Andrew B. Holmes

Light-emitting diodes based on conjugated polymers

CONJUGATED polymers are organic semiconductors, the semiconducting behaviour being associated with the π molecular orbitals delocalized along the polymer chain. Their main advantage over non-polymeric organic semiconductors is the possibility of processing the polymer to form useful and robust structures. The response of the system to electronic excitation is nonlinear—the injection of an electron and a hole on the conjugated chain can lead to a self-localized excited state which can then decay radiatively, suggesting the possibility of using these materials in electroluminescent devices. We demonstrate here that poly(p-phenylene vinylene), prepared by way of a solution-processable precursor, can be used as the active element in a large-area light-emitting diode. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green–yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large-area light-emitting displays.

Electroluminescent Conjugated Polymers—Seeing Polymers in a New Light

A new exciting interdisciplinary research field has evolved following the discovery that conjugated polymers can emit light when put into light-emitting diodes. A myriad of light-emitting polymers (examples of which are shown below) shining in various colors have been developed through chemical intuition and structural design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

School of Chemistry, Bio21 Institute, University of Melbourne, 30 Flemington Road, Victoria 3010, Australia; School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Republic of Singapore 639798; Institute of Materials Research and Engineering (IMRE) and the Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602; and F. Hoffmann-La Roche Ltd., Pharmaceuticals Division, Discovery Chemistry, CH-4070 Basel, Switzerland

High-efficiency dye-sensitized solar cells with ferrocene-based electrolytes

Dye-sensitized solar cells based on iodide/triiodide (I(-)/I(3)(-)) electrolytes are viable low-cost alternatives to conventional silicon solar cells. However, as well as providing record efficiencies of up to 12.0%, the use of I(-)/I(3)(-) in such solar cells also brings about certain limitations that stem from its corrosive nature and complex two-electron redox chemistry. Alternative redox mediators have been investigated, but these generally fall well short of matching the performance of conventional I(-)/I(3)(-) electrolytes. Here, we report energy conversion efficiencies of 7.5% (simulated sunlight, AM1.5, 1,000 W m(-2)) for dye-sensitized solar cells combining the archetypal ferrocene/ferrocenium (Fc/Fc(+)) single-electron redox couple with a novel metal-free organic donor-acceptor sensitizer (Carbz-PAHTDTT). These Fc/Fc(+)-based devices exceed the efficiency achieved for devices prepared using I(-)/I(3)(-) electrolytes under comparable conditions, revealing the great potential of ferrocene-based electrolytes in future dye-sensitized solar cells applications. This improvement results from a more favourable matching of the redox potential of the ferrocene couple with that of the new donor-acceptor sensitizer.

PtdIns(3)P Regulates the Neutrophil Oxidase Complex by Binding to the PX Domain of p40phox

The production of reactive oxygen species (ROS) by neutrophils has a vital role in defence against a range of infectious agents, and is driven by the assembly of a multi-protein complex containing a minimal core of five proteins: the two membrane-bound subunits of cytochrome b558 (gp91phox and p22phox) and three soluble factors (GTP–Rac, p47phox and p67phox (refs 1, 2). This minimal complex can reconstitute ROS formation in vitro in the presence of non-physiological amphiphiles such as SDS. p40phox has subsequently been discovered as a binding partner for p67phox (ref. 3), but its role in ROS formation is unclear. Phosphoinositide-3-OH kinases (PI(3)Ks) have been implicated in the intracellular signalling pathways coordinating ROS formation but through an unknown mechanism. We show that the addition of p40phox to the minimal core complex allows a lipid product of PI(3)Ks, phosphatidylinositol 3-phosphate (PtdIns(3)P), to stimulate specifically the formation of ROS. This effect was mediated by binding of PtdIns(3)P to the PX domain of p40phox. These results offer new insights into the roles for PI(3)Ks and p40phox in ROS formation and define a cellular ligand for the orphan PX domain.

Bcl-2 functionally interacts with inositol 1,4,5- trisphosphate receptors to regulate calcium release from the ER in response to inositol 1,4,5-trisphosphate

Inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are channels responsible for calcium release from the endoplasmic reticulum (ER). We show that the anti-apoptotic protein Bcl-2 (either wild type or selectively localized to the ER) significantly inhibited InsP3-mediated calcium release and elevation of cytosolic calcium in WEHI7.2 T cells. This inhibition was due to an effect of Bcl-2 at the level of InsP3Rs because responses to both anti-CD3 antibody and a cell-permeant InsP3 ester were decreased. Bcl-2 inhibited the extent of calcium release from the ER of permeabilized WEHI7.2 cells, even at saturating concentrations of InsP3, without decreasing luminal calcium concentration. Furthermore, Bcl-2 reduced the open probability of purified InsP3Rs reconstituted into lipid bilayers. Bcl-2 and InsP3Rs were detected together in macromolecular complexes by coimmunoprecipitation and blue native gel electrophoresis. We suggest that this functional interaction of Bcl-2 with InsP3Rs inhibits InsP3R activation and thereby regulates InsP3-induced calcium release from the ER.

Impaired Stress-Coping and Fear Extinction and Abnormal Corticolimbic Morphology in Serotonin Transporter Knock-Out Mice

A lesser-expressing form of the human 5-HT transporter (5-HTT) gene has been associated with increased fear and anxiety and vulnerability to the effects of stress. These phenotypic abnormalities are linked to functional and anatomical disturbances in a neural pathway connecting the prefrontal cortex (PFC) and amygdala. Likewise, rodent and nonhuman primate studies indicate a major role for PFC and amygdala in the mediation of fear- and stress-related behaviors. We used a 5-HTT knock-out (KO) mouse to examine the effects of genetically driven loss of 5-HTT function for the following: (1) depression-related behavior in response to repeated stress, and pavlovian fear conditioning, extinction, and extinction recall; and (2) dendritic morphology and spine density of Golgi-stained pyramidal neurons in the infralimbic cortex (IL) and the basolateral amygdala (BLA). 5-HTT KO mice exhibited increased depressive-like immobility after repeated exposure to forced swim stress, compared with wild-type (WT) controls. Whereas fear conditioning and fear extinction was normal, 5-HTT KO mice exhibited a significant deficit in extinction recall. The apical dendritic branches of IL pyramidal neurons in 5-HTT KO mice were significantly increased in length relative to WT mice. Pyramidal neurons in BLA had normal dendritic morphology but significantly greater spine density in 5-HT KO mice compared with WT mice. Together, the present findings demonstrate a specific phenotypic profile of fear- and stress-related deficits in 5-HTT KO mice, accompanied by morphological abnormalities in two key neural loci. These data provide insight into the behavioral sequelae of loss of 5-HTT gene function and identify potential neural substrates underlying these phenotypes.

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

The photovoltaic response in poly(p-phenylene vinylene) thin-film devices

We report measurements of the photovoltaic effect in diode structures formed with thin films (100 nm) of the conjugated polymer poly(p-phenylene vinylene), PPV, sandwiched between electrodes of indium/tin oxide, ITO, and either aluminium, magnesium or calcium. Under illumination incident through the ITO contact, large open-circuit voltages were measured, which saturated at approximately 1.2 V for Al and Mg devices, and approximately 1.7 V for Ca devices. Quantum efficiencies (short-circuit current/incident photon flux) of order 1% were measured at low intensities (0.1 mW cm-2). The spectral response of the photocurrent demonstrates that photon absorption near the electron-collecting electrode optimizes the photocurrent, indicating that device performance is limited by low electron mobilities in the bulk PPV. The photocurrent exhibits a weak temperature dependence, with an activation energy that is a function of the electric field in the polymer. We have used these measurements to estimate an exciton binding energy in PPV of approximately 0.4 eV.

Identification of ARAP3, a novel PI3K effector regulating both Arf and Rho GTPases, by selective capture on phosphoinositide affinity matrices

We show that matrices carrying the tethered homologs of natural phosphoinositides can be used to capture and display multiple phosphoinositide binding proteins in cell and tissue extracts. We present the mass spectrometric identification of over 20 proteins isolated by this method, mostly from leukocyte extracts: they include known and novel proteins with established phosphoinositide binding domains and also known proteins with surprising and unusual phosphoinositide binding properties. One of the novel PtdIns(3,4,5)P3 binding proteins, ARAP3, has an unusual domain structure, including five predicted PH domains. We show that it is a specific PtdIns(3,4,5)P3/PtdIns(3,4)P2-stimulated Arf6 GAP both in vitro and in vivo, and both its Arf GAP and Rho GAP domains cooperate in mediating PI3K-dependent rearrangements in the cell cytoskeleton and cell shape.