Voltage-induced swelling and deswelling of weak polybase brushes

We have investigated a novel method of remotely switching the conformation of a weak polybase brush using an applied voltage. Surface-grafted polyelectrolyte brushes exhibit rich responsive behavior and show great promise as "smart surfaces", but existing switching methods involve physically or chemically changing the solution in contact with the brush. In this study, high grafting density poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grown from silicon surfaces using atom transfer radical polymerization. Optical ellipsometry and neutron reflectivity were used to measure changes in the profiles of the brushes in response to DC voltages applied between the brush substrate and a parallel electrode some distance away in the surrounding liquid (water or D(2)O). Positive voltages were shown to cause swelling, while negative voltages in some cases caused deswelling. Neutron reflectometry experiments were carried out on the INTER reflectometer (ISIS, Rutherford Appleton Laboratory, UK) allowing time-resolved measurements of polymer brush structure. The PDMAEMA brushes were shown to have a polymer volume fraction profile described by a Gaussian-terminated parabola both in the equilibrium and in the partially swollen states. At very high positive voltages (in this study, positive bias means positive voltage to the brush-bearing substrate), the brush chains were shown to be stretched to an extent comparable to their contour length, before being physically removed from the interface. Voltage-induced swelling was shown to exhibit a wider range of brush swelling states in comparison to pH switching, with the additional advantages that the stimulus is remotely controlled and may be fully automated.     

4He+ ion beam irradiation induced modification of poly(dimethylsiloxane). Characterization by infrared spectroscopy and ion beam analytical techniques

In this study we investigated the chemical and surface wettability changes of poly(dimethylsiloxane) (PDMS) induced by a 2.0 MeV He(+) beam irradiation. The chemical changes created in PDMS were characterized by universal attenuated total reflectance infrared (UATR-FTIR) spectroscopy, while the changes of the wettability were determined by contact angle measurements. In a separate analysis, hydrogen depletion was also investigated with a 1.6 MeV He(+) beam by applying the elastic recoil detection analysis (ERDA) and Rutherford backscattering spectrometry techniques simultaneously. The ERDA results showed that the hydrogen content of PDMS decreased irreversibly, which means that volatile products were formed under radiolysis, such as hydrogen or methane. The results were completed with UATR-FTIR measurements. We propose a complete reaction mechanism for the processes taking place in PDMS. These ion beam induced processes, such as chain scissions, cross-linking, and depletion of small molecular weight fragments, lead to the formation of a silica-like final product (SiO(x)). The significant chemical changes at the surface influence the wettability of PDMS, making it considerably more hydrophilic. The penetration depth of the 2.0 MeV He(+) ions is significantly higher compared to that of other surface modification techniques, which makes the modified layer thick and homogeneous; on the other hand, it is easily controllable by the energy of the incident ions.     

Aspects of heterogeneous enantioselective catalysis by metals

Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific reference to four different systems where the phenomena that control enantioselection appear to be very different. In the case of glucose electro-oxidation, it is clear that any intrinsic chirality present at the metal surface plays a vital role. With α-keto hydrogenation, achiral surfaces modified by the adsorption of chiral agents become effective enantioselective catalysts and the formation of extended arrays of chiral species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. Hydrogen bonding also plays a central role in β-ketoester hydrogenation, but here fundamental studies indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. Asymmetric C═C hydrogenation, though relatively little studied, has the potential for major impact in synthetic organic chemistry both on the laboratory scale and in the manufacture of fine chemicals and pharmaceuticals. The structural attributes that determine whether a given chiral ligand is effective have been identified; the ability to form strong covalent bonds with the metal surface while also resisting hydrogenation and displacement by the strongly adsorbing reactant under reaction conditions is an essential necessary condition. Beyond this, ligand rigidity in the vicinity of the chirality center coupled with resistance to SAM formation is a critically important factor whose absence results in racemic chemistry.     

Sensitivity improvement of 1H-15N cross-polarization at high MAS frequency applied to NMR structural characterization of organic solids

(15)N CP/MAS solid state NMR should be a method of choice to obtain essential structural information on organic materials containing nitrogen atoms. However, the technique is generally not selected for the characterization of non-labelled chemical compounds, which represents the most common situation encountered by chemists. Actually, due to the poor sensitivity of (15)N the method is time-consuming and a very fine calibration is often a prerequisite to reach a sufficient signal/noise. The main drawback comes from the weakness of (15)N-(1)H dipolar couplings which leads to a splitting of the static Hartman Hahn condition into very narrow sideband conditions under MAS. Practically, it is more difficult to obtain a high enough CP transfer level on (15)N for the entire spectrum than on other more conventional nuclei like (13)C. An experimental investigation of the CP efficiency using the ramp and adiabatic CP transfer experiments is here proposed. Preliminary adjustments of experimental settings were first made on an (15)N-labeled substituted heterocyclic model system, and then applied to several other organic compounds. Particular attention was paid to the detection of non-protonated nitrogen atoms with a significant chemical shift anisotropy, which represented the least favourable case. It was experimentally demonstrated that, for these atoms, the adiabatic passage provided a much higher transfer level than the more conventional ramp sequence leading to an enhancement factor of up to 3.5 at a MAS frequency of 30 kHz. The resulting sensitivity rendered possible the detection of non-protonated nitrogen atoms at natural abundance with 2.5-mm rotors at 9.4 T.     

Synthesis of N-alkylated amino acids using fluorous-tagged hydroxylamines

The development of a new fluorous-tagged ammonia-equivalent for the synthesis of N-alkylated amino acids is described. The required building blocks were readily accessed in high yield and purity using F-SPE purification technique. Coupling of the fluorous-tagged hydroxylamines with a selection of boronic acids and glyoxalic acid gave the desired N-alkylated amino acids. Subsequent removal of the fluorous tag via catalytic hydrogenation was investigated using a number of different catalysts and solvents. A more robust de-tagging procedure involves the transformation of the amino acid to the corresponding methyl ester followed by a Mo(CH₃CN)₃(CO)₃ mediated N-O bond cleavage.     

Liquid chromatography/tandem mass spectrometry for analysis of 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) and 1,2,5,6-tetrabromocyclooctane (TBCO)

Although the two flame retardants 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) and 1,2,5,6-tetrabromocyclooctane (TBCO) have been widely used, a selective instrumental method of analysis for these compounds has not been developed to date. In this study, we demonstrate the feasibility to utilize liquid chromatography/tandem mass spectrometry (LC/MS/MS) for the separation and analysis of α- and β-TBCO and α-, β-, γ-, and δ-TBECH. Acetone was initially used in a tetrahedron solvent system for LC optimization. A simple isocratic elution allowed near-baseline separation of these compounds. Different ionization approaches and mechanisms were investigated. The mass spectrometric transition of [M + O(2)](-) => Br(-) (459.8 => 78.9) was a selective detection method for the target analytes. Good instrument detection limits (5 pg for γ-/δ-TBECH, 125 pg for α-/β-TBECH, and 30 pg for α-/β-TBCO with 2.0 μL injection) were obtained. Excellent linearity up to 50 ng/μL (R(2) >0.999) was also achieved. This method has been applied to environmental samples (surface water) for screening purposes with recoveries ranging from 76-92% (CV%: 5-8%). This method shows significant improvement over previous methods.     

Tuning main group redox chemistry through steric loading: subvalent Group 13 metal complexes of carbazolyl ligands

The ability of substituted carbazol‐9‐yl systems to ligate in σ fashion through the amido N‐donor, or to adopt alternative coordination modes through the π system of the central five‐membered ring, can be tuned by systematic variation in the steric demands of substituents in the 1‐ and 8‐positions. The differing affinities of the two modes of coordination for hard and soft metal centres can be shown to influence not only cation selectivity, but also the redox properties of the metal centre. Thus, the highly sterically sterically demanding 1,3,6,8‐tetra‐tert‐butylcarbazolyl ligand can be used to generate the structurally characterised amido‐indium(I) complex, [{(tBu₄carb)In}_n], (together with its isostructural thallium counterpart) in which the metal centre interacts with the central pyrrolyl ring in η³ fashion [d(In? N)=2.679(3) Å; d(In? C)=2.819(3), 2.899(3) Å]. By contrast, the smaller 3,6‐di‐tert‐butylcarbazolyl system is less able to restrict the metal centre from binding at the anionic nitrogen donor in the plane of the carbazolyl ligand (i.e. in σ fashion). Analogous chemistry with In^I precursors therefore leads to disproportionation to the much harder In^II [and In⁰], and the formation of the mixed‐valence product, [In₂{In₂(tBu₂carb)₆}], a homoleptic molecular [In₄(NR₂)₆] system. This chemistry reveals a flexibility of ligation for carbazolyl systems that contrasts markedly with that of the similarly sterically encumbered terphenyl ligand family.     

Highly efficient visible-light driven photochromism: developments towards a solid-state molecular switch operating through a triplet-sensitised pathway

We introduce a new highly efficient photochromic organometallic dithienylethene (DTE) complex, the first instance of a DTE core symmetrically modified by two Pt(II) chromophores [Pt(PEt(3))(2)(C≡C)(DTE)(C≡C)Pt(PEt(3))(2)Ph] (1), which undergoes ring-closure when activated by visible light in solvents of different polarity, in thin films and even in the solid state. Complex 1 has been synthesised and fully photophysically characterised by (resonance) Raman and transient absorption spectroscopy complemented by calculations. The ring-closing photoconversion in a single crystal of 1 has been followed by X-ray crystallography. This process occurs with the extremely high yield of 80%--considerably outperforming the other DTE derivatives. Remarkably, the photocyclisation of 1 occurs even under visible light (>400 nm), which is not absorbed by the non-metallated DTE core HC≡C(DTE)C≡CH (2) itself. This unusual behaviour and the high photocyclisation yields in solution are attributed to the presence of a heavy atom in 1 that enables a triplet-sensitised photocyclisation pathway, elucidated by transient absorption spectroscopy and DFT calculations. The results of resonance Raman investigation confirm the involvement of the alkynyl unit in the frontier orbitals of both closed and open forms of 1 in the photocyclisation process. The changes in the Raman spectra upon cyclisation have permitted the identification of Raman marker bands, which include the acetylide stretching vibration. Importantly, these bands occur in the spectral region unobstructed by other vibrations and can be used for non-destructive monitoring of photocyclisation/photoreversion processes and for optical readout in this type of efficiently photochromic thermally stable systems. This study indicates a strategy for generating efficient solid-state photoswitches in which modification of the Pt(II) units has the potential to tune absorption properties and hence operational wavelength across the visible range.     

Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H₄L, has led to an Fe^II spin‐crossover (SCO) complex, [Fe(H₄L)₂][ClO₄]₂ ? H₂O ? 2 (CH₃)₂CO ( 1 ), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T_0.5 of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high‐spin state may also be thermally trapped, exhibiting a very large T_TIESST (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO₄^? and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe? N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order–disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising‐like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.     

Mild route to generate gaseous metal anions

Gaseous metal anions such as Na(-), K(-), Cs(-), and Ag(-) can be generated at ambient temperatures by the collision-induced dissociation of the anions of several dicarboxylic acid salts, including oxalate, maleate, fumarate, succinate, and glutamate salts. The formation of gaseous metal anions in this way is unprecedented because the metal is initially present in its cationic form. The mild process described here could facilitate novel applications of metal anions as selective reagents for gas-phase ion-molecule and ion-ion reactions. Ab initio calculations were used to describe the dissociation process for anions of the oxalate salts. The formation of alkalides occurs via production of a metal-carbon dioxide anion intermediate with a bidentate three-center two-electron bond to the metal. The metal atom acquires a partial negative charge in the intermediate structure.