Germanium analogs of carbenium ions

The review summarizes advances in the production of germanium analogs of carbenium ions both per se in solutions and crystals and as intermediates in reactions of organogermanium compounds. The data on the structures of germylium cations of different types are considered. Emphasis is given to the reactivity of germylium ions and their ion-molecule reactions with nucleophilic reagents in the gas and condensed phases.     

Synthesis and supramolecular self-assembly study of a novel porphyrin molecule in Langmuir and Langmuir-Blodgett films

The self-assembly and supramolecular engineering of porphyrins into ordered arrays have recently attracted much interest because of their promising application potential in molecular and electronic devices, spintronics, energy harvesting and storage, catalysis, and sensor development. We herein report the synthesis and supramolecular self-assembly study of a novel porphyrin molecule, 2Por-TAZ, in Langmuir and Langmuir-Blodgett films. The 2Por-TAZ molecule contains two porphyrin macrocycles attached to a triaminotriazine headgroup. Triaminotriazines are known to form a highly ordered linear supramolecular self-assembly through complementary hydrogen bonding with barbituric acid molecules at the air-water interface. Surface pressure-area isotherm measurements and polarized UV-vis absorption spectroscopic studies indicate that the 2Por-TAZ molecules adopted an edge-on orientation at the air-water interface. Polarized UV-vis absorption study also revealed that the 2Por-TAZ molecules formed linear supramolecular networks on pure water and barbituric acid subphase with porphyrin flat planes facing toward the compression direction. The binding of barbituric acid with 2Por-TAZ molecules was observed from the expansion of the Langmuir monolayer film. Compared to the transferred LB film from pure water subphase, both the UV-vis absorbance and fluorescence emission intensity of the LB film transferred from barbituric acid subphase increased significantly.     

Langmuir-Blodgett films as aligning layers for homeotropic alignment of liquid crystal molecules

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

Surface-based molecular self-assembly: Langmuir-Blodgett films of amphiphilic Ln(III) complexes

The unique photophysical properties of the Ln(III) series has led to significant research efforts being directed towards their application in sensors. However, for “real-life” applications, these sensors should ideally be immobilised onto surfaces without loss of function. The Langmuir-Blodgett (LB) technique offers a promising method in which to achieve such immobilisation. This mini-review focuses on synthetic strategies for film formation, the effect that film formation has on the physical properties of the Ln(III) amphiphile, and concludes with examples of Ln(III) LB films being used as sensors.     

Stable mechanistically-relevant aromatic-based carbenium ions in zeolite catalysts

The existence of carbenium ion species is assumed in many zeolite catalysis mechanisms. Using computational techniques that include environmental effects, a benzenium-type carbenium ion is identified in zeolite catalysts for the first time. Localization of nearby transition states indicate that this species may play an important role as an intermediate in the bimolecular m-xylene disproportionation reaction. The barrier to back-donation of the proton from the benzenium ion is at least 50 kJ/mol, meaning that this species may be spectroscopically observable. An additional carbenium ion intermediate, formed by abstraction of a hydride from m-xylene, is also predicted. The stability of this second new carbenium ion suggests that aromatic-based carbenium ions are likely to be intermediates in many zeolite-catalyzed reactions. Two types of fundamentally different fully periodic calculations support the stability predictions.     

Orbital interactions in selenomethyl-substituted pyridinium ions and carbenium ions with higher electron demand

Computational, solution phase, and crystal structure analysis of 2- and 4-organoselenylmethyl-substituted pyridinium ions (10a-c and 11a-c) provides strong evidence for C-Se hyperconjugation (σ(C-Se)-π*) between the C-Se σ-bond and the π-deficient aromatic ring and a through-space interaction (n(Se)-π*) between the selenium p-type lone pair and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (n(Se)-σ*(CC)) involving the selenium p-type lone pair electrons and the polarized CH(2)-C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) show that C-Se hyperconjugation (σ(C-Se)-π*) is the predominant mode of stabilization in the weakly electron-demanding pyridinium ions (10d, 11d, 14, and 15); however, the through-space (n(Se)-π*) interaction becomes more important as the electron demand of the β-Se-substituted carbocation increases. The anomeric interaction (n(Se)-σ*(CC)) is relatively weak in all ions.     

Micro- and nanopatterned copper structures using directed self-assembly on templates fabricated from phase-separated mixed Langmuir-Blodgett films

We report a versatile method to confine metal thin films in micro- and nanopatterns using directed self-assembly on the templates fabricated from phase-separated mixed Langmuir-Blodgett (LB) films. The pattern of the mixed LB films can be tuned by adjusting intermolecular interaction between the film-forming molecules in the LB films and by varying the fabrication conditions of the films such as the mixing ratio, subphase temperature, and surface pressure. We use the patterned LB films for templates to confine metal in patterned regions, taking advantage of the difference between the surface free energy of the patterned regions and that of the self-assembled monolayer of the silane coupling agent. Au nanoparticles are confined onto the patterned films as a catalyst for the succeeding Cu electroless deposition. The atomic force microscopic images, Auger electron spectra, and scanning Auger electron maps of a Cu-deposited film show that Cu is selectively deposited on the patterns of phase separation of the original mixed LB films.     

Investigation of alignment mechanism of liquid crystal on polyimide Langmuir-Blodgett films by scanning tunnelling microscopy

The homogeneous alignment of a liquid crystal material, 4'-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.     

Investigation of alignment mechanism of liquid crystal on polyimide Langmuir-Blodgett films by scanning tunnelling microscopy

Abstract

The homogeneous alignment of a liquid crystal material, 4′-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.     

Alkylation of benzyl chloride by gaseous carbenium ions

The alkylation of benzyl chloride has been studied in the gas phase using radiolytically formed carbenium ions as the charged reagents. The intramolecular selectivity of the electrophilic attack, deduced from the composition of the neutral products, is characterized by the comparable reactivity of the aromatic ring and of the halogen atom of the substrate toward the gaseous cations. As to ring alkylation, the reactivity of the ortho positions of benzyl chloride is considerably lower than those of chlorobenzene. The results are compared with pertinent mass-spectrometric data, and discussed in connection with existing models of gas phase aromatic substitution by charged electrophiles.     

Polyoxometalate monolayers in Langmuir-Blodgett films

Langmuir and Langmuir-Blodgett (LB) films of a variety of polyoxometalates of different shapes, sizes, and charges were prepared by taking advantage of the adsorption properties of these polyanions on a positively charged monolayer of an organic surfactant spread on water. Three different aspects were investigated. 1) The electrochemical and electrochromic properties of LB films containing the easily reducible polyoxoanion [P2Mo18O62]6-. Absorbance changes of these LB films deposited onto an ITO substrate have been induced by repeated switching of the applied potential. These changes are due to the formation of the colored reduced forms of the polyanion. Coloration and bleaching of the LB film occur very quickly and are reversible. 2) The preparation of LB films based on magnetic polyoxometalates, such as the Keggin anions, [CoW12O40]6- and [SiMn(OH2)W11O39]6-, or containing magnetic clusters of increasing nuclearities such as [Co4(H2O)2(PW9O34)2]10- and [Co4(H2O)2(P2W15O62)2]16- based on a Co4O16 ferromagnetic cluster, and the polyoxometalates [Co9(OH)3(H2O)6(HPO4)2(PW9O34)3]16- and [Ni9(OH)3-(H2O)6(HPO4)2(PW9O34)3]16- based on a nonanuclear M9O36 cluster. 3) The preparation of LB films of the giant heteropolyoxomolybdate, [Na3(NH4)12][Mo57Fe6(NO)6O174(OH)3-(H2O)24]76 H2O.     

Langmuir-Blodgett films of diazobenzene molecules

Langmuir-Blodgett (LB) films from diazobenzene Sudan III have been investigated using surface potential measurements as a function of number of layers and deposition pressures, with the surface potential data being related to molecular dipole moments obtained from theoretical electronic structure calculations. The surface potential increased with the number of layers for SIII LB films, and then tended to saturate. Results from density functional theory (DFT) and UV-vis spectroscopy indicated that the increase is due to addition of layers with oriented molecular dipoles, with the saturation tendency being attributed to a decrease in the amount of material deposited in each layer. The surface potential increased with the surface pressure used for deposition, probably owing to a higher contribution from the vertical component of the dipole moment as a closer molecular packing, which is associated with decreasing conformational entropy, was reached.     

Triplet-triplet energy transfer in Langmuir-Blodgett films

The temperature effect on the efficiency of the triplet energy transfer between different molecules included in molecular layers by the Langmuir-Blodgett (LB) procedure was studied. The efficiency of the triplet energy transfer from the LB film of the donor to the LB film of the acceptor is determined by the homogeneous broadening of the energy donor levels.     

A novel nanogold multilayer constructed by Langmuir-Blodgett and self-assembly techniques

A multilayer comprising nanogold particles and organic molecules was constructed by self-assembly of the small organic molecule 1-phenyl-5-mercaptotetrazole onto the octadecylamine-protected nanogold monolayer in the course of transferring the monolayer, layer by layer, from a Langmuir-Blodgett trough to solid substrates. The structure and properties of the constructed multilayer were characterized by TEM images and ATR-IR, XPS, and UV-vis spectra. It is evidenced for the first time that the combination of Langmuir-Blodgett and self-assembly techniques provides a new way to organize functional nanoparticles such as organic-nanogold complexes.     

Langmuir-Blodgett films of octadecanethiol--properties and potential applications

Octadecanethiol (ODT) is known to form self-assembled monolayer on noble metal surfaces which has potential technological applications. Langmuir-Blodgett (LB) technique is another useful method of obtaining highly ordered assembly of molecules. It is of interest to find whether ODT molecules can also form a stable Langmuir monolayer which facilitates the preparation of LB films. In literature, it has been reported that ODT molecules form an unstable Langmuir monolayer. We have studied the stability of the monolayer of the ODT molecules at air-water interface using surface manometry and microscopy techniques. We find the monolayer to be stable on ultrapure water of resistivity greater than 18MOmega cm. However, the behavior changes in the presence of even small amount of additives like NaOH or CdCl2 in the subphase. Our AFM studies on the LB films of ODT deposited from ion-free ultrapure water showed streak-like bilayer domains. The LB films of ODT deposited from CdCl2 containing aqueous subphase yield dendritic domains of the complexed unit grown over ODT monolayer. These nanostructures on surfaces may have potential applications in molecular electronics.     

Monolayers and Langmuir-Blodgett films of crown-substituted phthalocyanines

Tetra-15-crown-5-phthalocyanine ligand and its ruthenium complex with axial CO and MeOH groups were synthesized. The properties of their monolayers and Langmuir-Blodgett films were studied. In the case of the ligand, monolayer films of molecular associates are formed. The compatibility of the ligand and stearic acid in a mixed binary monolayer was established. Stearic acid improves the ligand distribution over the water surface and results in the formation of monolayer associates immobilized in its matrix. The condensation effect of Na⁺ cations on the mixed monolayers was found. The ruthenium complex (R₄Pc)Ru(MeOH)(CO) forms stable true monolayers. The macrocycle planes in stacking are inclined relatively to the normal to the subphase surface by an angle of 25°. The Langmuir-Blodgett films of the complex were established to have redox peaks. A high electrochemical stability of the Langmuir-Blodgett films and a high electroactivity of phthalocyanine rings were demonstrated. It was shown by impedance spectroscopy that the binding of Na⁺ and K⁺ ions by Langmuir-Blodgett films of the (R₄Pc)Ru(MeOH)(CO) complex results in an increase in the impedance values in a region of medium frequencies by three and five times, respectively.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2426–2435, November, 2004.     

Excimer laser ablation of Langmuir-Blodgett films

Photoablation of several Langmuir-Blodgett (LB) films on Silicon substrates was performed at the excimer laser wavelength of 248 nm. This is a fast, solvent-free, one-step method for structuring thin organic films. Structures were produced both by projection of a mask and also using direct writing of the laser beam, yielding feature sizes on the order of microns. Spectral analysis of the remaining material showed no change compared to the unexposed material, suggesting that no degradation has occurred. This technique appears to be viable for use in the lithography of LB films.