Browsing by Author "Eccher, Juliana"

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    Caracterização de derivados de dimetoxidicinamalacetona, difluroboro flavanonas e quinoxalinas e das interações com Buckminsterfulereno (C60) para aplicação em dispositivos eletrônicos orgânicos.
    (2022) Curcio, Sergio Fernando; Cazati, Thiago; Valaski, Rogério; Cazati, Thiago; Eccher, Juliana; Soares, Jaqueline dos Santos; Postachini, Bruna Bueno; Merlo, Aloir Antônio
    Compostos derivados das quinoxalinas (5e, 5g), difluroboro flavanonas (4c, 4e) e dimetoxidicinamalacetona (AGC101), foram estudados em decorrência de suas propriedades fotofísicas. Para executar o estudo, foram preparados soluções e filmes finos desses compostos com diferentes quantidades de C60, com intuito de verificar o potencial desses materiais como doadores de elétrons em dispositivos eletrônicos orgânicos. Os materiais foram caracterizados por espectroscopia de absorção óptica, fluorescência estacionária e fluorescência resolvida no tempo. Para todos os compostos, os decaimentos do estado excitado mostraram redução do tempo de vida no estado sólido em comparação com os tempos de vida das soluções, indicando assim, agregação das moléculas. A aplicação da teoria de Kasha indica agregação tipo H (empilhamento) para a AGC101 e a 5e e tipo J (lado a lado) para 5g, 4c e 4e. Assumindo que estes compostos se comportam como doadores de elétrons e o C60 como aceitador de elétrons, foi analisada a supressão da fluorescência em função da concentração de C60, tanto em solução como em filme. O estudo dos gráficos de Stern-Volmer das soluções dos compostos mostrou predominância de supressões estáticas (do tipo por esfera efetiva). A mesma análise foi realizada para os filmes e, aplicando a teoria de Förster para a transferência de energia ressonante (FRET), foi interpretado o resultado como distância de difusão excitônica LD, obtendo distâncias de 5,3nm a 7,8nm em acordo com os valores encontrados nos artigos de referência. Os coeficientes de difusão D foram também calculados obtendo valores entre 2,8x10-3 cm 2 /s e 5,4x10-3 cm 2 /s na ordem de grandeza dos encontrados na literatura. As estruturas e valores obtidos permitem afirmar que os materiais estudados são de interesse para a fabricação de dispositivos eletrônicos orgânicos.
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    Delayed fluorescence by triplet−triplet annihilation from columnar liquid crystal films.
    (2022) Franca, Larissa Gomes; Santos, Paloma Lays dos; Pander, Piotr; Cabral, Marília Gabriela Belarmino; Cristiano, Rodrigo; Cazati, Thiago; Monkman, Andrew P.; Bock, Harald; Eccher, Juliana
    Delayed fluorescence (DF) by triplet−triplet annihila- tion (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.
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    High rectification in organic diodes based on liquid crystalline phthalocyanines.
    (2015) Apostol, Petru; Eccher, Juliana; Dotto, Marta Elisa Rosso; Costa, Cassiano Batesttin; Cazati, Thiago; Hillard, Elizabeth A.; Bock, Harald; Bechtold, Ivan Helmuth
    The optical and electrical properties of mesogenic metal-free and metalated phthalocyanines (PCs) with a moderately sized and regioregular alkyl periphery were investigated. In solution, the individualized molecules show fluorescence lifetimes of 4–6 ns in THF. When deposited as solid thin films the materials exhibit significantly shorter fluorescence lifetimes with bi-exponential decay (1.4–1.8 ns; 0.2–0.4 ns) that testify to the formation of aggregates via p–p intermolecular interactions. In diode structures, their pronounced columnar order outbalances the unfavorable planar alignment and leads to excellent rectification behavior. Field-dependent charge carrier mobilities are obtained from the J–V curves in the trap-limited space-charge-limited current regime and demonstrate that the metalated PCs display an improved electrical response with respect to the metal-free homologue. The excited-state lifetime characterization suggest that the p–p intermolecular interactions are stronger for the metal-free PC, confirming that the metallic centre plays an important role in the charge transport inside these materials.
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    Highly luminescent liquid crystals by connecting 1,3,4-oxadiazole with thiazolo[5,4-d]thiazole units.
    (2021) Santos, Arthur Bernardo de Souza; Manfredi, Alex Molina; Salla, Cristian Andrey Momoli; Farias, Giliandro; Girotto, Edivandro; Eccher, Juliana; Westphal, Eduard; Curcio, Sergio Fernando; Cazati, Thiago; Malvestiti, Ivani; Falcão, Eduardo Henrique Lago; Bechtold, Ivan Helmuth; Gallardo, Hugo
    The direct bonding between a thiazolo[5,4-d]thiazole and two 1,3,4-oxadiazole units allowed us to create a new and versatile rigid core for luminescent liquid crystal, which showed interesting and variable mesomorphic and photophysical properties. From the 5-bis(5-phenyl-1,3,4-oxadiazol-2-yl)thiazolo[5,4-d]thiazole new core, three molecules with different number of alkoxy chains were synthesized and had their properties correlated with the molecular structure. The molecule with two chains showed a smectic C mesophase, while the mesogens with four and six chains presented hexagonal columnar mesomorphism, which was confirmed by POM and XRD measurements. In addition, the molecule with six chains presented liquid crystalline behavior close to room temperature. In solution, the molecules presented strong photoluminescence ranging from blue to yellow, with quantum yields higher than 0.6. Excited state lifetimes allowed to correlate the fluorescence component associated to the different emitting species to the molecular organization in spin coated films. The molecular energy levels, together with thermal stability and possible charge carrier transport due to molecular packing, suggest that these molecules are promising for optoelectronic applications. Overall, this work contributes to the development of the use of thiazolo[5,4-d]thiazole in liquid crystals, demonstrating its great efficiency and versatility.
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    Strongly luminescent and liquid-crystalline π-conjugated 2-methyl[1,2,3]benzotriazoles with a linear donor-acceptor-donor structure.
    (2020) Silva, Elias Regis da; Hinojosa, Abad Roger Castillo; Eccher, Juliana; Tonet, Michele Duarte; Brondani, Daniela; Zapp, Eduardo; Curcio, Sergio Fernando; Postacchini, Bruna Bueno; Cazati, Thiago; Vieira, André Alexandre
    Symmetrical 2-methyl[1,2,3]benzotriazole (BZT) derivatives with elongated peripheral units connected via acetylenic triple bonds present calamitic thermotropic mesomorphism with nematic and smectic phases. They show intense photoluminescence with near-unity quantum yields in solution. The peripheral groups significantly influence the excited-state lifetime. Fluorescence quenching is observed in the presence of C60, testifying of charge transfer to the fullerene acceptor. The varying sterical demand of the different substituents considerably influences the efficiency of the charge transfer induced fluorescence quenching. HOMO, LUMO and band gap energies ranged from −5.15 to −5.97 eV (ionization potential), −2.47 to −2.96 eV (electron affinity) and 2.68 to 3.08 eV (optical band gap).
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    Triplet exciplex electroluminescence from two columnar liquid crystal perylene derivatives.
    (2016) Eccher, Juliana; Almeida, Ana Cláudia Batista; Cazati, Thiago; Seggern, Heinzvon; Bock, Harald; Bechtold, Ivan Helmuth
    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 electro luminescence is observed between single-material and two-material devices, assigned as resulting from triplet exciplexes in the electroluminescence of the two- material films, where as singlet exciplex states contribute to the photoluminescence of the mixture and double layer films.