Browsing by Author "Monkman, Andrew P."

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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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    Dynamics of aggregated states resolved by gated fluorescence in films of room temperature phosphorescent emitters.
    (2019) Santos, Paloma Lays dos; Silveira Júnior, Orlando José; Huang, Rongjuan; Jardim, Guilherme Augusto de Melo; Matos, Matheus Josué de Souza; Silva Júnior, Eufrânio Nunes da; Monkman, Andrew P.; Dias, Fernando B.; Cury, Luiz Alberto
    Phenazine derivative molecules were studied using steady state and time resolved fluorescence techniques and demonstrated to lead to strong formation of aggregated species, identified as dimers by time dependent density functional theory calculations. Blended films in a matrix of Zeonexs, produced at different concentrations, showed different contributions of dimer and monomer emissions in a prompt time frame, e.g. less than 50 ns. In contrast, the phosphorescence (e.g. emission from the triplet state) shows no significant effect on dimer formation, although strong dependence of the phosphorescence intensity on concentration is observed, leading to phosphorescence being quenched at higher concentration.
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    Exciton diffusion in polyfluorene copolymer thin films : kinetics, energy disorder and thermally assisted hopping.
    (2009) Dias, Fernando B.; Kamtekar, Kiran T.; Cazati, Thiago; Williams, Geoff; Bryce, Martin R.; Monkman, Andrew P.
    A series of {(9,9-dioctylfluorene)0.7x-(dibenzothiophene-S,S-di-oxide)0.3-[4,7-bis(2-thienyl)-2,1,3-benzothiadiazole]x}(PFS30-TB Tx),where x represents the minor percentage of the red emitter4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) randomly incor-porated into the copolymer backbon e, is investigated in orderto follow the energy transfer from PFS30to TBT moieties. Theemission of the donor poly[(9 ,9-dioctylfluorene)0.7-(dibenzothio-phene-S,S-dioxide)0.3identified by PFS30and peaking at450 nm, is clearly quenched by the presence of the red TBTchromophore emitting at 612 nm, with an isoemissive pointobserved when the spectra are collected as a function of tem-perature. A plot of the ratio between the TBT and PFS30emis-sions as a function of the reciprocal of temperature gives aclear linear trend between 290 and 200 K, with an activationenergy of 20 meV and showing a turn over to a non-activated egime below 200 K. Picosecond time-resolved fluorescencedecays collected at the PFS30and TBT emission wavelengths,show a decay of the PFS30emission and a fast build-in, fol-lowed by a decay, of the TBT emission, confirming that thepopulation of the TBT excited state occurs during the PFS30lifetime(~600 ps). The population of the TBT excited stateoccurs on a time regime around 150 ps at 290 K, showing anenergy barrier of 20 meV that turns over to a non-activatedregime below 200 K in clear agreement with the steady-statedata. The origin of the activation barrier is attributed to thepresence of physical and energetic disorder, affected by fastthermal fluctuations that dynamically change the energy land-scape and control the exciton migration through the polymerdensity of states.
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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