Biomedical subjects
Publications and source records attributed to Dieter Neher.
A pentathiophene-based swivel cruciform, which allows rotation between the cruciform arms, was synthesized. Homogeneous microcrystalline films were processed from solution, and field-effect transistors utilizing this dimer gave hole mobilities up to 0.012 cm2/V.s.
It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements.
An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated.
The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported.
Polymer layers can exhibit significantly improved performances if they possess a multicomponent phase-separated morphology. We present two approaches to control the dimensions of phase separation in thin polymer-blend layers; both rely on polymer nanospheres prepared by the miniemulsion process. In the first approach, heterophase solid layers are prepared from an aqueous dispersion containing nanoparticles of two polymers, whereas in the second approach, both polymers are already contained in each individual nanoparticle. In both cases, the upper limit for the dimension of phase separation is determined by the size of the individual nanoparticles, which can be adjusted down to a few tens of nanometres. We also show that the efficiencies of solar cells using two-component particles are comparable to those of devices prepared from solution at comparable illumination conditions, and that they are not affected by the choice of solvent used in the miniemulsion process.
A series of chirally substituted poly (p-phenyleneethynylene) copolymers was prepared by alkyne metathesis of mixtures of two different 2,5-dialkyl-1,4-dipropynylbenzenes. One of the monomers was the chiral bis-2,5-(S)-3,7-dimethyloctyl-1,4-dipropynylbenzene, and the second one was an achiral dipropynyl monomer. If the content of chiral monomer is 25-50 mol %, unusually large chiroptical effects, that is, optical dissymmetries, result in absorption (g = -0.38) and emission (g = -0.19) of these copolymers. The large dissymmetries can be explained by a supramolecular ordering of the PPEs into stranded features that are visualized by dark-field transmission electron microscopy. The strands of chirally substituted PPEs display a striated structure that suggests that the whole feature is a single chirally twisted crystallite.
Recently, a novel class of photoaddressable polymers (PAPs) has been reported, which allows for reversible photorecording with values of the photoinduced birefringence in excess of 0.2. These polymers are copolymers of methacrylate-type monomers with a mesogenic and a nonmesogEnic photoactive azobenzene in the side chain. This report presents a detailed analysis of the three-dimensional photoinduced reorientation of side chains within thin layers of PAP materials, utilizing polarized UV/Visible spectroscopy, infrared, and photothermal deflection spectroscopy (PDS). The three-dimensional orientation distribution before and after illumination with 514 nm polarized light was investigated by angle-dependent UV/Vis absorption spectroscopy. As-prepared samples possess an uniaxial orientation with the photoactive units being preferentially aligned within the plane of the substrate layer. These films become biaxial upon photoalignment with polarized light incident normal to the substrate plane. For PAP materials containing mesogenic side chains, the out-of-plane absorption stays nearly unchanged, proving that the photoinduced reorientation occurs mainly within the layer plane. For a homopolymer bearing only a nonmesogenic azobenzene in the side chain, photobleaching is observed, associated with a disease in absorption in all three principle directions. This finding can be understood in terms of an orientational coupling between the photodegradation product and the active azobenzene units. Photodegradation was well established by infrared spectroscopy. Photothermal deflection spectroscopy has been performed to manifest photoinduced changes in the transparent wavelength region of the PAP material, below the absorption edge. In contrast to the pronounced optical changes in the UV/Vis and IR spectra, the absorption in this range remains nearly unchanged. This result provides evidence that the polymers can be well applied for holographic storage in thick-layer devices.