1,002 research outputs found
Transport Properties of Highly Aligned Polymer Light-Emitting-Diodes
We investigate hole transport in polymer light-emitting-diodes in which the
emissive layer is made of liquid-crystalline polymer chains aligned
perpendicular to the direction of transport. Calculations of the current as a
function of time via a random-walk model show excellent qualitative agreement
with experiments conducted on electroluminescent polyfluorene demonstrating
non-dispersive hole transport. The current exhibits a constant plateau as the
charge carriers move with a time-independent drift velocity, followed by a long
tail when they reach the collecting electrode. Variation of the parameters
within the model allows the investigation of the transition from non-dispersive
to dispersive transport in highly aligned polymers. It turns out that large
inter-chain hopping is required for non-dispersive hole transport and that
structural disorder obstructs the propagation of holes through the polymer
film.Comment: 4 pages, 5 figure
Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy
have been used to study the well-known order-disorder transition (ODT) to the
beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through
combination of time-dependent and temperature-dependent measurements. The ODT
is well described by a simple Avrami picture of one-dimensional nucleation and
growth but crystallization, on cooling, proceeds only after molecular-level
conformational relaxation to the so called beta phase. Rapid thermal quenching
is employed for PF8 studies of pure alpha phase samples while extended
low-temperature annealing is used for improved beta phase formation. Low
temperature PL studies reveal sharp Franck-Condon type emission bands and, in
the beta phase, two distinguishable vibronic sub-bands with energies of
approximately 199 and 158 meV at 25 K. This improved molecular level structural
order leads to a more complete analysis of the higher-order vibronic bands. A
net Huang-Rhys coupling parameter of just under 0.7 is typically observed but
the relative contributions by the two distinguishable vibronic sub-bands
exhibit an anomalous temperature dependence. The PL studies also identify
strongly correlated behavior between the relative beta phase 0-0 PL peak
position and peak width. This relationship is modeled under the assumption that
emission represents excitons in thermodynamic equilibrium from states at the
bottom of a quasi-one-dimensional exciton band. The crystalline phase, as
observed in annealed thin-film samples, has scattering peaks which are
incompatible with a simple hexagonal packing of the PF8 chains.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure
2-Amino-6-[(2,6-dichlorophenyl)imino]-3-oxocyclohexa-1,4-dienecarbaldehyde
The title compound, C13H8Cl2N2O2, was obtained by the oxidation of diclofenac {systematic name: 2-[2-(2,6-dichlorophenylamino)phenyl]acetic acid}, an anti-inflammatory drug, with hydrogen peroxide catalysed by chlorido[5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinato]manganese(III), using ammonium acetate as co-catalyst. The asymmetric unit contains two crystallographically independent molecules of the title compound (Z′ = 2). The close packing of individual molecules is mediated by a series of strong and rather directional N—H⋯Cl and N—H⋯O hydrogen bonds, plus weak π–π [distance between the individual double bonds of symmetry-related iminoquinone rings = 3.7604 (13) Å] and Cl⋯O interactions [3.0287 (18) Å]
Population change in breeding boreal waterbirds in a 25-year perspective : What characterises winners and losers?
Understanding drivers of variation and trends in biodiversity change is a general scientific challenge, but also crucial for conservation and management. Previous research shows that patterns of increase and decrease are not always consistent at different spatial scales, calling for approaches combining the latter. We here explore the idea that functional traits of species may help explaining divergent population trends. Complementing a previous community level study, we here analyse data about breeding waterbirds on 58 wetlands in boreal Fennoscandia, covering gradients in latitude as well as trophic status. We used linear mixed models to address how change in local abundance over 25 years in 25 waterbird species are associated with life history traits, diet, distribution, breeding phenology, and habitat affinity. Mean abundance increased in 10 species from 1990/1991 to 2016, whereas it decreased in 15 species. Local population increases were associated with species that are early breeders and have small clutches, an affinity for luxurious wetlands, an herbivorous diet, and a wide breeding range rather than a southern distribution. Local decreases, by contrast, were associated with species having large clutches and invertivorous diet, as well as being late breeders and less confined to luxurious wetlands. The three species occurring on the highest number of wetlands all decreased in mean abundance. The fact that early breeders have done better than late fits well with previous research about adaptability to climate change, that is, response to earlier springs. We found only limited support for the idea that life history traits are good predictors of wetland level population change. Instead, diet turned out to be a strong candidate for an important driver of population change, as supported by a general decrease of invertivores and a concomitant increase of large herbivores. In a wider perspective, future research needs to address whether population growth of large-bodied aquatic herbivores affects abundance of co-occurring invertivorous species, and if so, if this is due to habitat alteration, or to interference or exploitative competition.Peer reviewe
Evolutionary History of the HAP2/GCS1 Gene and Sexual Reproduction in Metazoans
The HAP2/GCS1 gene first appeared in the common ancestor of plants, animals, and protists, and is required in the male gamete for fusion to the female gamete in the unicellular organisms Chlamydomonas and Plasmodium. We have identified a HAP2/GCS1 gene in the genome sequence of the sponge Amphimedon queenslandica. This finding provides a continuous evolutionary history of HAP2/GCS1 from unicellular organisms into the metazoan lineage. Divergent versions of the HAP2/GCS1 gene are also present in the genomes of some but not all arthropods. By examining the expression of the HAP2/GCS1 gene in the cnidarian Hydra, we have found the first evidence supporting the hypothesis that HAP2/GCS1 was used for male gamete fusion in the ancestor of extant metazoans and that it retains that function in modern cnidarians
The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation
The ability to ensure continuous availability of energy despite highly variable
supplies in the environment is a major determinant of the survival of all
species. In higher organisms, including mammals, the capacity to efficiently
store excess energy as triglycerides in adipocytes, from which stored energy
could be rapidly released for use at other sites, was developed. To orchestrate
the processes of energy storage and release, highly integrated systems operating
on several physiological levels have evolved. The adipocyte is no longer
considered a passive bystander, because fat cells actively secrete many members
of the cytokine family, such as leptin, tumor necrosis factor-alpha, and
interleukin-6, among other cytokine signals, which influence peripheral fuel
storage, mobilization, and combustion, as well as energy homeostasis. The
existence of a network of adipose tissue signaling pathways, arranged in a
hierarchical fashion, constitutes a metabolic repertoire that enables the
organism to adapt to a wide range of different metabolic challenges, such as
starvation, stress, infection, and short periods of gross energy excess
Search for Doubly-Charged Higgs Boson Production at HERA
A search for the single production of doubly-charged Higgs bosons H^{\pm \pm}
in ep collisions is presented. The signal is searched for via the Higgs decays
into a high mass pair of same charge leptons, one of them being an electron.
The analysis uses up to 118 pb^{-1} of ep data collected by the H1 experiment
at HERA. No evidence for doubly-charged Higgs production is observed and mass
dependent upper limits are derived on the Yukawa couplings h_{el} of the Higgs
boson to an electron-lepton pair. Assuming that the doubly-charged Higgs only
decays into an electron and a muon via a coupling of electromagnetic strength
h_{e \mu} = \sqrt{4 \pi \alpha_{em}} = 0.3, a lower limit of 141 GeV on the
H^{\pm\pm} mass is obtained at the 95% confidence level. For a doubly-charged
Higgs decaying only into an electron and a tau and a coupling h_{e\tau} = 0.3,
masses below 112 GeV are ruled out.Comment: 15 pages, 3 figures, 1 tabl
Tuning intermolecular interactions in di-octyl substituted polyfluorene via hydrostatic pressure
Polyfluorenes (PFs) represent a unique class of poly para-phenylene based
blue-emitting polymers with intriguing structure-property relationships. Slight
variations in the choice of functionalizing side chains result in dramatic
differences in the inter- and intra-chain structures in PFs. We present
photoluminescence (PL) and Raman scattering studies of bulk samples and thin
films of dioctyl-substituted PF (PF8) under hydrostatic pressure. The bulk
sample was further thermally annealed at 1.9 GPa. The PL vibronics of the as-is
sample red-shift at an average rate of 26 meV/GPa. The thermally annealed
sample is characterized by at least two phase transitions at 1.1 GPa and 4.2
GPa, each of which has a different pressure coefficient for PL vibronics. The
Huang-Rhys factor, a measure of the electron-phonon interaction, is found to
increase with increasing pressures signaling a higher geometric relaxation of
the electronic states. The Raman peaks harden with increasing pressures; the
intra-ring C-C stretch frequency at 1600 cm has a pressure coefficient
of 7.2 cm/GPa and exhibits asymmetric line shapes at higher pressures,
characteristic of a strong electron-phonon interaction. The optical properties
of PF8 under high pressure are further contrasted with those of a branched side
chain substituted PF.Comment: 22 pages, 10 figure
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