O poli (p-fenilenevinileno), ou PPV, é um polímero de grande relevância tecnológica devido a suas propriedades eletroluminescentes, que têm sido exploradas em diodos emissores de luz orgânicos, displays flexíveis e outros dispositivos optoeletrônicos. Embora o PPV seja um material de importância para muitas aplicações, a sua síntese na nano/microescala não pode ser obtida através do método padrão, o qual utiliza o aquecimento de um polímero precursor poli (cloreto de xileno tetrahidrotiofenio) (PTHT). Este trabalho mostra como a microestruturação com pulsos de femtosegundo pode ser empregada para a síntese de PPV em regiões pré-determinadas, empregando três diferentes abordagens, permitindo uma nova metodologia para a fabricação precisa de microcircuitos poliméricos complexos, (i) na primeira abordagem, o processo de conversão é obtido pela irradiação de filmes de PTHT com pulsos laser ultracurtos em regiões previamente determinadas, o que leva ao controle espacial da formação de PPV em microescala, (ii) na segunda abordagem, microestruturas tridimensionais dopadas com PTHT foram fotopolimerizadas por absorção de dois fótons. A conversão de PTHT para PPV nestas microestruturas dopadas foi obtida após um tratamento térmico, (iii) na terceira abordagem, a transferência direta induzida por laser (LIFT) com pulsos de femtossegundos permite a deposição controlada de PPV com alta resolução espacial, fornecendo micropadrões 2D, preservando sua estrutura e propriedades ópticas. As estruturas foram caracterizadas por microscopia eletrônica de varredura, microscopia óptica de transmissão, microscopia de fluorescência e microscopia confocal de fluorescência. Suas propriedades ópticas foram analisadas através de sistemas de micro-fotoluminescência e micro-absorção implementadas em um microscópio invertido. Medidas de espectroscopia Raman, microscopia de força atômica e medidas elétricas também foram realizadas. Este trabalho mostra como a microestruturação com laser de fs pode ser explorada para a síntese de PPV em regiões pré-determinadas para fabricar uma variedade de microdispositivos, abrindo novos caminhos na optoeletrônica baseada em polímeros. / Poly(p-phenylenevinylene), or PPV, is a polymer of great technological relevance due to its electroluminescent properties, which have been exploited in organic light emitting diodes, flexible displays and other optoelectronic devices. Although PPV is a material of foremost importance for many applications, its synthesis at the nano/micro scale cannot be achieved through the standard method that uses heating of a precursor polymer poly(xylene tetrahydrothiophenium chloride)(PTHT). This work demonstrates the use of direct laser writing with femtosecond pulses to obtain the synthesis of PPV in pre-determined regions, by applying three different approaches, allowing the precise fabrication of complex polymeric microcircuits, (i) in the first approach the conversion process is achieved by irradiating PTHT films with ultra-short laser pulses in previously determined regions, which leads to the spatial control of PPV formation at microscale, (ii) in the second approach, three-dimensional microstructures doped with PTHT were photopolymerized by two photons absorption. The conversion of PTHT to PPV in these doped microstructures was obtained by a subsequent thermal treatment, (iii) in the third approach, laser-induced forward transfer (LIFT) with femtosecond pulses enables the controlled deposition of PPV with high spatial resolution, providing 2D micropatterns, while preserving its structure and optical properties. The structures were characterized by scanning electron, fluorescence, transmission and confocal fluorescence microscopies. Their optical properties were analyzed by micro-photoluminescence and micro-absorption setups assembled on an inverted microscope. Raman spectroscopy, electrical measurements and atomic force microscopy were also performed. This thesis shows the use of fs-laser writing methods for the synthesis of PPV in pre-determined regions, to fabricate a variety of microdevices, thus opening new avenues in polymer-based optoelectronics.

O poli (p-fenilenevinileno), ou PPV, é um polímero de grande relevância tecnológica devido a suas propriedades eletroluminescentes, que têm sido exploradas em diodos emissores de luz orgânicos, displays flexíveis e outros dispositivos optoeletrônicos. Embora o PPV seja um material de importância para muitas aplicações, a sua síntese na nano/microescala não pode ser obtida através do método padrão, o qual utiliza o aquecimento de um polímero precursor poli (cloreto de xileno tetrahidrotiofenio) (PTHT). Este trabalho mostra como a microestruturação com pulsos de femtosegundo pode ser empregada para a síntese de PPV em regiões pré-determinadas, empregando três diferentes abordagens, permitindo uma nova metodologia para a fabricação precisa de microcircuitos poliméricos complexos, (i) na primeira abordagem, o processo de conversão é obtido pela irradiação de filmes de PTHT com pulsos laser ultracurtos em regiões previamente determinadas, o que leva ao controle espacial da formação de PPV em microescala, (ii) na segunda abordagem, microestruturas tridimensionais dopadas com PTHT foram fotopolimerizadas por absorção de dois fótons. A conversão de PTHT para PPV nestas microestruturas dopadas foi obtida após um tratamento térmico, (iii) na terceira abordagem, a transferência direta induzida por laser (LIFT) com pulsos de femtossegundos permite a deposição controlada de PPV com alta resolução espacial, fornecendo micropadrões 2D, preservando sua estrutura e propriedades ópticas. As estruturas foram caracterizadas por microscopia eletrônica de varredura, microscopia óptica de transmissão, microscopia de fluorescência e microscopia confocal de fluorescência. Suas propriedades ópticas foram analisadas através de sistemas de micro-fotoluminescência e micro-absorção implementadas em um microscópio invertido. Medidas de espectroscopia Raman, microscopia de força atômica e medidas elétricas também foram realizadas. Este trabalho mostra como a microestruturação com laser de fs pode ser explorada para a síntese de PPV em regiões pré-determinadas para fabricar uma variedade de microdispositivos, abrindo novos caminhos na optoeletrônica baseada em polímeros. / Poly(p-phenylenevinylene), or PPV, is a polymer of great technological relevance due to its electroluminescent properties, which have been exploited in organic light emitting diodes, flexible displays and other optoelectronic devices. Although PPV is a material of foremost importance for many applications, its synthesis at the nano/micro scale cannot be achieved through the standard method that uses heating of a precursor polymer poly(xylene tetrahydrothiophenium chloride)(PTHT). This work demonstrates the use of direct laser writing with femtosecond pulses to obtain the synthesis of PPV in pre-determined regions, by applying three different approaches, allowing the precise fabrication of complex polymeric microcircuits, (i) in the first approach the conversion process is achieved by irradiating PTHT films with ultra-short laser pulses in previously determined regions, which leads to the spatial control of PPV formation at microscale, (ii) in the second approach, three-dimensional microstructures doped with PTHT were photopolymerized by two photons absorption. The conversion of PTHT to PPV in these doped microstructures was obtained by a subsequent thermal treatment, (iii) in the third approach, laser-induced forward transfer (LIFT) with femtosecond pulses enables the controlled deposition of PPV with high spatial resolution, providing 2D micropatterns, while preserving its structure and optical properties. The structures were characterized by scanning electron, fluorescence, transmission and confocal fluorescence microscopies. Their optical properties were analyzed by micro-photoluminescence and micro-absorption setups assembled on an inverted microscope. Raman spectroscopy, electrical measurements and atomic force microscopy were also performed. This thesis shows the use of fs-laser writing methods for the synthesis of PPV in pre-determined regions, to fabricate a variety of microdevices, thus opening new avenues in polymer-based optoelectronics.

The study of conjugated polymers, in particular PPV derivatives, continues to fascinate researchers both from the standpoint of developing new routes to control their structure and photophysical properties; this is also driven by the immense potential since this class of polymers has demonstrated in the context of various devices, such as LEDs, photovoltaics, FETs, etc. Also, there has been great interest in utilizing conjugated polymers in developing sensory devices. This thesis has examined a few interesting aspects that could be used to control the structure and consequently the photophysical properties of PPV derivatives. The first chapter of this thesis provides a brief introduction to conjugated polymers, with an emphasis on PPV based polymers, different synthetic methodologies for their preparation, previous attempts to obtain PPV with controlled conjugation length, and also a brief discussion of conjugated polymer nanoparticles (CPNs) and their preparation by various methods. The second chapter deals with the preparation of conjugated polymer (MEHPPV) nanoparticles by reprecipitation method and utilization of these nanoparticles in detection of nitro explosives in aqueous medium. Nanoparticles of MEHPPV with different sizes were prepared by a simple precipitation method from a THF solution into water. Although these nanoparticles were prepared from very hydrophobic MEHPPV, these nanoparticles were reasonably stable in aqueous medium, especially when their sizes were relatively small; their UV-visible and fluorescence spectra could be readily recorded using simple solution methods. The sizes could be controlled by varying the concentration of the polymer solution used. The CPNs are spherical particles as confirmed by atomic force microscopy (AFM). The emission maximum of the nanoparticles is red shifted compared to a solution of the polymer. The fluorescence spectrum of this aqueous nanoparticle dispersion exhibited very high sensitivity to electron-deficient aromatic compounds, in particular the explosive TNT; the CPNs were able to sense nanomolar concentrations of the explosives. Stern-Volmer constant (KSV) is higher for 2,4,6-trinitro toluene (TNT) than any other analytes studied. Among the different sized nanoparticles studied the bigger one showed highest quenching efficiency. Electron-deficient aromatic molecules were shown to quench the fluorescence of the nanoparticles, possibly by excited state electron transfer mechanism; this hypothesis was supported by quenching experiments carried out using a variety of nitro-aromatic molecules with varying reduction potentials, in addition to a few electron-rich aromatic molecules. A fairly good correlation between the quenching efficiency of the analyte and its reduction potential was noticed; however, in a few instances this correlation failed. This suggested that a second factor, namely the solubility of the analyte in water (its hydrophobicity) also is a key factor as this governs the tendency of the analyte to adsorb on the nanoparticle surface, which clearly is the first step in the quenching process. In the third chapter synthesis and characterization of MEHPPV with reduced conjugation length by utilizing the concept of conjugation breaking using non-coplanar entities are presented. MEHPPV with reduced conjugation length was prepared by incorporating non-coplanar entities, such as biphenyl and binaphthyl units, along the polymer backbone. Both Gilch and Witting-Horner methods were successfully utilized to prepare copolymers; the former approach permitted the variation of the level of the twisted comonomer incorporation, whereas the latter approach only provided an alternating copolymer. Although biphenyl based monomers could not be homopolymerized by Gilch method, it was possible to prepare copolymers using xylylene type comonomers. The polymers prepared by the Gilch method are random copolymers; the solubility decreases with increase in the amount of biphenyl incorporation, which was ascribed to the presence of a substantial fraction of longer conjugated segments in such random copolymers. The non-coplanar entities cause truncation of conjugation length in MEHPPV, as evident from their absorption spectra. The copolymers prepared via the Gilch method exhibited a blue shift of about 57 and 20 nm in the absorption and emission, respectively; while the alternating copolymers prepared by Wittig-Horner-Emmons method exhibited a larger blue-shift of about 84 and 54 nm clearly implying a significantly larger reduction in the conjugation length. The copolymers prepared by Wittig-Horner-Emmons method are alternating in nature and therefore leads to a greater reduction in conjugation length; this was evident from the substantially higher blue-shift in the absorption and fluorescence spectra. An interesting feature in the alternating copolymers is the distinct difference in the relative changes in the absorption and emission spectra of the biphenyl and binaphthyl containing copolymers, which appear to suggest in the latter case there is a greater extent of planarization of the excited state. Both these systems provided some useful insights into the various factors that govern the photophysical properties of this class of truncated conjugated polymers. Further examination of this aspect could reveal some other unique features of these copolymers. In the fourth chapter, an approach to prepare copolymer precursors to unsubstituted PPV that holds the potential to control conjugation length is described. The precursor copolymers were prepared by changing the monomer feed ratio of the two monomers, viz. dithiocarbamate (DTC) and xanthate, using the Gilch copolymerization to generate the precursors having varying extents of DTC and xanthate groups. The percentage composition of the precursor copolymers was calculated using 1H NMR and compared with the values calculated from thermogram; the copolymer composition varies linearly with monomers feed suggesting that any desired composition can be readily accessed. Thermogravimetric analysis (TGA) of the precursors, as a preliminary study to examine the possibility of selective elimination of one of these groups to generate conjugated polymers with varying conjugation lengths, demonstrates that a certain level of selectivity in thermal elimination can be achieved because of the distinctly different thermal labilities of the xanthate and DTC groups. These studies clearly suggest that fine-tuning of the thermally eliminatable groups, specifically using xanthate and DTC, could serve as a useful approach to vary the conjugation length of unsubstituted PPVs, which could have important implication in device fabrication. Clearly further work is needed to characterize the selectively eliminated polymers using other spectroscopic methods, such as UV-visible and fluorescence, before device work could be taken up.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / In this work we made a theoretical study of absorption line-shape of poly (pphenylene vinylene) (PPV). The model was applied in films processed by techniques: spin-coating (SC), layer-by-layer (LbL), and Langmuir-Blodgett (LB). The probability of electronic transitions was calculated based on the molecular model of excitons, Franck-Condon states, Gaussian distribution of non-entangled chains with conjugate degree n, semi-empirical parameterization of energy gap, electric dipole moment, and electron-vibrational mode coupling. Based on the approach of the energy gap functional dependence 1/n, the inclusion of the non-homogeneity energy dispersion 1/n2 is essential to obtain good experimental data agreement, mainly, where the absorption spectra display peaks width of about 65 meV. For unresolved absorption spectra, such as those observed for a large number of conjugated polymers processed via spin-coating technique, for example, the non-homogeneity energy dispersion parameterization is not significant. We develop a model to describe the possible chemical reactions of conversion of poly(xylyliden tetrahydrothiophenium chloride) (PTHT) monomers to PPV monomers and PPV monomers in others monomers of different degrees of conjugation. The dependence 1/n of the functional energy gap was compared with experimental data oligomers and calculations optimized geometry by Density Functional Theory (DFT). Effects of exciton confinement were considered in the transition dipole moment functional. Simulations to calculate the oscillator strength of the absorption spectra in PPV oligomers indicate that the HOMO-LUMO state prevails over other excited states. Results obtained from the use of the distribution of the random walk are as good as those achieved by the use of Gaussian distribution of conjugated segments in simulations of absorption spectra of the PPV. The use of transition dipole moment functional, which includes the effect of exciton confinement together with the distribution of the random walk provide good results in adjustments theoretical absorption spectra of PPV with more consistent physical parameters. / Nesse trabalho fizemos um estudo teórico da forma de linha de absorção do poli(p-fenileno vinileno) (PPV). O modelo foi aplicado em filmes processados pelas técnicas: spin-coating (SC), layer-by-layer (LbL) e Langmuir-Blodgett (LB). A probabilidade de transição eletrônica foi calculada com base no modelo do exciton molecular, estados de Franck-Condon, distribuição Gaussiana de cadeias não emaranhadas com grau de conjugação n, parametrização semi-empírica do gap de energia, do momento de dipolo elétrico e do acoplamento eletrônico vibracional. Baseamos na aproximação do funcional do gap de energia de dependência 1/n, a inclusão da dispersão de energia não homogênea 1/n2, que é essencial para se obter uma boa concordância com os dados experimentais, principalmente em casos, onde o espectro de absorção apresenta picos com largura de aproximadamente 65 meV. Em espectros de filmes pouco definidos, tais como os processados pela técnica spin-coating, a dispersão de energia não homogênea não é significativa. Desenvolvemos um modelo para descrever as possíveis reações químicas de conversão de monômeros de poli- (cloreto de xilideno tetrahidrotiofeno) (PTHT) em PPV e de monômeros de PPV em outros de diferentes graus de conjugação. A dependência 1/n do funcional do gap de energia foi comparada com dados experimentais de oligômeros de PPV e cálculos otimizados da geometria pela Teoria do Funcional da Densidade (DFT). Efeitos de confinamento de exciton foram considerados no funcional do momento de dipolo de transição eletrônica. Simulações feitas para calcular a força de oscilador no espectro de absorção em oligômeros de PPV indicam que o estado HOMO-LUMO prevalece sobre outros estados excitados. Resultados obtidos pelo uso da distribuição do passeio aleatório são tão bons quanto aos realizados pelo uso da distribuição Gaussiana de segmentos conjugados em simulações de espectros de absorção do PPV. O uso do funcional do momento de dipolo de transição, que inclui efeito de confinamento de excitons, junto com a distribuição do passeio aleatório propiciam bons resultados nos ajustes teóricos de espectros de absorção do PPV com parâmetros físicos mais consistentes. / Doutor em Física