Understanding of the intrinsic difference between normal- and perfluoro-alkyl compounds toward total understanding of material properties

Molecular symmetry change of perfluoro-n-alkanes in 'Phase I' monitored by infrared spectroscopy

Phase diagram of polytetrafluoroethylene (PTFE) comprises four regions. Phases II and IV are characterized by twisted perfluoroalkyl (Rf) chains having different twisting rate of 13/6 and 15/7, respectively, while Phase III is characterized by a planer trans-zigzag molecular skeleton like a normal alkyl chain. These are confirmed by X-ray and electron diffraction and have already been established. Unlike these, Phase I is left an unresolved matter. This phase is complicated indeed and is not symbolized by a single molecular structure. At an ambient pressure, Phase I is the temperature region above 30 ºC (303 K), and the helical molecular structure is supposed to be gradually untwisted with an elevating temperature. This untwisting image is roughly suggested by the diffraction, neutron scattering, and thermal expansion techniques, but the conventional approaches have all experimental limitations because the untwisting accompanies disorder (or defect) in the twist along the chain. To explore the transition between two different helical structures of the Rf chain having disordered structures, vibrational spectroscopic techniques are expected to be an alternative approach. For infrared spectroscopy, for example, the twisting rate of the molecule is simply recognized as a degree of molecular symmetry. Here, we show that the band progression peaks of the CF2 symmetric stretching vibration mode are quite sensitive and useful for pursuing the molecular symmetry change in Phase I for both peak intensity and position using perfluoro-n-alkanes having different chain length covering both even and odd number of the CF2 groups.

Imparting Selective Fluorophilic Interactions in Redox Copolymers for the Electrochemically Mediated Capture of Short-Chain Perfluoroalkyl Substances

With increasing regulations on per- and polyfluoroalkyl substances (PFAS) across the world, understanding the molecular level interactions that drive their binding by functional adsorbent materials is key to effective PFAS removal from water streams. With the phaseout of legacy long-chain PFAS, the emergence of short-chain PFAS has posed a significant challenge for material design due to their higher mobility and hydrophilicity and inefficient removal by conventional treatment methods. Here, we demonstrate how cooperative molecular interactions are essential to target short-chain PFAS (from C4 to C7) by tailoring structural units to enhance affinity while modulating the electrochemical control of capture and release of PFAS. We report a new class of fluorinated redox-active amine-functionalized copolymers to leverage both fluorophilic and electrostatic interactions for short-chain PFAS binding. We combine molecular dynamics (MD) simulations and electrosorption to elucidate the role of the designer functional groups in enabling affinity toward short-chain PFAS. Preferential interaction coefficients from MD simulations correlated closely with experimental trends: fluorination enhanced the overall PFAS uptake and promoted the capture of less hydrophobic short-chain PFAS (C ≤ 5), while electrostatic interactions provided by secondary amine groups were sufficient to capture PFAS with higher hydrophobicity (C ≥ 6). The addition of an induced electric field showed favorable kinetic enhancement for the shortest PFAS and increased the reversibility of release from the electrode. Integration of these copolymers with electrochemical separations showed potential for removing these contaminants at environmentally relevant conditions while eliminating the need for chemical regeneration.

Design of discotic liquid crystal enabling complete switching between and memory of two alignment states over a large area

The alignment control of discotic columnar liquid crystals (LCs), featuring a low motility of the constituent molecules and thus having a large viscosity, is a challenging task. Here we show that triphenylene hexacarboxylic ester, when functionalized with hybrid side chains consisting of alkyl and perfluoroalkyl groups in an appropriate ratio, gives a hexagonal columnar (Col_h) LC capable of selectively forming large-area uniform homeotropic or homogeneous alignments, upon cooling from its isotropic melt or upon application of a shear force at its LC temperature, respectively. In addition to the alignment switching ability, each alignment state remains persistent unless the LC is heated to its melting temperature. In situ X-ray diffraction analysis under the application of a shear force, together with polarized optical microscopy observations, revealed how the columnar assembly is changed during the alignment-switching process. The remarkable behavior of the discotic LC is discussed in terms of its rheological properties.

A columnar liquid crystal consisting of a triphenylene hexacarboxylic ester mesogen and semifluoroalkyl side chains shows complete switching between homeotropic and homogeneous alignments, each of which remains persistent up to its melting point.

Perfluoroalkanes remain on water surface even after volatilization: Affinity analysis of fluorinated solvent with water surface

Perfluoroalkyl (R_f) compounds are known to have a poor solubility for most solvents except fluorinated solvents, which is known as a fluorous property. In Langmuir (L) film studies of R_f compounds, fluorinated solvents such as perfluoro-n-alkanes are generally used as a good solvent for depositing a sample monolayer on the water surface. On the other hand, a single R_f chain with a short length such as C₆F₁₃- is known to exhibit a totally different character from a condensed matter to have a strong affinity to a water molecule on the water surface via the dipole-dipole interaction, which is known as the dipole interactive (DI) property. On considering the DI property, the solvents of perfluoro-n-alkanes would remain on water for a long time, which may disturb the formation of L film on water. In the present study, details of a liquid layer of perfluoro-n-alkanes on water are investigated by using infrared external reflection (IR ER) spectrometry. Although the perfluoro-n-alkanes are highly volatile, the relevant vibration bands did not disappear even after two hours, which means that they remain on the water surface. Fortunately, however, the remained solvent, C₆F₁₄, has been found no disturbing factor for preparation of L films.

Well-Defined NHC-Ni Complexes as Catalysts: Preparation, Structures and Mechanistic Studies in Cross-Coupling Reactions

Developmental studies are ongoing to discover a way to utilise new N-heterocyclic carbene (NHC)-Ni complexes as catalysts. Using a bulky NHC ligand, it is possible to synthesise an NHC/phosphine-mixed heteroleptic Ni(II) complex, which can serve as an excellent catalyst for various cross-coupling reactions. During the study of the reaction mechanisms using these Ni complexes, NHC-Ni(I) complexes were accidentally discovered, and it was observed that they exhibit excellent catalytic activity for cross-coupling reactions. The possibility of the presence of NHC-Ni(I) intermediates in these catalytic reaction pathways has been experimentally demonstrated. Depending on the type of reaction, dinuclear Ni(I) and mononuclear Ni(I) complexes can function as intermediates. The results of the investigation of each reaction mechanism are summarised, and the prospects are described.

Targeted modifications in ionic liquids - from understanding to design

Ionic liquids are extremely versatile and continue to find new applications in academia as well as industry. This versatility is rooted in the manifold of possible ion types, ion combinations, and ion variations. However, to fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituents. In this work, we discuss targeted modifications as a powerful tool to provide understanding and to enable design. A 'targeted modification' is a deliberate change in the structure of an ionic liquid. This includes chemical changes in an experiment as well as changes to the parameterisation in a computer simulation. In any case, such a change must be purposeful to isolate what is of interest, studying, as far as is possible, only one concept at a time. The concepts can then be used as design elements. However, it is often found that several design elements interact with each other - sometimes synergistically, and other times antagonistically. Targeted modifications are a systematic way of navigating these overlaps. We hope this paper shows that understanding ionic liquids requires experimentalists and theoreticians to join forces and provides a tool to tackle the difficult transition from understanding to design.

Infrared active surface modes found in thin films of perfluoroalkanes reveal the dipole-dipole interaction and surface morphology

Infrared (IR) spectra of an organic thin film are mostly understood by considering the normal modes of a single molecule, if the dipole-dipole (D-D) interaction is ignorable in the film. When the molecules have a chemical group having a large permanent dipole moment such as the C=O and C-F groups, the D-D interaction induces vibrational couplings across the molecules, which produces an extra band as a surface phonon or polariton band because of the small thickness. Since the dipole moment of an organic compound is much less than that of an inorganic ionic crystal, we have a problem that the extra band looks like a normal-mode band, which are difficult to be discriminated from each other. In fact, this visual similarity sometimes leads us to a wrong direction in chemical discussion because the direction of the transition moment of the extra band is totally different from those of the normal modes. Here, we show useful selection rules for discussing IR spectra of a thin film without performing the permittivity analysis. The apparent change in the spectral shape on decrease in the thickness of the sample can be correlated with the morphological change in the film surface, which can also be discussed with changes in the molecular packing. This analytical technique has effectively been applied for studying the chemical properties of perfluoroalkanes as a chemical demonstration, which readily supports the stratified dipole-array theory for perfluoroalkyl compounds.

Fluorine-Substituted Arylphosphine for an NHC-Ni(I) System, Air-Stable in a Solid State but Catalytically Active in Solution

Monovalent NHC-nickel complexes bearing triarylphosphine, in which fluorine is incorporated onto the aryl groups, have been synthesized. Tris(3,5-di(trifluoromethyl)-phenyl)phosphine efficiently gave a monovalent nickel bromide complex, whose structure was determined by X-ray diffraction analysis for the first time. In the solid state, the Ni(I) complex was less susceptible to oxidation in air than the triphenylphosphine complex, indicating greatly improved solid-state stability. In contrast, the Ni(I) complex in solution can easily liberate the phosphine, high catalytic activity toward the Kumada-Tamao-Corriu coupling of aryl bromides.

Hypervalent Iodine(III)-Catalyzed Balz-Schiemann Fluorination under Mild Conditions

An unprecedented hypervalent iodine(III) catalyzed Balz-Schiemann reaction is described. In the presence of a hypervalent iodine compound, the fluorination reaction proceeds under mild conditions (25-60 °C), and features a wide substrate scope and good functional-group compatibility.