Polarization modification of PZT thin films by means of electric fields and stress in scanning force microscopy

Polarization switching in scanning force microscopy (SFM) is influenced by both electric fields and stress, whereby the latter can arise inherently from Maxwell stress. We discuss the influence of electric charges and of the polarization asymmetry on the switching behaviour. For single crystallites of PZT(53/47) thin films, the sectors for ferroelectric, ferroelastoelectric and ferroelastic switching are represented in a field-stress map. The influence of stress on the second harmonic of the SFM is also discussed.     

Nonlinear and bistable properties of doped PMMA lightguides

Polymer lightguides of PMMA doped with the azo dyes methyl-red and N, N-dihexyl-4-amino-4'-nitro-azobenzene (DHANA) are investigated with respect to their opto-optical properties. The trans-cis photoisomerisation of the dyes leads to reversible polarization-dependent optically induced refractive index changes. The index changes are anisotropic. Birefringence is measured using index matched lightguides. The changes with respect to each polarization are determined by leaky mode spectroscopy and by using a half-integrated interferometer. These changes are intensity dependent and, with a suitable geometry, bistability at low intensities (3 mW/mm³) can be obtained.     

An integrated organic photoconductive detector for optoelectronics

In the system (poly-(N-vinyl carbazole) (PVK): trinitrofluorenone (TNF) a photoconductive complex is formed. Thin metal films are sputtered on BK7 glass substrates, forming two electrodes with 1 mm separation. The photoconductive complex is deposited from a solution in chlorobenzene/tetrahydrofurane on the glass substrate between the electrodes, forming a photoconductive detector. Photoconductivity of these planar detectors is studied using He-Ne laser light (=633 nm) as a function of electric field and for different TNF concentrations. An increase of photosensitivity is found for high TNF concentrations.Time resolution of the photoconductive PVK:TNF detector is investigated using a ruby pulse laser (=694 nm). The possible use of such detectors in combination with other polymer lightguides is demonstrated. Compared to most polymer waveguide materials PVK exhibits a rather high refractive index ofn=1.7. Thus the coupling of light into the photoconductive film is achieved directly. The planar structure allows further integration of polymeric components for optoelectronics.     

Local elevation: A method for improving the searching properties of molecular dynamics simulation

Summary The concept of memory has been introduced into a molecular dynamics algorithm. This was done so as to persuade a molecular system to visit new areas of conformational space rather than be confined to a small number of low-energy regions. The method is demonstrated on a simple model system and the 11-residue cyclic peptide cyclosporin A. For comparison, calculations were also performed using simulated temperature annealing and a potential energy annealing scheme. Although the method can only be applied to systems with a small number of degrees of freedom, it offers the chance to generate a multitude of different low-energy structures, where other methods only give a single one or few. This is clearly important in problems such as drug design, where one is interested in the conformational spread of a system.     

Equilibrium of the reaction between dissolved sodium sulfide and biologically produced sulfur

The equilibrium of the heterogeneous reaction between dissolved sodium sulfide and biologically produced sulfur particles has been studied. Biologically produced sulfur was obtained from a bioreactor of a hydrogen sulfide removal process in which the dominating organism is Thiobacillus sp. W5. Detailed knowledge of this reaction is essential to understand its effect on the process. The results were compared with the equilibrium of the reaction of sulfide with ‘inorganic’ elemental sulfur. The equilibrium between dissolved sodium sulfide and biologically produced sulfur particles can be described by an equilibrium constant, K_x, which consists of a weighted sum of constants for polysulfide ions of different chain length, rather than a true single equilibrium constant. For biologically produced sulfur pK_x = 9.10 ± 0.08 (21 °C) and 9.17 ± 0.09 (35 °C) with an average polysulfide chain length x = 4.91 ± 0.32 (21 °C) and 4.59 ± 0.31 (35 °C). The pK_x value for biologically produced sulfur is significantly higher than for reaction of dissolved sodium sulfide with inorganic sulfur (pK_x = 8.82; 21 °C). This difference is probably caused by the negatively charged polymeric organic layer, which is present on biologically produced sulfur but absent with “inorganic” sulfur. Specific binding of polysulfide ions to the organic layer results in a higher polysulfide concentration at the reaction site compared to the bulk concentration. This results in an apparent decrease of the measured equilibrium constant, K_x.     

Preparation of quinoxalines, dihydropyrazines, pyrazines and piperazines using tandem oxidation processes

Alpha-hydroxyketones undergo MnO2-mediated oxidation followed by in situ trapping with aromatic or aliphatic 1,2-diamines to give quinoxalines or dihydropyrazines, respectively, in a one pot procedure which avoids the need to isolate the highly reactive 1,2-dicarbonyl intermediates. Modifications of the procedure allow the formation of pyrazines and piperazines.     

Chiroptical properties of 2-substituted pyrrolidines

The CD spectra of L-2-methylpyrrolidine, L-prolinol, and their N-methylated derivatives have been determined. As in the 2-substituted piperidines, N-methylation results in an inversion of the sign of the Cotton effects (CE). However, the sign of the long-wavelength CE does not follow the simple helicity rule found for 2-substituted piperidines, since the pyrrolidine ring is itself chiral and makes its own contributions to the observed CE's. The rotational contribution due to pyrrolidine ring chirality appears to be opposite in sign to and larger in magnitude than that due to the 2-substituent in both the secondary and the tertiary amines.     

Synthesis of Cannabinoid Model Compounds. Part 3. (6aR, 10aR)-N-ethyl-Δ8-tetrahydrocannabinol-18-amide, (6aR, 10aR, 17 RS)-N-ethyl-17-methyl-Δ8- tetrahydrocannabinol-18-amide and (6aR, 10aR)-17,18-didehydro-Δ8-tetrahydrocannabinol

The novel cannabinoids (6aR, 10aR)-N-ethyl-Δ⁸-tetrahydrocannabinol-18-amide (15) and (6aR, 10aR, 17 RS)-N-ethyl-17-methyl-Δ⁸- tetrahydrocannabinol-18-amide (16) , designed as cannabinoid affinity ligands, were synthesized from the corresponding acids 11 and 12 via the N-hydroxysuccinimide esters. Amide 16 was tested in the rat and was generalized to Δ⁹-tetrahydrocannabinol, being 5 times less potent than the training drug. An improved synthesis of (6aR, 10aR)-17,18-didehydro-Δ⁸-tetrahydrocannabinol (23) is reported. As model reaction for the preparation of a tritiated Δ⁸-tetrahydrocannabinol, compound 23 was selectively deuterated at C(17) and C(18) in benzene/Et₃N using [(C₆H₅)₃P]₃RuCl₂ as catalyst.