Wavelength and metal dependence in the photofragmentation of a gas-phase lanthanide beta-diketonate complex

The time-of-flight mass spectra of tris(2,2,6,6-tetramethyl-3,5-heptanedionato) lanthanide(III) [or Ln(thd)3 with Ln = Eu, Tb, Gd] produced by laser-induced multiphoton ionization in a supersonic expansion were studied as a function of laser excitation wavelength. Resonance-enhanced multiphoton ionization (REMPI), monitoring the Eu(I) ion signal from gas-phase Eu(thd)3, was observed in three distinct visible-excitation regions, corresponding to electronic absorption transitions on neutral Eu(0) atoms. The confirmation of the presence of Eu(0) atoms in the beam supports the proposed mechanism for the production of Ln atoms through sequential dissociation of neutral thd ligands from the metal following photoexcitation into ligand-to-metal charge-transfer (LMCT) states. Evidence is also presented that the LnO+ and LnOH+ fragments observed in the mass spectrum are produced via a separate, competing fragmentation pathway. The branching ratios between the two fragmentation pathways are compared for Ln(thd)3 (Ln = Eu, Tb, Gd). The ligand-dissociation pathway that produces Ln atoms appears to be more favorable in Ln(thd)3 complexes with low-lying LMCT states. Finally, the observation of the Tb2(thd)6+ dimer and its associated fragmentation pattern, as well as the presence of metal carbides, which are relevant to carbon contamination in chemical vapor deposition, is discussed.     

Selective metal substitution for the preparation of heterobimetallic microporous coordination polymers

The designed synthesis of heterobimetallic microporous coordination polymers (MCPs) is reported by a strategy employing the selective replacement of a single metal in homometallic MCPs with two unique metal coordination environments: octahedral and tetrahedral. This strategy is successful in the preparation of six mixed-metal MCPs, where Co/Zn and Ni/Zn versions of MOF-4, MOF-39, and a Zn-BTEC MCP are reported.     

Selective retention of explosives and related compounds on gas-chromatographic capillary columns coated with lanthanide(III) β-diketonate polymers

The purpose of this research was to investigate coating capillary columns with lanthanide(III) β-diketonate polymers to provide gas-chromatographic columns with selectivity toward explosives and related compounds. Capillary columns were statically coated with a mixture of a lanthanide(III) β-diketonate polymer, which utilized p-di(4,4,5,5,6,6,6-heptafluoro-1,3-hexanedionyl)benzene (dihed) as a ligand, and SE-30. A novel coating technique was developed that used a methanol-containing azeotrope to provide solubility for both polymers while maintaining a low boiling point compatible with static coating. As based on temperature-programmed retention indices, the experimental columns displayed selective retention of nitroaromatic, nitroalkane, and nitrate ester test probes relative to an appropriate SE-30 control column. Selective retention was more pronounced for strongest Lewis base analyte contained in the test mixture (i.e., cyclohexanone). Relative analyte retention on the La(dihed), Tb(dihed), and Eu(dihed) columns are discussed relative to Lewis acidity and solubility of the polymer in the azeotropic coating solution.     

Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles

The synthesis, spectroscopic characterization, and fluorescence quenching efficiency of polymers and copolymers containing tetraphenylsilole or tetraphenylgermole with Si-Si, Ge-Ge, and Si-Ge backbones are reported. Poly(tetraphenyl)germole, 2, was synthesized from the reduction of dichloro(tetraphenyl)germole with 2 equivs of Li. Silole-germole alternating copolymer 3 was synthesized by coupling dilithium salts of tetraphenylsilole dianion with dichloro(tetraphenyl)germole. Other tetraphenylmetallole-silane copolymers, 4-12, were synthesized through the Wurtz-type coupling of the dilithium salts of the tetraphenylmetallole dianion and corresponding dichloro(dialkyl)silanes. The molecular weights (M(w)) of these metallole-silane copolymers are in the range of 4000 approximately 6000. Detection of nitroaromatic molecules, such as nitrobenzene (NB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and picric acid (PA), has been explored. A linear Stern-Volmer relationship was observed for the first three analytes, but not for picric acid. Fluorescence spectra of polymetalloles or metallole-silane copolymers obtained in either toluene solutions or thin polymer films displayed no shift in the maximum of the emission wavelength. This suggests that the polymetalloles or metallole-silanes exhibit neither pi-stacking of polymer chains nor excimer formation. Fluorescence lifetimes of polymetalloles and metallole-silanes were measured both in the presence and absence of TNT, and tau(o)/tau is invariant. This requires that photoluminescence quenching occurs by a static mechanism.     

Selective metal cation capture by soft anionic metal-organic frameworks via drastic single-crystal-to-single-crystal transformations

Flexible anionic metal-organic frameworks (MOFs) are transformed into neutral heterobimetallic systems via single-crystal-to-single-crystal processes invoked by cation insertion. These transformations are directed by cooperative bond breakage and formation, resulting in expansion or contraction of the 3D framework by up to 33% due to the flexible nature of the organic linker. These MOFs displays highly selective uptake of divalent transition-metal cations (e.g., Co(2+) and Ni(2+)) over alkali-metal cations (Li(+) and Na(+)).     

Energetic polymers as binders in composite propellants and explosives

The principles behind the use of polymeric binders in composite propellants and explosives are described with emphasis on the properties which they should possess in order to satisfy the requirements for inclusion in a composition. The desirability of using energetic polymers as binders in terms of both performance and safety, and the problems associated with their preparation and properties, are discussed. The contributions of chemical synthesis within DRA to overcome these problems will be shown. Preparation of energetic polymers both by polymer modification and by polymerization of an energetic monomer is described. We have developed three energetic polymers: poly-3-nitratomethyl-3-methyloxetane (polyNIMMO), polyglycidyl nitrate (polyG-LYN) and nitrated hydroxy-terminated polybutadiene (NHTPB). Two of these (polyNIMMO and polyGLYN) have shown excellent properties in propellant and explosive formulations and proved that low-vulnerability, high-performance compositions are possible. The properties of the products from our work are compared with those of other groups and a glimpse of the future in this area is given to show the potential for new energetic polymers.     

Precious metal polymers

The synthesis and properties of polymers containing precious metal centres in the backbone are described. Polymers with labile transition metal centres can be prepared by ring-opening polymerization of macrocycles, and examples are given with silver, gold and palladium as the metal centres. In some cases, the polymers can be further organized by using ligands with hydrogen bonding substituents, and self-assembly into sheet or network structures can then occur. Secondary bonding between inorganic centres can also lead to ordered self-assembly. Bicyclic precursors can ring open to form either chains or sheets.     

Selective gas-phase oxidation of pyridine derivatives with hydrogen peroxide

The interfering kinetics of the synchronous reactions of hydrogen peroxide decomposition and the oxidation of pyridine derivatives have been studied experimentally. The regions of the selective oxidation of the pyridine derivatives have been found, and the optimal conditions for the production of 4-vinylpyridine, 4-vinylpyridine N-monoxide, 2,2-dipyridyl, and pyridine have been determined. The most probable synchronization mechanism is suggested for hydrogen peroxide decomposition and the free-radical chain oxidation of pyridine derivatives. The HO ₂ ^· radical plays the key role in this mechanism. The activation energies are calculated for the elementary steps of 4-ethylpyridine dehydrogenation.     

Electron capture ionization of explosives with a microflow rate particle beam interface

A method for the analysis of four widely used explosives based on reversed-phase liquid chromatography coupled to a quadrupole mass spectrometer is presented. A microflow rate particle beam interface was employed that offers simplified operation procedures and improved interfacing performance. A positive role played by the reduced size of the aerosol droplets generated by the microflow rate interface is outlined in this work. Greater vaporization efficiency and negligible thermal decomposition were observed for the selected compounds in the ion source of the mass spectrometer. Electron capture ionization allowed specific and sensitive determination of the analytes. Detection limits that ranged between 60 and 200 pg and had a signal-to-noise ratio of 5:1 were obtained in selected ion monitoring mode after column elution.     

Selective adsorption of metronidazole on conjugated microporous polymers

Conjugated microporous polymers (CMPs) have recently received extensive attention in oil/organic solvent-water separation field as a kind of ideal porous absorbents with tunable porosity, large surface areas, and super-hydrophobicity. However, reports on the application of CMPs in adsorption of hydrophilic contaminants from water are very few. In this work, we studied the adsorption of metronidazole (MNZ), a polar antibiotic, by two kinds of CMPs. The adsorption characteristics of MNZ by the CMPs, including adsorption kinetics, mechanism, and isotherm parameters were calculated. The adsorption kinetics of MNZ was well expressed by the pseudo-second-order model, and the adsorption process was found to be mainly controlled by film diffusion. The adsorption isotherm data agreed well with the Langmuir isotherm model, and the values of free energy E indicated that the adsorption nature of MNZ on the CMPs was physisorption. Increasing dispersion degree of the CMPs in MNZ solution resulted in greater adsorption. This work may provide fundamental guidance for the removal of antibiotics by CMPs.     

Synthetic polymers for capture and detection of microorganisms

Methods for the detection of pathogens and their deactivation are of paramount importance for public health and security. In this review we highlight some recent examples of sophisticated synthetic polymers and macromolecular constructs that can be used to interact selectively with target pathogens. The biomimetic properties of these materials lend themselves not just to high specificity interactions with target pathogens, but also towards novel capture and deactivation mechanisms. These are the first steps towards combined diagnostics/therapeutics against pathogens.     

Selective extraction of nitroaromatic explosives by using molecularly imprinted silica sorbents

Two molecularly imprinted silicas (MISs) were synthesized and used as selective sorbents for the extraction of nitroaromatic explosives in post-blast samples. The synthesis of the MISs was carried out with phenyltrimethoxysilane as monomer, 2,4-dinitrotoluene (2,4-DNT) as template and triethoxysilane as cross-linker by a sol–gel approach in two molar ratios: 1/4/20 and 1/4/30 (template/monomer/cross-linker). Non-imprinted silica sorbents were also prepared following the same procedures without introducing the template. An optimized procedure dedicated to the selective treatment of aqueous samples was developed for both MISs for the simultaneous extraction of the template and other nitroaromatic compounds commonly used as explosives. The capacity of the MISs was measured by the extraction of increasing amounts of 2,4-DNT in pure water and is higher than 3.2 mg/g of sorbent for each MIS. For the first time, four nitroaromatic compounds were selectively extracted and determined simultaneously with extraction recoveries higher than 79%. The potential of these sorbents was then highlighted by their use for the clean-up of post-blast samples (motor oil, post-mortem blood, calcined fragments, etc.). The results were compared to those obtained using a conventional sorbent, thus demonstrating the interest of the use of these MISs as selective sorbents.     

Engineering porous organic polymers for carbon dioxide capture

As atmospheric CO₂ levels rise, the development of physical or chemical adsorbents for CO₂ capture and separation is of great importance on the way towards a sustainable low-carbon future. Porous organic polymers are promising candidates for CO₂ capture materials owing to their structural flexibility, high surface area, and high stability. In this review, we highlight high-performance porous organic polymers for CO₂ capture and summarize the strategies to enhance CO₂ uptake and selectivity, such as increasing surface area, increasing interaction between porous organic polymers and CO₂, and pore surface functionalization.     

Selective pyrolysis of bifunctional compounds: gas-phase elimination of carbonate-ester functionalities

Compounds containing both carbonate and ester functionalities were synthesized and then subjected to online-GC gas-phase pyrolysis. The carbonate groups were cleaved selectively in all elimination reactions. The end products of the reaction were found to be affected by the nature of the substrate. The presence of hydrogen and carbonyl substituents on the carbon β to the carbonate group resulted in further product decomposition through a concerted six-membered transition state. Results from flash vacuum pyrolysis (FVP) and analysis of the GC data indicate that the cleavage of the carbonate group is fast, and that the slower secondary decomposition reactions are independent of the presence of the carbonate group. Spectroscopic analyses of the products are reported.     

Selective metal deposition on photoswitchable molecular surfaces

We report here a novel phenomenon: selective metal deposition on photoswitchable diarylethene (DAE) surfaces. Magnesium vapor was deposited by vacuum evaporation on the colored DAE but not on the uncolored surface. The selective deposition originates in the change of the glass transition temperature of the amorphous DAE film resulting from photoisomerization and therefore from changes of surface molecular motion. We clarified that Mg atoms on the uncolored surface actively migrated on the surface and were desorbed from the surface. The possibility of depositing other metals is also discussed. Light-controllable metal-integrated deposition was demonstrated as a new function of the photoswitchable molecular surfaces. This study reveals new features of the photoswitchable molecular surfaces, and their potential suggests bright prospects for future applications in organic electronics.     

Selective sampling with direct ion mobility spectrometric detection for explosives analysis

This study investigates the potential and limitations of a field-deployable analytical approach that involves selective capture of explosive materials with direct analysis by ion mobility spectrometry (IMS). Selective capture of explosives was performed on deactivated quartz fiber filters impregnated with metal β-diketonate polymers. These Lewis acidic polymers selectively interact with Lewis base analytes such as explosives. The filters were directly inserted into an IMS instrument for analysis. The uptake kinetics of 2,4,6-trinitrotoluene (TNT) from a saturated atmosphere were characterized, and based on these studies, passive equilibrium sampling was applied to estimate the TNT concentration within an ammunition magazine that contained bulk TNT. Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) uptake from a saturated environment also was examined over a one-month period. Each incremental sampling period showed increasing quantities of RDX culminating with collection of approximately 5 ng of RDX on the filter at the end of one month. This is the first time that gas-phase uptake of RDX has been demonstrated.