Thierry Pigot

Photo-oxidation of di-n-butylsulfide by various electron transfer sensitizers in oxygenated acetonitrile

The selective activation of different photosensitizers has been carried out under comparable conditions and their efficiency towards di-n-butylsulfide oxidation in oxygenated acetonitrile compared from the product distribution after 150 minutes of irradiation. As expected, the best selectivity towards sulfoxide is obtained with a conventional energy transfer sensitizer such as Rose Bengal (RB), but also with a quinone with a low-lying triplet state, 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil or CHLO) and with 9,10-dicyanoanthracene (DCA). More significant yields in sulfonic and sulfuric acids are obtained under sensitization with 9,10-anthraquinone (ANT) or a derivative of benzophenone, 4-benzoyl benzoic acid (4-BB), with which additional experiments were carried out in order to discuss the involvement of either singlet oxygen or superoxide radical anion. Triphenyl pyrylium tetrafluoroborate (TPT+) is inefficient under the selected conditions and sulfide photo-oxidation can only be achieved with higher TPT+ concentrations with simultaneous total TPT+ bleaching. With TPT+, 1,2,4,5-tetracyanobenzene (TCNB) and TiO2, the product distribution and the low selectivity as well as the formation of numerous common by-products are indicative of radical mechanisms. All these results are discussed according to the possible formation of activated oxygen species, such as singlet oxygen, superoxide radical anion or alkylperoxy radicals.

Solvent-Free Production of Singlet Oxygen at the Gas-Solid Interface: Visible Light Activated Organic-Inorganic Hybrid Microreactors Including New Cyanoaromatic Photosensitizers

We synthesized new cyanoaromatics, benzo[b]triphenylene-9,14-dicarbonitrile (DBTP) 1a and a graftable derivative, 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid (DBTP-COOH) 1b, easily prepared from commercial reagents. Their photosensitizing properties were investigated. Hybrid porous silica monoliths loaded with encapsulated 1a or grafted 1b were prepared, and their adsorption, spectroscopic and photosensitizing properties, as well as stability, were compared. Solvent-free, efficient oxidation of dimethylsulfide (DMS) was observed at the gas-solid interface under visible light irradiation. Quantum yields of formation of 1O2 inside the porous monoliths are comparable to those of phenalenone. Singlet oxygen lifetimes (approximately 25 micros) were found to be longer in silica monoliths than in usual polar solvents such as methanol or ethanol. This new class of hybrid materials work as porous, transparent, and highly efficient microreactors for oxidation reactions under visible light.

A new cyanoaromatic photosensitizer vs. 9,10-dicyanoanthracene: systematic comparison of the photophysical properties

The cyanoanthracene derivative, benzo[b]triphenylene-9,14-dicarbonitrile (1) can be prepared readily with a graftable function while maintaining (1)O(2) photosensitizing properties comparable to those of the standard compound 9,10-dicyanoanthracene (DCA). In view of the high potential of the derivatives of 1 for photooxidation reactions under heterogeneous conditions, we compared the photophysical properties of 1 in solution with those of DCA. In pursuing the comparison of 1 and DCA, we observed small but significant changes of the vibronic bands in the electronic absorption spectra of DCA in different solvents, which were well correlated with solvent polarity, similar to the pyrene polarity scale. The main difference between 1 and DCA is in the emission properties: we observed a much stronger sensitivity of the fluorescence emission spectrum to the electron-donating ability of the solvent than for DCA. The emission spectrum of 1 is in general structureless with a large Stokes shift. The ability of the singlet state of 1 to participate in charge transfer interactions with electron-donating solvents is proposed to account for these results. It makes 1 a highly sensitive probe to the surrounding medium. Reversible reduction was observed for both photosensitizers, with a small shift to more negative potentials for 1 compared to DCA. The reduction potential of the first singlet excited state is of the same order of magnitude in both cases. Several photo-oxidation reactions sensitized by 1 and DCA are compared in homogeneous solution and at the gas-solid interface by embedding 1 and DCA in silica monoliths. Our results confirmed the dual character of both cyanoanthracene derivatives as electron transfer and energy transfer sensitizers, highly efficient for singlet oxygen production.

New materials for sensitized photo-oxygenation

A variety of photosensitizers has been included, adsorbed or grafted onto different supports. This review focuses on original new materials designed for photoinduced oxygenation reactions, using light as the activation method and oxygen as the reactant. Such materials find widespread applications for the oxidation of fine chemicals under mild conditions, for water treatment, for photodynamic bactericidal inactivation or photodynamic therapy. The generation, diffusion and reactivity of Reactive Oxygen Species (ROS), and particularly of singlet oxygen 1O2 in these heterogeneous media are discussed.

Gas-phase generation and UV photoelectron spectroscopy studies of diaminogermylenes

Ionization potentials (IPs) are the real fingerprint of the electronic structure of molecules, and the combination of flash vacuum thermolysis reactions with ultraviolet photoelectron spectroscopy, supported by theoretical study, is a very efficient method of using these IPs to characterize different reactive intermediates in the gas phase. This approach was applied to elucidate diaminosubstituted germylene chemistry, which is an interesting example of germylene electron deficiency reduced by the donation of two nitrogen lone pairs, while the germylene lone pair is stabilized by the inductive effect of two electronegative nitrogen atoms (thermodynamic stabilization). We report here the first direct evidence by UV photoelectron spectroscopy of saturated, cyclic diaminogermylene 2, generated by FVT of germacyclopentene 1.

Mechanism for the oxidation of phenol by sulfatoferrate(VI): Comparison with various oxidants

The oxidative action of a solid and stable potassium sulfatoferrate(VI) material on phenol was studied in aqueous solution under different stoichiometries. The performance towards phenol and the total organic carbon is compared to that of potassium permanganate and calcium hypochlorite. The total mineralization of phenol is not completely achieved by the studied chemical oxidants, and some oxidation products have been identified by gas chromatography-mass spectrometry and gas chromatography-flame ionization detector analysis. A radical reaction pathway, involving the formation of oxidation intermediates or by-products such as benzoquinone, phenoxyphenol and ring opening products, is proposed for the decomposition of phenol by ferrate(VI). Phenoxyphenol is also involved in the oxidation mechanism for permanganate whereas chlorinated phenols are produced by hypochlorite. The role of the chloride anion impurity of the potassium sulfatoferrate(VI) material has been highlighted in this study; no negative impact on the removal of phenol and its mineralization is observed compared to the use of a pure commercial ferrate(VI). The efficiency of sulfatoferrate(VI) for the oxidative removal of phenol from industrial wastewater is also confirmed.

Immobilized organic photosensitizers with versatile reactivity for various visible-light applications.

Various photosensitizers were grafted by conventional peptide coupling methods to functionalized silica with several macroscopic shapes (powders, films) or embedded in highly transparent and microporous silica xerogel monoliths. Owing to the transparency and free‐standing shape of the monoliths, the transient species arising from irradiation of the PSs could be analyzed and were not strikingly different from those observed in solutions. The observed reactivity for either liquid–solid (α‐terpinene oxygenation vs dehydrogenation) or gas–solid (dimethylsulfide, DMS, solvent‐free oxidation) reactions was consistent with the properties of the excited states of the PSs under consideration. Immobilized anthraquinone‐derived materials preferentially react in both cases by electron transfer from the substrate to the triplet state of the sensitizer, in spite of an efficient singlet oxygen production. The recently developed 9,14‐dicyanobenzo[b]triphenylene‐3‐carboxylic acid, DBTP‐COOH, efficiently reacts via energy transfer to yield singlet oxygen from its triplet state. It was shown to perform better than 9,10‐dicyanoanthracene and rose bengal for DMS oxidation and α‐terpinene photooxygenation to ascaridole, respectively. Thus, by a proper choice of the organic immobilized photocatalyst, it is possible to develop efficient and reusable materials, activated under visible light, for various applications and to tune the reaction pathway, opening the way to green oxidation processes.

Efficient cyanoaromatic photosensitizers for singlet oxygen production: synthesis and characterization of the transient reactive species

In order to graft cyanoaromatic molecules onto various inert supports, we designed two new cyanoanthracene derivatives of benzo[b]triphenylene-9,14-dicarbonitrile (DBTP, 1), which already demonstrated good photosensitizing properties. We synthesized 3-(N-hydroxypropyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 3 and 3-(N-N0-Boc-aminohexyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 4 and compared their photophysical properties in acetonitrile relative to those of the parent compound 1 and its carboxylic derivative 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid, 2. The transient species were analysed and the quantum yields of singlet oxygen production (ΦΔ) determined in acetonitrile. The effect of chemical functionalization can be considered negligible, since absorption spectra, fluorescence emission spectra and fluorescence lifetimes do not significantly change with the substituent. The triplet-triplet absorption spectra and the triplet excited state lifetimes are similar for the whole series. For compounds 1-4 high values of ΦΔ, close to that of the standard sensitizer 1H-phenalen-1-one (PN, ΦΔ ≈ 1), and higher than that of the well-known photosensitizer 9,10-dicyanoanthracene (DCA), are due to very efficient intersystem crossing from the singlet to the triplet excited state and subsequent energy transfer to ground state oxygen ((3)O2). They belong to a class of very efficient photosensitizers, absorbing visible light and stable under irradiation, they may be functionalized without significant changes to their photophysical behaviour, and grafted onto various supports.

Synthesis, complexing properties and molecular modelling of open chain receptors of barbiturates derived from 2,6-diamino pyridine

Three new derivatives of 2,6-diacyldiaminopyridine are reported. NMR shift titrations were performed in CDCl3 with barbiturates. The diamide1 affords a greater complexation energy (−13.00 kJ mol−1) with bemegride than the dithioamide2 (−9.15 kJ mol−1). This result, unexpected on the basis of the proton acidities, is explained by the great torsion energy induced in2 by the bulky sulfur atom. Compounds3 and4 present unusual four and five H-bond features with barbital and relatively weak complexation energies (−9.53 and −16.34 kJ mol−1, respectively). Molecular mechanics indicates that ligand4 displays a helical secondary structure which is disrupted by complexation. Calculations of the H-bond energies (ΔEcalc.) of the intermolecular assemblies with barbital or phenobarbital and other host-guest complexes given in the literature give a good correlation (r=0.98) with experimental values: ΔEcalc.=1.07 ΔGa−42.0. Limitations of this relation are discussed.

Assessment of erosion and sedimentation dynamic in a combined sewer network using online turbidity monitoring

Eroded sewer sediments are a significant source of organic matter discharge by combined sewer overflows. Many authors have studied the erosion and sedimentation processes at the scale of a section of sewer pipe and over short time periods. The objective of this study was to assess these processes at the scale of an entire sewer network and over 1 month, to understand whether phenomena observed on a small scale of space and time are still valid on a larger scale. To achieve this objective the continuous monitoring of turbidity was used. First, the study of successive rain events allows observation of the reduction of the available sediment and highlights the widely different erosion resistance for the different sediment layers. Secondly, calculation of daily chemical oxygen demand (COD) fluxes during the entire month was performed showing that sediment storage in the sewer pipe after a rain period is important and stops after 5 days. Nevertheless, during rainfall events, the eroded fluxes are more important than the whole sewer sediment accumulated during a dry weather period. This means that the COD fluxes promoted by runoff are substantial. This work confirms, with online monitoring, most of the conclusions from other studies on a smaller scale.