ADVANCED MATERIALS Communications Molecular Rectification in Oriented Polymer Structures""

International audiencePolymeric semiconductor devices are receiving increasing attention in view of potential applications requiring low-cost processing over large areas.'"*] In this respect, unlike with evaporated molecules, the wet-processing capability of polymers offers total compatibility with other complemen-tary technologies. The concepts from which organic-semi-conductor devices are designed are mostly derived from in-organic-semiconductor physics and technology.''] In order to build efficient organic-semiconductor devices such as electroluminescent or photovoltaic solar cells, a rectifying junction is required. Such junction can be of two main typed4] Schottky junctions between an organic semi-conductor and a metal, and p-n junctions between two p-and n-type organic-semiconductors. The Schottky junctions often suffer from degradation problems originating from electrochemical potential differences at the rectifying con-tacts, which induce diffusion of electrode material into the polymer.'" Such a drawback is corrected with p-n junctions in which metal to polymer contacts are ohmic. However, p-n junctions require the use of two polymer layers with wet-processing compatibility. Moreover n-type organic-semiconductors are less common than p-type ones owing to their lower stability under oxygen which is a consequence of their reductive behavior. We report here a different approach in which oriented diode-like molecules, the so-called push-pull molecules used in nonlinear optics,[6] are contained inside a polymer binder. Oriented molecules induce a rectifying effect, be-having as a distributed homojunction within a single poly-mer thin film. In order to demonstrate the principle, we start with an initially symmetric structure: the polymer film is sandwiched between two identical electrodes. The cur-rent-voltage characteristics of the device are also symme-trical. By application of a static electric field through the polymer film, while heating near the glass transition tem-perature (T,,) dopant molecules are oriented in the field with an order parameter (cosO).['] The current-voltage characteristics then become strongly asymmetric. Experimentally, molecular order is controlled using sec-ond harmonic generation (SHG): it appears tha

Les populations néoteniques de Triturus helveticus des causses et du Bas-Languedoc I. Répartition et caractéristiques

Neoteny is common in some populations of Palmate newts Triturus helveticus of Southern France. Population samples have been collected from 117 pools, at three different seasons of the year. Percentages of neotenic individuals have been calculated for 29 of these pools where neotenic newts were found. Neoteny is not evenly distributed over the whole study area. It is more prevalent in the S.E. part of the Causse du Larzac and in the Causse de Blandas, than in the coastal plains of Languedoc. On the “ causses ” (arid limestone tablelands), neoteny only occurs among palmate newts, no neotenic individual having ever been found among the sympatric Triturus marmoratus population. Morphological differences between neotenic and normal popu lations of Triturus helveticus are described. None of them is consistent enough to allow sub-specific status to be given to the neotenic population. Breeding takes place later in the season among the Larzac population and the duration of the aquatic phase of the yearly cycle is also longer in the plateau population than among the newts of the coastal plains. All these differences, morphological as well as biological, disappear when the newts are kept in labo ratory conditions. Metamorphosis occurs normally when the progeny of both newt populations are bred in the laboratory. However mortality is much higher among the progeny of neotenic females, during their first three months of life

Les populations néoténiques de Triturus helveticus Raz. des Causses et du Bas-Languedoc. II. Ecologie

The physical, chemical, and biotic characteristics of the ponds where neotenic populations of Palmate newts are found in Sou thern France are described. The frequency of neoteny is high in permanent ponds, at an altitude ranging from 400 to 700 m, on arid limestone plateaux with a yearly rainfall of 1500-2000 mm. In such areas there are large seasonal variations of water temperature, ranging from + 1 to + 28 °C, water remaining cold (below + 15°C) for 8 to 10 months during the year. The saline content of water is low, as shown by conductivity figures (less than 200 pmhos/cm/cm2) . No iodine deficiency was ever found in the ponds of the study area. A correlation has been found between the frequency of neoteny and the water concentration in Ca, Mg and particularly K (which may account for 35-40 % of the cations) . The aridity of the area surrounding the pools can also contri bute to the selection of neotenic individuals, paedogenesis being of obvious adaptive value in such an environment

Etude de la structure electronique de molecules organiques conjuguees par generation de 3eme harmonique

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 78383 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Les populations néoténiques de Triturus helveticus des Causses et du Bas-Languedoc. I. Répartition et caractéristiques

Neoteny is common in some populations of Palmate newts Triturus helveticus of Southern France. Population samples have been collected from 117 pools, at three different seasons of the year. Percentages of neotenic individuals have been calculated for 29 of these pools where neotenic newts were found. Neoteny is not evenly distributed over the whole study area. It is more prevalent in the S.E. part of the Causse du Larzac and in the Causse de Blandas, than in the coastal plains of Languedoc. On the “causses” (arid limestone tablelands), neoteny only occurs among palmate newts, no neotenic individual having ever been found among the sympatric Triturus marmoratus population. Morphological differences between neotenic and normal populations of Triturus helveticus are described. None of them is consistent enough to allow sub-specific status to be given to the neotenic population. Breeding takes place later in the season among the Larzac population and the duration of the aquatic phase of the yearly cycle is also longer in the plateau population than among the newts of the coastal plains. All these differences, morphological as well as biological, disappear when the newts are kept in laboratory conditions. Metamorphosis occurs normally when the progeny of both newt populations are bred in the laboratory. However mortality is much higher among the progeny of neotenic females, during their first three months of life.Gabrion J., Sentein P., Gabrion C. Les populations néoténiques de Triturus helveticus des Causses et du Bas-Languedoc. I. Répartition et caractéristiques. In: La Terre et La Vie, Revue d'Histoire naturelle, tome 31, n°3, 1977. pp. 489-506

Les populations néoténiques de Triturus helveticus Raz. des Causses et du Bas-Languedoc. II. Écologie

The physical, chemical, and biotic characteristics of the ponds where neotenic populations of Palmate newts are found in Southern France are described. The frequency of neoteny is high in permanent ponds, at an altitude ranging from 400 to 700 m, on arid limestone plateaux with a yearly rainfall of 1500-2000 mm. In such areas there are large seasonal variations of water temperature, ranging from + 1 to + 28°C, water remaining cold (below + 15°C) for 8 to 10 months during the year. The saline content of water is low, as shown by conductivity figures (less than 200 μmhos/cm/cm²). No iodine deficiency was ever found in the ponds of the study area. A correlation has been found between the frequency of neoteny and the water concentration in Ca, Mg and particularly K (which may account for 35-40 % of the cations). The aridity of the area surrounding the pools can also contri¬ bute to the selection of neotenic individuals, paedogenesis being of obvious adaptive value in such an environment.Gabrion J., Sentein P., Gabrion C. Les populations néoténiques de Triturus helveticus Raz. des Causses et du Bas-Languedoc. II. Écologie. In: La Terre et La Vie, Revue d'Histoire naturelle, tome 32, n°4, 1978. pp. 577-610

Michel / [par François Sentein]

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