Energy Transfer in Nanostructured Films Containing Poly(p-phenylene vinylene) and Acceptor Species

The combination of luminescent polymers and suitable energy-accepting materials may lead to a molecular-level control of luminescence in nanostructured films. In this study, the properties of layer-by-layer (LbL) films of polyp-phenylene vinylene) (PPV) were investigated with steady-state and time-resolved fluorescence spectroscopies, where fluorescence quenching was controlled by interposing inert polyelectrolyte layers between the PPV donor and acceptor layers made with either Congo Red (CR) or nickel tetrasulfonated phthalocyanine (NiTsPc). The dynamics of the excited state of PPV was affected by the energy-accepting layers, thus confirming the presence of resonant energy transfer mechanisms. Owing to the layered structured of both energy donor and acceptor units, energy transfer varied with the distance between layers, r, according to 1/r(n) with n = 2 or 3, rather than with 1/r(6) predicted by the Forster theory for interacting point dipoles.

Modeling Adsorption Processes of Poly-p-phenylenevinylene Precursor and Sodium Acid Dodecylbenzenesulfonate onto Layer-by-Layer Films Using a Langmuir-type Metastable Equilibrium Model

The adsorption kinetics curves of poly(xylylidene tetrahydrothiophenium chloride) (PTHT), a poly-p-phenylenevinylene (PPV) precursor, and the sodium salt of dodecylbenzene sulfonic acid (DBS), onto (PTHT/DBS)(n) layer-by-layer (LBL) films were characterized by means of UV-vis spectroscopy. The amount of PTHT/DBS and PTHT adsorbed on each layer was shown to be practically independent of adsorption time. A Langmuir-type metastable equilibrium model was used to adjust the adsorption isotherms data and to estimate adsorption/desorption coefficients ratios, k = k(ads)/k(des), values of 2 x 10(5) and 4 x 10(6) for PTHT and PTHT/DBS layers, respectively. The desorption coefficient has been estimated, using literature values for poly(o-methoxyaniline) desorption coefficient, as was found to be in the range of 10(-9) to 10(-6) s(-1), indicating that quasi equilibrium is rapidly attained.

MEH-PPV photobleaching control by femtosecond pulse shaping

In the work reported here we were able to control the photobleaching of poly[2-methoxy-5-(2`-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), excited by two-photon absorption, using femtosecond pulse shaping. By applying a cosine-like spectral phase mask, we observe a reduction of three times in the photobleaching rate, while the fluorescence intensity decreases by 20%, in comparison to the values obtained with a Fourier-transform-limited pulse. These results demonstrate an interesting trade-off between photobleaching rate and nonlinear fluorescence intensity. The possible mechanism behind this process is discussed in terms of the pulse spectral profile and the absorbance band of MEH-PPV. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Controlling the absorption and emission properties of polyparaphenylenevinylene films

In this work we used the conversion process of a precursor polymer into polyparaphenylenevinylene (PPV) at low temperatures in order to control the effective conjugation degree of spin-casted PPV films. The absorption and emission spectra of the films were studied by following a partial substitution of chloride counterions from poly(xylylidene tetrahydrothiophenium chloride) (PTHT), used as a precursor, by sodium acid dodecyl benzenesulfonate (DBS), added as a surfactant salt. Upon controlling the DBS amount and conversion temperature (T-c) of PTHT/DBS to PPV films, the band gap of PPV changed from 409 to 506 nm, and 505 to 532 nm, values obtained from absorbance and emission measurements, respectively. Based on these experimental data, we proposed a physical model which represents the chemical structure of PPV as a distribution of conjugated chain segments (like oligomers) alternated by non-conjugated segments (structural defects and/or from the precursor polymer). (C) 2008 Elsevier B.V. All rights reserved.

Molecular Ordering in Layer-by-Layer Polyelectrolyte Films Studied by Sum-Frequency Vibrational Spectroscopy: The Effects of Drying Procedures

Sum-Frequency Vibrational Spectroscopy (SFVS) has been used to investigate the effect of nitrogen-flow drying on the molecular ordering of Layer-by-Layer (LbL) films of poly(allylamine hydrochloride) (PAH) alternated with poly(styrene sulfonate) (PSS). We find that films dried by spontaneous water evaporation are more ordered and homogeneous than films dried by nitrogen flow. The latter are quite inhomogeneous and may have regions with highly disordered polymer conformation. We propose that drying by spontaneous water evaporation reduces the effect of drag by the drying front, while during nitrogen-flow drying the fast evaporation of water ""freezes"" the disordered conformation of adsorbed polyelectrolyte molecules. These findings are important for many applications of LbL films, since device performance usually depends on film morphology and its molecular structure.

Optical birefringence induced by two-photon absorption in polythiophene bearing an azochromophore

We carried out experiments of induced birefringence via two-photon absorption in spin-coated films of the conjugated polymer poly[2-[ethyl-[4-(4-nitro-phenylazo)-phenyl] -amino]-ethane (3-thienyl)ethanoate], PAzT, at 680 and 775 nm. This process allows recording in the bulk because of the spatial confinement of the bireffingence provided by the two-photon absorption. The induced birefringence is associated with molecular reorientation caused by the two-photon induced isomerization of the azochromophores attached to the polymer backbone. In addition, the two-photon absorption spectrum of PAzT was measured to help selecting the excitation wavelength for two-photon absorption induced birefringence. (c) 2008 Elsevier Ltd. All rights reserved.

Thickness and Annealing Temperature Effects on the Optical Properties and Surface Morphology of Layer-by-Layer Poly(p-phenyline vinylene) plus Dodecylbenzenesulfonate Films

The fabrication of controlled molecular architectures is essential for organic devices, as is the case of emission of polarized light for the information industry. In this study, we show that optimized conditions can be established to allow layer-by-layer (LbL) films of poly(p-phenylene vinylene) (PPV)+dodecylbenzenesulfonate (DBS) to be obtained with anisotropic properties. Films with five layers and converted at 110 degrees C had a dichroic ratio delta = 2.3 and order parameter r = 34%, as indicated in optical spectroscopy and emission ellipsometry data. This anisotropy was decreased with the number of layers deposited, with delta = 1.0 for a 75-layer LbL PPV + DBS film. The analysis with atomic force microscopy showed the formation of polymer clusters in a random growth process with the normalized height distribution being represented by a Gaussian function. In spite of this randomness in film growth, the self-covariance function pointed to a correlation between clusters, especially for thick films. In summary, the LbL method may be exploited to obtain both anisotropic films with polarized emission and regular, nanostructured surfaces. (c) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 206-213, 2011

Photoluminescence of MEH-PPV with ultraviolet excitation

The basic optical properties of PPV-based polymers have been extensively studied due to their potential technological applications. However, a detailed investigation of electronic processes following photoexcitation in the ultraviolet is still lacking. We report photoluminescence measurements on poly(1-methoxy-4-ethylhexyloxy-paraphenylenevinylene) - MEH-PPV in the 2.0-5.6 eV range, with excitation up to 5.6 eV. The photoluminescence spectra lineshape is independent of excitation energy. The photoluminescence efficiency is high for energies well below the absorption maximum due to near-resonant excitation of the longest conjugated segments which are responsible for the PL It decreases strongly for excitation energies in the range 2.1-2.5 eV (up to the absorption maximum) and slightly from 2.5 to 5.6 eV. The results indicate that states excited in the ultraviolet rapidly relax nonradiatively to the lowest state, from where the usual luminescence occurs. (C) 2010 Elsevier B.V. All rights reserved.

Detection of microwaves using the organic semiconductor poly(p-phenylenevinylene)

We report a new procedure to convert the polymer precursor poly(xylylidene tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylenevinylene) (PPV) using microwave irradiation. Spin-coated PTHT films were irradiated at room temperature under ambient conditions in a commercial microwave oven, with varying power from 20W to 100W. Complete conversion was reached within only 5 min of irradiation for powers above 50W, yielding PPV films with absorption and photoluminescence spectra that are practically indistinguishable from the spectra of thermally converted PPV films, which require ca. 2 h of a high temperature (similar to 200 degrees C) thermal treatment. In addition to a much faster conversion procedure, the irradiation with microwaves led to a red shift in the absorption spectrum of a PTHT film, which varied linearly with the time of irradiation. These films can then be used as low-cost, easy-to-use detectors of microwaves. (C) 2010 Elsevier B.V. All rights reserved.

Influence from the sulfonate group (RSO(3)(-)) on the conversion process and emission efficiency of poly(p-phenylene vinylene)

The discovery of an alternative route to convert poly(xylyliden tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylene vinylene) (PPV) using dodecylbenzenesulfonate (DBS) has allowed the formation of ultrathin films with unprecedented control of architecture and emission properties. In this work, we show that this route may be performed with several sufonated compounds where RSO(3)(-) replaces the counter-ion (Cl(-)) of PTHT, some of which are even more efficient than DBS. Spin-coating films were produced from PTHT and azo-dye molecules, an azo-polymer and organic salts as counter-ions of PTHT. The effects of the thermal annealing step of PTHT/RSO(3)(-) films at 110 and 230 degrees C were monitored by measuring the absorption and emission spectra. The results indicate that the exchange of the counterion Cl(-) of PTHT by a linear long chain with RSO(3)(-) group is a general procedure to obtain PPV polymer at lower conversion temperature (ca. 110 degrees C) with significant increase in the emission efficiency, regardless of the chemical position and the number of sulfonate groups. With the enhanced emission caused by Congo Red and Tinopal as counter-ions, it is demonstrated that the new synthetic route is entirely generic, which may allow accurate control of conversion and emission properties. (C) 2010 Elsevier B.V. All rights reserved.

De-aggregation of polyfluorene derivative by blending with a series of poly(alkyl methacrylate)s with varying sidegroup sizes

A polyfluorene derivative of the PPV, poly(9,9`-n-dihexyl-2,7-fluorenedilvinylene-alt-1,4-phenylenevinylene), with a strong tendency to aggregation was blended with several members of a series composed by poly(alkyl methacrylate)s with the following substituents in the ester position: methyl, ethyl, isopropyl, isobutyl, n-butyl, and cyclohexyl. The de-aggregation effect in blends was studied by steady-state photoluminescence spectroscopy using several blend compositions. The efficiency of each dispersing phase was discussed in terms of the polymer miscibility, controlled by interaction parameters between the polyfluorene and every poly(alkyl methacrylate)s, here described using Hilde-brand solubility parameters. (c) 2009 Elsevier Ltd. All rights reserved.

Temperature dependence of molecular dynamics and supramolecular aggregation in MEH-PPV films: A solid-state NMR, X-ray and fluorescence spectroscopy study

This article presents an investigation of the temperature induced modification in the microstructure and dynamics of poly[2-methoxy-5-(2`-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) cast films using Wide-Angle X-ray Scattering (WAXS), solid-state Nuclear Magnetic Resonance (NMR), and Fluorescence Spectroscopy (PL). MEH-PPV chain motions were characterized as a function of temperature by NMR. The results indicated that the solvent used to cast the films influences the activation energy of the side-chain motions. This was concluded from the comparison of the activation energy of the toluene cast film, E(a) = (54 +/- 8) kJ/mol, and chloroform cast film, E(a) = (69 +/- 5) kJ/mol, and could be attributed to the higher side-chain packing provided by chloroform, that preferentially solvates the side chain in contrast to toluene that solvates mainly the backbone. Concerning the backbone mobility, it was observed that the torsional motions in the MEH-PPV have average amplitude of similar to 10 degrees at 300 K, which was found to be independent of the solvent used to cast the films. In order to correlate the molecular dynamics processes with the changes in the microstructure of the polymer, in situ WAXS experiments as a function of temperature were performed and revealed that the interchain spacing in the MEH-PPV molecular aggregates increases as a function of temperature, particularly at temperatures where molecular relaxations occur. It was also observed that the WAXS peak associated with the bilayer spacing becomes narrower and its intensity increases whereas the peak associated with the inter-backbone planes reduces its intensity for higher temperatures. This last result Could be interpreted as a decrease in the number of aggregates and the reduction of the interchain species during the MEH-PPV relaxation processes. These WAXS results were correlated with PL spectra modifications observed upon temperature treatments. (C) 2008 Elsevier Ltd. All rights reserved.

Absorption and emission properties of LBL PPV films deposited on ITO electrodes

In this work we studied the properties of absorption and emission line shape of layer-by-layer (LBL) poly(p-phenylene vinylene) (PPV) on indium-tin oxide (ITO) electrode. To minimize the PPV thermal conversion effects during the polymer processing, we used a less aggressive leaving group in the precursor polymer; minimizing electrode degradation. LBL ITO/PPV films showed the same absorption and emission line shape compared with LBL PPV films deposited on non-metallic substrates (glass). With this analysis we indirectly observe the decrease in the ITO degradation. Atomic force microscopy (AFM) technique was used to analyze quantitatively the microscopic morphology of the film surface. Results indicated that the substrate topology is not affected, to a large extent, by the use of dodecylbenzensulfonate (DBS) ion. (C) 2008 Elsevier B.V. All rights reserved.

A new approach to organomanganese compounds: the tellurium/manganese exchange reaction

Diorganomanganese compounds react with aryl, vinyl, and alkynyl tellurides in a tellurium/manganese exchange reaction. The new mixed organomanganese reagents react selectively with electrophiles. (C) 2010 Published by Elsevier Ltd.

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.