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PubMed2006

High frequency faradaic rectification voltammetry at microelectrodes.

An experimental setup for carrying out faradaic rectification measurements at micrometer-sized electrodes under potential control is described. A new method of data analysis is proposed that allows the determination of the standard rate constant and the electron-transfer coefficient of a fast charge transfer process without knowing the impedance of the microelectrode. This method is based on the frequency dependence of the shape of the faradaic rectification voltammograms (i.e., the average width of the peaks and the ratio of the peak heights) rather than on the magnitude of the faradaic rectification signal. The method was tested in the determination of heterogeneous electron transfer kinetics of Fe(CN)6(3-/4-) and Ru(NH3)6(2+/3+) in aqueous solutions on a platinum microelectrode (12.5 microm in radius) and ferrocene/ferrocinum redox couple in a dimethylformamide solution on a gold microelectrode (12.5 microm in radius).

Journal Article↗
PubMed2006

Prolongation of the lifetime of the charge-separated state at low temperatures in a photoinduced electron-transfer system of [60]fullerene and ferrocene moieties tethered by rotaxane structures.

A rotaxane tethering both fullerene (C60) and ferrocene (Fc) moieties (abbreviated as (C60;Fc)rotax+) was synthesized in a good yield by the urethane end-capping of pseudorotaxane based on the crown ether-secondary amine motif. In (C60;Fc)rotax+, the C60 group serving as an electron acceptor is attached to the crown ether wheel, through which the axle with a Fc group acting as an electron donor on its end penetrates. The intrarotaxane photoinduced energy-transfer and electron-transfer processes between C60 and Fc in (C60;Fc)rotax+ have been investigated by time-resolved transient absorption and fluorescence measurements with changing solvent polarity. Nanosecond transient absorption measurements of the rotaxane demonstrated that the charge-separated state (C60*-;Fc*+)rotax+ is formed mainly via the excited triplet state of C60 in polar solvents. The lifetime of (C60*-;Fc*+)rotax+ was evaluated to be 20 ns in dimethylformamide (DMF) at room temperature. With lowing temperature, the lifetime of (C60*-;Fc*+)rotax+ extends to 270 ns in DMF at -65 degrees C, due to the structural changes leaving C60*- and Fc*+ at a relatively long distance in the low-temperature region.

Journal Article↗
PubMed2006

What can you learn from a molecular probe? New insights on the behavior of C343 in homogeneous solutions and AOT reverse micelles.

The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (pi) and the H-bond acceptor (beta) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (alpha) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the beta than to the pi polarity parameter; this sensitivity increases in the excited state, while the dependence on alpha vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high beta) and low H-bond donor character (low alpha). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a "polar-solvent pool" with macroscopic properties similar to the bulk.

Journal Article↗
PubMed2006

Preparation and characterization of silver-poly(vinylidene fluoride) nanocomposites: formation of piezoelectric polymorph of poly(vinylidene fluoride).

In situ Ag nanoparticles are produced on reduction of Ag+ with N,N-dimethylformamide in the presence of poly(vinylidene fluoride) (PVF2). The plasmon band transition is monitored with time in the reaction mixture for three sets of experiments by UV-vis spectroscopy. The plasmon band absorbance increases sigmoidally with log(time). Analysis of the data with the Avrami equation yields an exponent n value between 1.5 and 2.0, indicating two-dimensional nucleation with linear or diffusion controlled growth. The TEM study of the polymer nanocomposites (PNC) indicates both spherical and rodlike morphology for PNC0.5 and PNC2.5 samples, whereas the PNC11 sample has spherical and agglomerated structures (the numerical number associated with PNC indicates percentage (w/w) of Ag in the nanocomposite). The WAXS and FTIR studies indicate the formation of piezoelectric beta-polymorphic PVF2 in the nanocomposites. The DSC study indicates some increase of the melting point and enthalpy of fusion of PVF2 in the nanocomposite, although with increase in Ag nanoparticle concentration the increase is smaller. The crystallization temperatures of PNCs also increased, indicating nucleating effect of Ag nanoparticles in the composite. In the TGA curves, the PNCs exhibit a three-step degradation process. The degradation temperatures of PNCs are lower than that of PVF2. The storage modulus data indicate a significant reinforcement of the mechanical property in the PNCs where also the reinforcement effect decreases with increasing nanoparticle concentration. Both the loss modulus and tan delta plots indicate two peaks; the lower temperature peak has been attributed for glass transition temperature, whereas the higher one has been attributed to a similar type relaxation process for the crystalline-amorphous interface. The increase in the glass transition is marginal for the PNCs, but the increase of later transition temperature is somewhat higher. The FTIR study shows that the dipolar interaction of the >CF2 dipole with the surface charges of Ag nanoparticle stabilizes the nanoparticle in the nanocomposite.

Journal Article↗
PubMed2006

Cation/anion recognition by a partially substituted lower rim calix[4]arene hydroxyamide, a ditopic receptor.

The complexation ability of a partially substituted lower rim calix[4]arene hydroxyamide derivative, 25,27-bis[N-(2-hydroxy-1,1-bishydroxymethylethyl)amino- carbonylmethoxy]calix[4]arene-26,28-diol, 1, for cations and anions was investigated through (1)H NMR, conductometry, spectrophotometry, and calorimetry in dipolar aprotic media. (1)H NMR studies of 1 in the deuterated solvents (acetonitrile, methanol, and dimethylsulfoxide) reflect ligand-solvent interactions in methanol and dimethylsulfoxide. As far as the cations are concerned, a selectivity peak is found when standard Gibbs energies of complexation of 1 with cations (alkaline-earth, zinc, and lead) are plotted against corresponding data for cation hydration. This finding reflects the key role played by the desolvation and binding processes in the overall complexation of this receptor and these cations in acetonitrile. This is also interpreted in terms of enthalpy and entropy data. Factors such as, the nature and the arrangement of donor atoms in the hydrophilic cavity of the ligand on cation complexation process, are discussed. This paper also addresses anion complexation processes. It is found that 1 interacts through hydrogen bond formation with fluoride, dihydrogen phosphate, and pyrophosphate in acetonitrile and N,N-dimethylformamide. The thermodynamics associated with these processes is fully discussed taking into account literature data involving calix[4]pyrroles and these anions in these solvents. Previous work regarding the water solubility of these ligands is discussed. It is concluded that 1 behaves as a ditopic ligand in dipolar aprotic media.

Journal Article↗
PubMed2006

Electron-transfer reactions with significant changes in structure. Unsymmetrical crowded ethylenes.

The electrochemical reduction mechanisms of xanthylideneanthrone, 6, thioxanthylideneanthrone, 7, 10-(diphenylmethylene)anthrone, 8, and 9-(diphenylmethylene)-9H-fluorene, 9, have been studied in dimethylformamide. The reduction of the first two compounds proceeds from folded forms of the neutral to twisted forms of the anion radical according to a square scheme. The data for reduction of 8 can be well accounted for by the same square scheme. However, one-step reduction with concerted electron transfer and structural change cannot be ruled out. Compound 9, whose fluorene ring system cannot fold, exists only in twisted forms in the neutral, anion radical, and dianion. Consequently, there are no major changes in structure upon reduction, and the compound is reduced in two reversible steps with the second complicated by rapid loss of the dianion that is probably due to protonation by components of the medium.

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PubMed2004

Effect of the interfacial chemical environment on in-plane electronic conduction of indium tin oxide: role of surface charge, dipole magnitude, and carrier injection.

Reaction of the indium tin oxide (ITO) surface with Brønsted acids (bases) leads to increases (decreases) in its in-plane conductance as measured by a four-point probe configuration. The conductance varies monotonically with pH, suggesting that the degree of surface protonation or hydroxylation controls the surface charge density, which in turn affects the width of the n-type depletion layer and ultimately the in-plane conductance. Measurements at constant pH with a series of tetraalkylammonium hydroxide species of varying cation size indicate that surface dipoles also affect ITO conductance by modulating the magnitude of the surface polarization. Modulating the double layer with varying aqueous salt solutions also affects ITO conductance, though not to the same degree as strong Brønsted acids and bases. Solvents of varying dielectric constant and proton donating ability (ethanol, dimethylformamide) decrease ITO conductance relative to H2O. In addition, changing solvent gives rise to thermally derived conductance transients, which result from exothermic solvent mixing. The self-assembly of alkanethiols at the surface increases the conductance of ITO films, most likely through carrier population effects. In all cases examined the combined effects of surface charge, adsorbed dipole layer magnitude, and carrier injection are responsible for altering the ITO conductance. Besides being directly applicable to the control of electronic properties, these results also point to the use of four-point probe resistance measurements in condensed phase sensing applications.

Journal Article↗
PubMed2005

Electrospun poly(styrene-block-dimethylsiloxane) block copolymer fibers exhibiting superhydrophobicity.

Block copolymer poly(styrene-b-dimethylsiloxane) fibers with submicrometer diameters in the range 150-400 nm were produced by electrospinning from solution in tetrahydrofuran and dimethylformamide. Contact angle measurements indicate that the nonwoven fibrous mats are superhydrophobic, with a contact angle of 163 degrees and contact angle hysteresis of 15 degrees . The superhydrophobicity is attributed to the combined effects of surface enrichment in siloxane as revealed by X-ray photoelectron spectroscopy and surface roughness of the electrospun mat itself. Additionally, the fibers are shown by transmission electron microscopy to exhibit microphase-separated internal structures. Calorimetric studies confirm the strong segregation between the polystyrene and poly(dimethylsiloxane) blocks.

Journal Article↗
PubMed2005

Light-directed movement of polymer microstructures.

Light has been used to induce photochemical changes in the surface chemistry of porous polymer microstructures giving rise to a substantial change in volume. When illumination is asymmetric, this results in light-directed motion of the structure. Swellable trimethylolpropane trimethacrylate cross-linked poly(2-hydroxylethyl methacrylate) microstructures were constructed by azo-bis-isobutyronitrile photopolymerization using a 20 x 0.5 NA microscope objective and 365 nm laser excitation. Structures were aminiated with glycine and protected with the photolabile group 4-nitroveratryloxycarbanyl (NVOC). Addition of NVOC resulted in a volume increase >10% when performed in the solvent N,N'-dimethylformamide. Photochemical cleavage of NVOC using asymmetric illumination of a cone-shaped microstructure with a 365 nm laser induced polymer shrinkage in excess of 4% at the base of the cone and resulted in a maximum velocity of 1 mm/s at the tip of the cone. Symmetric illumination gave rise to displacement of solvent from the microstructure due to shrinkage with a velocity in excess of 0.01 mm/s. This system could in principle be used for light-directed movement of micromechanical systems, optical control of microfluidics, or light activated chemical delivery.

Journal Article↗
PubMed2005

Programmed hyperhelical supramolecular assembly of nickel phthalocyanine bearing enantiopure 1-(p-tolyl)ethylaminocarbonyl groups.

The present paper reports uniqueness of a simple, programmed design of disk-shaped homochiral nickel phthalocyanine (Pc) molecules bearing four enantiomerically pure 1-(p-tolyl)ethylaminocarbonyl groups at their peripheral positions, (Pc-(R) and Pc-(S)), and their controlled self-organization into mesoscopic supramolecular helical fibers with a preferential handedness in solution and onto solid surfaces. A combination of four fundamental intermolecular interactions, including quadruple hydrogen bonding, pi-pi stacking, homochiral interactions of the enantiopure bulky aralkyl entities, and noncoordinating nature of nickel ion of the Pc molecules afforded a high thermal stability of the Pc self-assembly in chloroform (CHCl(3)), tetrahydrofuran, and o-dichlorobenzene and onto hydrophilic mica and hydrophobic HOPG surfaces. A higher-ordered helical self-assembly of Pc disks was observed in these solutions (approximately 200 Pc molecules), while the self-assembly was completely dissociated into monomeric species in N,N-dimethylformamide due to a loss of hydrogen-bonding interactions between Pc molecules. Supramolecular chirality in the hierarchical self-assembly of Pc molecules originated from the presence of (R)- or (S)-chiral centers in the peripheral tails, which rotate noncovalently linked molecular building blocks to effectively form the helical architectures. The helical Pc nanofibers dissolved in CHCl(3), estimated to be ca. 70 nm from peak molecular weight obtained by SEC analysis, acts as a building block for higher-order helical fibers (ca. 1 microm) at single molecular level on the solid surfaces, as demonstrated by the dynamic force mode atomic force microscopy. Regardless of hydrophilic and hydrophobic substrates, the interaction between these Pc molecules and the solid surfaces could not affect the morphology of helical assemblies, indicating a unique robustness of these assemblies.

Biopolymers↗
PubMed2005

pH-responsive core-shell particles and hollow spheres attained by macromolecular self-assembly.

According to our "block-copolymer-free" strategy for self-assembly of polymers, noncovalently connected micelles (NCCM) with poly(epsilon-caprolactone) (PCL) as the core and poly(acrylic acid) (PAA) as the shell in aqueous solutions were attained due to specific interactions between the component polymers. The micellar structure was then locked in by the reaction of PAA with diamine. Afterward, hollow spheres based on PAA network were obtained by either core degradation with lipase or core dissolution with dimethylformamide of the cross-linked micelles. The cavitation process was monitored by dynamic light scattering, which indicated a mass decrease and size expansion. The hollow structure is confirmed by transmission electron microscopy observations. The resultant hollow spheres are pH- and salt-responsive: there is a substantial volume increase when pH changes from acid to base, and vice versa. The volume change takes place dramatically over the pH-range from 5.8 to 7.5. Furthermore, this volume-pH-dependence is found to be completely reversible provided the effect of ionic strength is excluded. The volume change can be adjusted by changing the shell thickness and the cross-linking degree of the hollow spheres. The salt effect on the hollow sphere size depends on pH: with increasing salt concentration the size shows an increase, a decrease, and a little change in acidic, basic, and neutral media, respectively.

Journal Article↗
PubMed2004

Hydrogen-bonding-directed layer-by-layer assembly of poly(4-vinylpyridine) and poly(4-vinylphenol): effect of solvent composition on multilayer buildup.

This paper describes the buildup of hydrogen-bonding-directed poly(4-vinylpyridine)/poly(4-vinylphenol) (PVPy/PVPh) multilayer film that was fabricated by layer-by-layer (LbL) assembly of PVPy and PVPh from an ethanol solution. UV-visible spectroscopy and Fourier transform infrared (FT-IR) spectroscopy revealed a uniform deposition process. The interaction between PVPy and PVPh was identified as hydrogen bonding through FT-IR spectroscopy and temperature-dependent IR spectral changes of the hydrogen-bonded multilayer. Notably, we discussed the effect of solvent conditions on the growth of PVPy/PVPh multilayer films monitored by UV-visible spectroscopy. It was found that increasing the ratio of N,N-dimethylformamide (DMF) in the mixed ethanol/DMF solvents resulted in a marked decrease of the amount of polymers adsorbed, which was attributed to the increased polarity of the adsorption solutions. Furthermore, the solvent stability of PVPy/PVPh multilayer film in mixed ethanol/DMF solvents with different DMF ratios was also investigated. As a result, a new method for tuning the structure of hydrogen-bonding-directed multilayer film was developed.

Ethanol↗
PubMed2004

Organogelation of diacetylene cholesteryl esters having two urethane linkages and their photopolymerization in the gel state.

Various diacetylene cholesteryl esters having two urethane linkages were synthesized to study the relationship between their gelation properties and chemical structures. Most of these compounds form organogels in cyclohexane, and some compounds gelatinized hexane, diethyl ether, N,N-dimethylformamide, and ethanol. The cholesteryl moieties play an important role in gel formation, but IR spectroscopic measurements show that the main driving force for gelation is hydrogen bonding of the urethane groups. Upon UV irradiation, most of the gels polymerized to give polydiacetylenes, with concomitant changes from colorless to a variety of hues, such as dark blue, orange, and pink. The polymerization proceeds efficiently in cases where the gels change color to dark blue. The polymerization reached 52% chemical yield, with the quantum yield estimated to be at least 54. Solid-state NMR spectroscopy confirmed that polymerization in the gel state proceeds via 1,4-addition.

Acetylene↗
PubMed2004

Molecular-level approach to inhibit degradations of alkanethiol self-assembled monolayers in aqueous media.

A molecular-level approach is developed to prevent or inhibit the degradation processes of alkanethiol self-assembled monolayers (SAMs). Previous studies revealed two degradation pathways: direct desorption and oxidation-desorption. By use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in situ and time-dependent imaging reveals and confirms that degradations of alkanethiol SAMs on gold mainly initiate at defect sites, such as domain boundaries and vacancy islands, and then propagate into the ordered domains. Our approach targets at attaching small molecules with preferred adhesion to the defects. The best candidates are aqueous media containing a small amount of amphiphilic surfactant molecules, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). High-resolution studies demonstrate that DMSO and DMF molecules attach to SAM surfaces and more favorably at defect sites, forming relatively stable adsorbates. This attachment increases the activation energy sufficiently to inhibit both degradation pathways. The robustness of this approach has been investigated as a function of surfactant concentration, solution temperature, and the stirring condition. Molecular-level mechanisms and energetics for degradation inhibition of SAMs are also discussed in detail.

Journal Article↗
PubMed2004

Preparation, characterization, and solution viscosity of polystyrene-block-polyisoprene nanofiber fractions.

A polystyrene-block-polyisoprene (PS-b-PI) sample with 130 styrene and 370 isoprene units was synthesized and characterized. The diblock formed mostly cylindrical micelles in N,N-dimethylacetamide with a PI core and a PS corona. The PI core of the micelles was cross-linked by S2Cl2 to yield nanofibers. The nanofibers were shortened by ultrasonication to yield fractions withweight-average length (Lw) between approximately 900 and approximately3400 nm. Transmission electron microscopy and light scattering were used to characterize the fractions. The zero-shear intrinsic viscosity data [eta] of the fractions were obtained in tetrahydrofuran (THF), THF/N,N-dimethylformamide (DMF), and THF/cyclohexane (CHX), where THF is a good solvent for both the corona and the core, DMF solubilizes only the corona, and CHX is a theta solvent for the corona chains at 34.5 degrees C. The [eta] data of the fractions were treated by the Bohdanecky method derived from the Yamakawa-Fujii-Yoshizaki (YFY) theory for wormlike polymer chains and yielded the persistence length lP and the hydrodynamic diameters dh for the nanofibers. The reasonable dh values and the reasonable trend of dh variation with solvent quality change establish unambiguously the validity of YFY theory.

Journal Article↗
PubMed2005

Growth and photoluminescence characterization of highly oriented CuI/beta-cyclodextrin hybrid composite film.

A highly (111) oriented CuI/beta-cyclodextrin inorganic/organic composite film on glass substrates was deposited from a dispersion of CuI and beta-cyclodextrin in a mixture of acetonitrile and dimethylformamide (DMF) solvents, in which DMF plays an important role in the orientation of CuI film. The composite film exhibits obviously improved band gap photoluminescence compared with that of the pure CuI film due to the passivation of iodine-related defect sites by beta-cyclodextrin matrix. This result indicates that the optical properties of CuI film can be easily adjusted by the formation of composite film with proper ligand agents.

Journal Article↗
PubMed2005

Star-block copolymers as templates for the preparation of stable gold nanoparticles.

Gold nanoparticles of improved stability against aggregation were prepared using poly(ethylene oxide)-block-poly(epsilon-caprolactone) (PEO-b-PCL) star-block copolymers. A five-arm star-shaped macroinitiator (PEO) was utilized for the automated parallel controlled ring-opening polymerization of epsilon-caprolactone to prepare a series of PEO-b-PCL star-block copolymers with a constant PEO core linked to PCL blocks of variable length. The PEO core was swelled with KAuCl4 in N,N-dimethylformamide (DMF), and gold nanoparticles were subsequently obtained by reduction with NaBH4. Since the process was always templated by the same PEO core for all investigated polymers, the average dimension of the formed gold nanoparticles was in the same range for all star-block copolymers. In sharp contrast, the size distribution and long-term stability against aggregation of the gold nanoparticles dispersed in DMF were strongly dependent on the PCL block length, confirming the role of PCL blocks as stabilizing blocks for these nanoparticles.

Journal Article↗
PubMed2005

Indirect reduction of aryldiazonium salts onto cathodically activated platinum surfaces: formation of metal-organic structures.

Platinum phases of general formula [Pt(n-), M+, MX] can be electrogenerated from cathodic polarization in dry dimethylformamide containing a supporting electrolyte, MX. The reaction of these electrogenerated Pt phases as reducing agent with aryldiazonium salts was investigated for preparing controlled metal-organic interfaces and characterizing the reactivity of the "reduced platinum phases". In a two-step process, the "reduced platinum phase" locally reacts with aryldiazonium salts, leading to the attachment of aryl groups onto the metal surface in the previously modified areas. Detailed experiments using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and in situ electrochemical atomic force microscopy (EC-AFM) were carried out to follow the reaction in solution with the example of NaI as supporting electrolyte (MX = NaI). These studies demonstrate the irreversible attachment of aryl groups onto the platinum electrode. Comparison between the direct electroreduction of aryldiazonium compounds (4-nitrophenyl- and 4-bromophenyldiazonium) on a platinum electrode and their reaction with [Pt2-, Na+, NaI] suggests that a similar general mechanism is responsible for the grafting. However in the second case, no applied potential is required to stimulate the binding thanks to the reductive properties of [Pt2-, Na+, NaI]. Competitive reduction of the organic layer and growth of the layer were observed and analyzed as a function of the injected charge used to initially produce [Pt2-, Na+, NaI]. Similar reactions are highly probable with other MX salts owing to the redox properties observed for this type of platinum phase ([Pt(n-), M+, MX]).

Journal Article↗