Browsing by Author "Souza, Bernardo de"

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    New boron(III) blue emitters for all-solution processed OLEDs : molecular design assisted by theoretical modeling.
    (2019) Salla, Cristian Andrey Momoli; Santos, Jéssica Teixeira dos; Farias, Giliandro; Bortoluzzi, Adailton João; Curcio, Sergio Fernando; Cazati, Thiago; Izsák, Róbert; Neese, Frank; Souza, Bernardo de; Bechtold, Ivan Helmuth
    Luminescent boron(III) complexes have recently been employed as emitters in organic light-emitting diodes (OLEDs) with reasonable success. They are easy to prepare and sufficiently stable to be used in such devices, being of great interest as a simple molecular emissive layer. Although emitters for this class with all colors have already been reported, highly efficient and stable blue emitters for applications in solution processed devices still pose a challenge. Here, we report the design, synthesis, and characterization of new boron complexes based on the 2-(benzothiazol-2-yl)phenol ligand (HBT), with different donor and acceptor groups responsible for modulating the emission properties, from blue to red. The molecular design was assisted by calculations using our newly developed formalism, where we demonstrate that the absorption and fluorescence spectra can be successfully predicted, which is a powerful technique to evaluate molecular photophysical properties prior to synthesis. In addition, density functional theory (DFT) enables us to understand the molecular and electronic structure of the molecules in greater detail. The molecules studied here presented fluorescence efficiencies as high as Φ = 0.88 and all solution processed OLEDs were prepared and characterized under an ambient atmosphere, after dispersion in the emitting layer. Surprisingly, even considering these rather simple experimental conditions, the blue emitters displayed superior properties compared to those in the present literature, in particular with respect to the stability of the current efficiency.
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    Reducing lifetime in Cu(I) complexes with thermally activated delayed fluorescence and phosphorescence promoted by chalcogenolate-diimine ligands.
    (2020) Farias, Giliandro; Salla, Cristian Andrey Momoli; Heying, Renata da Silva; Bortoluzzi, Adailton João; Curcio, Sergio Fernando; Cazati, Thiago; Santos, Paloma Lays dos; Monkman, Andrew P.; Souza, Bernardo de; Bechtold, Ivan Helmuth
    Luminescent copper(I) complexes have drawn attention due to their promising performance as alternative optoelectronic materials to the well-known heavy transition metals complexes. Herein, we report the synthesis of six luminescent Cu(I) complexes with phosphines and 1,10-phenanthroline-derived ligands with thiadiazole and selenodiazole groups in order to evaluate the effect of heavy atom on their photophysical properties. Steady-state and time-resolved spectroscopy confirmed delayed fluorescence emission via a thermally activated delayed fluorescence mechanism in all cases. The experimental spectroscopic data was analyzed with detailed quantum-chemical calculations. Interestingly, these complexes did not show the expected “heavy atom effect”, that enhances the spin-orbit coupling matrix elements, but nevertheless the addition of the heavier chalcogens contributed to reduce the photoluminescence lifetime to roughly 800 ns, which is the lowest reported so far for such TADF materials
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    Synthesis of 2,1,3-Benzoxadiazole derivatives as new fluorophores-combined experimental, optical, electro, and theoretical study.
    (2020) Frizon, Tiago Elias Allievi; Vieira, André Alexandre; Silva, Fabrícia Nunes da; Saba, Sumbal; Farias, Giliandro; Souza, Bernardo de; Zapp, Eduardo; Lôpo, Michell N.; Braga, Hugo de Campos; Grillo, Felipe Fardin; Curcio, Sergio Fernando; Cazati, Thiago; Rafique, Jamal
    Herein, we report the synthesis and characterization of fluorophores containing a 2,1,3-benzoxadiazole unit associated with a π-conjugated system (D-π-A-π-D). These new fluorophores in solution exhibited an absorption maximum at around ∼419 nm (visible region), as expected for electronic transitions of the π-π ∗ type (ε ∼2.7 × 107 L mol−1 cm−1 ), and strong solvent-dependent fluorescence emission (ΦFL ∼0.5) located in the bluish-green region. The Stokes’ shift of these compounds is ca. 3,779 cm−1 , which was attributed to an intramolecular charge transfer (ICT) state. In CHCl3 solution, the compounds exhibited longer and shorter lifetimes, which was attributed to the emission of monomeric and aggregated molecules, respectively. Density functional theory was used to model the electronic structure of the compounds 9a–d in their excited and ground electronic states. The simulated emission spectra are consistent with the experimental results, with different solvents leading to a shift in the emission peak and the attribution of a π-π ∗ state with the characteristics of a charge transfer excitation. The thermal properties were analyzed by thermogravimetric analysis, and a high maximum degradation rate occurred at around 300◦C. Electrochemical studies were also performed in order to determine the band gaps of the molecules. The electrochemical band gaps (2.48–2.70 eV) showed strong correlations with the optical band gaps (2.64–2.67 eV).