Pattern forming instability induced by light in pure and dye-doped nematic liquid crystals

We study theoretically the instabilities induced by a linearly polarized ordinary light wave incident at a small oblique angle on a thin layer of homeotropically oriented nematic liquid crystal with special emphasis on the dye-doped case. The spatially periodic Hopf bifurcation that occurs as the secondary instability after the stationary Freedericksz transition is analyzed.Comment: 8 pages, 7 figures, LaTeX, accepted to Phys. Rev.

Poly(thiophenes) derivatized with linear and macrocyclic polyethers: from cation detection to molecular actuation

The association of linear or macrocyclic polyethers with the electronic properties of the π-conjugated polythiophene backbone leads to functional conducting polymers that exhibit metal cation dependent electronic properties. Based on this concept, various classes of cation sensors have been proposed and investigated for almost two decades. The interactions of metal cations with linear or macrocyclic polyether functional groups lead to modifications of the electronic properties of the π-conjugated backbone through various mechanisms including direct electronic effects on a single conjugated chain, collective electrochemical processes, or conformational changes. Conjugated polymers and oligomers representative of these various processes are discussed with an emphasis on recent examples of derivatized conjugated systems in which the interactions between metal cations and polyether groups serve as driving force to create molecular motion in conjugated systems

ADDA and ADADA systems based on triphenylamine as molecular donors for organic photovoltaics

Three molecular donor (D) acceptor (A) systems of structure A–D–A–D–A as well as an A–D–D–A compound have been synthesized by spatial extension of reference D–A system containing a triphenylamine donor block (5). UV–Vis absorption spectroscopy, cyclic voltammetry and theoretical calculations show that the presence of a median acceptor group has limited effect on the internal charge transfer while direct dimerization leads to an increase of the effective conjugation length. A cursory evaluation of the new compounds as donor material in bilayer solar cells using fullerene C60 as the acceptor material shows that the presence of a median acceptor has deleterious effect on conversion efficiency while the simple dimerization of the molecule leads to a substantial improvement of the short-circuit current density and efficiency

Macrocycles with bithiophene units: synthesis, structure, and electrochemical properties

Bithiophene macrocycles with oligo(oxyethylene) loops were synthesized in good yields by reacting 4,4-bis(hydroxymethyl)-2,2-bithiophene with ditosylated oligoethyleneglycols. The structures of the macrocycles were elucidated by NMR spectroscopy and MS spectrometry. The electronic and electrochemical properties of the macrocyclic compounds were determined using cyclic voltammetry and UV-Vis spectroscopy. (C) 2013 Elsevier Ltd. All rights reserved

How will a drier climate change carbon sequestration in soils of the deciduous forests of Central Europe?

Global warming is accompanied by increasing water stress across much of our planet. We studied soil biological processes and changes in soil organic carbon (SOC) storage in 30 Hungarian oak forest sites in the Carpathian Basin along a climatic gradient (mean annual temperature (MAT) 9.6\u201312.1 C, mean annual precipitation (MAP) 545\u2013725 mm) but on similar gently sloped hillsides where the parent materials are loess and weathered dust inputs dating from the end of the ice age. The purpose of this research was to understand how a drying climate, predicted for this region, might regulate long-term SOC sequestration. To examine the effects of decreasing water availability, we compared soil parameters and processes in three categories of forest that represented the moisture extremes along our gradient and that were defined using a broken-stick regression model. Soil biological activity was significantly lower in the driest (\u2018\u2018dry\u2019\u2019) forests, which had more than double the SOC concentration in the upper 30 cm layer (3.28 g C/100 g soil \ub1 0.11 SE) compared to soils of the wettest (\u2018\u2018humid\u2019\u2019) forests (1.32 g C/100 g soil \ub1 0.09 SE), despite the fact that annual surface litter production in humid forests was * 37% higher than in dry forests. A two-pool SOM model constrained to fit radiocarbon data indicates that turnover times for fast and slow pools are about half as long in the humid soil compared to the dry soil, and humid soils transfer C twice as efficiently from fast to slow pools. Enzyme activity and fungal biomass data also imply shorter turnover times associated with faster degradation processes in the soils of humid forests. Thermogravimetry studies suggest that more chemically recalcitrant compounds are accumulating in the soils of dry forests. Taken together, our results suggest that the predicted climate drying in this region might increase SOC storage in Central European mesic deciduous forests even as litter production decreases

Synthesis and Metal Cation Complexing Properties of Crown-Annelated Terthiophenes Containing 3,4-Ethylenedioxythiophene

Date du colloque&nbsp;: 05/2009</p

Small Molecular Donors for Organic Solar Cells Obtained by Simple and Clean Synthesis

A small donor–acceptor molecule is synthesized in a two-step procedure involving reaction of N,N-diphenylhydrazine on 2,5-diformylthiophene and Knoevenagel condensation. Results of UV/Vis absorption spectroscopy and cyclic voltammetry show that replacement of the phenyl ring bridge of a reference compound 2 by an azo group produces a slight red-shift of λmax, an enhancement of the molecular absorption coefficient, and a decrease of the energy level of the frontier orbitals. A preliminary evaluation of the potentialities of compound 1 as donor material in a basic bilayer planar heterojunction cell of 28 mm2 active area using C60 as acceptor gave a short-circuit current density of 6.32 mA cm−2 and a power conversion efficiency of 2.07 %

Synthesis and electronic properties of terthienyls β-substituted by (thienyl)cyanovinylene groups

Terthienyls functionalized at their two outer β,β′-positions by 2- and 3-(thienyl)cyanovinyl groups have been synthesized by basic condensation. The analysis of their electronic properties by UV–vis spectroscopy and cyclic voltammetry shows that the mode of derivatization affects essentially the LUMO level of the conjugated system