Investigating the efficiency of silica materials with wall-embedded nitroxide radicals for dynamic nuclear polarisation NMR.

Dynamic nuclear polarisation (DNP) can significantly enhance the sensitivity of solid-state nuclear magnetic resonance (SSNMR) experiments by transferring the electron spin polarisation of paramagnetic species to nuclei through microwave irradiation of the sample at cryogenic temperatures. Paramagnetic species required for DNP can be provided in the form of mesoporous silica materials containing nitroxide radicals either located on the porous surface or embedded in the pore walls. The present study focuses specifically on porous materials with wall-embedded radicals that were synthesised using conventional molecular imprinting protocols. More remarkably, by changing the molecular structure of the TEMPO precursor, the theoretical distance between the oxygen atoms in a pair of wall-embedded face-to-face TEMPO radicals was increased stepwise (0.7, 0.9, 1.1, 1.3 and 1.5 nm). The thermal activation of these five series of materials led to 37 TEMPO-functionalised silica materials with different radical concentrations. Their efficiency as DNP polarising agents was subsequently investigated at 9.4 T and ∼110 K under magic-angle spinning conditions (10 kHz) after impregnating them at room temperature with an aqueous solution of isotopically enriched proline. Our results show that the highest DNP efficiency was obtained for the silica materials that exhibited the shortest theoretical oxygen-oxygen distance between the TEMPO rings, suggesting that the design rules accepted for soluble DNP polarising agents may not be transposed to these materials with wall-embedded pairs of nitroxides.

Monitoring Crystallization Processes in Confined Porous Materials by Dynamic Nuclear Polarization Solid-State Nuclear Magnetic Resonance.

Establishing mechanistic understanding of crystallization processes at the molecular level is challenging, as it requires both the detection of transient solid phases and monitoring the evolution of both liquid and solid phases as a function of time. Here, we demonstrate the application of dynamic nuclear polarization (DNP) enhanced NMR spectroscopy to study crystallization under nanoscopic confinement, revealing a viable approach to interrogate different stages of crystallization processes. We focus on crystallization of glycine within the nanometric pores (7-8 nm) of a tailored mesoporous SBA-15 silica material with wall-embedded TEMPO radicals. The results show that the early stages of crystallization, characterized by the transition from the solution phase to the first crystalline phase, are straightforwardly observed using this experimental strategy. Importantly, the NMR sensitivity enhancement provided by DNP allows the detection of intermediate phases that would not be observable using standard solid-state NMR experiments. Our results also show that the metastable ß polymorph of glycine, which has only transient existence under bulk crystallization conditions, remains trapped within the pores of the mesoporous SBA-15 silica material for more than 200 days.

Coniferyl Alcohol Radical Detection by the Dirigent Protein AtDIR6 Monitored by EPR.

Plant dirigent proteins (DIRs) control the stereoselectivity of the monolignol coniferyl alcohol radical coupling. The main mechanistic hypothesis on this chemo- and stereoselective reaction invokes a binding of coniferyl alcohol radical substrates in the dirigent protein active site so that only one enantiomeric form can be produced. We have studied the influence of the Arabidopsis thaliana AtDIR6 protein on the transient coniferyl alcohol radical by EPR. Herein, we show that AtDIR6 stabilizes coniferyl alcohol radicals prior to directing their coupling towards the formation of (-)-pinoresinol.

Modulating lifetimes and relaxation times of phenoxyl radicals through their incorporation into different hybrid nanostructures.

The development of new open shell systems is essential for advances in spin science. In this work, we report the synthesis and characterization of three nanostructured materials, namely SBA-15 silicas, periodic mesoporous organosilicas (PMOs) and lamellar polysilsesquioxanes, all functionalized with the same diazene-based phenoxyl radical precursor. The impact of the nature of the material, i.e. loading of radical precursor and structure, on half-lifetimes (t1/2) and relaxation times of phenoxyl radicals was investigated. Although phenoxyl radicals are transient in solution, their t1/2 range from hours to years at room temperature (RT) when they are embedded in nanostructured materials. The above mentioned functionalized materials were used to generate the corresponding phenoxyl radicals and their relaxation times were measured (〈T1e〉 and Tm) from 50 K to RT. The results were rationalized in terms of limited mobility of the radical as a result of supramolecular interactions and structure rigidity. All these data show that it is possible to design functionalized nanostructured material with radicals possessing specific electronic relaxation properties which can be of interest in fields like DNP, organic magnetism or spin qubit.

Embedding cyclic nitrone in mesoporous silica particles for EPR spin trapping of superoxide and other radicals.

The generation of superoxide radical anion in biological systems is one of the major initiating events in the redox biology of NADPH oxidases and mitochondrial redox signalling. However, the pallette of chemical tools for superoxide detection is very limited, hampering progress in understanding the chemical biology of superoxide. Although EPR spin trapping is regarded as the most rigorous technique for superoxide detection, rapid reduction of the EPR-active superoxide spin adducts to EPR-silent hydroxylamines, or to hydroxyl radical adducts by bioreductants, significantly limits the applicability of this technique in biological systems. To overcome these limitations, in this work, we report the synthesis and characterization of a new mesoporous silica functionalized with a phosphorylated cyclic spin trap (DIPPMPO nitrone). The DIPPMPO-grafted silica is a versatile spin-trap agent enabling the identification of a wide range of carbon or oxygen-centered transient radicals in organic and in aqueous media. Moreover, superoxide was efficiently trapped under in vitro conditions in both cell-free and cellular systems. The generated superoxide adduct exhibited an exceptional half-life of 3.5 h and a resistance toward bioreductant agents such as glutathione for several hours.

Investigating Unusual Organic Functional Groups to Engineer the Surface Chemistry of Mesoporous Silica to Tune CO2-Surface Interactions.

As the search for functionalized materials for CO2 capture continues, the role of theoretical chemistry is becoming more and more central. In this work, a strategy is proposed where ab initio calculations are compared and validated by adsorption microcalorimetry experiments for a series of, so far unexplored, functionalized SBA-15 silicas with different spacers (aryl, alkyl) and terminal functions (N3, NO2). This validation then permitted to propose the use of a nitro-indole surface functionality. After synthesis of such a material the predictions were confirmed by experiment. This confirms that it is possible to fine-tune CO2-functional interactions at energies much lower than those observed with amine species.

Diazene-Functionalized Lamellar Materials as Nanobuilding Blocks: Application as Light-Sensitive Fillers to Initiate Radical Photopolymerizations.

New polysilsesquioxane-based lamellar materials, functionalized with radical precursors, were synthesized. They play a double role in the preparation of composite materials: first, as filler homogeneously dispersed in the monomer after delamination, second as radical initiator in photopolymerization. These polysilsesquioxanes enable fast and efficient photopolymerization upon UV light for thick samples. High conversions in monomers as well as the formation of hybrid polymers covalently linked to the filler are observed. This strategy, based on a double bottom-up approach, avoids the solubility/dispersion problem encountered in the classical preparation of composite polymers from preformed organic polymers.

Silica materials with wall-embedded nitroxides provide efficient polarization matrices for dynamic nuclear polarization NMR.

Hybrid mesoporous silica materials with wall-embedded nitroxides are shown to efficiently polarize impregnated substrates in high-field dynamic nuclear polarization magic-angle spinning solid-state NMR experiments.

Mechanistic investigation of enediyne-connected amino ester rearrangement. Theoretical rationale for the exclusive preference for 1,6- or 1,5-hydrogen atom transfer depending on the substrate. A potential route to chiral naphthoazepines.

Memory of chirality (MOC) and deuterium-labeling studies were used to demonstrate that the cascade rearrangement of enediyne-connected amino esters 1a and 1b evolved through exclusive 1,5- or 1,6-hydrogen atom transfer, subsequent to 1,3-proton shift and Saito-Myers cyclization, depending on the structure of the starting material. These results were independently confirmed by DFT theoretical calculations performed on model monoradicals. These calculations clearly demonstrate that in the alanine series, 1,5-hydrogen shift is kinetically favored over 1,6-hydrogen shift because of its greater exergonicity. In the valine series, the bulk of the substituent at the nitrogen atom has a major influence on the fate of the reaction. N-Tosylation increases the barrier to 1,5-hydrogen shift to the benefit of 1,6-hydrogen shift. The ready availability of 1,6-hydrogen atom transfer was explored as a potential route for the enantioselective synthesis of naphthoazepines.

An efficient and recyclable hybrid nanocatalyst to promote enantioselective radical cascade rearrangements of enediynes.

Mesoporous silica grafted with a tertiary amine was used as a basic nanocatalyst to promote in confined medium the enantioselective cascade rearrangement of enediynes based on the phenomenon of memory of chirality; the multi-substrates recyclable catalytic reagent could easily be recovered by simple filtration, and reused without any decrease in activity even when changing the solvent.

Self-assembly of layered organosilicas based on weak intermolecular interactions.

New layered hybrid organic-inorganic materials were obtained directly by hydrolysis and polycondensation of monosilylated precursors of type (EtO)(3)Si(CH(2))(3)NH(C=O)NHC(n)H(2n+1) (n = 3, 8, 12 and 16). These precursors were easily prepared by reaction between commercially available triethoxysilylpropylisocyanate and primary aminoalkanes. The obtained materials were characterized by combining elemental analysis, X-ray powder diffraction, (13)C, (29)Si NMR and FT-IR spectroscopies. They were found to be well condensed, and stable in water. We showed that their formation results from a cooperative effect between hydrogen bonding interactions originating from ureido groups and hydrophobic interactions between the long alkyl chains. There is no formation of material when n = 2, or from (EtO)(3)SiC(n)H(2n+1) when n = 18, thus proving that both types of weak intermolecular interactions are required. The chelating property of these materials towards europium(III) ions was tested.

Selective lanthanides sequestration based on a self-assembled organosilica.

In this paper, we investigate the cation-exchange properties of a self-assembled hybrid material towards trivalent ions, lanthanides (La(3+), Eu(3+), Gd(3+), Yb(3+)) and Fe(3+). The bis-zwitterionic lamellar material was prepared by sol-gel process from only 3-aminopropyltriethoxysilane (APTES), succinic anhydride, and ethylenediamine. In ethanol heated under reflux, the exchange ethylenediammonium versus Ln(3+) proved to be complete by complexometry measurements and elemental analyses, one Cl(-) ion per one Ln(III) remaining as expected for charge balance. In aqueous solution at 20 degrees C, the material was found to be selective towards lanthanide in spite of the similarity of their ionic radii. The cation uptake depends on the nature of the salt, the difference between two lanthanides reaching up to 20 % in some cases. Finally, ion-exchange reaction with FeCl(3) was chosen as a probe to get more information on the material after incorporation of trivalent ions. Based on Mössbauer spectroscopic investigations on the resulting material in conjunction with the XRD analysis of materials containing trivalent ions, a structural model was proposed to describe the incorporation of trivalent ions by exchange reaction within the original zwitterionic material.

A novel and simplified procedure for patterning hydrophobic and hydrophilic SAMs for microfluidic devices by using UV photolithography.

This work describes how selective patterning of hydrophobic and hydrophilic areas inside microchannels of microfluidic devices can be achieved by combining well-known chemical protocols and standard photolithography equipment (365 nm). Two techniques have been performed and compared. The first technique is based on the preparation of self-assembled monolayers of photocleavable organosilane and the second one on photoassisted grafting (365 nm) of self-assembled monolayers (SAMs) on a silicon or glass substrate. In the first case, we begin with monolayers carrying an o-nitrobenzyl function (hydrophobic area) that is photochemically cleaved, revealing a carboxylic acid group (hydrophilic area). The problem is that the energy necessary to cleave this monolayer is too high and the reaction time is more than 1 h with 50 mW/cm(2) irradiation flux. To overcome this practical disadvantage, we propose another approach that is based on the thiol-ene reaction with benzophenone as photoinitiator. In this approach, a monolayer of mercaptopropyltrimethoxysilane (MPTS) is prepared first. Subsequently, a hydrocarbon chain is photografted locally onto the thiol layer, forming a hydrophobic surface while the reminding unmodified thiol surface is oxidized into sulfonic acid (hydrophilic area). We demonstrated the feasibility of this approach and synthesized high-quality self-assembled monolayers by UV grafting with an irradiation time of 30 s at 365 nm (50 mW/cm(2)). The modified surfaces have been characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), AFM, and multiple internal reflection infrared spectroscopy (MIR-FTIR). The difference in the contact angles on the hydrophilic and hydrophobic surfaces reached a remarkable 77 degrees. We have also demonstrated that this method is compatible with selective surface grafting inside microfluidic channels.

One-step template-directed synthesis of multifunctionalised nanoporous silica: on the way to interactive nanomaterials.

The preparation of multifunctional mesoporous silica containing a NLO chromophore in the framework (bridged azobenzene phosphonium salts) and mercaptopropyl groups able to stabilize gold(0) nanoparticles in the channel pores was achieved in one step by using the direct liquid crystal templating approach.