Browsing by Author "Dias, Fernando B."

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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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    Energy transfer in nanostructured films containing poly(p-phenylene vinylene) and acceptor species.
    (2009) Postacchini, Bruna Bueno; Zucolotto, Valtencir; Dias, Fernando B.; Monkman, Andy; Oliveira Junior, Osvaldo Novais de
    The combination of luminescent polymers and suitable energy-accepting materials may lead to a molecularlevel control of luminescence in nanostructured films. In this study, the properties of layer-by-layer (LbL) films of poly(p-phenylene vinylene) (PPV) were investigated with steady-state and time-resolved fluorescence spectroscopies, where fluorescence quenching was controlled by interposing inert polyelectrolyte layers between the PPV donor and acceptor layers made with either Congo Red (CR) or nickel tetrasulfonated phthalocyanine (NiTsPc). The dynamics of the excited state of PPV was affected by the energy-accepting layers, thus confirming the presence of resonant energy transfer mechanisms. Owing to the layered structured of both energy donor and acceptor units, energy transfer varied with the distance between layers, r, according to 1/rn with n ) 2 or 3, rather than with 1/r6 predicted by the Fo¨rster theory for interacting point dipoles.
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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.