Spatially and temporally resolved emission from aggregates in conjugated polymers

Aggregation and Gelation of Aromatic Polyamides with Parallel and Anti-parallel Alignment of Molecular Dipole Along the Backbone

The understanding of macromolecular structures and interactions is important but difficult, due to the facts that a macromolecules are of versatile conformations and aggregate states, which vary with environmental conditions and histories. In this work two polyamides with parallel or anti-parallel dipoles along the linear backbone, named as ABAB (parallel) and AABB (anti-parallel) have been studied. By using a combination of methods, the phase behaviors of the polymers during the aggregate and gelation, i.e., the forming or dissociation processes of nuclei and fibril, cluster of fibrils, and cluster-cluster aggregation have been revealed. Such abundant phase behaviors are dominated by the inter-chain interactions, including dispersion, polarity and hydrogen bonding, and correlatd with the solubility parameters of solvents, the temperature, and the polymer concentration. The results of X-ray diffraction and fast-mode dielectric relaxation indicate that AABB possesses more rigid conformation than ABAB, and because of that AABB aggregates are of long fibers while ABAB is of hairy fibril clusters, the gelation concentration in toluene is 1 w/v% for AABB, lower than the 3 w/v% for ABAB.

Operating organic light-emitting diodes imaged by super-resolution spectroscopy

Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semiconducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packed chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. This points the way towards real-time analysis of materials design principles in devices as they actually operate.

There is a need to characterize devices during operation in real-time and at nanoscopic length scales. Here, King et al. perform electroluminescence-STED imaging with a polymer based light-emitting diode, revealing nanoscopic defects that would be unresolvable with traditional optical microscopy.

Recent advances in conjugated polymers for light emitting devices

A recent advance in the field of light emitting polymers has been the discovery of electroluminescent conjugated polymers, that is, kind of fluorescent polymers that emit light when excited by the flow of an electric current. These new generation fluorescent materials may now challenge the domination by inorganic semiconductor materials of the commercial market in light-emitting devices such as light-emitting diodes (LED) and polymer laser devices. This review provides information on unique properties of conjugated polymers and how they have been optimized to generate these properties. The review is organized in three sections focusing on the major advances in light emitting materials, recent literature survey and understanding the desirable properties as well as modern solid state lighting and displays. Recently, developed conjugated polymers are also functioning as roll-up displays for computers and mobile phones, flexible solar panels for power portable equipment as well as organic light emitting diodes in displays, in which television screens, luminous traffic, information signs, and light-emitting wallpaper in homes are also expected to broaden the use of conjugated polymers as light emitting polymers. The purpose of this review paper is to examine conjugated polymers in light emitting diodes (LEDs) in addition to organic solid state laser. Furthermore, since conjugated polymers have been approved as light-emitting organic materials similar to inorganic semiconductors, it is clear to motivate these organic light-emitting devices (OLEDs) and organic lasers for modern lighting in terms of energy saving ability. In addition, future aspects of conjugated polymers in LEDs were also highlighted in this review.

Structural control of the photoluminescence of silole regioisomers and their utility as sensitive regiodiscriminating chemosensors and efficient electroluminescent materials

We synthesized a group of silole regioisomers 1(x,y), whose photoluminescence varied dramatically with its regiostructure. By internally hindering the intramolecular rotation, we succeeded in creating a novel silole (1(3,4)) that is strongly luminescent in solutions and whose fluorescence quantum yield in acetone is as high as 83%. We revealed that 1(3,4) was a sensitive chemosensor capable of optically discriminating nitroaromatic regioisomers of p-, o-, and m-nitroanilines. Against general belief, crystal formation of 1(2,4) blue-shifted its emission color and boosted its emission efficiency. The light-emitting diode based on the crystal of 1(2,4) emitted a strong blue light (464 nm) in a high current efficiency (5.86 cd/A).

Near-field microscopy and lithography of light-emitting polymers

We describe the application of scanning near-field optical microscopy (SNOM) to the study of the photophysical and self-organization properties of thin films of blends of conjugated polymers, and also to the lateral nanoscale patterning of conjugated-polymer structures. Such thin-film plastic semiconductor nanostructures offer significant potential for use in opto-electronic devices. The implementation of SNOM we employ is the most established form in which a probe with a sub-wavelength aperture is scanned in close proximity to the sample surface. We consider the nature of the near-field optical distribution, which decays within the first ca. 100 nm of these semiconductor materials, and address the identification of topographic artefacts in near-field optical images. While the topographic information obtained simultaneously with optical data in any SNOM experiment enables an easy comparison with the higher-resolution tapping-mode atomic force microscopy, the spectroscopic contrast provided by fluorescence SNOM gives an unambiguous chemical identification of the different phases in a conjugated-polymer blend. Both fluorescence and photoconductivity SNOM indicate that intermixing of constituent polymers in a blend, or nanoscale phase separation, is responsible for the high efficiency of devices employing these materials as their active layer. We also demonstrate a scheme for nano-optical lithography with SNOM of conjugated-polymer structures, which has been employed successfully for the fabrication of poly(-phenylene vinylene) nanostructures with 160 nm feature sizes.

Supramolecular assembly of poly(phenylene vinylene) with crown ether substituents to form nanoribbons

Poly(para-phenylene vinylene) with crown ether substituents (C-PPV) could form nanoribbons through supramolecular assembly with K(+) in dilute chloroform solution. The length of the nanoribbons increased with an increase in the standing time of the C-PPV/K(+) solution. Experimental evidence to support the interaction between K(+) and crown ether substituents was provided, and the growth mechanism of C-PPV/K(+) nanorods to nanoribbons was proposed. The influence of the length of the nanoribbons on the photophysics of C-PPV was also investigated.

Conjugated polymers as molecular materials: how chain conformation and film morphology influence energy transfer and interchain interactions

The electronic structure of conjugated polymers is of current interest because of the wide range of potential applications for such materials in optoelectronic devices. It is increasingly clear that the electronic properties of conjugated polymers depend sensitively on the physical conformation of the polymer chains and the way the chains pack together in films. This article reviews the evidence that interchain electronic species do form in conjugated polymer films, and that their number and chemical nature depend on processing conditions; the chain conformation, degree of interchain contact, and rate of energy transfer can be controlled by factors such as choice of solvent, polymer concentration, thermal annealing, presence of electrically charged side groups, and encapsulation of the polymer chains in mesoporous silica. Taken together, the results reconcile many contradictions in the literature and provide a prescription for the optimization of conjugated polymer film morphology for device applications.

Structural control in thin layers of poly(p-phenyleneethynylene)s: photophysical studies of Langmuir and Langmuir-Blodgett films

We present the relationship between the spatial arrangement and the photophysical properties of fluorescent polymers in thin films with controlled structures. Eight surfactant poly(p-phenyleneethynylene)s were designed and studied. These detailed studies of the behavior of the polymers at the air-water interface, and of the photophysical properties of their transferred LB films, revealed key structure-property relationships. Some of the polymers displayed pi-aggregates that are characteristic of an edge-on structure at the air-water interface. Monolayer LB films of these polymers showed greatly reduced quantum yields relative to solution values. Other polymers exhibited a highly emissive face-on structure at the air-water interface, and did not form pi-aggregates. The combination of pressure-area isotherms and the surface pressure dependent in situ UV-vis spectra of the polymers at the air-water interface revealed different behavioral details. In addition, the UV-vis spectra, fluorescence spectra, and quantum yields of the LB films provide design principles for making highly emissive films.

Efficient light harvesting by sequential energy transfer across aggregates in polymers of finite conjugational segments with short aliphatic linkages

Interactions between lumophores have a critical influence on the photophysical properties of conjugated polymers. We synthesized a new series of light-harvesting polymers (poly-DSBs, I-IV) of dialkyloxy- or dialkyl-substituted distyrylbenzene (the substituents being methoxy, 2-ethylhexyloxy, and cyclohexyl) with short aliphatic linkage (methylene or ethylene) and examined the effects of interactions between lumophores and of chemical structures on the absorption, emission, and excitation spectra. The proximity between distyrylbenzene lumophores was shown to be critical to the interactions between lumophores and to the energy-transfer processes. In concentrated solutions and solid films, intermolecular aggregates exist resulting from different extents of interactions between lumophores and are found to involve at least three species: loose, compact, and the most aligned aggregates as observed by photoluminescence and excitation spectroscopies. We also found, for the first time, sequential energy transfer from individual lumophores to the most compact, aligned aggregates via the looser intermolecular aggregates, as observed directly by time-resolved fluorescence spectroscopy. Such a process mimics energy transfer in photosynthesis units and is so efficient such that the fluorescence color can be red-shifted drastically by the presence of comparatively few aggregates and that the light evolved from concentrated solutions and films of poly-DSBs I-IV is entirely or almost the aggregation emission. Although the sequential energy-transfer process in fully conjugated electro-/photoluminescent polymers due to inhomogenity other than distributed conjugation lengths has never been directly observed at room temperature, we suggest that events similar to those observed in poly-DSBs in conjugated polymers could occur but on a much shorter time scale, i.e., a few picoseconds.

Imaging phase-separated domains in conducting polymer blend films with near-field scanning optical microscopy

We present high-resolution images with near-field scanning optical microscopy to study phase separation in polymer films of poly(styrene) and poly(3-octyl-thiophene). Transmission and transmitted fluorescence near-field scanning optical microscope images were taken for direct visualization of the intermediate steps of phase separation in a regime where small domain sizes prevent investigation by conventional microscopy. The interpretation of near-field data on samples with large or varying film thickness or topography are also discussed, and a method for recognizing topographically induced artifacts in a quantitative way is suggested.

Unmasking electronic energy transfer of conjugated polymers by suppression of O(2) quenching

The photochemistry of poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) has been found to be highly dependent on the presence of O(2), which increases singlet exciton quenching dramatically. Spectroscopy on isolated single molecules of MEH-PPV in polycarbonate films that exclude O(2) reveals two distinct polymer conformations with fluorescence maxima near 555 and 580 nanometers wavelength, respectively. Time-resolved single-molecule data demonstrate that the 580-nanometer conformation exhibits a "landscape" for intramolecular electronic energy relaxation with a "funnel" that contains a 580-nanometer singlet exciton trap at the bottom. The exciton traps can be converted to exciton quenchers by reaction with O(2). Conformationally induced, directed-energy transfer is arguably a critical dynamical process that is responsible for many of the distinctive photophysical properties of conjugated polymers.

Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations

The optical, electronic and mechanical properties of synthetic and biological materials consisting of polymer chains depend sensitively on the conformation adopted by these chains. The range of conformations available to such systems has accordingly been of intense fundamental as well as practical interest, and distinct conformational classes have been predicted, depending on the stiffness of the polymer chains and the strength of attractive interactions between segments within a chain. For example, flexible polymers should adopt highly disordered conformations resembling either a random coil or, in the presence of strong intrachain attractions, a so-called 'molten globule'. Stiff polymers with strong intrachain interactions, in contrast, are expected to collapse into conformations with long-range order, in the shape of toroids or rod-like structures. Here we use computer simulations to show that the anisotropy distribution obtained from polarization spectroscopy measurements on individual poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] polymer molecules is consistent with this prototypical stiff conjugated polymer adopting a highly ordered, collapsed conformation that cannot be correlated with ideal toroid or rod structures. We find that the presence of so-called 'tetrahedral chemical defects', where conjugated carbon-carbon links are replaced by tetrahedral links, divides the polymer chain into structurally identifiable quasi-straight segments that allow the molecule to adopt cylindrical conformations. Indeed, highly ordered, cylindrical conformations may be a critical factor in dictating the extraordinary photophysical properties of conjugated polymers, including highly efficient intramolecular energy transfer and significant local optical anisotropy in thin films.