Paul L. Burn

Poly(p‐phenylenevinylene) light‐emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement

We have fabricated light‐emitting diodes with poly(p‐phenylenevinylene) as the emissive layer, and with an electron‐transporting layer formed from a solid state dispersion of 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low‐voltage regime and an eightfold improvement in the high‐voltage regime over devices without the electron‐transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron‐transport layer is to confine holes to the emissive layer.

Response of melanocortin-4 receptor-deficient mice to anorectic and orexigenic peptides.

Mutations reducing the functional activity of leptin, the leptin receptor, α–melanocyte stimulating hormone (α–MSH) and the melanocortin–4 receptor (Mc4r) all lead to obesity in mammals. Moreover, mutant mice that ectopically express either agouti (Ay/a mice) or agouti–related protein (Agrp), antagonists of melanocortin signalling, become obese. These data suggest that α–MSH signalling transduced by Mc4r tonically inhibits feeding; however, it is not known to what extent this pathway mediates leptin signalling. We show here that Mc4r–deficient (Mc4r–/–) mice do not respond to the anorectic actions of MTII, an MSH–like agonist, suggesting that α–MSH inhibits feeding primarily by activating Mc4r. Obese Mc4r–/– mice do not respond significantly to the inhibitory effects of leptin on feeding, whereas non–obese Mc4r–/– mice do. These data demonstrate that melanocortin signalling transduced by Mc4r is not an exclusive target of leptin action and that factors resulting from obesity contribute to leptin resistance. Leptin resistance of obese Mc4r–/– mice does not prevent their response to the anorectic actions of ciliary neurotrophic factor (CNTF), corticotropin releasing factor (CRF), or urocortin; or the orexigenic actions of neuropeptide Y (NPY) or peptide YY (PYY), indicating that these neuromodulators act independently or downstream of Mc4r signalling.

Green phosphorescent dendrimer for light-emitting diodes

Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

Optical spectroscopy of highly ordered poly(p-phenylene vinylene)

The authors report a study of the photophysical properties of poly(p-phenylene vinylene), PPV, prepared in a way that gives an especially high degree of intrachain order. Optical absorption, photoluminescence, photoinduced absorption, and photoconductivity excitation spectra are presented and compared to data reported for less well ordered PPV. Spectral red shifts, sharpening of spectral lines, and a transfer of oscillator strength into the vibronic ground states of the electronic transitions are observed. Photoinduced absorption due to long-lived charged excitations, previously reported for less ordered PPV, could not be detected in this material. Photoconductivity excitation spectra show a steep rise at the absorption edge with no appreciable offset between the onsets for photoconduction and absorption. A very slow photocurrent component is observed, which the authors associated with the trapping and subsequent thermal release of photocarriers.

High-efficiency green phosphorescence from spin-coated single-layer dendrimer light-emitting diodes

We demonstrate very high-efficiency green phosphorescence from a single-layer dendrimer organic light-emitting diode formed by spin-coating. A first generation fac-tris(2-phenylpyridine) iridium cored dendrimer doped into a wide-gap 4,4′-bis(N-carbazole) biphenyl host displays a peak external quantum efficiency of 8.1% (28 Cd/A) at a brightness of 3450 Cd/m2 and a current density of 13.1 mA/cm2. A peak power efficiency of 6.9 lm/W was measured at 1475 Cd/m2 and 5 mA/cm2. We attribute this exceptionally high quantum efficiency for a single-layer device to the excellent film forming properties and high photoluminescence quantum yield of the dendrimer blend and efficient injection of charge into the emissive layer. These results suggest that dendrimers are an effective method for producing efficient phosphorescent devices by spin-coating.

The mahogany protein is a receptor involved in suppression of obesity

Genetic studies have shown that mutations within the mahogany locus suppress the pleiotropic phenotypes, including obesity, of the agouti-lethal-yellow mutant,. Here we identify the mahogany gene and its product; this study, to our knowledge, represents the first positional cloning of a suppressor gene in the mouse. Expression of the mahogany gene is broad; however, in situ hybridization analysis emphasizes the importance of its expression in the ventromedial hypothalamic nucleus, a region that is intimately involved in the regulation of body weight and feeding. We present new genetic studies that indicate that the mahogany locus does not suppress the obese phenotype of the melanocortin-4-receptor null allele or those of the monogenic obese models (Lepdb, tub and Cpefat). However, mahogany can suppress diet-induced obesity, the mechanism of which is likely to have implications for therapeutic intervention in common human obesity. The amino-acid sequence of the mahogany protein suggests that it is a large, single-transmembrane-domain receptor-like molecule, with a short cytoplasmic tail containing a site that is conserved between Caenorhabditis elegans and mammals. We propose two potential, alternative modes of action for mahogany: one draws parallels with the mechanism of action of low-affinity proteoglycan receptors such as fibroblast growth factor and transforming growth factor-β, and the other suggests that mahogany itself is a signalling receptor.

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

Diacylglycerol in large α-actinin/actin complexes and in the cytoskeleton of activated platelets

The interaction of the cytoskeleton with plasma membranes may be mediated by vinculin1–4, α-actinin5,6 and other proteins7–11; α-actinin can interact specifically with model membranes only if they contain diacylglycerol and palmitic acid12. On stimulation of platelets by thrombin, which leads to a reorganization of the cytoskeleton13–15, diacylglycerol is produced rapidly16, simultaneously with the disappearance of phosphatidylinositol16–19. One important function of the diacylglycerol produced in platelets may be the activation of the Ca2+- and phospholipid-dependent protein kinase C20–22 We show here that, in the presence of diacylglycerol and palmitic acid, a supramolecular complex between α-actinin and actin is formed in vitro. In the electron microscope, this complex displays substructures similar to those of microfilament bundles in vivo. Furthermore, such α-actinin/lipid complexes can also be formed in situ during the stimulation of blood platelet aggregation. Thus, α-actinin may be one of the proteins directly involved in structures connecting the cytoskeleton to cell membranes.

Electroluminescence from multilayer conjugated polymer devices: Spatial control of exciton formation and emission

Abstract We have constructed electroluminescent diodes using several layers of conjugated polymers with differing band gaps; these provide a range of different colour light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene, PPV, and derivatives have been used, with indium/tin oxide as hole-injecting layer and calcium as electron-injecting contact layer. For this selection of materials, we show that the ordering of the polymer layers allows control of the colour of device emission. Emission can be produced in more than one layer.

Narrowband light detection via internal quantum efficiency manipulation of organic photodiodes

Spectrally selective light detection is vital for full-colour and near-infrared (NIR) imaging and machine vision. This is not possible with traditional broadband-absorbing inorganic semiconductors without input filtering, and is yet to be achieved for narrowband absorbing organic semiconductors. We demonstrate the first sub-100 nm full-width-at-half-maximum visible-blind red and NIR photodetectors with state-of-the-art performance across critical response metrics. These devices are based on organic photodiodes with optically thick junctions. Paradoxically, we use broadband-absorbing organic semiconductors and utilize the electro-optical properties of the junction to create the narrowest NIR-band photoresponses yet demonstrated. In this context, these photodiodes outperform the encumbent technology (input filtered inorganic semiconductor diodes) and emerging technologies such as narrow absorber organic semiconductors or quantum nanocrystals. The design concept allows for response tuning and is generic for other spectral windows. Furthermore, it is material-agnostic and applicable to other disordered and polycrystalline semiconductors.