Electroluminescence of phenylene-vinylene random copolymers with different conjugation lengths

PPV random derivates were synthesized and characterized. Polymer light emitting diodes (PLEDs) were assembled using the random copolymers as emissive layer and showed EL in the blue-green region in function of the method of preparation. The increase in the average conjugation degree in the polymer chain led to the reduction of the turn-on voltage of the device. The addition of Alq3 as ETL increased tenfold the luminescence efficiency. (C) 2009 Elsevier B.V. All rights reserved.

Correlations between Conjugation Length, Macromolecular Dynamics, and Photophysics of Phenylene-Vinylene/Aliphatic Multiblock Copolymers

This work reports a detailed spectroscopy study of a series of multiblock conjugated nonconjugated copolymers built by p-phenylene vinylene type units (PV) and octamethylene spacers, namely, poly(1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene) (LaPPS18). The relative proportions of the PV and aliphatic segments were estimated on the basis of solid-state NMR and Raman spectroscopy. The overall structure was characterized by wide angle X-ray diffraction; H-1 wide-line dipolar chemical shift correlation (DIPSHIFT), and centerband-only detection of exchange (CODEX) NMR data, that together with glass transition temperatures allowed us to identify the groups involved in the molecular dynamics. These different structural properties were used to explain the photoluminescence properties in terms of peak position and spectral profile

Temperature Effect on the Two-Photon Absorption Spectrum of All-trans-beta-carotene

In this report, we investigate the influence of temperature on the two-photon absorption (2PA) spectrum of all-trans-beta-carotene using the femtosecond white-light-continuum Z-scan technique. We observed that the 2PA cross-section decreases quadratically with the temperature. Such effect was modeled using a three-energy-level diagram within the sum-over-essential states approach, assuming temperature dependencies to the transition dipole moment and refractive index of the solvent. The results show that the transition dipole moments from ground to excited state and between the excited states, which governed the two-photon matrix element, have distinct behaviors with the temperature. The first one presents a quadratic dependence, while the second exhibits a linear dependence. Such effects were attributed mainly to the trans -> cis thermal interconversion process, which decreases the effective conjugation length, contributing to diminishing the transition dipole moments and, consequently, the 2PA cross-section.

Two-photon excitation and optical limiting in polyfluorene derivatives

Fluorene-based polymers are widely known materials due to a combination of features such as photoluminescence and electroluminescence, oxidative stability, and film-forming ability. However, studies reporting nonlinear optical properties in this class of conjugated polymer are scarce. Here, we report a new class of polyfluorene derivatives poly(9,9'-n-dihexyl-2,7-fluorenedilvinylene-alt-1,4-phenylenevinylene), poly(9,9'-n-dihexyl-2,7-fluorenedilvinylene-alt-2,5-thiophene), and poly[(9,9-di-hexylfluorenediylvinylene-alt-1,4-phenylenevinylene)-co-((9,9'-(3-t-butylpropanoate) fluorene-1,4-phenylene)] displaying high two-photon absorption (2PA) in the spectral range from a 490 to 1100 nm. The 2PA cross-section peak values for these materials are as high as 3000 Goppert Mayer (1 GM = 1 x 10-50 cm4 s/photon), which is related to the high degree of conjugation along the polymer backbone. The polymers that were used in this study presented a strong two-photon luminescence and also displayed optical limiting behavior, which, in combination with their well-established properties, make them highly suitable for nonlinear optical devices. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 148153, 2012

Enhancement of the Nonlinear Optical Absorption of the E7 Liquid Crystal at the Nematic-Isotropic Transition

We present an experimental study of the nonlinear optical absorption of the eutectic mixture E7 at the nematic-isotropic phase transition by the Z-scan technique, under continuous-wave excitation at 532 nm. In the nematic region, the effective nonlinear optical coefficient beta, which vanishes in the isotropic phase, is negative for the extraordinary beam and positive for an ordinary beam. The parameter , whose definition in terms of the nonlinear absorption coefficient follows the definition of the optical-order parameter in terms of the linear dichroic ratio, behaves like an order parameter with critical exponent 0.22 +/- 0.05, in good agreement with the tricritical hypothesis for the nematic-isotropic transition.

The influence of the degree of back-mixing on hydrogen production in an anaerobic fixed-bed reactor

This paper reports on results obtained from experiments carried out in an acidogenic anaerobic reactor aiming at the optimization of hydrogen production by altering the degree of back-mixing. It was hypothesized that there is an optimum operating point that maximizes the hydrogen yield. Experiments were performed in a packed-bed bioreactor by covering a broad range of recycle ratios (R) and the optimum point was obtained for an R value of 0.6. In this operating condition the reactor behaved as 8 continuous stirred-tank reactors in series and the maximum yield was 4.22 mol H-2 mol sucrose(-1). Such optimum point was estimated by deriving a polynomial function fitted to experimental data and it was obtained as the conjugation of three factors related to the various degrees of back-mixing applied to the reactor: mass transfer from the bulk liquid to the biocatalyst, liquid-to-gas mass transfer and the kinetic behavior of irreversible reactions in series. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Spectroscopic Study on the Structural Differences of Thermally Induced Cross-Linking Segments in Emeraldine Salt and Base Forms of Polyaniline

This paper reports the spectroscopic study on the structural differences of thermally induced cross-linking segments in polyaniline in its emeraldine salt (PANI-ES) and base (PANI-EB) forms. Casting films of PANI-ES (ES-film) and PANI-EB (EB-film) were prepared and heated at 150 degrees C under atmospheric air for 30 min. Raman spectra excited at 632.8 nm of heated ES-film presented the characteristic bands of phenazine-like structures at 1638, 1392, and 575 cm(-1), whereas EB-film showed lower relative intensities for these bands. The lower content of phenazine-like segments in heated EB-film is related to residual polaronic segments from preparation procedures, as revealed by Raman. This statement was confirmed by a sequence of thermal and doping experiments in both films. Quantum-chemical calculations by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) showed that the phenazine-like structure presents the intense Raman band at 1350 cm(-1) due to heterocycle breathing mode, and the non-phenazine-like structure (substituted hydrophenazine-type) presents higher energy for HOMO-LUMO transition, indicating the lack of conjugation in the heterocycle compared with the phenazine-like structure. According to experimental and theoretical data reported here, it is proposed that only thermally treated PANI-ES presents phenazine-like rings, whereas PANI-EB presents heterocyclic non-aromatic structures.

EVALUATION OF GLUTATHIONE S-TRANSFERASE GSTM1 AND GSTT1 POLYMORPHISMS AND METHYLMERCURY METABOLISM IN AN EXPOSED AMAZON POPULATION

Over the last decades, the presence of methylmercury (MeHg) in the Amazon region of Brazil and its adverse human health effects have given rise to much concern. The biotransformation of MeHg occurs mainly through glutathione (GSH) in the bile mediated by conjugation with glutathione S-transferases (GST). Epidemiological evidence has shown that genetic polymorphisms may affect the metabolism of MeHg. The aim of this study was to evaluate the association between GST polymorphisms, GSH, and Hg levels in blood (B-Hg) and in hair (H-Hg) of an Amazon population chronically exposed to the metal through fish consumption. Blood and hair samples were collected from 144 volunteers (71 men, 73 women). B-Hg and H-Hg levels were determined by inductively coupled plasma-mass spectrometry, and GSH levels were evaluated by a spectrophotometric method. GSTM1 and T1 genotyping evaluation were carried out by multiplex polymerase chain reaction (PCR). Mean levels of B-Hg and H-Hg were 37.7 +/- 24.5 mu g/L and 10.4 +/- 7.4 mu g/g, respectively; GSH concentrations ranged from 0.52 to 2.89 mu M/ml of total blood. Distributions for GSTM1/T1, GSTM1/GSTT1*0, GSTM1*0/T1, and GSTM1*0/GSTT1*0 genotypes were 35.4, 22.2, 25.0, and 17.4%, respectively. GSTT1 genotype carriers presented lower levels of B-Hg and H-Hg when compared to other genotypes carriers. In addition, GSTM1*0/GSTT1*0 individuals presented higher Hg levels in blood and hair than subjects presenting any other genotypes. There appeared to be no evidence of an effect of polymorphisms on GSH levels. Therefore, our data suggest that GST polymorphisms may be associated with MeHg detoxification.

Synthesis of self-assembled tubes in direct conjugated porphyrin-gold nanohybrid solution

We report on the formation of self-assembled meso-tetrakis (p-sulfonatofenyl) porphyrin (H2 TPP'S POT. 4-''IND. 4') tubes stabilized by gold nanoparticles (NPs) in basic solution and on their spectroscopic chareterization. The role of the gold NPs in the aggregation dynamics of free-base sulfonated porphyrin (H2TPP'S POT. 4-''IND. 4') is also investigated. The direct conjugation of the gold NPs to the H2TPPS4 molecule quenches the fluorescence intensity, while absorption peaks are blue-shifted, indicating predominant H-type aggregation. It is observed that porphyrin molecules adsorbed on the surface of the gold NP interact and form tubes of maximum diameter ∼1.5 μm and length >100 μm. Steady-state and time-resolved spectroscopic techniques confirm nonradiative energy transfer from porphyrin to gold NP.

Theoretical and experimental studies on the electronic, optical, and structural properties of poly-pyrrole-2-carboxylic acid films

A theoretical approach is used here to explain experimental results obtained from the electrosynthesis of polypyrrole-2-carboxylic acid (PPY-2-COOH) films in nonaqueous medium. An analysis of the Fukui function (reactivity index) indicates that the monomer (pyrrole-2-carboxylic acid, PY-2-COOH), and dimers and trimers are oxidized in the C4 or C5 positions of the heterocyclic ring of the PY-2-COOH structure. After calculating the heat of formation using semiempirical Austin Model 1 post-Hartree-Fock parameterization for dimer species, both C4 and C5 positions adjacent to the aromatic rings of PPY-2-COOH were considered the most susceptible ones to oxidative coupling reactions. The ZINDO-S/CI semiempirical method was used to simulate the electronic transitions typically seen in the UV-VIS-NIR range in monomer and oligomers with different conjugation lengths. The use of an electrochemical quartz crystal microbalance provides sufficient information to propose a polymerization mechanism of PY-2-COOH based on molecular modeling and experimental results.