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ABSTRACT: The preparation, characterization and electrical properties of Langmuir-Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4'-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good-quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X-ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one-layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently COOAu junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na+ counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H-bonds with neighbouring molecules give rise to a supramolecular structure. STM-based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single-molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in COOAu molecular junctions, but also in terms of the presence of lateral H-bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.
Chemistry 02/2013; · 5.93 Impact Factor
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ABSTRACT: Conductance across a metal|molecule|metal junction is strongly influenced by the molecule-substrate contacts, and for a given molecular structure, multiple conductance values are frequently observed and ascribed to distinct binding modes of the contact at each of the molecular termini. Conjugated molecules containing a trimethylsilylethynyl terminus, -C[triple bond, length as m-dash]CSiMe(3) give exclusively a single conductance value in I(s) measurements on gold substrates, the value of which is similar to that observed for the same molecular backbone with thiol and amine based contacting groups when bound to under-coordinated surface sites.
Dalton Transactions 10/2012; · 3.84 Impact Factor
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ABSTRACT: Aggregation of the ionic liquids 1-butylpyridinium tetrafluoroborate, 1-butylpyridinium triflate, 1-butyl-2-methylpyridinium
tetrafluoroborate, 1-butyl-3-methylpyridinium tetrafluoroborate, 1-butyl-4-methylpyridinium tetrafluoroborate, 1-butyl-3-methylpyridinium
dicyanamide, and 1-octyl-3-methylpyridinium tetrafluoroborate in aqueous solution has been characterized at 298.15 K through
density, ρ, speed of sound, u, and conductivity, σ, measurements. In addition, apparent molar volumes, V
φ
, and isentropic compressibilities, κ
s
, have been calculated from the experimental data. To characterize the formation of aggregates, the critical aggregation concentration
of the ionic liquids, cac, the degree of ionization of the aggregates, β, and the standard Gibbs energy of aggregation, DGm°\Delta G_{\mathrm{m}}^{\circ}, have been obtained, with good agreement between results derived from the different methods. The dependence on the structural
variation of these ions has been analyzed by comparing the results obtained for this series of ionic liquids.
Journal of Solution Chemistry 04/2012; 38(12):1622-1634. · 1.41 Impact Factor
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ABSTRACT: Langmuir films have been fabricated from 4-[4'-(4''-thioacetyl-phenyleneethynylene)-phenyleneethynylene]-aniline (NOPES) after cleavage of the thioacetyl protecting group. Characterization by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveal the formation of a high quality monolayer at the air-water interface. One layer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of the NOPES Langmuir film onto solid substrates. X-ray photoelectron spectroscopy (XPS), surface polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of one layer LB films in which the functional group associated with binding to the substrate can be tailored by the film transfer conditions. Using LB methods this molecule could be transferred to gold samples with either the amine or thiol group attached to the gold surface. The amine group is directly attached to the gold substrate (Au-NH(2)-OPE-SH) when the substrate is initially immersed in the subphase and withdrawn during the transfer process; in contrast, monomolecular films in which the thiolate group is attached to the gold substrate (Au-S-OPE-NH(2)) are obtained when the substrate is initially out of the subphase and immersed during the transfer process. The morphology of these films was analyzed by atomic force microscopy (AFM), showing the formation of homogeneous layers. Film homogeneity was confirmed by cyclic voltammetry, which revealed a large passivation of gold electrodes covered by NOPES monolayers. Electrical properties for both polar orientated junctions have been investigated by scanning tunnelling microscopy (STM), with both orientations featuring a nonrectifying behavior.
Langmuir 03/2011; 27(7):3600-10. · 4.19 Impact Factor
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ABSTRACT: Some of the possible roles that organometallic chemistry will play in the future are discussed and reflected upon against the backdrop of current global uncertainty. In particular, the manipulation of organometallic excited states, redox states, and magnetic properties for use in energy generation, photocatalysis, and molecular electronics is discussed.
01/2011;
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ABSTRACT: Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.
Chemistry 10/2010; 16(45):13398-405. · 5.93 Impact Factor
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ABSTRACT: This paper describes the preparation of Langmuir−Blodgett (LB) films comprised of an oligomeric phenylene−ethynylene (OPE) derivative, 4-[4-(phenylethynyl)-phenylethynyl] benzoic acid (BPEBA). Analysis of the surface pressure and surface potential vs area per molecule isotherms reveal that good quality monolayer films can be formed at surface pressures of 15 mN/m. The monolayers were transferred onto solid substrates with a Z-type deposition and a transfer ratio of 1. Raman and surface-enhanced Raman spectroscopy (SERS) studies reveal that the films are physisorbed onto silver metal substrates. The morphology of the deposited films was analyzed by means of atomic force microscopy (AFM), revealing the formation of homogeneous layers free of three-dimensional defects. The optical and emissive properties of the LB films were determined, with significant blue-shifted absorption spectra indicating the formation of two-dimensional H aggregates in the films. In addition, a significant Stokes shift in the excitation and emission spectra of the films is indicative of a distribution of molecular conformations around the long molecular axis in the solidlike monolayer environment. Scanning tunneling microscopy (STM) studies of single layer BPEBA LB films were performed. The tip−sample distance has been calibrated carefully to obtain I−V curves above the LB film. I−V curves are unexpectedly symmetrical in spite of the asymmetric contacts of the molecule with the tip and the substrate. Single molecule conductance for BPEBA has also been determined and the similarity of these results to I−V data for BPEBA incorporated in LB films indicates that lateral (intermolecular) conductance is negligible for electrical measurements using the STM configuration.
02/2010;
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ABSTRACT: Time-lapse atomic force microscopy is used in this contribution to directly watch the growth of nanofibers of a lipidated peptide on a mica surface. Specifically, the studied lipopeptide is the palmitoyl derivative of the fragment 505-514 of NS3 protein from the hepatitis G virus, abbreviated as Palmitoyl-NS3 (505-514). Data on the morphology, growth rate, and orientation of these peptide-amphiphile nanofibers have been obtained. From these data, it can be concluded that this synthetic lipopeptide forms two types of fiber-like aggregates: (i) half-spherical fibrous aggregates with lengths of hundreds of nanometers and (ii) spherical fibrous aggregates with lengths of several micrometers. In addition, when a fresh lipopeptide aqueous solution is deposited onto a mica surface, the aggregates spontaneously orient parallel to each other, yielding well-aligned nanofibers on large areas of the mica surface. A significant growth in both the length and the number of the fibers was observed during the first minutes after the solution deposition. Elongation of the fibrous aggregates from one end is more frequent, though elongation from both ends also occurs, with growth rates in the 4-5 nm/s range. The effects of dilution, mechanical perturbation, and pH on the aggregation behavior of Palmitoyl-NS3 (505-514) are also detailed in this paper.
The Journal of Physical Chemistry B 12/2009; 114(1):620-5. · 3.70 Impact Factor
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ABSTRACT: A new luminescent ionic liquid crystal (ILC) with a positive charge in the aromatic central core and two hydrophobic alkyl chains at the ends, called Ipz, has been synthesized and its behavior at the air−liquid (NaCl 2 × 10−6 M) interface has been characterized by combination of surface pressure and surface potential versus area per molecule (π−A and ΔV−A) isotherms, Brewster angle microscopy (BAM) and UV−vis reflection spectroscopy. The results indicate the formation of a supramolecular architecture at the air−liquid interface formed by a head-to-head bilayer on top of a flat-core monolayer. Langmuir films were transferred onto solid substrates, and atomic force microscopy (AFM) and X-ray reflection (XRR) have confirmed that well-ordered multilayer films are obtained. Langmuir films supramolecular structure is different from the molecular organization of Ipz in lamellar liquid-crystalline phases. Furthermore, fluorescence emission spectra reveal a blue shift of the emission band of about 18 nm when compared to the solution spectra.
10/2009;
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ABSTRACT: Docetaxel (DCT) is an antineoplastic drug for the treatment of a wide spectrum of cancers. DCT surface properties as well as miscibility studies with l-alpha-dipalmitoyl phosphatidylcholine (DPPC), which constitutes the main component of biological membranes, are comprehensively described in this contribution. Penetration studies have revealed that when DCT is injected under DPPC monolayers compressed to different surface pressures, it penetrates into the lipid monolayer promoting an increase in the surface pressure. DCT is a surface active molecule able to decrease the surface tension of water and to form insoluble films when spread on aqueous subphases. The maximum surface pressure reached after compression of a DCT Langmuir film was 13 mN/m. Miscibility of DPPC and DCT in Langmuir films has been studied by means of thermodynamic properties as well as by Brewster angle microscopy (BAM) analysis of the mixed films at the air-water interface, concluding that DPPC and DCT are miscible and they form non-ideally mixed monolayers at the air-water interface. Helmholtz energies of mixing revealed that no phase separation occurs. In addition, Helmholtz energies of mixing become more negative with decreasing areas per molecule, which suggests that the stability of the mixed monolayers increases as the monolayers become more condensed. Compressibility values together with BAM images indicate that DCT has a fluidizing effect on DPPC monolayers.
The Journal of Physical Chemistry B 11/2008; 112(44):13834-41. · 3.70 Impact Factor
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ABSTRACT: This article describes the synthesis and fabrication of Langmuir and Langmuir-Blodgett (LB) films incorporating a chiral azobenzene derivative, namely, ( S)-4- sec-butyloxy-4'-[5''-(methyloxycarbonyl)pentyl-1''-oxy]azobenzene, abbreviated as AZO-C4(S). Appropriate conditions for the fabrication of monolayers of AZO-C4(S) at the air-water interface have been established, and the resulting Langmuir films have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and UV-vis reflection spectroscopy. The results indicate the formation of an ordered trilayer at the air-water interface with UV-vis reflection spectroscopy showing a new supramolecular architecture for multilayered films as well as the formation of J aggregates. Films were transferred onto solid substrates, with AFM revealing well-ordered multilayered films without 3D defects. Infrared and UV-vis absorption spectroscopy indicate that the supramolecular architecture may be favored by the formation of H bonds between acid groups in neighboring layers and pi-pi intermolecular interactions. Circular dichroism spectra reveal chiro-optical activity in multilayered LB films.
Langmuir 10/2008; 24(18):10196-203. · 4.19 Impact Factor
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Soft Matter 01/2008; 4:1508-1514. · 4.39 Impact Factor
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ABSTRACT: This paper reports the assembly of well-defined molecular films from 4-[4-(4-hexyloxyphenylethynyl)-phenylethynyl]-aniline (HBPEB amine), an oligomeric phenylene ethynylene (OPE) derivative, by means of the Langmuir–Blodgett technique. Initially, Langmuir films of HBPEB amine were prepared at the air–water interface and characterized with surface pressure vs area per molecule isotherms. Brewster Angle Microscopy was used to map the different phases of the monolayer at the air–water interface. UV–vis reflection spectroscopy showed a blue shift of 20 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a chloroform solution of HBPEB amine, which indicates that two-dimensional H-aggregates are formed at the air–water interface. The monolayers were transferred onto solid substrates with a Y-type deposition and a transfer ratio of 1. The excellent transfer characteristics permitted the construction of films comprising up to 150 layers, with a uniform architecture. The LB films fabricated from HBPEB amine show a blue shift of ca. 30 nm with respect to the chloroform solution of HBPEB amine, likely arising from a change in the orientation of the molecular transition dipole with respect to the surface normal in the solid-supported films in comparison with the Langmuir films at the air–water interface. The current–voltage (I−V) characteristics of LB monolayers are unexpectedly symmetrical, the asymmetric contacts not being apparent from the observed I−V characteristics.
12/2007;
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ABSTRACT: The effects of the Hofmeister series of ions are ubiquitous in chemistry and biology. In this paper specific ion effects on the surface behavior of a viologen dication, namely 1,1(')-dioctadecyl-4,4(')-bipyridilium, are shown. Surface pressure and surface potential vs area isotherms were obtained on aqueous subphases containing potassium salts with several representative counterions in the Hofmeister series (C6H5O3-7, SO2 -4, HPO2-4, Cl-, Br-, NO-3, I-, and ClO-4). The parameters obtained from the compression isotherms (area per molecule, phase transitions, Young modulus, initial surface potential, and variation of the surface potential upon compression) are dependent on the nature of the counterion, indicating ion specificity. Aqueous subphases containing C6H5O3-7, SO2-4, and HPO2-4 anions yield more expanded viologen monolayers and these anions do not effectively penetrate into the monolayer. Brewster angle microscopy was used to map the different phases of the viologen monolayers at the air-water interface. The Langmuir films were also characterized by UV-vis spectroscopy, with quantitative analysis of the reflection spectra supporting an organizational model in which the viologen chromophore undergoes a gradual transition to a more vertical position with respect to the water surface upon compression. A comparison of the tilt angles of the viologen on the different subphases indicates that anions that can more easily penetrate in the monolayer permit the viologen moieties to adopt a slightly more vertical position with respect to the water surface.
Journal of Colloid and Interface Science 12/2007; 315(2):588-96. · 3.07 Impact Factor
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ABSTRACT: This paper reports the fabrication and characterization of Langmuir and Langmuir-Blodgett (LB) films incorporating an oligo(phenylene-ethynylene) (OPE) derivative, namely, 4-[4-(4-hexyloxyphenylethynyl)-phenylethynyl]-benzoic acid (HBPEB). Conditions appropriate for deposition of monolayers of HBPEB at the air-water interface have been established and the resulting Langmuir films characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. The Langmuir films are readily transferred onto solid substrates, and one-layer LB films transferred at several surface pressures onto mica substrates have been analyzed by means of atomic force microscopy, from which it can be concluded that 14 mN/m is an optimum surface pressure of transference, giving well-ordered homogeneous films without three-dimensional defects and a low surface roughness. The optical and emissive properties of the LB films have been determined with significant blue-shifted absorption spectra indicating formation of two-dimensional H aggregates and a Stokes shift illustrating the effects of the solid-like environment on the molecular chromophore.
The Journal of Physical Chemistry B 07/2007; 111(25):7201-9. · 3.70 Impact Factor
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ABSTRACT: This paper reports the preparation and characterization of pure Langmuir and Langmuir-Blodgett (LB) films of a stilbene derivative containing two alkyl chains, namely 4-dioctadecylamino-4'-nitrostilbene. Mixed films incorporating docosanoic acid and the stilbene derivative are also studied. Brewster angle microscopy (BAM) analysis has revealed the existence of randomly oriented three-dimensional (3D) aggregates, spontaneously formed immediately after the spreading process of the stilbene derivative onto the water surface. These 3D aggregates coexist with a Langmuir film that shows the typical gas, liquid, and solid-like phases in the surface pressure and surface potential vs area per molecule isotherms, indicative of an average preferential orientation of the stilbene compound at the air-water interface, and a gradual molecular arrangement into a defined structure upon compression. A blue shift of 55 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a chloroform solution of the nitrostilbene indicates that two-dimensional (2D) H-aggregates are formed at the air-water interface. The monolayers are transferred undisturbed onto solid substrates with atomic force microscopy (AFM) revealing that the one layer LB films are constituted by a monolayer of the stilbene derivative together with some 3D aggregates. When the nitrostilbene compound is blended with docosanoic acid, the 3D aggregation is avoided in the Langmuir and Langmuir-Blodgett films, but does not limit the formation of 2D H-aggregates, desirable for second-order nonlinear optical response in the blue domain. The AFM images of the mixed LB films show that they are formed by a docosanoic acid monolayer and, on the top of it, a bilayer of the stilbene derivative.
Journal of Colloid and Interface Science 05/2007; 308(1):239-48. · 3.07 Impact Factor
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ABSTRACT: The H+ acceptor activity of a proton sponge, namely, diphenyl bis(octadecylamino)phosphonium bromide, has been studied at the air-liquid interface using several subphases. Mixed Langmuir and Langmuir-Blodgett (LB) films containing the proton sponge and a fatty acid (behenic acid) in the whole composition range have been prepared. Surface pressure versus area per molecule isotherms were recorded and excess Gibbs energies of mixing calculated. The existence of strong interactions between the proton sponge and the fatty acid is observed when the subphase is either pure water or a NaOH aqueous solution. A stoichiometric 1:1 reaction between both molecules takes place at the air-water interface. This reaction has an efficiency close to 100% at high surface pressures, provided the majority anion present in the subphase is OH-. However, when the majority anion is another one, this complex is hardly formed. From the experimental results, we conclude that the acid-base reaction is highly dependent on the protonation state of the proton sponge at the air-liquid interface that is a function of the present counterion in the subphase. The floating films were also transferred onto solid substrates and characterized by means of IR spectroscopy, atomic force microscopy (AFM), and X-ray diffraction to investigate in more detail the complex formation. The interactions between the complex (when formed) and the excess component have been studied in terms of the subphase nature. It was found that the complex is immiscible with the proton sponge, yielding films made of different domains. Nevertheless, the complex is miscible with the fatty acid when the subphase used is an alkaline solution, presumably due to electrostatic interactions between the carboxylate group of the acid and the complex.
The Journal of Physical Chemistry B 04/2007; 111(11):2845-55. · 3.70 Impact Factor
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ABSTRACT: Spectroscopic characterization and fabrication of Langmuir and Langmuir-Blodgett (LB) films of an azopolymer-pyridine (PAzPy) are reported. UV-visible absorption and fluorescence spectra, Fourier transform infrared (FTIR) spectra, and Raman spectra were recorded. The vibrational assignment of the observed spectra is supported by a complete geometry optimization, followed by vibrational frequency and intensity computations of both the trans and cis forms of the monomer (AzPy) using density functional theory at the B3LYP 6-31G(d,p) level of theory. Langmuir monolayers of the polymer (PAzPy) were formed at the water-air interface, and LB films of high quality were formed onto solid substrates. The polymer LB films were investigated by surface-enhanced Raman scattering.
Langmuir 03/2007; 23(4):1804-9. · 4.19 Impact Factor
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ABSTRACT: Langmuir and Langmuir−Blodgett films of a nitrile-terminated tolan, namely 4-[4‘-decyloxyphenylethynyl]benzonitrile, have been fabricated and characterized at various surface pressures, with surface pressure and surface potential isotherms together with Brewster angle microscopy being used to map the different phases of the tolan monolayer at the air−water interface. The Langmuir films have been characterized by UV−vis spectroscopy, with quantitative analysis of the reflection spectra supporting an organizational model in which compression of the film leads to change in the tilt angle of the tolan molecules from an initial value of ca. 35° to one of ca. 60° before collapse of the monolayer. Moreover, a blue shift in the reflection spectrum of the Langmuir film of 30 nm with respect to the spectrum of a chloroform solution of the nitrile tolan indicates that two-dimensional H-aggregates are formed at the air−water interface. These structures represent a minimum free-energy conformation for the system, as they are observed even before the compression process starts. The monolayers are transferred undisturbed onto solid substrates, with atomic force microscopy revealing well-ordered films without three-dimensional defects.
01/2007;
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ABSTRACT: Densities and kinematic viscosities of the binary mixtures tetrahydrofuran + isomeric chlorobutanes have been measured at atmospheric pressure at the temperatures of 298.15 K and 313.15 K. Excess molar volumes, dynamic viscosities, and viscosity deviations have been obtained from experimental data and fitted by the Redlich−Kister equation. Kinematic viscosities have been correlated using the McAllister equation. Excess molar volumes are negative in the whole composition range, while viscosity deviations are small in absolute value and present an inversion of sign for two systems only at one temperature.
04/2006;
