Temperature dependence of the triplet diffusion and quenching rates in films of an Ir(ppy)(3)-cored dendrimer

We study photoluminescence and triplet-triplet exciton annihilation in a neat film of a fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)]-cored dendrimer and in its blend with a 4,4(')-bis(N-carbazolyl)biphenyl host for the temperature range of 77-300 K. The nearest neighbor hopping rate of triplet excitons is found to increase by a factor of 2 with temperature between 150 and 300 K and is temperature independent at lower temperature. The intermolecular quenching rate follows the Arrhenius law with an activation energy of 7 meV, which can be explained by stronger dipole-dipole interactions with the donor molecule in the higher triplet substate. The results indicate that energy disorder has no significant effect on triplet transport and quenching in these materials

Light-emitting poly(dendrimers)

Organic light-emitting diodes (OLEDs) have great potential for displays and lighting applications. For large area displays the ideal materials would be both phosphorescent and solution processible. These requirements mean that the materials need to be able to be patterned and the most advanced method for forming pixelated displays is inkjet printing. Light-emitting phosphorescent dendrimers have given high efficiency monochrome displays with the emitting layer deposited by spin-coating. However, the viscosity of the dendrimer solutions is insufficient for inkjet printing. We report the development of a new class of light-emitting materials, namely poly(dendrimers) in which a green emissive phosphorescent dendrimer is attached to a poly(styrene) backbone. Free radical polymerization of a dendrimer-styrene monomer gave a poly(dendrimer) with a weight average molecular weight of 24000 and a polydispersity of 3.6. A dilute Solution of the dendrimer had a viscosity 15% higher than the neat solvent. Comparison of the photophysical studies of the poly(dendrimer) versus a model monomer dendrimer showed that the PL spectrum was broader and red-shifted, and the PL quantum yield around 50% lower. This was attributed to intermolecular interactions of the emissive dendrimers, which are held closely together oil the polymer backbone

Triplet exciton diffusion and phosphorescence quenching in Iridium(III)-Centered dendrimers

A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films

Influence of the dendron chemical structure on the photophysical properties of bisfluorene-cored dendrimers

A detailed study of the photophysics of a family of bisfluorene-cored dendrimers is reported. Polarized time-resolved fluorescence, singlet-singlet exciton annihilation and fluorescence quantum yield measurements were performed and used to understand how the dendron structure affects the light-emitting properties of the materials. The exciton diffusion rate is similar in all films studied. An increase in the nonradiative deactivation rate by nearly one order of magnitude is observed in films of dendrimers with stilbenyl and carbazolyl based dendrons as compared to solutions, whereas the dendrimers with biphenyl and diphenylethylenyl dendrons showed highly efficient emission (photoluminescence quantum yields of 90%) in both solution and the solid state. The results of the materials that show fluorescence quenching can be explained by the presence of quenching sites at a concentration of just a fraction of a percent of all macromolecules. A possible explanation of this quenching is hole transfer from the emissive chromophore to the dendron in a face-to-face geometry. These results are important for the design of efficient blue emitters for optoelectronic applications. ©2008 American Institute of Physic

Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers

A study of the amplified spontaneous emission (ASE) properties of three bisfluorene-cored dendrimers in the solid state is reported. The results show that the dendron type has a strong impact on the photoluminescence quantum yield and affects the ASE threshold, the optical gain, and loss coefficients. Optically pumped distributed feedback lasers operating in the blue spectral region were fabricated by spin coating the dendrimer films on top of a two-dimensional corrugated fused silica substrate. A best lasing threshold of 4.5 mu J/cm(2) and a slope efficiency of 8.3% were obtained, which demonstrate the high potential of these materials for laser applications. (c) 2007 American Institute of Physics

Solvent-free fluidic organic dye lasers

We report on the demonstration of liquid organic dye lasers based on 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole, doped with green-and red-emitting laser dyes. Both waveguide and Fabry-Perot type microcavity fluidic organic dye lasers were prepared by capillary action under solvent-free conditions. Cascade Forster-type energy transfer processes from liquid carbazole to laser dyes were employed to achieve color-variable amplified spontaneous emission and lasing. Overall, this study provides the first step towards the development of solvent-free fluidic organic semiconducting lasers and demonstrates a new kind of optoelectronic applications for liquid organic semiconductors

Hybrid organic-inorganic liquid bistable memory devices

We demonstrate the realization of the first liquid nonvolatile rewritable memories relying on a blend of liquid carbazole and silver nanoparticles. When an external bias voltage is applied across this single active layer based structure, a bistable current with an ON/OFF ratio of about 2 x 10(2) is observed. Write-read-erase cycles and recorded information stability are also demonstrated. This study provides evidence that hybrid materials hold unexplored promises for original nonvolatile flash memories and should stimulate strong interest in the area of low-cost, large area, flexible data storage devices. (C) 2011 Elsevier B.V. All rights reserved.</p

Effects of solution processing and thermal annealing on the phosphorescence of iridium(III) complex-cored dendrimer films

We present evidence that the photoluminescence (PL) spectrum, PL kinetics and PL efficiency of solution-cast films of a phosphorescent fac-tris(2-phenylpyridyl)iridium(III)cored dendrimer are not affected by the choice of solvent and thermal annealing. In contrast, when the dendrimer is dispersed into a 4,4'-bis(N-carbazolyl)biphenyl (CBP) host, thermal annealing induces phase separation at a micrometer scale, which results in incomplete energy transfer from CBP to the dendrimer. The insensitivity of the dendrimer film morphology and PL properties to the casting conditions and thermal treatment shows the advantage of host-free electrophosphorescent dendrimers for the further development of light-emitting displays, whilst blended systems are potentially more suitable for photovoltaic applications. (C) 2009 Published by Elsevier B.V