Thermal behavior and high- and low-temperature phase structures of gemini fluorocarbon/hydrocarbon diblocks

A Confinement-Driven Nucleation Mechanism of Metal Oxide Nanoparticles Obtained via Thermal Decomposition in Organic Media

Thermal decomposition is a very efficient synthesis strategy to obtain nanosized metal oxides with controlled structures and properties. For the iron oxide nanoparticle synthesis, it allows an easy tuning of the nanoparticle's size, shape, and composition, which is often explained by the LaMer theory involving a clear separation between nucleation and growth steps. Here, the events before the nucleation of iron oxide nanocrystals are investigated by combining different complementary in situ characterization techniques. These characterizations are carried out not only on powdered iron stearate precursors but also on a preheated liquid reaction mixture. They reveal a new nucleation mechanism for the thermal decomposition method: instead of a homogeneous nucleation, the nucleation occurs within vesicle-like-nanoreactors confining the reactants. The different steps are: 1) the melting and coalescence of iron stearate particles, leading to "droplet-shaped nanostructures" acting as nanoreactors; 2) the formation of a hitherto unobserved iron stearate crystalline phase within the nucleation temperature range, simultaneously with stearate chains loss and Fe(III) to Fe(II) reduction; 3) the formation of iron oxide nuclei inside the nanoreactors, which are then ejected from them. This mechanism paves the way toward a better mastering of the metal oxide nanoparticles synthesis and the control of their properties.

Dipyrrolyldiketonato Titanium(IV) Complexes from Monomeric to Multinuclear Architectures: Synthesis, Stability, and Liquid-Crystal Properties

Dipyrrolyldiketone ligands (dpkH) are used with Ti(OⁱPr)₄ to afford monomeric titanium(IV) complexes displaying the general formula C₂-[Ti(dpk)₂(OⁱPr)₂]. The dpkH ligands employed consist of two dipyrrolyldiketone compounds (2H and 3H) and three diphenyl-substituted analogues (4H-6H). The behavior of these octahedral [Ti(dpk)₂(OⁱPr)₂] species in solution was investigated by ¹H NMR at variable temperatures. Dynamic phenomena were evidenced, and the activation parameters associated with these processes (ΔH^⧧, ΔS^⧧, and ΔG^⧧) were retrieved. [Ti(dpk)₂(OⁱPr)₂] complexes are precursors for the formation of high-nuclearity aggregates whose structures depend on the substituents on the diketone backbone. The crystal structures of monomeric ([Ti(1)₂(OⁱPr)₂]; 1 is the 1,3-diphenyl-1,3-propanedionato ligand) and [Ti(2)₂(OEt)₂]), dimeric ([Ti₂(1)₄(μ₂-O)₂]), and tetrameric ([Ti₄(4)₈(μ₂-O)₄]) species have been established, and the origin of this structural diversity is discussed. The solid-state optical properties of several complexes were determined and interpreted with the help of DFT calculations. Finally, the dinuclear complex [Ti(6)₂(μ₂-O)₂] was synthesized, where ligand 6 incorporates six long alkyl chains (C₁₆H₃₃). This complex shows rich mesomorphic properties, with an original room-temperature plastic crystal phase followed by a hexagonal columnar liquid-crystalline phase.

From nano-seggregation to mesophases: probing the liquid structure of perfluoroalkylalkanes with 129Xe NMR spectroscopy

In this work we demonstrate that pure perfluoroalkylalkane diblock molecules are not isotropic liquids and self organize forming domains at the nanometric scale. 129Xe NMR spectra were obtained as a function of temperature for seven liquid perfluoroalkylalkanes, covering a range of relative lengths of the hydrogenated and fluorinated segments. The results support the presence of domains richer in the hydrogenated groups, in which xenon is preferentially dissolved. The average local concentration within the xenon coordination sphere is estimated to be 0.05 mole fraction higher in hydrogenated groups than the stoichiometric proportion. Atomistic molecular dynamics simulations support this analysis and allow a detailed analysis of the liquid structure. Furthermore, 129Xe NMR spectra in perfluorohexylhexane (F6H6) and perfluorohexyloctane (F6H8) obtained as a function of temperature, clearly detect the existence of two distinct environments in the fluid, one richer in hydrogenated groups and another richer in fluorinated groups, consistent with the formation of mesophases. It is important to stress that nano-segregation is this case observed in liquids interacting exclusively through dispersion forces, unlike most common examples of segregation which are determined by hydrogen bonding and polarity. Given the simple molecular structure and interactions of the studied PFAA, we believe that the present results can have a general impact in understanding the early mechanisms of segregation, phase separation and self-assembly.

Long-Range Lateral Correlation between Self-Assembled Domains of Fluorocarbon-Hydrocarbon Tetrablocks by Quantitative GISAXS

The structure and lateral correlation of fluorocarbon-hydrocarbon tetrablock di(F10Hm) domains at the air/water interface have been determined by quantitative analysis of grazing incidence small-angle X-ray scattering (GISAXS) data. The measured GISAXS signals can be well represented by the full calculation of the form and structure factors. The form factor suggests that di(F10Hm) domains take a hemiellipsoid shape. Both major and minor axes of the hemiellipsoids monotonically increased in response to the elongation of the hydrocarbon blocks, which can be explained by the concominant increase in van der Waals interaction. The structure factor calculated from the GISAXS signals suggests that the domains take an orthorhombic lattice. Remarkably, the lateral correlation can reach over a distance that is more than 14 times longer than the distance to the nearest neighbors. Our data suggest that quantitative GISAXS enables the optimal design of mesoscopic self-assemblies at the air/water interface by fine-tuning of the structures of molecular building blocks.

From nano-emulsions to phase separation: evidence of nano-segregation in (alkane + perfluoroalkane) mixtures using 129Xe NMR Spectroscopy

In this work we demonstrate that mixtures of (hexane + perfluorohexane) above the upper critical solution temperature segregate by forming domains at the nanometric scale. 129Xe NMR spectra obtained for solutions of xenon in liquid mixtures of (hexane + perfluorohexane) as a function of temperature suggest the existence of domains richer in the hydrogenated component, in which xenon "prefers" to be solvated. The average local concentration within the xenon coordination sphere is at least 0.05 higher in hexane mole fraction than the nominal concentration of the mixture. Atomistic molecular dynamics simulations support this analysis in excellent agreement with the experimental data. Additionally, 129Xe NMR spectra in pure perfluoroalkanes allow a detailed analysis of the liquid structure, continuing that previously reported for the liquid alkanes. It should be emphasised that nano-segregation is here observed in fluids governed exclusively by dispersion interactions, in contrast to other examples in which hydrogen bonding and polarity play important roles. Given its simplicity, this case study is thus prone to have a general impact in understanding the early mechanisms of segregation, phase separation and self-assembly.

Engineering On-Surface Spin Crossover: Spin-State Switching in a Self-Assembled Film of Vacuum-Sublimable Functional Molecule

The realization of spin-crossover (SCO)-based applications requires study of the spin-state switching characteristics of SCO complex molecules within nanostructured environments, especially on surfaces. Except for a very few cases, the SCO of a surface-bound thin molecular film is either quenched or heavily altered due to: (i) molecule-surface interactions and (ii) differing intermolecular interactions in films relative to the bulk. By fabricating SCO complexes on a weakly interacting surface, the interfacial quenching problem is tackled. However, engineering intermolecular interactions in thin SCO active films is rather difficult. Here, a molecular self-assembly strategy is proposed to fabricate thin spin-switchable surface-bound films with programmable intermolecular interactions. Molecular engineering of the parent complex system [Fe(H₂ B(pz)₂ )₂ (bpy)] (pz = pyrazole, bpy = 2,2'-bipyridine) with a dodecyl (C₁₂ ) alkyl chain yields a classical amphiphile-like functional and vacuum-sublimable charge-neutral Fe^II complex, [Fe(H₂ B(pz)₂ )₂ (C₁₂ -bpy)] (C₁₂ -bpy = dodecyl[2,2'-bipyridine]-5-carboxylate). Both the bulk powder and 10 nm thin films sublimed onto either quartz glass or SiO_x surfaces of the complex show comparable spin-state switching characteristics mediated by similar lamellar bilayer like self-assembly/molecular interactions. This unprecedented observation augurs well for the development of SCO-based applications, especially in molecular spintronics.

Incompatibility-Driven Self-Organization in Polycatenar Liquid Crystals Bearing Both Hydrocarbon and Fluorocarbon Chains

The synthesis and liquid crystal properties are reported for tri- and tetra-catenar mesogens in which both hydrocarbon and semiperfluorocarbon chains have been incorporated. In the tricatenar mesogens, the lamellar spacing in the smectic C phase of the all-hydrocarbon mesogen almost doubles when the isolated hydrocarbon chain is replaced by a semiperfluorinated chain on account of the localized segregation in different sublayers between the two chain types. In the tetracatenar materials, the replacement of at least one hydrocarbon chain by semiperfluorocarbon chains is sufficient to promote columnar phase formation, but when the molecule has two hydrocarbon chains at one end and two semiperfluorocarbon chains at the other, the requirement for localized phase segregation leads to the formation of a rectangular phase with very large lattice parameters. The juxtaposition of terminal chains of different nature within the same molecular structure thus leads to a reduction in mesophase symmetry and the emergence of more complex supramolecular organization.

Insight into the polymerization mechanism of photoinduced step transfer-addition & radical-termination (START) polymerizations

The intrinsic polymerization mechanism of photoinduced step transfer-addition & radical-termination (START) has been revealed based on the successful construction of a catalytic system (Ru(bpy)₃Cl₂/RA) and a solvent system (DMC/MeCN).

The positive effect of water on photo-induced step transfer-addition & radical-termination (START) polymerization

In photo-induced Step Transfer-Addition & Radical-Termination (START) polymerization, the addition of water greatly enhanced the overall polymerization efficiency and inhibited the function loss (C–I).

Semifluorinated Alkanes at the Air-Water Interface: Tailoring Structure and Rheology at the Molecular Scale

Semifluorinated alkanes form monolayers with interesting properties at the air-water interface due to their pronounced amphi-solvophobic nature and the stiffness of the fluorocarbons. In the present work, using a combination of structural and dynamic probes, we investigated how small molecular changes can be used to control the properties of such an interface, in particular its organization, rheology, and reversibility during compression-expansion cycles. Starting from a reference system perfluor(dodecyl)dodecane, we first retained the linear structure but changed the linkage groups between the alkyl chains and the fluorocarbons, by introducing either a phenyl group or two oxygens. Next, the molecular structure was changed from linear to branched, with four side chains (two fluorocarbons and two hydrocarbons) connected to extended aromatic cores. Neutron reflectivity at the air-water interface and scanning force microscopy on deposited films show how the changes in the molecular structure affect molecular arrangement relative to the interface. Rheological and compression-expansion measurements demonstrate the significant consequences of these changes in molecular structure and interactions on the interfacial properties. Remarkably, even with these simple molecules, a wide range of surface rheological behaviors can be engineered, from viscous over viscoelastic to brittle solids, for very similar values of the surface pressure.

Semi fluorinated polymers as surface energy controlled layers for liquid crystal alignment

The effects of compounded hydrogenated–fluorinated surfaces formed by perfluorocyclobutane (PFCB)-containing polymers on the alignment of a liquid crystal, 4,4′-octylcycanobiphenyl (8CB), were investigated.

Nanostructured proton conductors formed via in situ polymerization of ionic liquid crystals

Ionic liquid crystals (ILCs) with hexagonal and lamellar phases were successfully fabricated by the self-assembly of a polymerizable amphiphilic zwitterion, which is formed by 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) and 4-dodecyl benzenesulfonic acid (DBSA) based on intermolecular electrostatic interactions. The microstructures and phase behaviors of ILCs were studied by polarized microscope (POM) and small-angle X-ray scattering (SAXS). The ILC topological structures can be considered as proton pathways and further fixed by photopolymerization to prepare nanostructured proton-conductive films. The introduction of highly ordered and well-defined ILC structures into these polymeric films radically improves the ionic conductivities.

Synthesis of Hydrophobic Carbohydrate Polymers and Their Formation of Thermotropic Liquid Crystalline Phases

The first synthesis of enantiopure glucose octyl ether polyamido-saccharides (GOE-PAS) with a defined molecular weight and narrow dispersity is reported using a controlled anionic ring-opening polymerization of a glucose-derived β-lactam sugar monomer possessing octyl ether chains. This new polymer structure is characterized by NMR, infrared (IR), optical rotation, gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). At room temperature, the polymers form lamellar (Lam) phases. Upon heating to mild temperatures (ca. 60 °C), the shortest polymer shows a direct transition to the isotropic (Iso) liquid state, while the longer polymers give rise to a hexagonal columnar (Col_h) phase before becoming isotropic at higher temperatures (ca. 120 °C).