A chiral poly(para-phenyleneethynylene) (PPE) derivative

We have investigated the chiroptical properties of novel PPEs containing chiral side chains. The synthesis of poly(2,5-bis[2-(S)-methylbutoxy]-1,4-phenyleneethynylene) (BMB-PPE) was achieved in a Heck-type aryl-aryl cross coupling (M _n = 10 000, D_p = 40). The very good solubility of BMB-PPE allows for a detailed study of its chiroptical properties. Our studies demonstrate that the bisignated CD spectra of BMB-PPE are associated with an intermolecular aggregation phenomena.     

Synthesis and characterization of a helical step‐ladder polyarylene

A helical step‐ladder polyarylene incorporating chiral (R)‐2,2′‐dioctoxy‐1,1′‐binaphthyl units was synthesized for the first time. The first step involved the preparation of a precursor poly(arylene ketone) via a palladium‐mediated Suzuki‐type cross‐coupling reaction with the aid of microwave heating. Two polymer‐analog reaction steps, the reduction of the keto groups to tertiary alcohol functionalities and subsequent intramolecular Friedel–Crafts cyclization, gave a step‐ladder polymer ( 6 ) in good yields with reasonable mean average molecular weights greater than 13,000. The regioselective cyclization pattern in the α position of the naphthalene core was confirmed by a comparison of the NMR data of the polymer with those of the corresponding model ladder oligomers, 12 and 13 , and also a single‐crystal structure of 13 . The optical spectra of the oligomers and polymers indicated that there was little electronic interaction across the binaphthyl units. The circular dichroism spectrum of 6 exhibited a strong bisignate Cotton effect in the π–π* absorption region of the planar chromophores, which reflected the strong exciton coupling within the helical polymer chain. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5533–5545, 2006     

Thiophene–phenylene/naphthalene‐based step‐ladder copolymers

A series of step‐ladder copolymers based on thiophene–phenylene–thiophene SL1 ‐ SL3 and thiophene–naphthylene–thiophene SL4 repeat units with varying lengths of the oligothiophene segment has been designed and synthesized via a microwave‐assisted Stille‐type cross‐coupling reaction followed by a polymer‐analogous cyclization reaction. The optical properties of the step‐ladder copolymers have been investigated in detail, in particular at low temperature and in the solid‐state. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7342–7353, 2008     

Numerical modeling and characterization of Electrolyte/Insulator/Semiconductor sensor systems

A model is proposed to describe integrated ionsensitive devices based on Electrolyte/Insulator/Semiconductor systems. In particular a special type of sensor will be discussed based on a pure Electrolyte/Insulator/Semiconductor structure in the constant capacitance mode. The following models will serve to describe the sensor effects: The site-binding model for pH-sensors and a single-site model for pF-sensors. The theory of the field effect has been integrated into the complete model. In this way pH/pF responses and quasistatic (lf) as well as rf C(V) characteristics can be simulated. A comparison between measurements and simulations can be made for the pH EIS structures with ZrO₂, SrTiO₃ and pF EIS structures with LaF₃.     

The atomistic origin of the inverse piezoelectric effect in α-quartz

Ab initio calculations have been carried out using the FP-APW+lo method in order to understand the atomic origin of the inverse piezoelectric effect in α-quartz. The external electric field was modelled by a saw-like potential V_ext in order to achieve translational symmetry within a supercell (SC) containing 72 atoms. The original trigonal quartz structure was repeated along the [110] direction, which corresponds to the direction of the external field. An electric field with 550 kV/mm was applied and the atomic positions of the SC were relaxed until the forces acting on the atoms vanished. In parts of the SC, V_ext changes almost linearly and thus the relaxed atomic positions can be used to determine the structural response due to the external electric field. The calculations provide the piezoelectric modulus of the correct order of magnitude. In contrast to previous models and in agreement with recent experimental results, the atomic origin of the piezoelectric effect can be described by a rotation of slightly deformed SiO₄ tetrahedra against each other. The change of the Si-O bond lengths and the tetrahedral O-Si-O angles is one order of magnitude smaller than that of the Si-O-Si angles between neighbouring tetrahedra. The calculated changes of X-ray structure factors are in agreement with experiment when the theoretical data are extrapolated down to the much smaller field strength that is applied in the experiment (E<10 kV/mm).     

Formation of self-supporting reversible cellular networks in suspensions of colloids and liquid crystals

In mixtures of thermotropic liquid crystals with spherical poly (methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transformation. To characterize the process of network formation in terms of morphology, phase transformation kinetics, and mechanical properties, we have combined data from polarization and laser scanning confocal microscopy with calorimetric, NMR, and rheological results. Our data suggest that the mechanism of network formation is dominated by a broadened temperature and time interval of phase transformation rather than by particle size or concentration. The observation that the width of the transformation interval strongly depends on sample preparation supports the hypothesis that a third component, most likely alkane remnants slowly liberated from the particles, plays a crucial role. In addition, calorimetric findings for liquid crystal/colloid mixtures, heated and cooled up to 13 times, point to separation of the liquid crystal into two compartments with different phase transformation kinetics. This could be explained by redistribution and enrichment of alkane in the particle-composed network walls. A further increase of the storage modulus, G', and incomplete dissolution of the networks in the isotropic state indicate that the formation of two compartments during repeated temperature cycles stabilizes the network and confers strong memory effects.     

Electrophosphorescence and delayed electroluminescence from pristine polyfluorene thin-film devices at low temperature

Intrinsic long-lived electrophosphorescence and delayed electroluminescence from a conjugated polymer (polyfluorene) thin film is observed for the first time at low temperature. From bias offset voltage dependent measurements, it is concluded that the delayed fluorescence is generated via triplet-triplet annihilation. A fast and efficient triplet exciton quenching by charge carriers is found to occur in the active polymer layer of the working devices.     

Relativistic electron correlation, quantum electrodynamics, and the lifetime of the 1s(2)2s(2)2p2p0(3/2) level in boronlike argon

The lifetime of the Ar13+ 1s(2)2s(2)2p2p0(3/2) metastable level was determined at the Heidelberg Electron Beam Ion Trap to be 9.573(4)(5). The accuracy level of one per thousand makes this measurement sensitive to quantum electrodynamic effects like the electron anomalous magnetic moment (EAMM) and to relativistic electron-electron correlation effects like the frequency-dependent Breit interaction. Theoretical predictions, adjusted for the EAMM, cluster about a lifetime that is approximately shorter than our experimental result.     

Radiation field and resonant two-center dielectronic transitions in relativistic ion-atom collisions

We consider projectile-electron excitation and loss in relativistic collisions of ionic projectiles with excited atoms. We show that under certain conditions electron transitions in the ion and atom can be resonantly coupled in the collision via the radiation field. The resonance becomes possible due to the Doppler effect, has a well-defined impact energy threshold, and clearly manifests itself in the cross sections. Since the range of the ion-atom interaction in the resonance case is very long, the presence of other atoms in the target medium as well as the size of the space occupied by the medium have to be taken into account. As a result, the cross section may become dependent on the density of the target atoms and/or the target size.