Sylvie Blanc

Self-Assembly of Tricuprous Double Helicates: Thermodynamics, Kinetics, and Mechanism

We report in this paper the coordination and kinetic properties of two oligobipyridine strands, which contain three 2,2′-bipyridine subunits separated by oxydimethylene bridges, the 4,4′-bis(CONET2)-substituted L and the 4,4′-bis(CO2Et)-substituted L′. Spectrophotometric measurements allowed the characterization of thermodynamic complexes and kinetic intermediates* which are involved in the self-assembly process of L2Cu3 and L2Cu3 helicates. The reaction presents positive cooperativity for the binding of two 2,2′-bipyridine strands to the cuprous cations. While reactive kinetic intermediates* present distorted coordination geometries around Cu1, the final rearrangement of the tricuprous bistranded helicates allows more closely tetrahedral coordination of each cation and reduces the interactions. Differences in the bulkiness and electronic properties of the L and L′ substituents do not affect significantly the stability of the corresponding helicates, but greatly influence binding rates in the self-assembly process.

Diffuse reflectance UV-Visible spectroscopy for the qualitative and quantitative study of chromophores adsorbed or grafted on silica

Diffuse reflectance UV-Visible spectroscopy (DRUV) is used to study the adsorption and grafting of the pyrene derivative 1 on different types of silicas. For the adsorbed silicas, the formation of dimeric aggregates is evidenced both by DRUV and by fluorescence spectroscopy. The processing of the UV spectrophotometric data allows, as in solution. the determination of apparent equilibrium constants and of the spectra of the dimer. From these results, two sets of silicas are distinguished according to the amount of dimer formed and to its UV spectrum. Lower amounts of dimeric species are found on silica with either large pore size or low particle size. The spectrum in this case is only slightly redshifted relative to that of the monomeric compound. When the pore size decreases, dimer formation is more significant and its spectrum is more characteristic. The results obtained for adsorbed silicas were quantitatively treated and the Kubelka-Munk function, F(R), plotted against the concentration. Attempts to estimate the loading of silicas grafted with 1 were undertaken. It appears that small particle size silica gives rise to the highest loading and to the best quality DRUV spectra as a consequence of a minor amount of fluorescent dimer. On the other hand, it is shown that the loading of grafted silica is difficult to control by the used grafting process and that milder conditions are required

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.

Formation of reactive nitrogen species including peroxynitrite in physiological buffer exposed to cold atmospheric plasma

Cold Atmospheric Plasmas (CAPs) or Non Thermal Plasmas (NTPs) are increasingly used for biomedical applications. Herein, we studied the interactions of such CAPs, typically atmospheric ionization waves produced in a helium–nitrogen mixture (He/1% N2) with a commonly used physiological liquid in biology, e.g. Phosphate Buffered Saline solution (PBS) at pH 7.4. Optical Emission Spectroscopy (OES) of the plasma phase revealed the formation in the He/1% N2 CAP of nitric oxide NO and hydroxyl HO˙ derivatives which can lead to numerous Reactive Oxygen and Nitrogen Species (RONS) after dissolution in the exposed PBS. Chemical changes in solution were first assessed by conductimetry and pHmetry; these experiments showed that an evaporation of the solution occurred under gas exposition and was amplified by the CAPs, being mostly related to the interaction between the ionization wave and the gas flow. Further, UV-visible absorption spectroscopy was used to identify and quantify long-lived RONS, namely nitrite (NO2−), nitrate (NO3−), as well as a short-lived species, i.e. peroxynitrite anion (ONOO−). The production in physiological solution of ONOO− under CAP exposure is demonstrated for the first time, based on experiments at two pH conditions (7.4 and 12) and on the analysis of decomposition kinetics of this unstable species. The combination of complementary physico-chemical techniques allows to decipher the complex reactivity of CAPs from the plasma phase to the liquid phase.

Iron(III) uptake and release by chrysobactin, a siderophore of the phytophatogenic bacterium Erwinia chrysanthemi.

The plant pathogenic enterobacterium Erwinia chrysanthemi causes important soft-rot disease on a wide range of plants including vegetables and ornamentals of economic importance. It produces a major mono(catecholate) siderophore, chrysobactin (alpha-N-(2,3-dihydroxybenzoyl)-D-lysyl-L-serine). To unravel the role of chrysobactin in the virulence of E. chrysanthemi, its iron(III) coordination properties were thus investigated in aqueous solutions using electrospray ionization mass spectrometric, potentiometric, and spectrophotometric methods. Moreover, kinetic experiments allowed us to determine the uptake and release mechanisms. The formation mechanism of the 1:1 complex reveals a key role of the terminal carboxylic group of chrysobactin in the binding of either FeOH(2+) or Fe2(OH)2(4+). The proton-driven dissociation of the ferric tris-, bis-, and mono(chrysobactin) complexes was also studied. For these three ferric complexes, a single protonation triggers the release of the bound chrysobactin molecule. Interestingly, the dissociation of the last ligand proceeded via the formation of an intermediate for which a salicylate-type mode of bonding was proposed.

Association of indigo with zeolites for improved color stabilization.

The durability of an organic color and its resistance against external chemical agents and exposure to light can be significantly enhanced by hybridizing the natural dye with a mineral. In search for stable natural pigments, the present work focuses on the association of indigo blue with several zeolitic matrices (LTA zeolite, mordenite, MFI zeolite). The manufacturing of the hybrid pigment is tested under varying oxidizing conditions, using Raman and ultraviolet–visible (UV-Vis) spectrometric techniques. Blending indigo with MFI is shown to yield the most stable composite in all of our artificial indigo pigments. In the absence of defects and substituted cations such as aluminum in the framework of the MFI zeolite matrix, we show that matching the pore size with the dimensions of the guest indigo molecule is the key factor. The evidence for the high color stability of indigo@MFI opens a new path for modeling the stability of indigo in various alumino-silicate substrates such as in the historical Maya Blue pigment.

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.

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.

A comparative study of nine berberine salts in the solid state: optimization of the photoluminescence and self-association properties through the choice of the anion

The arrangement of an ionic fluorophore in the crystalline state was regulated by the presence of various counter-ions and the effect on spectroscopic and self-association properties was studied. To do so, nine salts of berberine (i.e. a fluorescent natural alkaloid) were investigated. Most of them contained organic anions and were prepared using an ion-exchange process. Berberine chloride and hemisulfate were also used for the sake of comparison. The diffuse reflectance and photoluminescence spectra were recorded on powder compounds. All salts were emissive in the solid state and the emission efficiency was increased seven-fold with the nature of the anion. The optical properties were tentatively discussed on the basis of the crystal-packing mode. The possibility of implementing a bottom-up approach to generate microparticles was investigated using the reprecipitation method. Salts that contain the most hydrophobic anions gave a large number of homogeneous, elongated microparticles. This study showed that most of the berberine salts could be used as fluorescent materials, but proper choice of the anion allows using the self-association properties to best advantage.