Spatially and temporally resolved emission from aggregates in conjugated polymers

Blatchford, J. W.; Gustafson, Terry L.; Epstein, Arthur J.; Bout, David A. Vanden; Kerimo, Josef; Higgins, Daniel A.; Barbara, Paul F.; Fu, De Liang; Swager, Timothy M.; MacDiarmid, Alan G.

doi:10.1103/physrevb.54.r3683

Cited by 119 publications

(105 citation statements)

References 24 publications

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“…[6][7][8] The presence of interchain electronic species has important implications for the performance of devices based on these materials: interchain species may be responsible for quenching a conjugated polymer's luminescence but also may be beneficial for promoting charge transport. [9][10][11] There is also debate concerning the physical nature of interchain electronic species, which are often referred to in the literature as "excimers", 8,[12][13][14] "aggregrates", 9,[15][16][17][18][19][20] or "polaron pairs". [6][7][8][21][22][23] The way that we distinguish among these labels is to use "excimer" to denote a neutral excitation shared equally between two or more chromophores in the electronic excited state and "aggregate" to imply delocalization of the wave function over multiple chain segments in both the ground and excited states.…”

Section: Introductionmentioning

confidence: 99%

“…6,8,21 In films of some conjugated polymers, the presence of a weak absorption band that is red-shifted from that of a single chain provides evidence that interchain species (aggregates) also can form in the ground electronic state. 9,16,18,28 While all of these studies have provided some understanding regarding the dynamics of interchain species and their effects on luminescence quantum yields, there is still relatively little known concerning the electronic nature of the intermolecular excited state, and which of the various interchain labels, if any, is most appropriate.…”

Section: Introductionmentioning

confidence: 99%

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The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

Schaller¹,

Lee²,

Johnson³

et al. 2002

J. Phys. Chem. B

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The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is "excimer-like", exhibiting an ∼4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 µm in diameter), however, the interchain excited state exhibits a large (∼9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

Schaller¹,

Lee²,

Johnson³

et al. 2002

J. Phys. Chem. B

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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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“…Being a large class of fluorescent pigment, cyanine dye has been used extensively in photosensitization, nonlinear optics, biological fluorescent detection, biomedicine and photodynamic therapy of tumor [41][42][43][44][45]. It is reported that cyanine dye MTC, 3,3′-di(3-sulfo-propyl)-4,5,4′,5′-dibenzo-9-methylthiacarbocyanine triethylammonium salt ( as shown in Figure 2(a)), is an excellent G-quadruplex ligand and could present significant spectral changes and strong fluorescent enhancement when binding to specific G-quadruplex [46].…”

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confidence: 99%

Roles of flanking sequences in the binding between unimolecular parallel-stranded G-quadruplexes and ligands

et al. 2013

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G-quadruplexes attract more and more attention in recent years. Numerous small molecules which can induce or stabilize the formation of G-quadruplexes have been investigated on the purpose of anticancer drug development. As a motif existed in physiological condition, flanking sequences are an important part of G-quadruplexes but the study on the impact of flanking sequences on (G-quadruplex)-ligand binding is rarely reported. In this paper, the effects of flanking sequences on binding affinity between a series of unimolecular parallel-stranded G-quadruplex sequences derived from c-myc oncogene promoter (termed as c-myc G-quadruplexes) and their ligands are discussed in detail. The results showed that the flanking sequences on c-myc G-quadruplexes play key roles in (G-quadruplex)-ligand interaction. When a c-myc G-quadruplex is bound to its ligands, the flanking sequences might form a binding cavity above the terminal G-quartet, which could provide a suitable site for ligands to dock in. Moreover, the bases on flanking sequences could interact with ligand through - stacking, and finally form a sandwich-stacking mode (terminal G-quartet, ligand and bases on the flanking sequence). This mode could stabilize the (G-quadruplex)-ligand complex effectively and enhance the binding affinity dramatically. However, flanking sequences are also found to exhibit steric hindrance effect which could impede the (G-quadruplex)-ligand binding. G-quadruplex

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Spatially and temporally resolved emission from aggregates in conjugated polymers

Cited by 119 publications

References 24 publications

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

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

Roles of flanking sequences in the binding between unimolecular parallel-stranded G-quadruplexes and ligands

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