Nesta dissertação, visamos o entendimento de processos intermediários que regem a transferência de energia e determinam as propriedades ópticas de polímeros conjugados, a fim de encontrar meios para aumentar a eficiência de emissão e o tempo de vida dos ?LED?s? orgânicos. Estudamos especificamente o polímero PPV, poli(p-fenileno vinilideno), sintetizado através de novo método desenvolvido no Grupo de Polímeros do Instituto de Física de São Carlos, por Marletta e colaboradores[1], na forma de filmes finos depositados pelas técnicas ?spincast? e de automontagem (LbL). Para o trabalho utilizamos técnicas espectroscópicas de luminescência, excitação seletiva e absorbância. Por meio de técnicas de engenharia molecular e através do ordenamento induzido da rede polimérica, avaliamos o peso que têm os eventos intermediários à emissão. Estiramos gradualmente filmes de PTHT/PPV depositados sobre Teflon, sob um eixo preferencial, e constatamos a dependência da polarização da luz emitida com processos de transferência de energia e com a anisotropia da matriz polimérica. Realizamos a separação das cadeias do copolímero a fim de analisar separadamente a transferência de energia entre os segmentos conjugados pertencentes a cadeias distintas (processos intercadeia) ou ao longo da cadeia (processos intracadeia). Esta separação intercadeia e intracadeia resultaram em um significativo aumento da eficiência da emissão, assim como um deslocamento da banda de emissão para o azul, privilegiando a emissão de segmentos de menor conjugação e que participavam apenas nos processos de relaxação de energia e transferência da excitação e não dos estados emissivos finais. Separamos os segmentos conjugados dentro da própria cadeia do copolímero, regulando o comprimento de conjugação, através da incorporação de DBS. Aqui, verificamos, os processos de transferência intracadeia concorrem em igual peso com processos intracadeia na migração energética em polímeros conjugados. Concluímos que o processo limitante da eficiência de emissão em polímeros conjugados e dispositivos é a eficiente transferência de energia entre cadeias ou ao longo da cadeia para centros supressores de energia. / The main goal of this work is the understanding of intermediated energy relaxation processes that are governed by Energy Transfer. Such processes strongly affect the optical properties of conjugated polymers, reducing the emission efficiency and the life time of OLEDs. We study specifically the polymer poly(p-phenylene vinylene), PPV, synthesized through the new method developed by Marletta and co-workers [1]. The films were prepared by Spincast and self-assembly (Layer-by-Layer) techniques on different substrates. The optical characterization of the polymeric material was carried out by photoluminescence, selective excitation spectroscopy and UV-Vis absorbance measurements. With the use of molecular engineering techniques and by changing the polymeric order induced by uniaxial stretch of the films deposited on a flexible substrate, it was possible to infer the weight of intermediary events in final emission process. A special issue of this work was the study of the optical properties of thin PPV films deposited on flexible Teflon substrates under very low deformation levels. Optical data of low stretched conjugated polymers on transparent substrates were not previously available in the literature. Continuous axial stretch was applied at room temperature to very thin PPV films enabling simultaneous luminescence and absorbance polarization measurements. The optical properties of polymeric light emitting devices are strongly dependent on the structural order of the polymer chains and the energy transport mechanism between the conjugated segments. We demonstrate that such low stretched films presents a very efficient isotropic-to-polarized conversion regarding the fact we are dealing with energy transfer between luminescent chromophore of same species. We also carried out chain separation of conjugated segments of a PPV copolymer in order to analyze two Energy Transfer kinds: one that occurs among PPV segments in different chains (interchain processes) and the other involving the Energy Transfer between PPV segments along the same PPV chain (intrachain processes). To account for the interchain separation of the PPV segments we diluted the PPV chain in an inert polymeric matrix. The novelty, however, lies in the precise control of the separation of PPV segments along the chain by using a copolymer with sequences of conjugated PPV and non-conjugated PTHT distributed randomly along the polymer chain. The efficiency increase together with the spectral blue shif with the segment separation show that inter- and intrachain exciton mobility is enhanced by low range Förster Energy Transfer. For the case of samples without chain separation, it is easier for excitons to move around by energy transfer process and find quenching defects before their radioactive decay. For diluted or low conjugated samples, excitons will not reach defects before recombination since Energy Transfer is suppressed. Thus, we verify, the Intrachain Energy Transfer process have the same influence as for Interchain process produced by the energy migration among conjugated polymers segments. From a more fundamental point of view, the establishment of a controlled ordering and separation of the polymeric chains may reduce effects such as the luminescence quenching that strongly reduce the emission efficiency in organic devices.
| Identifer | oai:union.ndltd.org:usp.br/oai:teses.usp.br:tde-13112007-145818 |
| Date | 04 August 2006 |
| Creators | Maia, Francisco Carlos Barbosa |
| Contributors | Guimaraes, Francisco Eduardo Gontijo |
| Publisher | Biblioteca Digitais de Teses e Dissertações da USP |
| Source Sets | Universidade de São Paulo |
| Language | Portuguese |
| Detected Language | English |
| Type | Dissertação de Mestrado |
| Format | application/pdf |
| Rights | Liberar o conteúdo para acesso público. |
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