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.     

Infrared studies of pyroelectric Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and Horizontal Attenuated Total Reflectance (HATR) spectra have been recorded of arachidic acid (AA)/1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) Y-type alternate LB films deposited on an aluminium plate with 31 layers. It is well known that the frequencies of CH₂ stretching bands of a hydrocarbon chain are sensitive to the conformational ordering of the chain. Changes in frequency and intensity can be used to characterize film ordering and preferential molecular packing. The observed peak frequencies and intensities of these bands indicate that the alkyl chains are present in a mostly trans conformation and tilted from the normal direction with respect to the substrate in LB films. The FTIR–GAIR and HATR spectra of 31 layers alternate film show significant changes in the region 1700–1400 cm⁻¹ due to the partial proton transfer between acid and amine head groups. According to the HATR spectrum, the peak at 1731 cm⁻¹ is observed due to a proportion of the carboxylic acid groups forming sideways dimers indicating that if the carboxylic acid groups form sideways dimers, they are less likely to undergo proton transfer with the amino groups.     

Unimolecular rectifiers and prospects for other unimolecular electronic devices

We briefly review the progress towards useful one-molecule electronic devices, toward the ultimate reduction in integrated circuit sizes, and then describe five unimolecular rectifiers, or one-way conductors of electrical current: gamma-hexadecylquinolinium tricyanoquinodimethanide, an acetyl sulfoxide derivative of this compound, 2,6-di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, dimethylanilino-aza[C60]fullerene, and a new fullerene derivative.     

Photoreactions and lateral patterning in Langmuir and Langmuir-Blodgett films

Reversible morphological changes occur with photoisomerization of azobenzene in Langmuir-Blodgett (LB) films complexed with polycations, which contradicts an implicit assumption of the concept of free volume that two-dimensional film structures are preserved during the photoisomerization. J-aggregates of chromophores are formed by two processes. The first process is "light-induced J-aggregation" in which photoisomerized molecules form J-aggregates. The other process is "triggered J-aggregation," in which photoisomerization of one of the components triggers J-aggregation of another chemical species in the mixed films. Both processes of J-aggregation are in many cases accompanied by large morphological changes of the films. However, LB films fabricated using processes under isobaric conditions do not change their morphology during light-induced J-aggregation and are patterned with J-aggregates using ultraviolet illumination through a photomask. Phase separation in mixed LB films gives rise to two-dimensional patterns, which are used to fabricate templates by using an amphiphilic silane-coupling agent as one of the components in the mixed LB films. Nanopatterns are also fabricated.     

Interfacial Langmuir-Blodgett assembly of straight and parallel aligned nanoribbons.

The fabrication of one-dimensional nanomaterials is currently an important issue in nanoscience and nanotechnology. Herein, we report the construction of long, straight, and parallel aligned nanoribbons, by the in situ assembly of a novel, triangular amphiphilic molecule at an air-water interface. The 1,3,5-trimesyl-tri-L-glutamic acid hexaester, which has an aromatic core with three hydrophilic arms bearing three amide units and ending with six ester groups, was spread on a water surface, forming a stable, ultrathin film with a plateau region in its isotherm. When the film was compressed to a surface pressures, which exceeded the plateau region, nanoribbons were formed. Atomic force microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, electron diffraction, and circular dichroism were used to characterize the film and the nanoribbons. The nanoribbons were formed due to the in situ transformation of a two-dimensional monolayer into a three-dimensional nanoarchitecture at the air-water interface. When the compound was spread onto the water surface, it first formed a monolayer. Compressing the film led to the overlap of the second layer on the monomolecular sublayer. This second layer formed nanoribbons in which the triangular molecules stood vertically (using two corners) on the sublayer. The nanoribbons were stabilized by the ordered packing of the triangular molecules, which were held together by intermolecular hydrogen bonding. The results present a facile procedure for constructing three-dimensional, regular nanoarchitectures in a two-dimensional environment.     

Albumin and fibrinogen adsorption onto phosphatidylcholine monolayers investigated by Fourier transform infrared spectroscopy

Dipalmitoylphosphatidylcholine (DPPC) monolayers were deposited onto a germanium attenuated total reflectance (ATR) crystal using the Langmuir–Blodgett technique. The DPPC-coated crystal was then exposed to human serum albumin or human fibrinogen solutions while measuring the protein adsorption by recording FTIR spectra. The effect of the zwitterionic nature of the DPPC polar headgroup towards protein adsorption has been ascertained by exposing either the phospholipid headgroup or the acyl chains to the protein solution; this was possible by the use of a silanized or a bare germanium crystal. Calibration curves have been made to measure the protein surface concentrations. After 3 h, the albumin surface concentration on DPPC monolayers was about three times higher when the proteins were exposed to the lipid acyl chains instead of the polar headgroups (e.g. 3 vs. 1 μg cm⁻²). As for fibrinogen (FGN) adsorption, when the lipid polar headgroups were exposed to the protein solution, the FGN adsorption was low reaching a maximum value of 0.5 μg cm⁻². When interacting with the lipid acyl chains, the FGN adsorption reached a plateau at a value of 2.1 μg cm⁻² after 3 h. Clearly, both albumin and FGN showed a low tendency to adsorb on surfaces where the lipid polar headgroups are exposed toward the protein solution.     

Amphiphilic and magnetic properties of a new class of cluster-bearing [L2Cu4(mu4-O)(mu2-carboxylato)4] soft materials

A general approach toward amphiphilic systems bearing multimetallic clusters and their ability to form Langmuir-Blodgett films is presented. The synthetic strategy to stabilize these clusters involves the use of a ligand (HL) containing an N(2)O-donor set and long octadecanoic chains to obtain the carboxylate-supported [L(2)Cu(4)(mu(4)-O)(mu(2)-OAc)(4)]EtOH (1) and [L(2)Cu(4)(mu(4)-O)(mu(2)-OBz)(4)] (2) in which OAc(-) and OBz(-) represent acetate (1) and benzoate (2) co-ligands. These species were thoroughly characterized and had their structures solved by X-ray crystallography. We observed that the mu-oxo Cu(4) cluster is antiferromagnetically coupled and used broken-symmetry density functional theory (DFT) calculations to describe the main superexchange pathways of the tetracopper core. We also describe the amphiphilic properties of the ligand and the cluster-containing systems by means of area versus pressure isotherms and show that these cluster-bearing species can be transferred onto solid substrates yielding homogeneous Langmuir-Blodgett films, as characterized by atomic force microscopy and contact angle measurements.     

Subphase pH effect on the limiting molecular area of amphiphilic β-diketones in Langmuir monolayers

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Spin state transitions in Langmuir–Blodgett films of recombinant cytochrome P450scc and adrenodoxin

Langmuir–Blodgett (LB) films of recombinant cytochrome P450scc, of P450scc–adrenodoxin (Adx) complex and alternated layers of Adx and P450scc have been obtained. Spectral properties of these proteins in thin films were investigated by UV–Vis absorption spectroscopy. It has been found that cytochrome P450scc exists in LB films only in low-spin state while before the deposition it was in high-spin state. The data suggest that transferring the hemoprotein or its complex with redox partner results in the modification of the spin state by a conformational transition. In order to investigate further the P450scc and Adx interaction, the mass density of the films formed from these molecules has been studied by nanogravimetric measurements. Comparative study between nanogravimetric and spectral characterisation was performed. The results indicate that the protein–protein interaction is disrupted, when the complex is organised in thin film.     

Long-lived photoinduced charge separation in carbazole–pyrene–viologen system incorporated in Langmuir–Blodgett films of substituted diazacrown ethers

Diaza-18-crown-6 ethers appending two pyrenyl (Py) or two carbazolyl (Cz) groups were synthesized. These macrocyclic compounds form 1:1 host–guest complexes with methyl viologen chloride (MV²⁺), and these complexes were assembled into monolayers by Langmuir–Blodgett technique. The generated assembly involves the general structure of donor–sensitizer–acceptor (Cz–Py–MV²⁺) in space, although any of the photo- and redox-active components are not covalently bonded. Photoirradiation of the pyrenyl group resulted in the charge-separated pair Cz√⁺–Py–MV√⁺ which survived up to hours in a well anaerobic atmosphere. An electrode was fabricated by transferring the L–B film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in hydroquinone (H₂Q) solution to be ca. 168 mV when the light intensity was 218 mW/cm². This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in anodic photocurrent, and there is no net chemical change associated with the function of the cell which converts light to electricity.     

Dynamic organization of mixed Langmuir films of glucose oxidase and stearylamine at the air-water interface

The structure and the dynamic organization of a mixed Langmuir film of glucose oxidase and stearylamine at the air–water interface have been studied. The film has been first characterized at the air–water interface by surface pressure/area isotherms. The dynamics of the mixed film was studied by following the evolution of the film area at a constant pressure and the evolution of the pressure at a constant area. After transfer of the films on solid substrates, the chemical composition of the mixed film has been quantified by UV–vis and IR spectroscopies. These characterizations were carried out in order to study the incorporation of glucose oxidase into the stearylamine film, and its influence on the structural evolution of the film. From these results, the dynamic organization of this mixed film may be described. For short times, glucose oxidase molecules interact with stearylamine molecules in solution or at the interface; these interactions would lead to the formation of a complex between stearylamine and glucose oxidase molecules. For long times (at least 3 h), a homogeneous mixed film constituted essentially of this complex is obtained at the air–water interface. A detailed analysis by atomic force microscopy allowed us to support this model and the existence of the glucose oxidase/stearylamine complex.     

Electrochemical properties of protoporphyrin IX zinc(II) films

The electrochemical properties of protoporphyrin IX zinc(II) (ZnPP) films on indium-tin oxide (ITO) substrate have been studied for three types of films with different arrangements, which were an adsorbed film of ZnPP and LB films of ZnPP and its hybrid with hexadecyltrimethylammonium bromide. Cyclic voltammetry (CV) measurement showed that, as the adsorbed amount of ZnPP increases, an irreversible oxidation peak of ZnPP film is intensified. This reveals that electrochemical properties depend on the adsorbed amount rather than the orientation of porphyrin molecules. It was also supported from CV measurement and ultraviolet-visible absorption spectroscopy that porphyrins adsorbed on ITO substrate were desorbed after the single scan of potential. Additionally, photoresponse of these ZnPP films was investigated by photocurrent measurement. The photocurrent generation is due to carboxylic acid moieties but not ZnPP macrocycles.     

Langmuir–Blodgett films of pyridinium-dicyanomethanide dyes mixtures with photobleachable absorption bands

We report on Langmuir-Blodgett (LB) films characterization of 4-[5-dicyanomethanido)thien-2-y1]-N-(n-hexadecyl)pyridinium (C₁₆H₃₃-PDCNT), and 1-(N-(n-hexadecy]-4-pyridinio)-2-[5-(dicyanomethanido)thien-2-yl]ethene (C₁₆H₃₃-PDCNTE); LB films of the pure compounds and of the mixtures of the two compounds were prepared at 291 K: UV-vis investigation revealed the presence of photobleachable absorption bands, the ones at about 530 nm and 640 nm were due to charge transfer transitions of the monomer of C16H33-PDCNT and C₁₆H₃₃-PDCNTE, respectively; the sharp, photobleachable ones shifted to shorter wavelengths were due to H-aggregates of the two compounds. By changing the molar ratios of the two compounds in the mixtures and in other cases by annealing the LB films, the absorption maxima of the sharp, photobleachable bands due to H-aggregates could be tuned in the range 415–467 nm. These LB films are thus very promising in view of optical data storage applications.     

Micellization of economically viable surfactants in CO(2)

Stability and aggregation structures of various economically viable surfactants for CO(2) are reported. The compounds are either commercially available octylphenol nonionics (Triton X-100, X-100 reduced, and X-45) or custom-made analogues of aerosol-OT (J. Am. Chem. Soc. 123 (2001) 988). These were selected to reveal the influence of chain terminal group structure, namely highly methylated t-butyl units, on solubility and aggregation in CO(2). In addition the mean ethylene oxide block length is varied for the Triton surfactants (X-100 approximately EO(10), X-45 approximately EO(8)). High-pressure small-angle neutron scattering (SANS) experiments revealed the presence of aggregates, consistent with spheroidal reverse micelles. The nonionics show a temperature and pressure dependence on solubility. These results confirm the special affinity of highly methyl-branched tails for CO(2). However, none of these systems were able to disperse significant amounts of water or brine; therefore hydrated reversed micelles or microemulsion droplets were not stabilized. Hence the utility of these cheap methyl-branched surfactants in CO(2) is limited, and so groups of greater CO(2)-philicity are needed to achieve the goal of water-hydrocarbon surfactant-CO(2) dispersions.     

聚乙烯醇在强碱性水溶液中与Fe(Ⅲ)离子间的配位-交联反应

在不断搅拌1×10~(-4)m~3 pH>13的5％ PVA碱性水溶液中,缓慢地添加1×10~(-4)kgFeCl_36H_2O,溶液中即析出红棕色树胶状物.以XPS、ESR和IR等研究该析出物的组分及其形成过程.推定PVA部分羟基在上述条件下发生脱质子反应而提供L~-配位体,Fe(Ⅲ)盐水解生成初生Fe(OH)_3的Fe~(3+),它既有空轨道,又有较强的正电场,故能吸引PVA高分子配位体L~-上的氧而形成O→Fe配位键,从而发生L~-与Fe~(3+)离子间的交联反应,生成多核Fe(Ⅲ)-PVA配位聚合物而析出红棕色树胶状物。