Browsing by Author "Ramos, Rodrigo"

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    Polarized emission from stretched PPV films viewed at the molecular level.
    (2015) Ramos, Rodrigo; Savedra, Melissa Fabíola Siqueira; Cazati, Thiago; Faria, Roberto Mendonça; Caldas, Marília J.
    We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PPV), non-stretched and uniaxially stretched. The experimental study was carried out using linear polarized light as the excitation beam, oriented either parallel or perpendicular to the stretch axis (S). The results showed that when the excitation light source has polarization perpendicularly oriented to the stretch direction, the emitted PL presents maximum intensity in the orientation S, and a minimum in the direction orthogonal to S. In order to understand this interesting phenomenon, we employ theoretical simulations at the atomistic level. We use classical molecular dynamics to simulate amorphous PPV films, non-stretched and stretched, from which we find a tendency of overall alignment of PV units to S, and of local clustering in herring-bone and p-stacking partial symmetries. Our study of optical activity of these kinds of clusters, performed through a quantum semi-empirical method, allows us to explain this polarization conversion behavior, and indicates the possibility of using underivatized PPV as the active layer for polarized electroluminescent devices.
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    Structural dependence of MEH-PPV chromism in solution.
    (2017) Magalhães, Carlos Eduardo Tavares de; Savedra, Ranylson Marcello Leal; Dias, Karina da Silva; Ramos, Rodrigo; Savedra, Melissa Fabíola Siqueira
    The chromism observed in the MEH-PPV polymer in tetrahydrofuran (THF) solution is discussed as a function of the structural morphology of the backbone chains. To evaluate this phenomenon, we carried out simulations employing a hybrid methodology using molecular dynamics and quantum mechanical approaches. Our results support the hypothesis that the morphological order– disorder transition is related to the change from red to blue phase observed experimentally. The morphological disorder is associated with total or partial twisted arrangements in the polymer backbone, which induces an electronic conjugation length more confined to shorter segments. In addition, the main band of the MEH-PPV UV–Vis spectrum at the lower wavelength is related to the blue phase, in contrast to the red phase found for the more planar backbone chains.
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    Structural dependence of photochromism in MEH PPV solutions.
    (2015) Savedra, Ranylson Marcello Leal; Savedra, Melissa Fabíola Siqueira; Ramos, Rodrigo