are excimers of different distance or symmetry of the segmers or excimers with more than 2 segmers participating. The intensity of the G-peak in comparison to the P-peak increases by addition of low concentrations of hexachlorocyclohexane (c<10−5 mole/1). The dependence on the structure of the polymer and the influence of the electron scavenger suggests, that the appearance of the G-peak is connec...