2,082 research outputs found
Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study
Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which
has been studied during the last years for its efficient photo-luminescence. In
contrast, the molecular geometry, providing the basis for the electronic and
optical properties, has been hardly investigated. In this paper, we apply a
density-functional-theory based molecular-dynamics approach to reveal the
molecular structure of PDPA in detail. We find that oligomers of this material
are limited in length, being stable only up to eight repeat units, while the
polymer is energetically unfavorable. These facts, which are in excellent
agreement with experimental findings, are explained through a detailed analysis
of the bond lengths. A consequence of the latter is the appearance of
pronounced torsion angles of the phenyl rings with respect to the plane of the
polyene backbone, ranging from up to . We point out
that such large torsion angles do not destroy the conjugation of the
electrons from the backbone to the side phenyl rings, as is evident from the
electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Electronic and optical properties of families of polycyclic aromatic hydrocarbons: a systematic (time-dependent) density functional theory study
Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their
crystalline state are among the most promising materials for organic
opto-electronics. Following previous works on oligoacenes we present a
systematic comparative study of the electronic, optical, and transport
properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using
density functional theory (DFT) and time-dependent DFT we computed: (i)
electron affinities and first ionization energies; (ii) quasiparticle
correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied
molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv)
electronic absorption spectra of neutral and charged systems. The
excitonic effects are estimated by comparing the optical gap and the
quasiparticle corrected HOMO-LUMO energy gap. For each molecular property
computed, general trends as a function of molecular size and charge state are
discussed. Overall, we find that circumacenes have the best transport
properties, displaying a steeper decrease of the molecular reorganization
energy at increasing sizes, while oligorylenes are much more efficient in
absorbing low-energy photons in comparison to the other classes.Comment: 26 pages, 9 figures, 4 tables, accepted for pubblication in Chemical
Physics (14/04/2011
Onecut-dependent Nkx6.2 transcription factor expression is required for proper formation and activity of spinal locomotor circuits.
In the developing spinal cord, Onecut transcription factors control the diversification of motor neurons into distinct neuronal subsets by ensuring the maintenance of Isl1 expression during differentiation. However, other genes downstream of the Onecut proteins and involved in motor neuron diversification have remained unidentified. In the present study, we generated conditional mutant embryos carrying specific inactivation of Onecut genes in the developing motor neurons, performed RNA-sequencing to identify factors downstream of Onecut proteins in this neuron population, and employed additional transgenic mouse models to assess the role of one specific Onecut-downstream target, the transcription factor Nkx6.2. Nkx6.2 expression was up-regulated in Onecut-deficient motor neurons, but strongly downregulated in Onecut-deficient V2a interneurons, indicating an opposite regulation of Nkx6.2 by Onecut factors in distinct spinal neuron populations. Nkx6.2-null embryos, neonates and adult mice exhibited alterations of locomotor pattern and spinal locomotor network activity, likely resulting from defective survival of a subset of limb-innervating motor neurons and abnormal migration of V2a interneurons. Taken together, our results indicate that Nkx6.2 regulates the development of spinal neuronal populations and the formation of the spinal locomotor circuits downstream of the Onecut transcription factors
Second Record and DNA Barcode of the Ant Tyrannomyrmex rex Fernández (Hymenoptera: Formicidae: Myrmicinae)
Tyrannomyrmex is a rarely collected ant genus from Old World tropical forests comprising only three described species, all of them known from a single worker. Here we report the discovery of a second worker of Tyrannomyrmex rex from a selectively logged primary forest of Singapore, increasing the known distribution range of the species to nearly 250 km South-East. We also provide a DNA barcode for the species and a partial sequence of the wingless gene. Although insufficient evidence prevents us to draw any firm conclusion, the genus seems to be restricted to pristine or relatively undisturbed forests and, as a result, could be highly sensitive to habitat degradation
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