Estudo de mesofases líquido-cristalinas através de processamento digital de texturas ópticas

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Física, Florianópolis, 2010Os cristais líquidos são substâncias que podem exibir uma grande ariedade de texturas em cada uma das suas mesofases. A identificação das mesofases é essencial para a caracterização destes materiais, onde a microscopia óptica de luz polarizada (MOLP) é uma técnica largamente utilizada para este propósito. Como esta técnica requer apenas habilidades humanas, algumas transições relacionadas com mudanças nas texturas, podem ser de difícil percepção a olho nu. Entretanto, mudanças sutis podem ser percebidas através do processamento digital das imagens das texturas. Neste trabalho utilizamos um método computacional de processamento digital das texturas ópticas exibidas pelas mesofases líquido-cristalinas para identificar as transições de fase e determinar as temperaturas de transição. O conjunto de texturas que constitui uma certa região de variação de temperatura, na qual ocorrem as transições de fase, é processado. A partir deste processamento, nós obtemos o desvio padrão das tonalidades da imagem, onde o aparecimento de descontinuidades como uma função da temperatura está relacionado a uma transição de fase, consequência de mudanças nas texturas. Nós investigamos compostos que exibiram mesofases características de cristais líquidos termotrópicos e os resultados foram comparados com os resultados obtidos através das análises de calorimetria diferencial de varredura (DSC). A organização estrutural das mesofases foi confirmada através das medidas de difração de raios-x (DRX). Este estudo possibilitou determinar com precisão as temperaturas de transição de fase. Em especial, o método se mostrou bastante eficiente nas transições que envolvem entalpia reduzida, as quais são difíceis de serem identificadas pela técnica de DSC. Em um segundo momento, investigamos as texturas das mesofases através de outro método computacional que calcula o desvio padrão das tonalidades, considerando conceitos de leis de escala. O objetivo deste estudo foi identificar as mesofases líquido-cristalinas pelo padrão que a mesofase exibe.Liquid crystals are substances that can display a wide variety of textures in each of their mesophases. The identification of the mesophases is essential for the characterization of these materials, where the polarized light microscopy (MOLP) is a technique widely used for this purpose. Since this technique requires only human abilities, in some cases changes in the texture are not perceptible with the naked eye. However, subtle changes can be perceived with computacional image processing of the image textures. In this work we used a computational method of digital processing of optical textures exhibited by the liquid-crystalline mesophases to identify phase transitions and determine the transition temperatures. The set of textures that include a certain region of temperature where the phase transitions occur is processed and from the calculations we obtain the mean square deviation of the image tones, where the appearance of discontinuities as a function of the temperature is related to a phase transition, consequence of changes in the textures. We investigated compounds that exhibit characteristic mesophases of thermotropic liquid crystals and the results were compared with results obtained from analysis of differential scanning calorimetry (DSC). The structural organization of the mesophases was confirmed with x-ray diffraction experiments (XRD). This study enabled us to determine precisely the phase transition temperatures. The method becomes especially efficient in the identification of phase transitions with reduced enthalpy, which are difficult to be identified with the DSC technique. In a second moment, we investigated the textures of mesophases using another computacional method that calculates the mean square deviation of the image tones by considering scaling concepts. Our purpose was the identification of the liquid crystal mesophases by the pattern that the mesophase exhibes

Cristais líquidos discóticos como semicondutores orgânicos para aplicações eletrônicas

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Física, Florianópolis, 2014Nota: Prêmio UFSC de tese 2015Os cristais líquidos (CLs) têm sido reconhecidos recentemente como promissores semicondutores para aplicações na área de eletrônica orgânica. O presente trabalho apresenta a proposta de caracterizar e investigar a potencialidade de novos materiais orgânicos com propriedades líquido-cristalinas para aplicações em dispositivos eletrônicos. Moléculas discóticas constituídas de um centro rígido aromático e cadeias laterais flexíveis vêm sendo amplamente estudada se aplicadas em dispositivos orgânicos tais como os diodos emissores de luz (OLEDs), os transistores de efeito de campo (OFETs) e os fotovoltaicos. Os CLs aliam a organização molecular dos sistemas cristalinos, necessária ao trânsito das cargas, à fluidez de um líquido, facilitando o processamento do material. Eles podem ser processados tanto a partir de solução quanto por evaporação térmica. A principal vantagem na utilização destes materiais é a possibilidade de modificar e controlar a orientação molecular através de estímulos externos, como por exemplo, pela ação de temperatura, aplicação de campos elétrico ou magnético e através de tratamentos de superfície. Com isso pode-se otimizar as suas propriedades ópticas e elétricas. Os CLs discóticos estudados neste trabalho são derivados do centro aromático perileno. Os materiais com o centro perileno vêm sendo empregados como semicondutores tipo-n e têm apresentado elevada mobilidade de carga quando aplicados em dispositivos. Neste trabalho as propriedades mesomórficas, fotofísicas, eletroquímicas e elétricas dos compostos foram investigadas. O comportamento termotrópico dos compostos foi analisado por microscopia óptica de luz polarizada(MOLP), calorimetria diferencial de varredura (DSC) e difração de raio x (DRX). As propriedades fotofísicas foram investigadas através de espectroscopia UV-Vis e espectroscopia de fluorescência. A caracterização morfológica dos filmes finos produzidos pela técnica despin-coating e evaporação térmica foi realizada usando um microscópio de força atômica (AFM). Os níveis de energia LUMO dos compostos foram estimados através da técnica de voltametria cíclica. As propriedades fotofísicas em função da temperatura mostraram uma significativa supressão da fotoluminescência na mesofase colunar causada pela forte agregação molecular do empacotamento p-stacking. As propriedades elétricas foram investigadas através da deposição dos filmes em uma típica estrutura de diodo, ITO/PEDOT:PSS/CL/Ca/Al. A mobilidade de carga foi estudada através da aplicação de um modelo teórico às curvas experimentais de densidade de corrente em função da voltagem aplicada (J/V) e comparada com a mobilidade eletrônica obtida através das técnicas de tempo de voo (TOF) e Foto-CELIV. O alinhamento homeotrópico induzido por tratamento térmico para o composto com o centro perileno diimida resultou em propriedades elétricas aprimoradas, onde foi observado um ganho de quatro ordens de grandeza para a densidade de corrente e cinco ordens de grandeza para a mobilidade. O filme fino produzido pelo processo de evaporação térmica deste composto também apresentou propriedades elétricas notáveis na estrutura de diodo, sendo superior ao desempenho do filme produzido por spin-coating antes do alinhamento. A mobilidade para o filme evaporado foi medida através da estrutura de um transistor e mostrou boa concordância com a mobilidade obtida a partir do modelo aplicado às curvas J/V. A mobilidade para o filme evaporado foi superior a mobilidade encontrada para o filme spin-coating antes do alinhamento, mas inferior a mobilidade exibida pelo filme spin-coating alinhado homeotropicamente na estrutura de diodo. Os dispositivos produzidos a partir da combinação dos dois CLs, em heterojunções de bicamada e de volume, apresentaram um melhor desempenho do que os dispositivos fabricados para os compostos individualmente.Abstract: Liquid crystals (LCs) have been recently recognized as promising semiconductors for applications in the field of organic electronics. This work presents a proposal to characterize and investigate the potential application of new organic materials with liquid-crystalline properties inelectronic devices. Discotic molecules comprising a rigid aromatic core and flexible side chains have been widely studied and applied to organic devices such as light emitting diodes (OLEDs), field effect transistors (OFETs) and photovoltaics. LCs combine the molecular organization of the crystalline systems necessary to the transit of the charge and the fluidity of a liquid, facilitating processing of the material. They can be processed either from solution or by thermal evaporation. The main advantage of using these materials is the possibility of modifying and controlling the molecular orientation by external stimuli, for example, by the action of temperature, applying electric or magnetic fields and through surface treatments. Being possible to optimize their optical and electrical properties.The discotic LCs studied in this work are derivatives of the perylene aromatic core. Materials with perylene core have been used as n-type semiconductors and have shown high mobility of charge when applied to devices. In this work the mesomorphic, photophysical, electrochemical and electrical properties of the compounds were investigated. The thermotropic behavior of the compounds was analyzed by optical polarized light microscopy (MOLP), differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The photophysical properties were investigated by UV-Vis and fluorescence spectroscopy. Morphological characterization of the thin films produced by the technique of spin-coating and thermal evaporation was performed using an atomic force microscope (AFM). The LUMO energy levels of the compounds were estimated using the technique of cyclic voltammetry. The photophysical properties as a function of temperature showed a significant suppression of photoluminescence in the columnar mesophase due to the molecular aggregates caused by the strong p-stacking interactions. The electrical properties were investigated by depositing the films in a typical diode structure, ITO/PEDOT:PSS/CL/Ca/Al. The charge mobility was investigated by applying a theoretical model to experimental curves of the current density as a function of applied voltage (J/V) and compared with the electron mobility obtained by the techniques of time of flight (TOF) and Photo-CELIV. The homeotropic alignment induced by annealing for the compound perylene diimide resulted in improved electrical properties, where a gain of four orders of magnitude for the current density and five orders of magnitude in the mobility was observed. The thin film produced by thermal evaporation process of this compound also showed remarkable electrical properties in a diode structure. It was superior to the performance of the film produced by spin-coating before alignment. Mobility for the evaporated film was measured in a transistor structure and showed good agreement with the mobility obtained from the model applied to curves J/V. The mobility for the evaporated film was higher than the mobility for the spin-coating before alignment, but it was lower than the mobility exhibited by the spin-coating film after annealing in a diode structure. The devices produced from the combination of the two LCs in bilayer and bulk heterojunctions showed better performance than devices manufactured for the compounds individually

Delayed Fluorescence by Triplet–Triplet Annihilation from Columnar Liquid Crystal Films

Delayed fluorescence (DF) by triplet–triplet annihilation (TTA) is observed in solutions of a benzoperylene-imidoester mesogen that shows a hexagonal columnar mesophase at room temperature in the neat state. A similar benzoperylene-imide with a slightly smaller HOMO–LUMO gap, that also is hexagonal columnar liquid crystalline at room temperature, does not show DF in solution, and mixtures of the two mesogens show no DF in solution either, because of collisional quenching of the excited triplet states on the imidoester by the imide. In contrast, DF by TTA from the imide but not from the imidoester is observed in condensed films of such mixtures, even though neat films of either single material are not displaying DF. In contrast to the DF from the monomeric imidoester in solution, DF of the imide occurs from dimeric aggregates in the blend films, assisted by the imidoester. Thus, the close contact of intimately stacked molecules of the two different species in the columnar mesophase leads to a unique mesophase-assisted aggregate DF. This constitutes the first observation of DF by TTA from the columnar liquid crystalline state. If the imide is dispersed in films of polybromostyrene, which provides an external heavy-atom effect facilitating triplet formation, DF is also observed. Organic light-emitting diodes (OLEDs) devices incorporating these liquid crystal molecules demonstrated high external quantum efficiency (EQE). On the basis of the literature and to the best of our knowledge, the EQE reported is the highest among nondoped solution-processed OLED devices using a columnar liquid crystal molecule as the emitting layer

Síntese e caracterização de copolímeros de cadeia lateral derivados de acrilatos de 4,5-di-hidroisoxazol e do (-)-mentol

Five monomers 5-[4-(5-cyano-4,5-dihydroisoxazol-3-yl)phenoxy]undecyl acrylate (7a); n-alkyl 3-{4-[5-(acryloyloxyundecyl)oxyphenyl]}-4,5-dihydroisoxazole-5-carboxylate (7b,c for n-butyl and n-hexyl, respectively); 3-{4-[5-(acryloyloxyundecyl) oxyphenyl]}-4,5-dihydroisoxazole-5-carboxylic acid (7d) and (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl acrylate (9) and the corresponding copolymers 10a-d,11 and homopolymers 12 from 7a and 13 from 9 were designed and synthesized. Except for acrylate 9 which is derived from (-)-menthol, all of the monomers belong to the series containing the isoxazoline ring linked to the acrylate unit by a flexible spacer chain of eleven methylene units. They presented low glass temperature and despite birefringence behavior, these copolymers showed no mesomorphic properties

An unusual plank-shaped nematogen with a graphene nanoribbon core

A [12]phenacene exclusively decorated with four lateral hexylester substituents self-assembles into a nematic liquid crystal glass on cooling after melting at high temperature. This uniaxial nematic organization of a plank-shaped nanographene is unprecedented and in strong contrast to the common design rules for liquid crystals. Highly birefringent samples emitting polarized fluorescence can be obtained in homogeneously planar or twisted waveguiding configurations that are stable against crystallization at and below room temperature and up to 100 °C

The 2:1 cycloadducts from [3+2] 1,3-dipolar cycloaddition of nitrile oxide and vinylacetic acid. Synthesis and liquid crystal behaviour

Four liquid crystals (LC) 3,7a-bis(4-alkyloxyphenyl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,4]oxadiazol-6-yl)acetic acid (7a-d) were synthesised and the mesomorphic behaviour reported. The LCs were characterised as 2: 1 bisadducts, which were obtained from a double [3+2] 1,3-dipolar cycloaddition. In the first step, the cycloaddition of 4-alkyloxyphenylnitrile oxide (4a-d) and vinylacetic acid (5) gave the initial unobserved 1:1 cycloadducts 2-[3-(4-alkyloxyphenyl)-4,5-dihydroisoxazol-5-yl]acetic acid (6a-d). In the second step, the addition of a second equivalent of 4 to 6 yielded the 2: 1 bisadducts 7a-d without any traces of 6. All compounds 7a-d were unstable during the transition from the mesophase to the isotropic state upon first heating as evidenced by the large peaks in the differential scanning calorimetry traces. Due to the chemical instability of the compounds upon heating, the transition temperature related to the smectic C to smectic A transitions was acquired by means of an image processing method. X-Ray diffraction experiments were also used to analyse the liquid-crystalline phases. A theoretical calculation was performed using density functional theory (DFT) methods at the PBE1PBE/6-311+G(2d,p) level (with solvent effect) in order to get information about the energetic profile of the 2: 1 cycloaddition. DFT studies revealed that the cycloaddition process is controlled by the HOMO(dipolarophile) - LUMO(1,3-dipole), and that the double [3+2] 1,3-dipolar cycloaddition reaction is quite possible.CNPq [471194-2008-5]CAPESINCT-Catalise[PROCAD-2007/CAPES

Síntese e caracterização de copolímeros de cadeia lateral derivados de acrilatos de 4,5-di-hidroisoxazol e do (-)-mentol

Five monomers 5-[4-(5-cyano-4,5-dihydroisoxazol-3-yl)phenoxy]undecyl acrylate (7a); n-alkyl 3-{4-[5-(acryloyloxyundecyl) oxyphenyl]}-4,5-dihydroisoxazole-5-carboxylate (7b,c for n-butyl and n-hexyl, respectively); 3-{4-[5-(acryloyloxyundecyl) oxyphenyl]}-4,5-dihydroisoxazole-5-carboxylic acid (7d) and (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl acrylate (9) and the corresponding copolymers 10a-d, 11 and homopolymers 12 from 7a and 13 from 9 were designed and synthesized. Except for acrylate 9 which is derived from (-)-menthol, all of the monomers belong to the series containing the isoxazoline ring linked to the acrylate unit by a flexible spacer chain of eleven methylene units. They presented low glass temperature and despite birefringence behavior, these copolymers showed no mesomorphic properties

Triplet exciplex electroluminescence from two columnar liquid crystal perylene derivatives

Double layers and mixed single layers of two electronically complementary columnar liquid crystals show enhanced performances in light emitting diodes compared to devices made from single layers of only one of the materials. A shift from green to red electroluminescence is observed between single-material and two-material devices, assigned as resulting from triplet exciplexes in the electroluminescence of the two material films, whereas singlet exciplex states contribute to the photoluminescence of the mixture and double layer films. (C) 2016 Elsevier B.V. All rights reserved

Thermal Evaporation versus Spin-Coating: Electrical Performance in Columnar Liquid Crystal OLEDs

The electrical responses of a columnar liquid crystal (a diimidodiester derivative of benzo[ghi]perylene) depositedeither by spin-coating or by thermal evaporation into a typical OLED device are compared. For the spin-coated film, homeotropic alignment was induced by thermal annealing, which enhanced the charge carrier mobility significantly. For the evaporated films, homeotropic alignment could not be obtained by annealing. However, a degree of rectification higher than 3 orders of magnitude was achieved, even without annealing, with an electrical response similar to the response of the aligned spin-coated film. A trap-limited space−charge-limited current model was used to extract the charge carrier mobility directly from the current−voltage curves. Grazing incidence wide-angle X-ray scattering confirmed the homeotropic alignment of the annealed spin-coated film, whereas the columns are mostly oriented parallel to the surface in the evaporated case. In a field-effect transistor with bottom-gate bottom-contact geometry, the evaporated film exhibited a typical behavior of an n-type transistor. The degree of intermolecular order is thereby strongly dependent on the deposition method where vacuum deposition leads to a higher order. This higher order, however, impedes reorientation by annealing of the evaporated film but leads to improved charge transport between the electrodes even without homeotropic alignment of columnar liquid crystal