Chemo- and Regioselective Additions of Nucleophiles to Cyclic Carbonates for the Preparation of Self-Blowing Non-Isocyanate Polyurethane Foams

Polyurethane (PU) foams are indisputably daily essential materials found in many applications, notably for comfort (for example, matrasses) or energy saving (for example, thermal insulation). Today, greener routes for their production are intensively searched for to avoid the use of toxic isocyanates. An easily scalable process for the simple construction of self-blown isocyanate-free PU foams by exploiting the organocatalyzed chemo- and regioselective additions of amines and thiols to easily accessible cyclic carbonates is described. These reactions are first validated on model compounds and rationalized by DFT calculations. Various foams are then prepared and characterized in terms of morphology and mechanical properties, and the scope of the process is illustrated by modulating the composition of the reactive formulation. With impressive diversity and accessibility of the main components of the formulations, this new robust and solvent-free process could open avenues for construction of more sustainable PU foams, and offers the first realistic alternative to the traditional isocyanate route.     

Third-Generation Solar Cells: Toxicity and Risk of Exposure

Solar energy is considered clean energy, and its use is predicted to increase in the near future. Most installed units today are crystalline solar cells, but the field is in constant development, and when the first dye sensitized solar cell was published by Grätzel and O'Reagan a new, third-generation, solar power was born. Highly toxic metals are used to produce the photovoltaic units today, and with the predicted increase in solar cell installation, the human health hazards of these panels could become an issue. Additionally, many of these materials are used in their nanoform, which is associated with an additional risk. In this article, we discuss the technology behind the third-generation solar cells with its valuable use of nanotechnology as well as the possible health hazard when such nanomaterials are used in solar power units. We will show that the main exposure will occur either during the development and production phases or at the end-of-life stage of the solar cells, where toxic material can leach into landfills, and subsequently into the environment and impact the ecosystem directly, or humans indirectly through edible plants or drinking water.     

Environmentally Compatible Access to α-Trifluoromethylseleno-Enones

α-Trifluoromethylselenolated enones constitute valuable building-blocks for further synthesis of innovative fluorinated compounds. Herein, we described an easy access to such compounds in green conditions through a Morita-Baylis-Hillman like reaction. These conditions have also been extended to higher fluorinated homologs.     

Spectroscopic and structural impact of a stem base-pair change in DNA hairpins: GTTC-ACA-GAAC versus GTAC-ACA-GTAC

Successive investigations over the last decade have revealed and confirmed a stable loop closure in a family of d-[GTAC-5Pur6N7N-GTAC] hairpins, where 5Pur6N7N is a AAA, GAG and AXC loop (X being any nucleotide). The trinucleotide loop is characterized by a well defined 5Pur-7N mispairing mode, and by upfield chemical shifts for three sugar protons of the apical nucleotide 6N. The GTTC-ACA-GAAC DNA hairpin, of interest for its likely involvement in Vibrio cholerae genome mutations, has now been investigated. The GTAC-ACA-GTAC DNA hairpin has also been studied because it is intermediate between the other structures, as it contains the loop of the hairpin under consideration and the stem of the above family. The two hairpins with the ACA loop are stable. They show the same mispairing mode and similar upfield shifts as the previous family, but GTTC-ACA-GAAC seems to be slightly less compact than any other. GTTC-ACA-GAAC is remarkable in that it exhibits a B(II) character for the phosphate-ester conformation at 8Gp9A, together with a swing of the upper hairpin into the major groove that, in particular, brings 6CH1' roughly as close to 7AH2 as to 6CH6. These unexpected structural features are qualitatively deduced from (1)H and (31)P NMR spectra, and confirmed by Raman spectroscopy. This comparative study shows that not only the loop sequence but also the stem sequence may control hairpin structures.     

Efficient synthesis of calix[6]tmpa: a new calix[6]azacryptand with unique conformational and host-guest properties

A new C(3v)-symmetrical calix[6]azacryptand, that is, calix[6]tmpa (11), was synthesized by efficient [1+1] macrocyclization reactions. Remarkably, both linear and convergent synthetic strategies that were applied lead to equally good overall yields. Calix[6]tmpa behaves as a single proton sponge and appeared reluctant to undergo polyprotonation, unlike classical tris(2-pyridylmethyl)amine (tmpa) derivatives. It also acts as a good host for ammonium ions. Interestingly, it strongly binds a sodium ion and a neutral guest molecule, such as a urea, an amide, or an alcohol, in a cooperative way. A (1)H NMR study indicated that the ligand, as well as its complexes, adopt a major flattened cone conformation that is the opposite of that observed with the previously reported calix[6]cryptands. Characterization of the monoprotonated derivative 11H(+) by X-ray diffraction also revealed the presence of a 1,3-alternate conformation, which is the first example of its kind in the calix[6]arene family. This conformer is probably also present in solution as a minor species. The important covalent constraint induced by the polyaromatic tmpa cap on the calixarene skeleton, and conversely from the calix core onto the tmpa moiety, is the likely basis for the unique conformational and chemical properties of this host.     

Growth kinetics of heterostructured GaP-GaAs nanowires

We have studied the vapor-liquid-solid (VLS) growth dynamics of GaP and GaAs in heterostructured GaP-GaAs nanowires. The wires containing multiple GaP-GaAs junctions were grown by the use of metal-organic vapor phase-epitaxy (MOVPE) on SiO(2), and the lengths of the individual sections were obtained from transmission electron microscopy. The growth kinetics has been studied as a function of temperature and the partial pressures of the precursors. We found that the growth of the GaAs sections is limited by the arsine (AsH(3)) as well as the trimethylgallium (Ga(CH(3))(3)) partial pressures, whereas the growth of GaP is a temperature-activated, phosphine(PH(3))-limited process with an activation energy of 115 +/- 6 kJ/mol. The PH(3) kinetics obeys the Hinshelwood-Langmuir mechanism, indicating that the dissociation reaction of adsorbed PH(3) into PH(2) and H on the catalytic gold surface is the rate-limiting step for the growth of GaP. In addition, we have studied the competitive thin layer growth on the sidewalls of the nanowires. Although the rate of this process is 2 orders of magnitude lower than the growth rate of the VLS mechanism, it competes with VLS growth and results in tapered nanowires at elevated temperatures.     

Method development and inter-laboratory comparison about the determination of titanium from titanium dioxide nanoparticles in tissues by inductively coupled plasma mass spectrometry

Nanosized titanium dioxide (TiO₂) is one of the most interesting and valuable nanomaterials for the construction industry but also in health care applications, food, and consumer goods, e.g., cosmetics. Therefore, the properties associated with this material are described in detail. Despite its widespread use, the analytical determination and characterization of nanosized metal oxides is not as straightforward as the comparatively easy-to-detect metallic nanoparticles (e.g., silver or gold). This study presents the method development and the results of the determination of tissue titanium (Ti) levels after treatment of rats with the nanosized TiO₂. Total Ti levels were chosen to evaluate the presence and distribution of TiO₂ nanoparticles. A procedure consisting of incubation with a mixture of nitric acid (HNO₃) and hydrofluoric acid (HF), and heating was developed to digest tissues and TiO₂ nanomaterials in order to determine the total Ti content by inductively coupled plasma mass spectrometry (ICPMS). For the inter-laboratory comparison, altogether four laboratories analyzed the same samples upon digestion using the available ICPMS equipment. A major premise for any toxicokinetic study is the possibility to detect the chemical under investigation in biological samples (tissues). So, the study has to be performed with a dose high enough to allow for subsequent tissue level measurement of the chemical under investigation. On the other hand, dose of the chemical applied should not induce over toxicity in the animal as this may affect its absorption, distribution, metabolism, and excretion. To determine a non-toxic TiO₂ dosage, an acute toxicity study in rats was performed, and the organs obtained were evaluated for the presence of Ti by ICPMS. Despite the differences in methodology and independent of the sample preparation and the ICPMS equipment used, the results obtained for samples with Ti concentrations >4 μg Ti/g tissue agreed well.

Aromatic foldamers as scaffolds for metal second coordination sphere design

As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe–2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex–foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.

Wrapping a [2Fe–2S] metal complex in an aromatic foldamer helix is introduced as a new approach to tailor a second coordination sphere.     

Hyperpositive non-linear effects: enantiodivergence and modelling

The chiral ligand N-methylephedrine (NME) was found to catalyse the addition of dimethylzinc to benzaldehyde in an enantiodivergent way, with a monomeric and a homochiral dimeric complex both catalysing the reaction at a steady state and giving opposite product enantiomers. A change in the sign of the enantiomeric product was thus possible by simply varying the catalyst loading or the ligand ee, giving rise to an enantiodivergent non-linear effect. Simulations using a mathematical model confirmed the possibility of such behaviour and showed that this can lead to situations where a reaction gives racemic products, although the system is composed only of highly enantioselective individual catalysts. Furthermore, depending on the dimer''s degree of participation in the catalytic conversion, enantiodivergence may or may not be observed experimentally, which raises questions about the possibility of enantiodivergence in other monomer/dimer-catalysed systems. Simulations of the reaction kinetics showed that the observed kinetic constant k_obs is highly dependent on user-controlled parameters, such as the catalyst concentration and the ligand ee, and may thus vary in a distinct way from one experimental setup to another. This unusual dependency of k_obs allowed us to confirm that a previously observed U-shaped catalyst order vs. catalyst loading-plot is linked to the simultaneous catalytic activity of both monomeric and dimeric complexes.

An asymmetric reaction consisting of competing monomeric and dimeric catalysts may explain enantiodivergent non-linear effects.     

Computational design of new organic (D–π–A) dyes based on benzothiadiazole for photovoltaic applications,especially dye-sensitized solar cells

Research on Chemical Intermediates - A theoretical study on four organic dyes based on bis(4-hexyloxy)triphenylamine as donor and electron acceptor cyanoacrylic acid with a...