Philippe Fornier de Violet

A photochemical study of an O-methylated α-carbonyl β-1 lignin model dimer: 1,2-di(3′,4′-dimethoxyphenyl)ethanone (deoxyveratroin)

Abstract The photochemistry of 1,2-di(3′,4′-dimethoxyphenyl)ethanone (deoxyveratroin) ( 1 ) (an O-methylated α-carbonyl β-1 lignin model dimer) was examined. Its photochemical reactivity was studied in both liquid (benzene and ethanol) and solid (adsorbed on lignin-free fibres) states in degassed or aerated media. In degassed benzene and fibres, 1 is photochemically transformed into several identified compounds (monomeric to tetrameric units) and a coloured mixture of higher molecular weight fractions. The structures and titration of the photoproducts are well explained by Norrish type 1 cleavage and photoreduction of the carbonyl group. The presence of oxygen inhibits the reductive mechanism and leads to veratric acid by trapping the ketyl and desyl radicals. The observation of a low disappearance quantum yield of 1 in degassed ethanol is attributed to the presence of a low-lying π,π* triplet state as shown by phosphorescence.

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.

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.

Electron transfer from the excited singlet and triplet states of aromatic hydrocarbons to methyl viologen

Abstract Evidence is presented that the excited singlet and triplet states of aromatic hydrocarbons are oxidised by methyl viologen (MV 2+ ). Rate constants for electron transfer from triplet naphthalene, anthracene, phenanthrene and fluoranthene are close to diffusion-controlled whereas those for triplet 1-cyanonaphthalene and 1-cyanoanthracene are about one order of magnitude lower.

Solid Phase Enhanced Direct Spectrofluorometric Determination of Polychlorinated Biphenyls (PCBs) in Natural Waters

Abstract A method of detection of PCBs in water, based on extraction/concentration with chips cut from an SPE disk-shaped C18 inverse phase coupled with a fluorescence detection and quantification directly on the phase has been evaluated. Experiments were performed with PCBs congeners IUPAC N° 3, 35, 67 and 169. Within the experimental conditions used : Hitachi F-4500 fluorometer, band-paths: 2.5 nm and extraction time: 60 min. the limits of detection were estimated to be respectively : 0.07, 0.4, 0.3 and 0.2 ppb. These values are within the range of PCB concentrations found in highly polluted waters and thus confer some interest to the method. However its sensitivity is not sufficient for application to moderately polluted natural water and the method needs some further improvements. Moreover, fluorescence quenching by humic substances was shown to substantially affect PCBs fluorescence intensity and must be taken into consideration for quantitative analysis.

The effect of changing the electronic configurations of the excited states on the rate constant of photocyclization of aryl ethylenes

Abstract The addition of the phenyl group in the para position of diphenyl in the 1-( o -diphenyl)-1-phenylethylene (DPE) molecule was carried out in order to induce a modification of the configuration of the lowest excited states. Spectral data and theoretical calculations showed that the S 1 state of the resulting 1-(2′- p -terphenyl)-1-phenylethylene corresponds to an allowed transition 1 L a ← 1 A 1 . The lowest excited state of DPE has a mainly 1 L a character (84%) with a weak 1 B b contribution (5%). The moment of the corresponding transition is weak. The modification of the configuration of the lowest excited states changes both the electronic distribution and the flexibility of the molecule. The differences in the photochemical reactivity are best explained by geometric factors.

Laser flash photolysis of iodine-iodide mixture in tertiobutanol solution. Evidence for the existence of the radical anion

Abstract Laser flash photolysis of a concentrated iodine solution in the presence of I − ions in tertiobutanol gives rise to a transient absorption band with a maximum at 540 nm. At equilibrium the solution contains mainly I 2 , I 5 − and to a lesser extent I 3 − . The actinic flash at 529 nm photodissociates the iodine into iodine atoms which can be scavenged by I 5 − to give the transient species absorbing at 540 nm. As the I 5 − concentration is increased the radical anion is also formed by direct photodissociation of I 5 − since its absorption reaches the 529 nm range. Analysis of the kinetic data is given.