U. Rohr

Efficient white light-emitting diodes realized with new processable blends of conjugated polymers

An alternative method for producing efficient white light-emitting polymer diodes based on a blend of two polymers is reported. The white light emission is composed of a broad blue emission of laddertype (polyparaphenylene) (m-LPPP) and a red-orange emission of a new polymer, poly(perylene-co-diethynylbenzene) (PPDB). The red-orange electroluminescence emission is promoted by an excitation energy and charge transfer from m-LPPP to the PPDB. A concentration of 0.05% PPDB in the polymer blend is required in order to obtain white light emission. By inserting an insulating material in the blend, so that a maximum external quantum efficiency of 1.2% is obtained.

Liquid crystalline coronene derivatives with extraordinary fluorescence properties

Tempering of polystyrene films containing the novel liquid crystalline coronenebis(dicarboximide)s 2, which are formed by an easy route from perylene-3,4;9,10-tetracarboxylic dianhydride (1), leads to a shift of the emission within a few seconds. This could be used for dot-by-dot coloring and thus optical displays. The photoluminescence properties of 2 in the solid matrix are dependent on the nature of the aggregates formed.

Efficient full-colour electroluminescence and stimulated emission with polyphenylenes

Abstract Soluble and stable polyparaphenylene-type ladder polymers (LPPPs) with an extraordinarily high degree of intrachain order and exceptionally low concentration of defects belong to the class of best-defined conjugated low-dimensional organic semiconductors currently available. Parahexaphenyl, the highly stable oligomer of polyparaphenylene, can be synthesized in the form of single crystals, disordered thin films and highly ordered epitaxially grown thin films. We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light-emitting devices with these electroactive materials. These RGB devices are fabricated based on a new technique, which allows the realization of full-colour flat-panel displays. Using this new technique, we are able to produce devices having any desired emission colour in the visible and near-infrared spectrum. An efficient white-light emission is generated by an internal excitation energy (Forster-type) transfer from the blue LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present an optically pumped methyl-substituted LPPP waveguide, laser structure, which shows a spectrally very narrow and highly directional blue-green light output.

One-Dimensional Confinement of Organic Molecules via Selective Adsorption on CaF1 versus CaF2

We have integrated one-dimensional organic structures into semiconductor/insulator nanostructures by controlled self-assembly. Stepped Si(111) surfaces with self-organized CaF1/CaF2 stripe patterns serve as mask for selective adsorption of 3,10-di(propyl)perylene. These molecules adsorb preferentially on CaF1 nanostripes, rather than on CaF2. This property is used to fabricate large assemblies of parallel, equidistant stripes 1–15 nm wide. We discuss the selectivity in terms of a mechanism where the HOMO and LUMO of the molecule interact with the valence band and conduction band of CaF1 and CaF2.

Photophysics of excitation energy transfer in highly fluorescent polymers

Abstract We report the optical properties of highly fluorescent guest host systems of two conjugated polymers. The blue emitter laddertype poly(para-phenylene) (LPPP) is blended as a host with the red emitter poly(perylene-co-diethynylbenzene) (PPDB) as a guest at sub-percent and percent level concentrations. We use transient and steady-state photoluminescence as well as near-steady-state photoinduced absorption to show that an efficient excitation energy transfer of Forster type occurs between the blue emitting host and the red emitting guest. The spectral signatures of emissive and absorptive photogenerated species in both polymers are presented. In addition, we describe analytical relations to determine the lifetime of these species from photomodulation spectroscopy.

Charge transport in mesomorphic derivatives of perylene

Abstract Charge carrier mobilities have been measured in the solid and liquid crystalline phases of peripherally alkyl-chain substituted perylene derivatives using the pulse-radiolysis time-resolved microwave conductivity technique, PR-TRMC. By varying the alkyl substituents, the structure of the materials can be changed from a two-dimensional layered arrangement of the perylene units to one-dimensional, hexagonally packed columnar stacks. In all cases the pseudo-isotropic mobility within organised domains, σμ TRMC , is found to lie within the range 0.01 to 0.1 cm 2 /V/s and is only slightly dependent on temperature. In contrast to other discotic materials previously studied, no pronounced sharp decrease in σμ TRMC is found at the crystalline solid to liquid crystal phase transition.

Efficient color tuning (blue, red-orange, white) of light emitting diodes by excitation energy transfer

Abstract We present a color tuning technique allowing to control the emission colors of polymer light emitting diodes (PLEDS), which is of considerable interest for flat panel display applications. The emission color variation of the PLED from blue to red-orange is achieved by blending small amounts of a red light emitting guest polymer with the active PLED layer consisting of a blue emitting laddertype poly (paraphenylene) (LPPP). Using this new technique we realized highly efficient stable single layer PLEDs emitting different colors including white light emission. Besides the color tuning, which is established by an efficient excitation energy transfer (EET) and charge transfer from the host, m-LPPP, to the guest polymer, a significant increase of the photoluminescence (PL) and the electroluminescence (EL) quantum efficiency is observed.

Easy synthesis of liquid crystalline perylene derivatives

Mesophase forming chromophores are an important challenge of material science. This article describes the synthesis of new perylene mesogenes 10. The materials were obtained by twofold Diels-Alder reactions of our recently published 1:1 isomeric mixture of 3,9- and 3,10-dialkylperylenes with N-heptadecyltriazoline-3,5-diones. In contrast to the analogous derivatives 1 published earlier by our group (see reference 3), the new compounds 10 are prepared by a more efficient synthetic route and on a larger scale. Furthermore the characteristics of the liquid crystalline behavior of both materials are compared.

Efficient full-color electroluminescence and stimulated emission with polyphenylenes

We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light emitting devices based on poly(phenylene) type materials as the hexaphenyl and the methyl substituted laddertype poly(para phenylene) (m-LPPP). The RGB-devices are fabricated with an external color conversion technique based on PHP, whereas the white light emission is generated by an internal excitation energy transfer from the blue m-LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present photophysical properties, like spectral line-shape site selectivity of photoluminescence (PL), and electroluminescence of bulk poly(para-phenylenevinylene) PPV films and isolated PPV chains incorporated into a self- assembled matrix material, which leads to the formation of a regular hexagonal array of channels with a diameter of about 15 angstrom, in which the conjugated polymer chains are contained. The structure of the nano-composite in organic- light-emitting-diodes. A suitably structured m-LPPP waveguide shows a spectrally very narrow high directional blue-green light output when optically pumped. The high optical gain of m-LPPP is a results of the spectral separation of stimulated emission and photoinduced absorption bands, thus spectral narrowing is even observable in below cut-off waveguides. Under resonant excitation conditions, we observe strong stimulated Raman scattering.

Dynamics of photoexcitations in highly fluorescent organic guest-host-systems

A guest-host-system based on a few hundred ppm of the red emitting poly(perylene-co-diethynylbenzene) (PPDB) in blue emitting ladder-type poly(para-phenylene) (LPPP) can be used to build efficient white light emitting devices. Upon photoexcitation of the films a very efficient excitation energy transfer (EET). involving the diffusion of an electron hole pair from the higher energy gap LPPP to the lower-gap PPDB, is observed. The results of optically detected magnetic resonance and photoinduced absorption of these blends, all of which are sensitive to charged spin 1/2 states and spin I triplet excitons, are described and discussed.