Syntheses and phase-transfer properties of dendritic nanocarriers that contain perfluorinated shell structures

Perfect dendrimers that contain perfluorinated shells have recently attracted attention because they have been shown to encapsulate polar molecules in supercritical CO(2) and catalytically active metal nanoparticles in perfluorinated solvents. Moreover, they can then be easily separated after reaction from the biphasic organic/fluorous system. In this paper several dendritic architectures that contain perfluorinated shells were derived by covalent modification of glycerol dendrimers ([G0.5]-[G3.5]), hyperbranched polyglycerol, and polyethyleneimine. These core-shell architectures show interesting physicochemical properties. For example, they are soluble in fluorinated solvents, they are able to transport different guest molecules, and they display thermomorphic behavior. The transport capacity of these molecular nanocarriers increases significantly when amino groups are present in the core. Certain functionalized polyethyleneimines that contain perfluorinated shells show high transport capacities (up to 3 dye molecules per nanocarrier) in perfluorinated solvents. Moreover, these perfluoro-functionalized dendritic polyethyleneimines can act as templates that stabilize nanoparticles; for example, encapsulation and subsequent chemical reduction of Ag(I) ions. Silver nanoparticles with a narrow size distribution (3.9+/-1 nm) have been prepared and characterized by transmission electron microscopy. Furthermore, it has been demonstrated that the encapsulated guest molecules remain accessible to small molecules after transport into the fluorous phase. Therefore, dendritic nanocarriers that contain perfluorinated shells are currently being investigated as polar environments in nonpolar reaction media such as fluorous phases and supercritical CO(2), in particular, for application in homogenous catalysis.     

Perfluoroadamantane and its negative ion

As a general rule, saturated hydrocarbons are unable to bind an electron, i.e., their electron affinities are negative, but the corresponding perfluorinated molecules can have significant electron affinities, especially in the case of branched and ring systems. Four different density functional theory (DFT) methods in conjunction with double-zeta plus polarization function augmented diffuse function basis sets (DZP++) have been employed to study the equilibrium geometries, electron affinities, and vibrational frequencies of the adamantane (C10H16) and perfluoroadamantane (C10F16) molecules. Three types of neutral-anion separations reported are the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy. The adiabatic electron affinity predicted at the DZP++ B3LYP level of theory for adamantane is, as expected, negative (-0.58 eV), while that for perfluoroadamantane is distinctly positive, namely, 1.06 eV (or 1.31 eV after correction for zero-point vibrational energies).     

Surface ordering of a perfluorinated, self-assembled, dendrimer on a water subphase

We have investigated the surface ordering of a synthetic, asymmetric, fan-shaped dendrimer containing a carboxyl core and perfluorinated tails which was obtained by the esterification of the intermediary. X-ray diffraction patterns and transmission electron microscopy (TEM) images show the molecules self-assemble into a hexagonal, cylindrical mesophase. Surface pressure-area isotherms and Brewster angle microscopy measurements show the molecule forms a stable monolayer at the air-water interface with a single phase transition. As a condensed monolayer, the perfluorinated tails are well-packed with hexagonal symmetry with (10) spacing of approximately 0.5 nm from molecular-scale atomic force microscopy (AFM) images. Such dense molecular-scale packing has not been observed in other dendritic molecules thus far. Compared to the case of conventional dendritic molecules with alkyl tails, these molecules occupy a much smaller molecular area due to the strong microphase separation between the carboxylic core and perfluorinated tails at the air-water interface. After monolayer collapse, the irregular islands with terrace morphology are observed in contrast with conventional alkyl-terminated self-assembled dendritic molecules where irregular islands do not appear. The interfacial and internal structure of every terrace shows planar columnar morphology from AFM and TEM imaging. From these results, we discuss the stability of perfluorinated, self-assembled dendrimers on water, as well as how to generate planar morphology on a hydrophilic surface.     

The radiation degradation of polytetrafluoroethylene resulting in low-molecular and functionalized perfluorinated compounds

The degradation of polytetrafluoroethylene (PTFE) by high energy radiation with a high dose and in presence of oxygen forms perfluorinated carboxylic acids, among other compounds. In an inert atmosphere (in nitrogen) mixtures of perfluorinated olefines and paraffins of different chain length ranges are obtained. This process represents a new alternative of the synthesis of special active components, such as fluorocarbon surfactants, fluorine containing textile finishing agents, special dielectrics and others. Irradiation of the resulting perfluorinated paraffins (in nitrogen) and of gaseous degradation products of PTFE leads to a significant increase in the yield of perfluorinated olefines. Reaction mechanisms are discussed. Recombinations of radicals obtained by irradiation form branched molecules. The reactions are diffusion controlled. Irradiation of high molecular PTFE leads to low branching. The degree of branching of molecules increases in correlation with a decrease of viscosity of the reaction medium. An apparatus conception pertaining to the process of a continuous degradation of PTFE to perfluoroolefines and perfluoroparaffins in the favourable chain length ranges from six to 14 carbon atoms, according to application, is described. An essential component of this conception is the use of a target which consists of a tempered thin layer which is moved into an inertly processed reactor for irradiation. The action of high energy radiation of relatively low dose in air in presence of reactive substances leads to a finely grained PTFE powder with functional groups, which can be mixed easily with liquids, solutions and polymers.     

Semifluorinated alcohols in Langmuir monolayers--a comparative study

A series of semifluorinated alcohols differing in the proportion of the perfluorinated to hydrogenated chains length was synthesized and investigated in Langmuir monolayers using surface pressure and surface potential measurements. All the investigated semifluorinated alcohols were found to be capable of stable floating monolayer formation. The stability of monolayers was found to be higher upon increasing the length of the perfluorinated segment. A lower stability of the monolayers from alcohols having shorter perfluorinated fragment was attributed to the aggregation process, which was visualized with Brewster angle microscopy (BAM). Most condensed monolayers were formed by compounds with longer perfluorinated moiety, whereas monolayers composed by molecules with an iso-branched perfluorinated segment were found to be more expanded. The change of electric surface potential was negative along the whole compression. The maximum absolute values of DeltaV varies, depending on the number of CF(2) groups, from ca. -400 mV for F6H10OH to ca. -700 mV for F10H10OH. The dipole moments of free molecules were calculated with Hyperchem, and the obtained values were approximately the same (within the experimental error), i.e., 2.8D for all the investigated molecules, independently on the perfluorinated fragment length. The dipole moment vector was found to be virtually aligned to the main molecular axis for the studied compounds. Therefore, the observed differences in the measured values of DeltaV can result from a different dielectric permittivity of a particular monolayer.     

Prediction of molecular flexibility in halogenated alkanes via fractal dimensionality

The topological connectedness and experimentally determined boiling point for each of a series of halogenated normal alkane molecules were used to derive fractal dimensionalities of these molecules by a simplified version of the method of Rouvray and Pandey. The results, which relate to the time-averaged molecular conformations of the species in the bulk liquid phase, were then used to obtain estimates of the flexibility of the molecules in question. The effects of the halogen atoms are pronounced and reveal a number of interesting trends. Such analysis is of particular value in the medical context, since perfluorinated alkanes and other perfluorinated species are now being employed widely in medical operations as substitutes for body fluids.     

1,3,6,8-Tetraazapyrenes: synthesis, solid-state structures, and properties as redox-active materials

A series of new tetraazapyrene (TAPy) derivatives has been synthesized by reducing 1,4,5,8-tetranitronaphthalene to its corresponding tin salt (I) and reacting it with perfluorinated alkyl or aryl anhydrides. The resulting 2,7-disubstituted TAPy molecules and the known parent compound 1,3,6,8-tetraazapyrene (II) have been further derivatized by core chlorination and bromination. The brominated compounds served as starting materials for Suzuki cross-coupling reactions with electron-poor arylboronic acids. Single-crystal X-ray analyses established polymorphism for some TAPy compounds. The ground-state geometries of all new TAPy derivatives were modeled with DFT methods [B3PW91/6-31 g(d,p) and B3PW91/6-311+g(d,p)], especially focusing on the energies of the lowest unoccupied molecular orbital (LUMO) and the electron affinities (EA) of the molecules. The results of the calculations were confirmed experimentally by cyclic voltammetry to evaluate the substitution effects at the 2 and 7 position and the core positions, respectively, and gave LUMO energy levels that range from -3.57 to -4.14 eV. Fabrication of organic field-effect transistors (OFETs) with several of these tetraazapyrenes established their potential as organic n-type semiconductors.     

Recent trends in perfluorinated sulfoximines

The search for original perfluorinated moieties is a very modern and attractive challenge. Among the emergent groups, the S-perfluoroalkylated sulfoximines are very peculiar because of their structural diversity and promising properties. A literature survey shows that interest in these molecules is strongly increasing. This short review summarizes the recent works devoted to this topic.     

Heterocyclic analogs of pleiadiene. 42. Acylation of perimidines and 2,3-dihydroperimidines with derivatives of perfluorocarboxylic acids

2-Perfluoroalkylperimidines, 2,3-dihydroperimidines, perimidones, and thioxoperimidines are acylated by anhydrides and chlorides of perfluorinated acids without catalysts. It was established that this reaction proceeds considerably more readily than in the case of perimidine and its alkyl-substituted derivatives, since 2-perfluoroalkylperimidines, 2,3-dihydroperimidines, perimidones, and thioxoperimidines, because of their low basicities, are acylated in the form of neutral molecules, whereas simple perimidines form cations with low reactivities under the same conditions.See [1] for communication 41.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 418–421, March, 1979.     

MOF-Stabilized Perfluorinated Palladium Cages Catalyze the Additive-Free Aerobic Oxidation of Aliphatic Alcohols to Acids

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd²⁺ coordinate cages within the channels of an anionic tridimensional metal-organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd²⁺ to dehydrogenate primary alcohols, without any base, and also to activate O₂ for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.     

Molecular interactions and CO2-philicity in supercritical CO2. A high-pressure NMR and molecular modeling study of a perfluorinated polymer in scCO2

The carbon and fluorine chemical shifts of mixtures of carbon dioxide and Krytox, a carboxylic acid end-capped perfluorinated polyether used as stabilizer for the dispersion polymerization of methyl methacrylate, have been studied using high-pressure, high-resolution nuclear magnetic resonance. 13C and 19F spectra were measured in the density region between 0.54 and 0.73 g.cm(-3) at 334 K for different solutions of Krytox in scCO2 (0.22, 1.13 and 1.72 w/w %). An in-house developed high-pressure apparatus with the capability to change in situ the sample composition was used for this purpose using a 10 mm polyether ketone NMR tube. The nature of CO2-Krytox interaction was assessed both by comparing the CO2 deltaC variation of neat CO2 with that of mixtures with increasing surfactant composition and by the analysis of Krytox 19F corrected chemical shifts in terms of medium magnetic susceptibility. Ab initio calculations, at the second-order M?ller-Plesset level of theory to include the effects of electron correlation, were performed to access and compare the nature of the interactions between CO2 and perfluorinated and nonfluorinated analogue model molecules. Both experimental 13C and 19F HP-NMR results and molecular modeling studies support a F...CO2 site-specific Lewis acid-Lewis base interaction model. A positive entropic variation for the formation of CO2-fluorinated solute complex is advanced as an explanation for the higher solubility of perfluorinated molecules when compared to the nonfluorinated analogues.     

Structure of salted-out, solubilized micelles and microemulsions on the perfluorinated anionic surfactant tetraethylammonium perfluorooctyl sulfonate studied by cryogenic transmission electron microscopy

 Tetraethylammonium perfluorooctyl sulfonate (TEAFOS; critical micelle concentration, 1 mM), which forms a threadlike micelle in its pure solution, was adopted to study the structure of salted-out, solubilized micelles and microemulsions by cryogenic transmission electron microscopy. The concentration of the surfactant was kept constant at 60 mM. The micelle solution salted out with LiNO₃ provided a surfactant phase in the presence of a clear interface. The surfactant phase was studded, being formed of homogeneously dispersed spherical micelles, and had no obvious threadlike forms. The micelles, which solubilized the maximum amount of perfluorinated oil, were spherical and had the same size as isolated spherical micelles in pure TEAFOS solution. The microemulsions were formed in the presence of perfluorinated alcohol as cosurfactant and the particles were rotund even when the concentration of the perfluorinated oil was equivalent to that for solubilization and the sizes increased with increasing oil content. The difference in size between the solubilized micelles and microemulsions with the same amount of oil suggested that the oil molecules had been solubilized between palisades of perfluorinated alkyl chains in the micelles and had dissolved in the cores of the microemulsions. Received: 10 September 1999/Accepted: 2 December 1999     

An ab initio molecular orbital study of the C2F2 species: The difluorovinylidene → difluoroacetylene rearrangement

An ab initio MO study, using medium size Gaussian basis sets has been made of vinylidene carbene, acetylene and the isomeric cyclic intermediate presumed to represent the transition state of their interconversion reaction, along with their perfluorinated analogs. The total energies of the acetylenes are lower than the vinylidenes and the estimated activation energy of their interconversion is considerably higher for the fluorinated molecules. The results are in line with experimental observations.     

Predicting the biodegradation products of perfluorinated chemicals using CATABOL

Perfluorinated chemicals (PFCs) form a special category of organofluorine compounds with particularly useful and unique properties. Their large use over the past decades increased the interest in the study of their environmental fate. Fluorocarbons may have direct or indirect environmental impact through the products of their decomposition in the environment. It is a common knowledge that biodegradation is restricted within non-perfluorinated part of molecules: however, a number of studies showed that defluorination can readily occur during biotransformation. To evaluate the fate of PFCs in the environment a set of principal transformations was developed and implemented in the simulator of microbial degradation using the catabolite software engine (CATABOL). The simulator was used to generate metabolic pathways for 171 perfluorinated substances on Canada's domestic substances list. It was found that although the extent of biodegradation of parent compounds could reach 60%, persistent metabolites could be formed in significant quantities. During the microbial degradation a trend was observed where PFCs are transformed to more bioaccumulative and more toxic products. Perfluorooctanoic acid and perfluorooctanesulfonate were predicted to be the persistent biodegradation products of 17 and 27% of the perfluorinated sulphonic acid and carboxylic acid containing compounds, respectively.     

Toward electron encapsulation: polynitrile approach

This study seeks an answer to the following question: Is it possible to design a supramolecular cage that would "solvate" the excess electron in the same fashion in which several solvent molecules do that cooperatively in polar liquids? Two general strategies are outlined for this "electron encapsulation", viz. electron localization using polar groups arranged on the (i) inside of the cage or (ii) outside of the cage. The second approach is more convenient from the synthetic standpoint, but it is limited to polynitriles. We demonstrate, experimentally and theoretically, that this second approach faces a problem: the electron attaches to the nitrile groups, forming molecular anions with bent C-C-N fragments. Because the energy cost of this bending is high, for dinitrile anions in n-hexane, the binding energies for the electron are low and, for mononitriles, these binding energies are lower still, and the entropy of electron attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the solute molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups. We argue that such species would be more correctly viewed as multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, rather than cavity electrons. The way in which the excess electron density is shared by such molecules is similar to the way in which this sharing occurs in large di- and polynitrile anions, such as 1,2,4,5,7,8,10,11-octacyanocyclododecane(-). Only in this sense is the electron encapsulation possible. The work thus reveals limitations of the concept of "solvated electron" for organic liquids: it is impossible to draw a clear line between such species and a certain class of radical anions.     

Photoluminescence properties of a perfluorinated supramolecular columnar liquid crystal with a pyrene core: effects of the ordering and orientation of the columns

We investigated the effects of the alignment and ordering of pi-conjugated perfluorinated dendrimers containing pyrene moieties in their cores on their photoluminescence (PL) properties. The pyrene molecules are stacked in columns surrounded by aromatic and semifluorinated tails, which can be conjugated and act as chromophores. Polarized light microscopy (PLM), cross-sectional scanning electron microscopy (SEM), and atomic force microscopy (AFM) results show that variation of the cooling rate of the dendrimers produces variation in their orientation and ordering: Slow cooling (approximately <0.5 degrees C/min) of the isotropic melt in a sandwich glass cell results in a high degree of ordering and the vertical alignment of the columns on the substrate, in which the stacked pyrene molecules are oriented parallel to the surface over large areas. In contrast, rapid cooling (approximately >10 degrees C/min) leads to the planar alignment of the columns with significant disorder on the same substrate. UV-vis, PL, SEM, and AFM results show that the quenched columns with a planar orientation produce a broad emission band and a second weak shoulder, which indicates the presence of isolated molecules. However, the high degree of ordering of the columns with a vertical alignment produces a red-shift in the PL spectrum, with very few isolated molecules. By comparing two films with different alignments but similar ordering, we show that the ordering of this material has a greater influence on the PL spectrum than the alignment. This effect of the ordering of the columns was further verified by comparing the optical properties of the isolated dendrimers with those of small pi-conjugated molecules in solution and solid films.     

Enhanced thermal response of the SAd1 layer thickness in highly fluorinated thermotropic liquid crystals

X-ray studies of a homologous series of rod-shaped liquid crystal molecules with one tail perfluorinated and the other protonated, reveal large decreases in the smectic A layer spacing with increasing temperature. These materials form unique dimer phases in which the smectic layer spacing is dependent on the length of the perfluorinated tail and independent of the length of the protonated tail. The chain statistics of the perfluorinated tail significantly influence the thermal expansion coefficient since the length of the fluorinated tail defines the smectic layer spacing. Thermal expansion coefficients for the layer spacing observed here are negative and nearly an order of magnitude greater than for typical protonated rod-shaped thermotropic liquid crystals in the SA phase.     

Size‐Tunable Micron‐Bubbles Based on Fluorous–Fluorous Interactions of Perfluorinated Dendritic Polyglycerols

This paper describes the behavior of various generations of polyglycerol dendrimers that contain a perfluorinated shell. The aggregation in organic solvents is based on supramolecular fluorous–fluorous interactions, which can be described by means of ¹⁹F NMR spectroscopy. In order to study the interaction and aggregation phenomena of dendrimers with perfluorinated shell and perfluoro‐tagged guest molecules we investigated [G3.5]‐dendrimer with a perfluorinated shell in the presence of perfluoro‐tagged disperse red. Noteworthy, the interaction intensities varied in an unexpected manner depending on the equivalents of perfluoro‐tagged guest molecules added to the dendrimers in solution which then formed supramolecular complexes based on fluorous–fluorous interactions. We found that these complexes aggregated around residual air in the solvent to form stable micron‐sized bubbles. Their sizes correlated with the interaction intensities measured for certain dendrimer–guest molecule ratios. Degassing of the solutions led to a quasi phase separation between organic and fluorous phase, whereby the dendrimers formed the fluorous phases. Regassing the sample with air afforded bubbles of the initial size again.     

Proton-exchange membranes for hydrogen-air fuel cells

The review is devoted to recent advances in the development of polymer electrolytes for low-temperature (operating temperatures ～80°C) and medium-temperature (operating temperatures 160–180°C) fuel cells. At present the most used are perfluorinated polymer membranes, such as Nafion®, Aciplex®, Flemon®, and Dow®, owing to their high chemical stability and proton conductance, as well as good machinability. However, successful commercialization of fuel cells with such membranes is prevented by their high cost, as well as low proton conductance at low humidities and temperatures above 100°C. Much effort is underway to develop membranes alternative to perfluorinated ones, with emphasis on aromatic hydrocarbon polymers.