Nesse trabalho se estudou sistematicamente o efeito de dois bons solventes sobre os espectros de excitação e de fluorescência em condições fotoestacionárias do MEH-PPV com três massas molares diferentes, -M n = 51 kg mol ) e em filmes produzidos por espalhamento de soluções, mostrando diferenças que decorrem da diferente forma de solvatação das cadeias pelos diferentes solventes. Um deslocamento espectral para o vermelho foi observado em concentrações mais altas 10 -6 mol L -1 e pode ser explicado pela regra de Kasha para uma orientação anti-paralela dos momentos de transição dos dois cromóforos. As conformações em solução são parcialmente mantidas no estado sólido alem do fato de que os espectros se deslocam para o vermelho e são atribuídos à formação de agregados. Recozimento dos filmes em temperaturas acima da transição vítrea elimina as conformações mais tensionadas das cadeias, apaga o efeito de memória e leva a espectros de fluorescência mais finos.Here we systematically studied the excitation and the fluorescence steady-state spectroscopy of poly(2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) with three molecular weights, -M n = 51 kg mol -1 , -M n = 86 kg mol -1 and -M n = 125 kg mol -1 , in two equally good solvents and several concentrations, from dilute solutions to solid-state films produced by casting. The appropriateness of the two solvents was established by comparing their solubility parameters and the solubility parameter of the MEH-PPV estimated using the Small, the Van Krevelen and the Hoy models. Thus, chloroform and toluene were chosen. Fluorescence spectra were recorded for solutions in several concentrations (10 -8 mol L -1 to 10 -4 mol L -1 ) and films produced by casting, showing that chloroform and toluene solvate the polymer chain differently. Diluted solutions (10 -8 mol L -1 ) in chloroform exhibit broader fluorescence spectra. A red-shift of the fluorescence spectra was observed for concentrations higher than 10 -6 mol L -1 that can be explained using Kasha's rule for the sandwich anti-parallel orientation of the transition moments of the two chromophores. The conformations observed in solutions are partially retained in the solid films in addition to the broader red-shift spectra attributed to aggregated forms of the macromolecular segments. Annealing of the polymer films at the glass transition temperature eliminates the more stressed conformations, erases the memory and leads to sharper fluorescence spectra.

A detailed description of the thermal relaxation processes in MEH−PPV is reported. Bulk methods such as DMTA were employed in conjunction with other techniques that probe molecular motions, such as fluorescence spectroscopy, thermal stimulated current, and 13C NMR. From the two main transitions observed (glass transition process at 340 K and β-relaxation between 200 and 220 K), it was demonstrated that the first is strongly correlated with the dissociation of a fluorescent emissive interchain complex and that the second relaxation involves movements of the lateral substituents of the polymer backbone and, more specifically, their CH2 groups. NMR dipolar chemical shift correlation experiments pointed an increasing gain in mobility through the side chain, the lateral carbons close to the aromatic ring being more rigid than those located more distant from the main polymer chain. A kinetic model involving the dissociation of interchains to re-form intrachain excitons was proposed to explain the profiles of the photoluminescence spectra at higher temperatures.

This article describes the microstructure and dynamics in the solid state of polyfluorene-based polymers, poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), a semicrystalline polymer, and poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene}, a copolymer with mesomorphic phase properties. These structures were determined by wide-angle X-ray scattering (WAXS) measurements. Assuming a packing model for the copolymer structure, where the planes of the phenyl rings are stacked and separated by an average distance of approximately 4.5 A and laterally spaced by about approximately 16 A, we followed the evolution of these distances as a function of temperature using WAXS and associated the changes observed to the polymer relaxation processes identified by dynamical mechanical thermal analysis. Specific molecular motions were studied by solid-state nuclear magnetic resonance. The onset of the side-chain motion at about 213 K (beta-relaxation) produced a small increase in the lateral spacing and in the stacking distance of the phenyl rings in the aggregated structures. Besides, at about 383 K (alpha-relaxation) there occurs a significant increase in the amplitude of the torsion motion in the backbone, producing a greater increase in the stacking distance of the phenyl rings. Similar results were observed in the semicrystalline phase of PFO, but in this case the presence of the crystalline structure affects considerably the overall dynamics, which tends to be more hindered. Put together, our data explain many features of the temperature dependence of the photoluminescence of these two polymers.

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Solid-state nuclear magnetic resonance methods were used to study molecular dynamics of MEH-PPV at different frequency ranges varying from 1 Hz to 100 MHz. The results showed that in the 213 to 323 K temperature range, the motion in the polymer backbone is predominantly slow ͑Hz-kHz͒ involving small angle librations, which occurs with a distribution of correlation times. In the side chain, two motional regimes were identified: Intermediate regime motion ͑1-50 kHz͒ for all chemical groups and, additionally, fast rotation ͑ϳ100 MHz͒ for the terminal CH 3 group. A correlation between the motional parameters and the photoluminescent behaviors as a function of temperature was observed and is discussed.

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Recebido em 29/9/04; aceito em 21/6/05; publicado na web em 1/12/05 POLYMERIC LIGHT EMITTING DEVICES. Here we present an overview of electroluminescent devices that use conjugated polymers as the active media. The principal components of the devices are described and we show some examples of conjugated polymers and copolymers usually employed in polymeric light emitting devices (PLED). Some aspects of the photo and electroluminescence properties as well as of the energy transfer processes are discussed. As an example, we present some of the photophysical properties of poly(fluorene)s, a class of conjugated polymers with blue emission.Keywords: polymer; photoluminescence; electroluminescence. INTRODUÇÃOOs primeiros trabalhos sobre condução eletrônica em polímeros conjugados datam de 1970 1,2 sendo que a eletroluminescência (EL) em polímeros foi relatada pela primeira vez em 1989 3 . O uso de polímeros conjugados com um sistema de elétrons π delocalizados em dispositivos optoeletrônicos, conhecidos como OPLEDs ("Organic Polymeric Light Emission Devices"), tem crescido desde então. Na atualidade, monitores coloridos na forma de matriz ativa, fabricados com polímeros orgânicos emissores de luz (OPLEDs) estão na fronteira de serem comercializados. As razões para o sucesso dos OPLEDs devem-se às múltiplas possibilidades de estruturas químicas dos compostos possíveis, às várias possibilidades de formas de montagem dos dispositivos, aliadas à alta performance e ao menor custo de produção 4 . A tecnologia de exibição não é, entretanto, o único campo em que materiais orgânicos em geral, e poliméricos conjugados em particular, aparecem com um futuro promissor. Entre esses novos campos aparecem as novas tecnologias de células solares 5-8 , transistores 9-11 , emissores tipo laser, sistemas de armazenamento e circuitos integrados poliméricos [12][13][14][15][16][17] . O uso de materiais orgânicos, poliméricos ou não, nos vários tipos de dispositivos tem, entre suas principais vantagens, a imensa possibilidade de preparação de estruturas químicas diferentes, além de poderem ser misturados, formando sistemas com outras propriedades. Têm, entretanto, alguns problemas tecnológicos importantes ainda não resolvidos, relacionados com a menor estabilidade química quando comparados com materiais inorgânicos ou organo-metálicos. Entretanto, a possibilidade de mistura física entre componentes permitindo a obtenção de sistemas com diferentes propriedades é, sem dúvida, uma forma atraente e de custo reduzido de preparação de novos materiais. Essa flexibilidade nas formas de preparação de sistemas é particularmente interessante no caso de polímeros eletroluminescentes para os quais pequenas alterações nas rotas de síntese podem levar a materiais com diferentes estruturas e, o fato de apresentarem diferentes estruturas químicas, leva a que os materiais passem a emitir em diferentes regiões do espectro [18][19][20][21] . As diferenças nos processos sintéticos podem, também, envolver reações de copolimerização, permitindo a geração de materi...