Browsing by Author "Bock, Harald"

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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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    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.