J. Joussot-Dubien

pH dependence of singlet oxygen production in aqueous solutions using thiazine dyes as photosensitizers.

Abstract— The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of 1O2* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the pKs of the triplet dyes, by evaluating the yields of 1O2* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.

Singlet-singlet absorption and intersystem crossing from the 1B3u− state of naphthalene

Abstract By flash photolysis using a pulsed laser, we have examined the following intramolecular processes occuring from the lowest excited singlet state of naphthalene in cyclohexane at room temperature: fluorescence lifetime, excited singlet-singlet absorption and intersystem crossing kinetics.

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS‐II. pH DEPENDENCE OF ELECTRON ABSTRACTION RATE CONSTANT OF THE DYES IN THEIR TRIPLET STATE

Abstract— The pH dependence of the apparent reactivity of thiazine dyes in their triplet states has been studied in aqueous solutions, using as electron donor HY‐3, the trianionic species of ethylene diamine tetraacetic acid (EDTA), in the pH range 4–8. The pH dependence is found to be related to a change in the degree of protonation of the triplet excited dye. The apparent reactivity and lifetime of two differently protonated forms of thionine, azur B and methylene blue were determined by classical and dye‐laser flash techniques, making it possible to evaluate the rate constant for electron abstraction of these molecules in their triplet states. It is found that: (a) protonation on the ring nitrogen increases the electron‐abstraction rate constant of the triplet‐state species about twenty‐fold, and (b) methylation on the side amino groups decreases it.

Multiplet structure of the emission bands of coronene and perylene in n-heptane single crystal

Abstract Highly resolved emission spectra of coronene and perylene molecules included in single crystals of n -heptane have been observed at 4.2 K. Comparison with spectra obtained in polycrystallized solutions indicates that solute molecules are trapped in similar substitutional sites in both matrices. Polarization study of the multiplet components and ESR measurements reveal that multiplets are due to molecules differing only slightly in orientation.

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH-PHOTOLYSIS-I

Abstract— The photoreduction of thiazine dyes by ethylene diamine tetraacetic acid (EDTA) was investigated by Rash photolysis. This reaction was found to occur according to a three‐step mechanism. the first being the formation of the dye triplet state followed, in weakly acid solutions, by protonation. During the second step, the triplet state of the dye disappears through two competing processes: spontaneous deactivation and reaction with EDTA, which leads to the semireduced dye. The third step leads to the leucodye. It is shown that the overall quantum yield of photoreduction is governed by the second step and can be calculated from the ratio of the rate constants of the two elementary processes involved in this step. This ratio was measured over a wide pH range.

Laser flash photolysis of iodine dissolved in hydroxylic solvents. Identification of iodine atom charge transfer complexes

Sub-microsecond transient absorptions observed by laser flash photolysis of iodine in water, in methanol and in isopropanol in the 250-400 nm spectral range are attributed to charge transfer complexes of iodine atoms with solvent molecules. The 385 nm transient absorption previously attributed to these charge transfer species is reassigned to I−2 radical anions. A good correlation is found between the frequency of maximum absorption and the ionization potential of the solvent molecules.

Dispersion states and orientation of aromatic solutes in monocrystalline matrices of n ‐heptane

Solid solutions of coronene, perylene, and pyrene in monocrystalline n ‐heptane have been prepared by slow cooling. The solutes exhibit dichroic absorption, fluorescence, and phosphorescence spectra that make evident an orientation of aromatic molecules in the paraffinic crystal lattice. Molecular aromatic planes are found to be nearly perpendicular to the growth axis, while their largest axis is oriented in the direction of alkane chains. The sites in which the molecules are trapped appear to be similar to the ones obtained with low concentrated quickly frozen solutions. Although aggregates could be present, they are not observed.

Laser flash photolysis of iodine-iodide mixture in hydroxylic solvent. Evidence for the existence of the radical anion I

Abstract The laser flash photolysis of iodine-iodide mixtures in alcoholic solvents produces transient species absorbing in the red and near infrared region with maxima at 740 and 590 nm. The 740 nm absorption band has already been attributed to the well known I ion. In this work the 590 nm absorption band is assigned to the I radical anion which presumably could be formed by the equilibrium reaction: I + I 3 - ⇌ I . The variations of the intensities of the 590 and 740 nm transient absorption bands as a function of I 3 − concentration, temperature and solvent, support this assignment. The kinetics of formation of I and I has been studied in solutions of tertiary butanol. An analysis of the experimental results shows that the I ion could be formed via a mechanism involving an I intermediate.

Oriented interaction between naphthalene and alkane molecules: Theoretical interpretation of spectral shifts of ππ* bands of aromatic hydrocarbons in alkane solvents

Abstract Several features of the spectral shifts of ππ * bands for aromatic hydrocarbons dissolved in n -alkane solvents have been explained by the existence of preferred relative orientations between the aromatic hydrocarbon and the n -alkane molecules. Theoretical evaluation of the interaction energy between naphthalene and n -pentane or n -octane actually display two preferred orientations. Moreover, these orientations are identical with those proposed, on steric grounds, for explaining the adsorption of alkanes on graphitic basal planes as proposed by Groszek.

Reactive scattering using pulsed crossed supersonic molecular beams. Example of the C+NO→CN+O and C+N2O→CN+NO reactions

The dynamics of the C+NO→CN+O and C+N2O→CN+NO reactions are reinvestigated using a pulsed supersonic crossed molecular beam apparatus. Laser vaporization of graphite at the exit of a pulsed nozzle is used to produce the atomic carbon beam. Laser induced fluorescence spectra of the CN scattered product have been obtained for both reactions.