Christelle Absalon

Ferrocenyl-terminated redox stars: synthesis and electrostatic effects in mixed-valence stabilization.

A family of rigid ferrocenyl-terminated redox stars has been synthesized--by Negishi coupling, including hexa(ferrocenethynyl)benzene complexes, a dodecaferrocenyl star, and stars with extended rigid tethers--and fully characterized. Cyclic voltammetry (CV) studies of the parent complex hexa(ferrocenylethynyl)benzene, 1, show a single wave for the six-electron oxidation of 1 using Nn-Bu(4)PF(6) as the supporting electrolyte on a Pt anode in CH(2)Cl(2), whereas three distinct two-electron reversible CV waves are observed using Nn-Bu(4)BAr(F)(4) (Ar(F) = 3,5-C(6)H(3)-(CF(3))(2)). The CV of 1,3,5-tris(ferrocenylethynyl)benzene, 11, also shows only one wave for the three-electron transfer with Nn-Bu(4)PF(6) and three one-electron waves using Nn-Bu(4)BAr(F)(4). This confirms the lack of electronic communication between the ferrocenyl groups and a significant electrostatic effect among the oxidized ferrocenyl groups. This effect is not significant between para-ferrocenyl groups in 1,4-bis(ferrocenylethynyl)benzene for which only a single wave is observed even with Nn-Bu(4)BAr(F)(4) as the supporting electrolyte. The para-ferrocenyl substituents are quite independent, which explains that two para-ferrocenyl groups are oxidized at about the same potential in a single CV wave of 1. With the additional steric bulk introduced with a methyl substituent on the ferrocenyl group, however, even the para-methylferrocenyl groups are submitted to a small electrostatic effect splitting the six-electron transfer into six single-electron waves, probably because of the overall steroelectronic constraints. Contrary to 11, 1,3-bis(ferrocenylethynyl)benzene and related complexes with a third, different substituent in the remaining meta position different from a ferrocenylethynyl only show a single two-electron wave using Nn-Bu(4)BAr(F)(4), which is attributed to the transoïd conformation of the ferricinium groups minimizing the electrostatic effect. This shows that, in 11, it is the steric frustration that is responsible for the electrostatic effect, and the same occurs in 1. In several cases, ΔE(p) is much larger than the expected 60 mV value, characterizing a quasi-reversible (i.e., relatively slow) redox process. It is suggested that this slower electron transfer be attributed to conformational rearrangement of the ferrocenyl groups toward the transoïd position in the course of electron transfer. Thus both the thermodynamic and kinetic aspects of the electrostatic factor (isolated from the electronic factor), including the frustration effect, are characterized. The distinction between the electronic communication and through-space electrostatic effect was made possible in all of these complexes in which the absence of wave splitting using a strongly ion-pairing electrolyte shows the absence of significant electronic communication, and was confirmed by the new frustration phenomenon.

N-Heterocyclic Carbene-Induced Zwitterionic Ring-Opening Polymerization of Ethylene Oxide and Direct Synthesis of α,ω-Difunctionalized Poly(ethylene oxide)s and Poly(ethylene oxide)-b-poly(ε-caprolactone) Block Copolymers

An N-heterocyclic carbene (NHC), namely, 1,3-bis-(diisopropyl)imidazol-2-ylidene (1), was demonstrated to bring about the metal-free ring-opening polymerization of ethylene oxide at 50 degrees C in dimethyl sulfoxide, in absence of any other reagents. Poly(ethylene oxide) (PEO) of polydispersities <1.2 and molar masses perfectly matching the [monomer]/[(1)] ratio could thus be obtained in quantitative yields, attesting to the controlled/living character of such carbene-initiated polymerizations. It is argued that (1) adds to ethylene oxide to form a zwitterionic species, namely 1,3-bis-(diisopropyl)imidazol-2-ylidinium alkoxide, that further propagates by a zwitterionic ring-opening polymerization (ZROP) mechanism. Through an appropriate choice of terminating agent NuH or NuSiMe(3) at the completion of the polymerization, a variety of end-functionalized PEO chains could be generated. In particular, alpha,omega-bis(hydroxy)-telechelic PEO, alpha-benzyl,omega-hydroxy, and alpha-azido,omega-hydroxy-difunctionalized PEOs were synthesized by NHC (1)-initiated ZROP, using H(2)O, PhCH(2)OH, and N(3)SiMe(3) as terminating agent, respectively. Characterization of these alpha,omega-difunctionalized PEOs by techniques such as (1)H NMR spectroscopy, MALDI-TOF spectrometry, and size exclusion chromatography confirmed the quantitative introduction of functional groups at both alpha and omega positions of the PEO chains and the formation of very narrow molar mass polymers. Finally, the synthesis of a poly(ethylene oxide)-b-poly(epsilon-caprolactone) diblock copolymer by sequential ZROP of the corresponding monomers was successfully achieved using (1) as organic initiator without isolation of the PEO block intermediate.

Dendritic Carrier Based on PEG: Design and Degradation of Acid-sensitive Dendrimer-like Poly(ethylene oxide)s

Degradable dendrimer-like PEOs were designed using an original ABC-type branching agent featuring a cleavable ketal group, following an iterative divergent approach based on the anionic ring opening polymerization (AROP) of ethylene oxide and arborization of PEO chain ends. A seventh generation dendrimer-like PEO carrying 192 peripheral hydroxyls and exhibiting a molar mass of 446 kg · mol(-1) was obtained in this way. The chemical degradation of these dendritic scaffolds was next successfully accomplished under acidic conditions, forming linear PEO chains of low molar mass (≈2 kg · mol(-1)), as monitored by (1)H NMR, SEC, and MALDI-TOF mass spectrometry as well as by AFM.

Lipid components of olive oil from Tunisian Cv. Sayali: Characterization and authenticity

The analysis of the total lipid fraction from the Sayali variety of olive oil was accomplished in the present investigation. Glyceridic, unsaponifiable and flavour fractions of the oil were isolated and identified using several analytical methods. Chromatographic techniques have proven to be suitable for these determinations, especially capillary gas chromatography. Gas chromatography coupled to mass spectrometry was successfully used to identify sterols, triterpenes alcohols, 4-monomethylsterols, aliphatic alcohols and aroma compounds in our samples. Furthermore, solid phase microextraction was used to isolate volatiles from the total lipid fraction. Results from the quantitative characterization of Sayali olive oil showed that oleic acid (77.4%) and triolein (47.4%) were the dominant glyceridic components. However, the main compounds of the unsaponifiable fraction were beta-sitosterol (147.5mg/100g oil), 24-methylene cycloartenol (146.4mg/100g oil) and hexacosanol (49.3mg/100g oil). Moreover, results showed that the aldehydic compounds were the major flavours present in Sayali olive oil.

New strategies to study the chemical nature of wine oligomeric procyanidins

AbstractTannins represent a key element in red wine flavors, so researchers have made a lot of effort to try to understand the role of their structure in wine taste in recent decades. We report some new routes to achieve a true structure–taste relationship for the major tannins found in wine, which are procyanidins in their monomeric or oligomeric state. All these routes use synthetic standards. New advances in their synthesis and their analyses using chromatographic methods, NMR spectroscopy, and mass spectrometry to obtain more precise information about their chemical structure, including their stereochemistry and their precise degree of polymerization and galloylation, are described. FigureTannins, mainly procyanidins, represent a key element in red wine taste and flavors. They are especially involved in the sensation of astringency. Identifying them to achieve a true structure-taste Relationship remains a real challenge for researchers. This review reports on new routes to collect more and more information about the chemical nature of procyanidins present in wine.

Formation of Flavanol-aldehyde Adducts in Barrel-aged White Wine – Possible Contribution of These Products to Colour

This paper describes the formation and diversity of new compounds resulting from the polymerisation of furanic and phenolic flavanol-aldehydes with HPLC‑DAD and LC‑ES/MS analysis. Polymerisation, resulting from nucleophilic reactions, formed dimers, trimers, soluble and insoluble polymers. Reactions in hydroalcoholic solution with pure aldehydes (phenolic and furanic) and flavanols (catechin) were studied. The study was repeated with different aldehydes in white wine. This research focused particularly on the colour properties of the released products and their potential impact on the colour of white wine. Some products were purified and isolated; these were mainly catechinfurfuraldehyde, catechin-methyl-5-furfuraldehyde, catechin-hydroxymethyl-furfuraldehyde, catechin-vanillin, and catechin-syringaldehyde dimers. The most powerful coloured products resulted from furanic aldehydes. Over the course of the experiment, the reaction produced dimers, trimers and oligomers. After 50 to 60 days, the colour of the solution was mainly due to soluble polymeric forms. In addition, the role of SO2, generally used during vinification and ageing, was studied. The influence of SO2 on the kinetics of the reaction was limited.

Isolation, characterization, and determination of a new compound in red wine.

Chromatographic separation using fluorescence as a detection mode revealed, besides a series of flavan-3-ols, the recurrent presence of an undefined compound in Bordeaux red wine. Its isolation and structure characterization by complementary means (high-resolution mass spectrometry, nuclear magnetic resonance, and chemical synthesis) has permitted us to identify it as the nitrogen-containing glycoconjugate 3-indolyl-(2R)-O-β-d-glucosyl-lactic acid. Its quantification was performed for different wines of different vine varieties and terroirs with the aim to assess whether this compound may be used as a terroir, variety, or wine process tag.

Optical performances degradation of InGaN/GaN MQW LEDs related to fluorescence shift of copolymer-based silicone coating

Accelerated ageing tests (1500h/85°C/30mA) performed on packaged InGaN/GaN MQW LEDs have reported a fluorescence shift of silicone oil responsible for optical losses. Electrical and optical characteristics highlight a 65% loss of optical power. Through measurements of the copolymer silicone coating fluorescence emission spectra, we demonstrate that the polymer fluorescence (induced from the blue light emitted from the chip) enlarges the LED emission spectrum (7%) and shifts central wavelength (5 to 7 nm). To understand such a fluorescence shift, Attenuated Total Reflection, Nuclear Magnetic Resonance (NMR), mass spectrometry and Differential Scanning Calorimetry (DSC) have been performed. The copolymer molecular structure has been affected after ageing. Actually, NMR and Mass spectrometry evidences the disappearance of low molecular weight molecules and the presence of high molecular weight molecules after ageing. Such a mechanism is associated with the polymerization of the silicone oil after ageing. Indeed, DSC has confirmed that silicone oil polymerization process is activated by temperature. Finally, both polymerization of the silicone oil due to temperature and fluorescence shift activated by photothermal process have been identified as the main failure mechanisms responsible for optical power degradation.