Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-type ladder polymer

Pauck, T.; Hennig, Richard G.; Perner, M.; Lemmer, Uli; Siegner, U.; Mahrt, Rainer F.; Scherf, Ullrich; Müllen, Kläus; Bäßler, H.; Göbel, E. O.

doi:10.1016/0009-2614(95)00867-4

Cited by 73 publications

(31 citation statements)

References 19 publications

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“…Excitation of the aggregate band typically produces a broad, featureless and highly Stokes-shifted emission. Studies of laddertype poly(para-phenylene), [20][21][22] poly(para-pyridyl vinylene), 23,24 and polyfluorene 25 deriviatives show strong evidence for aggregate formation in films. Near-field scanning optical microscopy (NSOM) shows that the optical properties of these films vary from location to location, consistent with the idea of spatially localized aggregate species.…”

Section: Introductionmentioning

confidence: 99%

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

et al. 1999

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The presence of interchain species in the photophysics of conjugated polymer films has been the subject of a great deal of controversy. In this paper, we present strong evidence that interchain species do form in conjugated polymer films, and that the degree of interchain interactions can be controlled by varying the solvent and polymer concentration of the solution from which the films are cast. Thus, much of the controversy in the literature can be resolved by noting that the polymer samples in different studies had different side groups or were prepared in different ways and thus have different degrees of interchain interaction. The photoluminescence (PL) of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene), MEH-PPV, changes both its spectral shape and quantum yield when the films are prepared from different solutions or when the morphology is varied by annealing. Increasing the amount of interchain interactions enhances the red portion of the film's PL, a result assigned to a combination of changes in the vibronic structure of the PL of the exciton and increased numbers of weakly emissive interchain species. Photoluminescence excitation spectroscopy shows that excitation to the red edge of the absorption band preferentially enhances the red emission, suggesting that the interchain species are aggregates with a distinct ground state absorption. Scanning force microscopy shows topographic features that correlate with the degree of interchain interactions, verifying that the morphology of conjugated polymer films changes with polymer concentration, choice of solvent, and spin-casting speed. Even at low excitation intensities, photooxidative damage occurs quickly in MEH-PPV films excited in air, and the rate at which damage occurs is sensitive to the packing of the polymer chains. For samples under vacuum at low excitation intensity, a long-lived emissive tail, in combination with excitedstate absorption dynamics that do not match those of the emissive species, provide direct evidence for the production of interchain aggregates. Annealing an MEH-PPV film produces a photophysical signature similar to photooxidation, implying that defects in conjugated polymer films are intrinsic and depend on the details of how the chains are packed. At higher excitation intensities, we find that exciton-exciton annihilation occurs, and that the probability for annihilation can vary by an order of magnitude depending on the degree of interchain contact in the film. Finally, we show that changing the film morphology has a direct effect on the performance of MEH-PPV-based light-emitting diodes. Higher degrees of interchain interaction enhance the mobility of carriers at the expense of lower quantum efficiencies for electroluminescence. Taken together, the results reconcile much of the contradictory literature and provide a prescription for the optimization of conjugated polymer films for particular device applications.

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Section: Introductionmentioning

confidence: 99%

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

et al. 1999

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“…A way to achieve this is through the introduction and modification of the side chains/ functional group structure, which in turn results in an impact on the optical and transport properties. This is best exemplified with the various LEPs within the poly͑phenylene-vinylene͒ ͑PPV͒ 1-3 and the polyfluorenes [4][5][6] that have been synthesized to date. With structural variations within these two classes of polymers, markedly different transport properties have been observed.…”

Section: Introductionmentioning

confidence: 99%

Polarized optical spectroscopy applied to investigate two poly(phenylene-vinylene) polymers with different side chain structures

Pålsson

Vaughan

Monkman

2006

The Journal of Chemical Physics

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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Two related poly͑phenylene-vinylene͒ ͑PPV͒ light-emitting polymers have been investigated by means of polarized optical spectroscopy. The purpose of the investigation was to investigate the nature of the interactions in thin films and to examine what impact the difference in side chain structure and molecular weight in poly͑2Ј-methoxy-5-2-ethyl-hexoxy͒-1,4-phenylene vinylene ͑MEH-PPV͒ and poly͑2-͑3Ј ,7Ј-dimethyloctyloxy͒-5-methoxy-1,4-phenylene-vinylene͒ ͑OC 1 C 10 -PPV͒ has on the electronic and optical properties of the two polymers. Aligning the polymers by dispersing them in anisotropic solvents and stretched films shows that the side chains have an impact on the relative orientations of the transition dipole moments. In anisotropic solvents the linear dichroism is larger for MEH-PPV than for the related polymer OC 1 C 10 -PPV, while in stretched films the opposite situation prevails. A lower polarization of the luminescence from OC 1 C 10 -PPV, relative to MEH-PPV, was also obtained independent of alignment medium used. The data therefore suggest that while mechanical stretching may align the OC 1 C 10 -PPV to a greater degree, the emitting species is distinct from the absorbing species. The circular dichroism ͑CD͒ spectra of both polymers undergo dramatic changes when the liquid phase and the solid state ͑film͒ are compared. The solution CD spectra shows no evidence of interchain interactions; instead the spectra of both systems indicate a helical conformation of the polymers. The CD spectra of films are dramatically different with the strong Cotton effect being observed. This points to the formation of an aggregate in the film, with an associated ground state interaction, an interchain species such as a physical dimer, or a more complex higher aggregate.

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“…Electroluminescence efficiency of these species is depleted since they generate alternative pathways for light emission due to energy or charge migration to nonradiative states. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] In addition, films processed by different techniques are also morphologically different 22,23 and performance reproducibility requires further studies.…”

Section: Introductionmentioning

confidence: 99%

Solvent and molecular weight effects on fluorescence emission of MEH-PPV

Cossiello¹,

Akcelrud²,

Atvars³

2005

J. Braz. Chem. Soc.

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Nesse trabalho se estudou sistematicamente o efeito de dois bons solventes sobre os espectros de excitação e de fluorescência em condições fotoestacionárias do MEH-PPV com três massas molares diferentes, -M n = 51 kg mol ) e em filmes produzidos por espalhamento de soluções, mostrando diferenças que decorrem da diferente forma de solvatação das cadeias pelos diferentes solventes. Um deslocamento espectral para o vermelho foi observado em concentrações mais altas 10 -6 mol L -1 e pode ser explicado pela regra de Kasha para uma orientação anti-paralela dos momentos de transição dos dois cromóforos. As conformações em solução são parcialmente mantidas no estado sólido alem do fato de que os espectros se deslocam para o vermelho e são atribuídos à formação de agregados. Recozimento dos filmes em temperaturas acima da transição vítrea elimina as conformações mais tensionadas das cadeias, apaga o efeito de memória e leva a espectros de fluorescência mais finos.Here we systematically studied the excitation and the fluorescence steady-state spectroscopy of poly(2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) with three molecular weights, -M n = 51 kg mol -1 , -M n = 86 kg mol -1 and -M n = 125 kg mol -1 , in two equally good solvents and several concentrations, from dilute solutions to solid-state films produced by casting. The appropriateness of the two solvents was established by comparing their solubility parameters and the solubility parameter of the MEH-PPV estimated using the Small, the Van Krevelen and the Hoy models. Thus, chloroform and toluene were chosen. Fluorescence spectra were recorded for solutions in several concentrations (10 -8 mol L -1 to 10 -4 mol L -1 ) and films produced by casting, showing that chloroform and toluene solvate the polymer chain differently. Diluted solutions (10 -8 mol L -1 ) in chloroform exhibit broader fluorescence spectra. A red-shift of the fluorescence spectra was observed for concentrations higher than 10 -6 mol L -1 that can be explained using Kasha's rule for the sandwich anti-parallel orientation of the transition moments of the two chromophores. The conformations observed in solutions are partially retained in the solid films in addition to the broader red-shift spectra attributed to aggregated forms of the macromolecular segments. Annealing of the polymer films at the glass transition temperature eliminates the more stressed conformations, erases the memory and leads to sharper fluorescence spectra.

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Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-type ladder polymer

Cited by 73 publications

References 19 publications

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

Polarized optical spectroscopy applied to investigate two poly(phenylene-vinylene) polymers with different side chain structures

Solvent and molecular weight effects on fluorescence emission of MEH-PPV

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