03 Fakultät Chemie

Now showing 1 - 5 of 5

Open Access
The mechanism of intramolecular energy transfer in terminally substituted polyene molecules
(1993) Holl, Norbert; Port, Helmut; Wolf, Hans Christoph; Strobel, Hartmut; Effenberger, Franz
Absorption, fluorescence and excitation spectra of different terminally substituted polyenes have been recorded in the UV-Vis region. The observed intramolecular unidirectional energy transfer from a donor substituent to an acceptor at the other end of the chain is influenced but not inhibited by incorporation of a spacer into the polyene chain. In molecules without a spacer, internal conversion within the supermolecule can explain the observed transfer of energy. In molecules with a spacer interrupting the polyene chain the observed intramolecular energy transfer can be explained in terms of Förster transfer.
Open Access
Highly ordered LB films of DHAP : a donor acceptor substituted polyene
(1993) Schmelzer, Michael; Roth, Siegmar; Niesert, Claus-Peter; Effenberger, Franz; Li, Rui Cheng
The donor acceptor substituted polyene 5-(4-dihexadecylaminophenyl)-2-methyl-2,4-pentadienal (DHAP) is - due to its amphiphilic nature - a promising candidate for the Langmuir-Blodgett (LB) technique. The molecule was studied as a monolayer on a pure water surface under argon atmosphere. Monolayers and multilayers were transfered onto solid support. The films showed a high degree of order in the monolayer as well as in thick multilayers. LB films were investigated with polarisation dependent UV/VIS absorption spectroscopy. The film spectra showed additional absorption peaks which were not present in the spectra of the molecules in solution. The relative peak intensities were highly dependent on the orientation of the electric field vector with respect to the substrate normal. To get a detailed picture of the arrangement of the films on the molecular level, we studied the polarisation dependence of the FTIR spectra in different experimental set-ups. The spectra showed a perpendicular orientation of the conjugated system with respect to the substrate, whereas the backbone of the saturated hydrocarbon chains showed a medium tilt angle of 20.0° with respect to the substrate normal. X-ray small angle diffraction measurements were applied to investigate the thickness of the LB layers. From the position of the Bragg peaks, a thickness of 5.45 nm per bilayer can be calculated, in agreement with a bilayer model showing different molecular alignment of the molecules transferred during upstroke and downstroke.
Open Access
Localized states in anthrylpolyenes : influence of geometry
(1990) Heine, Barbara; Sigmund, Ernst; Maier, Stefan; Port, Helmut; Wolf, Hans Christoph; Effenberger, Franz; Schlosser, Hubert
Combining group theoretical arguments and (extended) Hückel calculations, it is shown that terminally anthryl-substituted polyenes exhibit states almost completely localized at the substituents. These localized states are related to characteristic optical transitions in anthracene which also are observed in anthrylpolyenes independent of the length of the polyene chain.
Open Access
Lowest excited singlet states of α-9-anthrylpolyenes
(1993) Holl, Norbert; Emele, Peter; Port, Helmut; Wolf, Hans Christoph; Strobel, Hartmut; Kesmarszky, Thomas; Effenberger, Franz
Properties of the electronically excited states of α-9-anthrylpolyenes are studied in n-hexane solutions by using optical absorption, emission and fluorescence excitation spectroscopy. The absorption spectra reveal a complex structure with contributions of both anthracene and polyene type bands but also of bands with mixed anthracence/polyene character. Small amounts of remaining impurities are chemically not separable but can be detected via fluorescence measurements. By computer-aided decomposition of spectra measured under variable selective excitation the actual, clearly chain-length dependent, fluorescences could be obtained.
Open Access
Deformation and fracture mechanical properties of precursor-derived Si-C-N ceramics
(2007) Janakiraman, Narayanan; Aldinger, Fritz (Prof. Dr.)
This thesis deals with the investigation of the deformation and fracture mechanical properties of precursor-derived (PDC) Si-C-N ceramics. The materials were synthesized from a liquid poly(ureamethylvinyl)silazane precursor. In order to access the intrinsic mechanical behavior of the materials, fully dense defect-free PDC specimens devoid of process induced extrinsic features were fabricated using a special casting and crosslinking process and controlled thermolysis procedures. The investigations were performed on a range of Si-C-N-(H) PDC with progressively varying material structures from hydrogenated amorphous to phase-separated nanocrystalline microstructures, from which the influence of material structure on the mechanical behavior was analyzed. The crack-tip fracture toughness (KI0) of the materials were evaluated using the novel crack opening displacement (COD) method. The estimated KI0 values ranged from 0.6 to 1.2 MPa m1/2. The variation in KI0 was well correlated with the structural evolution in the materials, effected by the progressive stripping of the one-fold coordinated hydrogen in the amorphous materials leading to increased network connectivity, and the nucleation and growth of turbostratic graphite (TG) and nanocrystalline SiC in the phase-separated materials. The net change in the resistance to fracture in these materials was effected by the change in the average fracture surface energy and crack deflection toughening. Crack deflection observed even in the amorphous materials revealed the presence of structural and compositional inhomogeneities. To further understand the cause and effect of crack deflection in terms of crack tip damage mechanisms, roughness analysis of fracture surfaces was carried out using the fractal approach. The analysis revealed self-affine scaling up to a correlation length of around 50 nm and a self-affine roughness exponent (ζ) of 0.8 ± 0.1 in all the materials, the latter in agreement with the universal roughness exponent conjectured in literature. However, no correlation was observed between the observed roughness exponents and the fracture toughness of the corresponding materials. Examination of the crack opening near the crack tip by high resolution AFM imaging revealed no persistent damage cavities along the crack, concluding that the fracture in the investigated Si-C-N ceramics proceeded in a brittle manner in the resolvable length scales, at crack velocities employed in the present experiments. The deformation behavior of the present Si-C-N ceramics under contact loading conditions was investigated using spherical as well as sharp (Vickers and Berkovich) indentation experiments. The elastic moduli and nanoindentation hardness evaluated from the analysis of the nanoindentation load-displacement curves correlated well with the evolution of material resistance to elastic and plastic deformation, commensurate with the structural and microstructural evolution in the materials. Analysis of the elastic and plastic deformation work quantities derived from the load-unload cycle in the Berkovich nanoindentation enabled the discrimination of the different plastic deformation characteristics of the amorphous and phase segregated materials. The equi-proportional variation of elastic and plastic deformation in the amorphous Si-C-N materials identical to vitreous silica indicated the anomalous character of plastic deformation in these materials that induced appreciable strain hardening under progressive densification. This was manifested in the load-dependant increase in hardness. The contrasting variation of plastic deformation work in the phase-separated materials indicated the emergence of an additional plastic deformation mechanism in these materials, that proceeded by a shear deformation promoted by the TG-phase. The anomalous densification behavior in amorphous Si-C-N materials also led to a load dependence of the strain rate sensitivity (m), also observed in vitreous silica, and controlled the evolution of indentation size effect (ISE). The magnitude and direction of ISE was determined by the relative dominance of the two concurrent effects, namely strain hardening and indentation creep deformation. The evolution of strain rate sensitivity in the range of investigated materials showed good agreement with the cluster model, which relates the increase in the number density of isolated regions in the microstructure to a corresponding increase in m. The non-densifying shear mode of plastic deformation in phase-separated materials led to a decrease in the strain hardening capability, increase in m and increased vulnerability to ISE.

03 Fakultät Chemie

Browse

Filters

Settings

Sort By

Results per page

Search Results