Spectroscopic investigation of the interactions between emeraldine base polyaniline and Eu(III) ions

The interactions of emeraldine base form of polyaniline (EB-PANI) and Eu(III) ions in 1-methyl-2-pyrrolidinone (NMP) solution and in films have been investigated by UV-vis-NIR, resonance Raman. luminescence and electron paramagnetic resonance (EPR) spectroscopies. These spectroscopic techniques allowed to characterize quinone and semiquinone segments in the polymeric chains. and the oxidation state of europium ions in Eu-PANI samples. For high values of Eu(III)/N molar ratio (24/1) the presence of a weak polaronic absorption band at 980 nm in UV-vis-NIR spectrum and the observation of bands at 1330 and 1378 (nu(center dot)(C-N+)) cm(-1) due to emeraldine salt in the Raman spectrum at 1064 nm indicate a low doping degree. Oxidation of EB-PANI to pernigraniline base (PB-PANI) occurs in diluted solutions. The experimental data showed that the solvent plays an important role on the nature of formed species. The narrow EPR signal at g = 2.006 (line width 8G) confirms the presence of PANI radical cations in Eu-PANI film. The absence of broad signal characteristic of Eu(II) in EPR spectrum suggested that europium ions are primarily at Eu(III) oxidation state. The luminescence spectra of Eu-PANI film presented emission bands at 405 and 418 nm assigned to PANI moieties and bands at 594,615 and 701 nm assigned to (5)D(0) -> (7)F(J) (J = 1, 2 and 4, respectively) transitions of Eu(III). EPR and photoluminescence data confirm that europium ions are mainly in Eu(III) oxidation state in Eu(III)/PANI films. (C) 2008 Elsevier B.V. All rights reserved.

The role of solvent on the doping of polyaniline with Fe(III) ions

The products formed from the reaction of emeraldine base polyaniline (EB-PANI) with Fe(III) ions in N-methyl-pyrrolidone (NMP), dimethylacetamide (DMA), dimethylformamide (DMF) and m-cresol media have been investigated using UV-VIS-NIR and resonance Raman (lambda(0) = 632.8 and 1064 nm) spectroscopies. Through these results it was verified that the different PANI forms in solution can be formed by the suitable choice of the solvent. The behavior of Fe(III)/EB-PANI in different solvents was rationalized in terms of the interactions among Fe(III) ions, EB-PANI and solvent. In basic NMP, DMA and DMF media, the reaction of Fe(III) with EB-PANI yields EB-PANI doping giving ES-PANI and/or the EB-PANI oxidation to PB-PANI. The formation of ES-PANI is favored in DMF while PB-PANI is formed in a greater extension in NMP and DMA. In acidic m-cresol, only ES-PANI is produced in Fe(III)/EB-PANI solutions indicating the important role played by the solvent in the nature of the product. (C) 2010 Elsevier B.V. All rights reserved.

Efficient and low cost devices for solar energy conversion: Efficiency and stability of some natural-dye-sensitized solar cells

Dye-sensitized solar cells, named by us Dye-Cells, are one of the most promising devices for solar energy conversion due to their reduced production cost and low environmental impact, especially those sensitized by natural dyes. The efficiency and stability of devices based on natural sensitizers such as mulberry (Morus alba Lam), blueberry (Vaccinium myrtillus Lam), and jaboticaba`s skin (Mirtus cauliflora Mart) were investigated. Dye-Cells prepared with aqueous mulberry extract presented the highest P(max) value (1.6 mW cm(-2)) with J(sc) = 6.14 mA cm(-2) and V(oc) = 0.49 V, Photoelectrochemical parameters of 16 cm(2) active area devices sensitized by mulberry dye were constant for 14 weeks of continuous evaluation. Moreover, the cell remained stable even after 36 weeks with a fairly good efficiency. Therefore, mulberry dye opens up a perspective of commercial feasibility for inexpensive and environmentally friendly Dye-Cells. (C) 2009 Elsevier B.V. All rights reserved.

The role of oxygen in the interaction of emeraldine base polyaniline with Cu(II) or Fe(III) ions in NMP solution

The influence of molecular oxygen in the interactions of emeraldine base form of polyaniline (EB-PANI) with Fe(III) or Cu(II) ions in 1-methyl-2-pyrrolidinone (NMP) solutions has been investigated by UV-vis-NIR, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies. Through the set of spectroscopic results it was possible to rationalize the role Of O(2) and to construct a scheme of preferential routes occurring in the interaction of EB-PANI with Fe(III) or Cu(II). Solutions of 4.0 mmol L(-1) EB-PANI with 0.8, 2.0 and 20 mmol L(-1) Fe(III) or Cu(II) ions in NMP were investigated and the main observed reactions were EB-PANI oxidation to pernigraniline (PB-PANI) and EB-PANI doping process by pseudo-protonation, or by a two-step redox process. In the presence Of O(2), PB-PANI is observed in all Fe(III)/EB solutions and EB-PANI doping only occurs in solutions with high Fe(III) concentrations through pseudo-protonation. On the other hand, emeraldine salt (ES-PANI) is formed in all Fe(III)/EB solutions under N(2) atmosphere and, in this case, doping occurs both by the pseudo-protonation and two-step redox mechanisms. In all Cu(II)/EB solutions PB-PANI is formed both in the presence and absence of O(2), and only for solutions with high Cu(II) concentrations doping process occurs in a very low degree. The most important result from EPR spectra was providing evidence for redox steps. The determined Cu(II) signal areas under oxygen are higher than under N(2) and, further. the initial metal proportions (1:2:20) are maintained in these spectra, indicating that Cu(I) formed are re-oxidized by O(2) and. so, Cu(II) ions are being recycled. Consistently, for the solutions prepared under nitrogen, the corresponding areas and proportions in the spectra are much lower, confirming that a partial reduction of Cu(II) ions actually occurs. (C) 2009 Elsevier B.V. All rights reserved.

Preferential location of prilocaine and etidocaine in phospholipid bilayers: A molecular dynamics study

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Kinetics of interface state-limited hole injection in alpha-naphthylphenylbiphenyl diamine (alpha-NPD) thin layers

Injection-limited operation is identified in thin-film, alpha-NPD-based diodes. A detailed model for the impedance of the injection process is provided which considers the kinetics of filling/releasing of interface states as the key factor behind the injection mechanism. The injection model is able to simultaneously account for the steady-state, current-voltage (J-V) characteristics and impedance response. and is based on the sequential injection of holes mediated by energetically distributed surface states at the metal-organic interface. The model takes into account the vacuum level offset caused by the interface dipole, along with the partial shift of the interface level distribution with bias voltage. This approach connects the low-frequency (similar to 1 Hz) capacitance spectra, which exhibits a transition between positive to negative values, to the change in the occupancy of interface states with voltage. Simulations based on the model allow to derive the density of interface states effectively intervening in the carrier injection (similar to 5 x 10(12) cm(-2)), which exhibit a Gaussian-like distribution. A kinetically determined hole barrier is calculated at levels located similar to 0.4 eV below the contact work function. (C) 2008 Elsevier B.V. All rights reserved.

Magnetic field effects on the conductivity of organic bipolar and unipolar devices at room temperature

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effect of the doping medium on blends of polyurethane and polyaniline

Flexible and free-standing films from blends of polyurethane, based on castor oil, and polyaniline were obtained with various compositions by casting. Significant increase on conductivity followed by a considerable decrease on doping time was obtained by doping the films in N,N-dimethylformamide (DMF) solution with p-toluene sulphonic acid (TSA) or HCl instead of the conventional doping in aqueous solution. This doping efficiency is proposed to be due to an improved swelling of the blend structure caused by the solvent. The electrical conductivity increases significantly upon polyaniline content increase reaching 10(-2) S/cm for a polyaniline content of about 10% (w/w).

Polyaniline mixed LB films exposed to X-rays

X-ray irradiation is shown to affect electronic properties of polyaniline (PANi) in composite Langmuir-Blodgett (LB) films of PANi and cadmium stearate, in a similar way to acid doping. The time it takes for the shift in the UV-vis spectra, characteristic of PANi doping, increases linearly with the film thickness, thus indicating a surface-controlled process. The humidity of the environment under which the films are irradiated is also of extreme importance. No shin is observed under vacuum or under dry atmospheres of N-2, O-2 and Ar. For humid environments the time for the shift decreases with increasing relative humidity.

Study of charge transport in blends of natural rubber and poly(o-methoxyaniline) based on a resistor network statistical model

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The use of Langmuir-Blodgett films of a perylene derivative and polypyrrole in the detection of trace levels of Cu2+ ions

Langmuir films of a tetracarboxylic perylene derivative and polypyrrole display condensed surface pressure isotherms that are shifted when Cu2+ ions are added to the ultrapure water subphase. These films were transferred onto interdigitated gold electrodes leading to Y-type Langmuir-Blodgett (LB) films. The electrodes modified with 5-layer LB films were immersed into a flask with ultrapure water and water containing Cu2+ ions at concentrations ranging from mM to muM. Impedance measurements indicated a distinct electrical response for the two types of films. Although the materials chosen have no specificity for ionic metals, they can be combined for detecting trace levels of Cu2+, which may be exploited in water quality monitoring. (C) 2004 Elsevier B.V. All rights reserved.

Electroactive layer-by-layer films of iron tetrasulfonated phthalocyanine

Electroactive films of iron tetrasulfonated phthalocyanine (FeTsPc) were assembled via the electrostatic layer-by-layer technique (LBL), in which FeTsPc layers were alternated with the polycationic poly(allylamine hydrochloride) (PAN). The multilayer formation was monitored via UV-Vis spectroscopy by measuring the increase in the Q Band of FeTsPc at 676 nm. Film thickness was estimated by profilometry as ca. 10 Angstrom per bilayer. Fourier transform infrared and UV-Vis absorption spectroscopy suggested specific interactions between FeTsPc and PAR Cyclic voltammograms showed reproducible pairs of oxidation-reduction peaks at 0.92 mV and 0.70 mV, respectively, for a 50-bilayer PAH/FeTsPc film at 50 mV/s (vs Ag/AgNO3).

Photoconductivity excitation spectrum for stretch-oriented PPV

We have obtained the photoconductivity (PC) excitation spectrum for a stretch-oriented poly(paraphenylene vinylene) film over a wide spectral range (up to 5 eV). The measurements were performed in the surface cell configuration with the electric field parallel or perpendicular to the stretch direction. Although the sample had a stretch ratio of similar to 4, the dark conductivity and the steady-state photoconductivity were both about 40 and 20 times higher with the electric field parallel to the average chain direction, respectively. However, the shape of the PC excitation spectrum was independent of field direction and did not show a significant rise in the ultraviolet, as is usually observed for measurements in the photodiode configuration. The implications of these results to the charge photogeneration mechanism in conjugated polymers are discussed.

Soliton clusters inducing insulator-to-metal transition in polyacetylene: a Su-Schrieffer-Heeger model study

The Su-Schrieffer-Heeger (SSH) Hamiltonian has been one of most used models to study the electronic structure of polyacetylene (PA) chains. It has been reported in the literature that in the SSH framework a disordered soliton distribution can not produce a metallic regime. However, in this work (using the same SSH model and parameters) we show that this is possible. The necessary conditions for true metals (non-vanishing density of states and extended wavefunctions around the Fermi level) are obtained for soliton concentration higher than 6% through soliton segregation (clustering). These results are consistent with recent experimental data supporting disorder as an essential mechanism behind the high conductivity of conducting polymers. (C) 2001 Elsevier B.V. B.V. All rights reserved.

Layer-by-layer hybrid films of polyaniline and vanadium oxide

Supramolecular structures of polyaniline (PANI) and vanadium oxide (V2O5) have been assembled via the electrostatic layer-by-layer (LBL) technique. The films were characterized by vibrational analyses which indicated that the interactions between the two components lead to different properties in the films when compared to sol-gel films. of the neat compounds. In particular, using surface enhanced Raman scattering we were able to probe LBL film properties that depend on which material comprises the topmost layer.