TY - JOUR A1 - Hoffmann, Katrin A1 - Dietzel, Birgit A1 - Schulz, Burkhard A1 - Reck, Guenter A1 - Hoffmann, Angelika A1 - Orgzall, Ingo A1 - Resch-Genger, Ute A1 - Emmerling, Franziska T1 - Combined structural and fluorescence studies of methyl-substituted 2,5-diphenyl-1,3,4-oxadiazoles - Relation between electronic properties and packing motifs JF - Journal of molecular structure N2 - Prerequisite for the rational design of functional organic materials with tailor-made electronic properties is the knowledge of the structure-property relationship for the specific class of molecules under consideration. This encouraged us to systematically study the influence of the molecular structure and substitution pattern of aromatically substituted 1,3,4-oxadiazoles on the electronic properties and packing motifs of these molecules and on the interplay of these factors. For this purpose, seven diphenyl-oxadiazoles equipped with methyl substituents in the ortho- and meta-position(s) were synthesized and characterized. Absorption and fluorescence spectra in solution served here as tools to monitor substitution-induced changes in the electronic properties of the individual molecules whereas X-ray and optical measurements in the solid state provided information on the interplay of electronic and packing effects. In solution, the spectral position of the absorption maximum, the size of Stokes shift, and the fluorescence quantum yield are considerably affected by ortho-substitution in three or four ortho-positions. This results in blue shifted absorption bands, increased Stokes shifts, and reduced fluorescence quantum yields whereas the spectral position and vibrational structure of the emission bands remain more or less unaffected. In the crystalline state, however, the spectral position and shape of the emission bands display a strong dependence on the molecular structure and/or packing motifs that seem to control the amount of dye-dye-interactions. These observations reveal the limited value of commonly reported absorption and fluorescence measurements in solution for a straightforward comparison of spectroscopic results with single X-ray crystallography. This underlines the importance of solid state spectroscopic studies for a better understanding of the interplay of electronic effects and molecular order. KW - Diphenyl-oxadiazoles KW - X-ray structure KW - Packing motif KW - Optical properties KW - Fluorescence quantum yield Y1 - 2011 U6 - https://doi.org/10.1016/j.molstruc.2010.11.071 SN - 0022-2860 VL - 988 IS - 1-3 SP - 35 EP - 46 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Emmerling, Franziska A1 - Orgzall, Ingo A1 - Dietzel, Birgit A1 - Schulz, Burkhard A1 - Larrucea, Julen T1 - Ordering the amorphous - Structures in PBD LED materials JF - Journal of molecular structure N2 - The class of 2,5 disubstituted-1,3,4-oxadiazoles containing a biphenyl unit on one side is intensively used as electron transport materials to enhance the performance of organic light emitting diodes (OLEDs). In contrast to the ongoing research on these materials insights in their structure-property relationships are still incomplete. To overcome the structural tentativeness and ambiguities the crystal structures of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, that of the related compound 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole and of 2-(4-biphenylyl)-5-(2,6-dimethylphenyl)-1,3,4-oxadiazole are determined. A comparison with the results of GAUSSIAN03 calculations and similar compounds in the Cambridge Structural Database leads to a profound characterization. KW - OLED KW - PBD KW - Diphenyl-1,3,4-oxadiazole KW - Crystallization Y1 - 2012 U6 - https://doi.org/10.1016/j.molstruc.2012.04.040 SN - 0022-2860 VL - 1030 IS - 23 SP - 209 EP - 215 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Schöne, Anne-Christin A1 - Richau, Klaus A1 - Kratz, Karl A1 - Schulz, Burkhard A1 - Lendlein, Andreas T1 - Influence of Diurethane Linkers on the Langmuir Layer Behavior of Oligo[(rac-lactide)-co-glycolide]-based Polyesterurethanes JF - Macromolecular rapid communications N2 - Three oligo[(rac-lactide)-co-glycolide] based polyesterurethanes (OLGA-PUs) containing different diurethane linkers are investigated by the Langmuir monolayer technique and compared to poly[(rac-lactide)-co-glycolide] (PLGA) to elucidate the influence of the diurethane junction units on hydrophilicity and packing motifs of these polymers at the air-water interface. The presence of diurethane linkers does not manifest itself in the Langmuir layer behavior both in compression and expansion experiments when monomolecular films of OLGA-PUs are spread on the water surface. However, the linker retard the evolution of morphological structures at intermediate compression level under isobaric conditions (with a surface pressure greater than 11 mN m(-1)) compared to the PLGA, independent on the chemical structure of the diurethane moiety. The layer thicknesses of both OLGA-PU and PLGA films decrease in the high compression state with decreasing surface pressure, as deduced from ellipsometric data. All films must be described with the effective medium approximation as water swollen layers. KW - Brewster angle microscopy KW - Langmuir monolayer KW - poly[(rac-lactide)-co-glycolide] KW - polyesterurethanes KW - spectroscopic ellipsometry Y1 - 2015 U6 - https://doi.org/10.1002/marc.201500316 SN - 1022-1336 SN - 1521-3927 VL - 36 IS - 21 SP - 1910 EP - 1915 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Sarauli, David A1 - Xu, Chenggang A1 - Dietzel, Birgit A1 - Stiba, Konstanze A1 - Leimkühler, Silke A1 - Schulz, Burkhard A1 - Lisdat, Fred T1 - Thin films of substituted polyanilines interactions with biomolecular systems JF - Soft matter N2 - We use substituted polyanilines for the construction of new polymer electrodes for interaction studies with the redox protein cytochrome c (cyt c) and the enzyme sulfite oxidase (SO). For these purposes four different polyaniline copolymers are chemically synthesized. Three of them are copolymers, containing 2-methoxyaniline-5-sulfonic acid with variable ratios of aniline; the fourth copolymer consists of 3-amino-benzoic acid and aniline. The results show that all polymers are suitable for being immobilized as thin stable films on gold wire and indium tin oxide (ITO) electrode surfaces from DMSO solution. This can be demonstrated by cyclic voltammetry and UV-Vis spectroscopy measurements. Moreover, cyt c can be electrochemically detected not only in solution, but also immobilized on top of the polymer films. Furthermore, the appearance of a significant catalytic current has been demonstrated for the sulfonated polyanilines, when the polymer-coated protein electrode is being measured upon addition of sulfite oxidase, confirming the establishment of a bioanalytical signal chain. Best results have been obtained for the polymer with highest sulfonation grade. The redox switching of the polymer by the enzymatic reaction can also be analyzed by following the spectral properties of the polymer electrode. Y1 - 2012 U6 - https://doi.org/10.1039/c2sm07261k SN - 1744-683X VL - 8 IS - 14 SP - 3848 EP - 3855 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Schöne, Anne-Christin A1 - Schulz, Burkhard A1 - Lendlein, Andreas T1 - Stimuli Responsive and Multifunctional Polymers: Progress in Materials and Applications JF - Macromolecular rapid communications Y1 - 2016 U6 - https://doi.org/10.1002/marc.201600650 SN - 1022-1336 SN - 1521-3927 VL - 37 SP - 1856 EP - 1859 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Schöne, Anne-Christin A1 - Kratz, Karl A1 - Schulz, Burkhard A1 - Lendlein, Andreas T1 - The relevance of hydrophobic segments in multiblock copolyesterurethanes for their enzymatic degradation at the air-water interface JF - Polymer : the international journal for the science and technology of polymers N2 - The interplay of an enzyme with a multiblock copolymer PDLCL containing two segments of different hydrophilicity and degradability is explored in thin films at the air-water interface. The enzymatic degradation was studied in homogenous Langmuir monolayers, which are formed when containing more than 40 wt% oligo(epsilon-caprolactone) (OCL). Enzymatic degradation rates were significantly reduced with increasing content of hydrophobic oligo(omega-pentadecalactone) (OPDL). The apparent deceleration of the enzymatic process is caused by smaller portion of water-soluble degradation fragments formed from degradable OCL fragments. Beside the film degradation, a second competing process occurs after adding lipase from Pseudomonas cepacia into the subphase, namely the enrichment of the lipase molecules in the polymeric monolayer. The incorporation of the lipase into the Langmuir film is experimentally revealed by concurrent surface area enlargement and by Brewster angle microscopy (BAM). Aside from the ability to provide information about the degradation behavior of polymers, the Langmuir monolayer degradation (LMD) approach enables to investigate polymer-enzyme interactions for non-degradable polymers. (C) 2016 Elsevier Ltd. All rights reserved. KW - Multiblock copolymer KW - Enzymatic polymer degradation KW - Oligo(omega-pentadecalactone) KW - Oligo(epsilon-caprolactone) KW - Langmuir monolayer degradation technique Y1 - 2016 U6 - https://doi.org/10.1016/j.polymer.2016.09.001 SN - 0032-3861 SN - 1873-2291 VL - 102 SP - 92 EP - 98 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Schöne, Anne-Christin A1 - Kratz, Karl A1 - Schulz, Burkhard A1 - Lendlein, Andreas T1 - Polymer architecture versus chemical structure as adjusting tools for the enzymatic degradation of oligo(epsilon-caprolactone) based films at the air-water interface JF - Polymer Degradation and Stability N2 - The enzymatic degradation of oligo(epsilon-caprolactone) (OCL) based films at the air-water interface is investigated by Langmuir monolayer degradation (LMD) experiments to elucidate the influence of the molecular architecture and of the chemical structure on the chain scission process. For that purpose, the interactions of 2D monolayers of two star-shaped poly(epsilon-caprolactone)s (PCLs) and three linear OCL based copolyesterurethanes (P(OCL-U)) with the lipase from Pseudomonas cepacia are evaluated in comparison to linear OCL. While the architecture of star-shaped PCL Langmuir layers slightly influences their degradability compared to OCL films, significantly retarded degradations are observed for P(OCL-U) films containing urethane junction units derived from 2, 2 (4), 4-trimethyl hexamethylene diisocyanate (TMDI), hexamethylene diisocyanate (HDI) or lysine ethyl ester diisocyanate (LDI). The enzymatic degradation of the OCL based 2D structures is related to the presence of hydrophilic groups within the macromolecules rather than to the packing density of the film or to the molecular weight. The results reveal that the LMD technique allows the parallel analysis of both the film/enzyme interactions and the degradation process on the molecular level. (C) 2016 Elsevier Ltd. All rights reserved. KW - Langmuir technique KW - Oligo(epsilon-caprolactone) KW - Enzymatic degradation KW - Polymer architecture Y1 - 2016 U6 - https://doi.org/10.1016/j.polymdegradstab.2016.07.010 SN - 0141-3910 SN - 1873-2321 VL - 131 SP - 114 EP - 121 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Sarauli, David A1 - Borowski, Anja A1 - Peters, Kristina A1 - Schulz, Burkhard A1 - Fattakhova-Rohlfing, Dina A1 - Leimkühler, Silke A1 - Lisdat, Fred T1 - Investigation of the pH-Dependent Impact of Sulfonated Polyaniline on Bioelectrocatalytic Activity of Xanthine Dehydrogenase JF - ACS catalysis N2 - We report on the pH-dependent bioelectrocatalytic activity of the redox enzyme xanthine dehydrogenase (XDH) in the presence of sulfonated polyaniline PMSA1 (poly(2-methoxyaniline-5-sulfonic acid)-co-aniline). Ultraviolet-visible (UV-vis) spectroscopic measurements with both components in solution reveal electron transfer from the hypoxanthine (HX)-reduced enzyme to the polymer. The enzyme shows bioelectrocatalytic activity on indium tin oxide (ITO) electrodes, when the polymer is present. Depending on solution pH, different processes can be identified. It can be demonstrated that not only product-based communication with the electrode but also efficient polymer-supported bioelectrocatalysis occur. Interestingly, substrate dependent catalytic currents can be obtained in acidic and neutral solutions, although the highest activity of XDH with natural reaction partners is in the alkaline region. Furthermore, operation of the enzyme electrode without addition of the natural cofactor of XDH is feasible. Finally, macroporous ITO electrodes have been used as an immobilization platform for the fabrication of HX-sensitive electrodes. The study shows that the efficient polymer/enzyme interaction can be advantageously combined with the open structure of an electrode material of controlled pore size, resulting in good processability, stability, and defined signal transfer in the presence of a substrate. KW - enzyme bioelectrocatalysis KW - sulfonated polyanilines KW - xanthine dehydrogenase KW - pH-dependent electrochemistry KW - macroporous ITO electrodes Y1 - 2016 U6 - https://doi.org/10.1021/acscatal.6b02011 SN - 2155-5435 VL - 6 SP - 7152 EP - 7159 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Hamaciuc, E. A1 - Bruma, Maria A1 - Köpnick, Thomas A1 - Kaminorz, Yvette A1 - Schulz, Burkhard T1 - Synthesis and Study of New Silicon-containing Polyoxadiazoles N2 - A series of new poly-1,3,4-oxadiazoles has been synthesized by polycondensation reaction of hydrazine sulfate with a mixture of a dicarboxylic acid containing unsaturated bonds and a dicarboxylic acid containing silicon, by using methanesulfonic acid/phosphorus pentoxide as a reaction medium. These polymers were highly thermostable but they were only soluble in strong inorganic acids such as sulfuric or methanesulfonic ones. An alternative way was followed by using the corresponding dihydrazides containing unsaturated bonds and the corresponding diacid chloride containing silicon that reacted in N-methylpyrrolidinone (NMP) to give soluble silicon-containing unsaturated polyhydrazides, which were cyclodehydrated either by thermal or chemical treatment to give the corresponding polyoxadiazoles. Very thin coatings of polyhydrazides and polyoxadiazoles were deposited onto silicon wafers and they showed a very smooth surface, free of pinholes, when studied by atomic force microscopy (AFM). Some polyoxadiazole films showed strong blue photoluminescence. Y1 - 2001 ER - TY - JOUR A1 - Kaminorz, Yvette A1 - Schulz, Burkhard A1 - Brehmer, Ludwig T1 - Optical and Electrical Properties of Substituted 2,5-Diphenyl-1,3,4-Oxadiazoles N2 - New substituted 2,5-diphenyl-1,3,4-oxadiazoles are reported as luminescent materials in light emitting diodes LEDs . The investigated new oxadiazoles show efficient blue and green emission in single layer devices. The combination with a hole transporting and red emitting polythiophene led to a white emission with higher quantum efficiency (QE). Y1 - 2000 ER - TY - JOUR A1 - Karageorgiev, Peter A1 - Stiller, Burkhard A1 - Prescher, Dietrich A1 - Dietzel, Birgit A1 - Schulz, Burkhard A1 - Brehmer, Ludwig T1 - Modification of the surface potential of azobenzene-containing langmuir-blodgett films in the near field of a scanning kelvin microscope tip by irradiation Y1 - 2000 ER - TY - JOUR A1 - Giebler, Rainer A1 - Schulz, Burkhard A1 - Reiche, Jürgen A1 - Brehmer, Ludwig A1 - Wühn, Mario A1 - Wöll, Christoph A1 - Smith, Andrew Phillip A1 - Urquhart, Steven G. A1 - Ade, Harald W. A1 - Unger, Wolfgang E. S. T1 - Near-edge x-ray absorption fine structure spectroscopy on ordered films of an amphiphilic derivate of 2,5- Diphenyl-1,3,4-oxadiazole N2 - The surfaces of ordered films formed from an amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole by the Langmuir-Blodgett (LB) technique and organic molecular beam deposition (OMBD) were investigated by the use of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. For the assignment of the spectral features of the C, N, and O K- edge absorption spectra, fingerprint spectra of poly(p-phenylene terephthalamide)(Kevlar), poly(ethylene terephthalate), poly(p-phenylene-1,3,4-oxadiazole), and 2,5-di- (pentadecyl)-1,3,4-oxadiazole, which contain related chemical moieties, were recorded. Ab initio molecular orbital calculations, performed with explicit treatment of the core hole, are used to support the spectral interpretations. Angle-resolved NEXAFS spectroscopy at the C, N, and O K-edges suggests a preferentially upright orientation of the oxadiazole derivative in the outermost layer of the films. X-ray specular reflectivity data and molecular modeling results suggest a similar interpretation. Y1 - 1999 ER - TY - JOUR A1 - Bruma, Maria A1 - Schulz, Burkhard A1 - Köpnick, Thomas A1 - Stiller, Burkhard A1 - Belomoina, Nataliya A1 - Mercer, Frank W. T1 - Synthesis and study of aromatic polyamides containing silicon and phenylquinoxaline rings in the main chain N2 - New poly(phenylquinoxaline-amide)s with silicon in the main chain have been prepared by polycondensation reaction of a diacid chloride, namely bis(p-chlorocarbonyl-phenyl)-diphenylsilane, with aromatic diamines containing one or two phenylquinoxaline rings separated by a flexible bridge such as ether or methylene. These polymers were easily soluble in polar aprotic solvents such as N-methylpyrrolidinone (NMP) and dimethylformamide (DMF) and showed high thermal stability with decomposition temperature being above 450°C and glass transition temperature in the range of 260- 304°C. Polymer solutions in NMP were processed into thin flexible films which exhibited very smooth surfaces, free of pinholes when studied by atomic force microscopy. The free-standing films showed a dielectric constant in the range of 3.6-3.7. Y1 - 1999 ER - TY - JOUR A1 - Bruma, Maria A1 - Hamciuc, Elena A1 - Schulz, Burkhard A1 - Köpnick, Thomas A1 - Stiller, Burkhard A1 - Mercer, Frank W. T1 - Synthesis of fluorinated poly(phenylquinoxaline-amide)s and study of thin films made therefrom N2 - A series of five fluorinated poly(phenylquinoxaline-amide)s were synthesized by a polycondensation reaction of a diacid chloride containing the hexafluoroisopropylidene (6F) group, namely 2,2-bis(p-chlorocarbonylphenyl)- hexafluoropropane, with various aromatic diamines incorporating two phenyl-substituted quinoxaline rings. These polymers were easily soluble in polar aprotic solvents such as N-methylpyrrolione (NMP), dimethylformamide (DMF), and tetrahydrofurane (THF), and showed a high thermal stability with decomposition temperatures above 400 °C and glass transition temperatures in the range of 260-290 Tg. Polymer solutions in NMP were processed into free-standing films that showed low dielectric constant values, in the range of 3.4-3.9, and good mechanical properties, with tensile strength in the range of 40-80 MPa and elongation to break in the range of 22-55%. Very thin films, in the range of tens of nanometer, which were deposited onto silicon wafers exhibited very smooth surfaces, free of pinholes when studied by atomic force microscopy (AFM). Y1 - 1999 ER - TY - JOUR A1 - Bruma, Maria A1 - Schulz, Burkhard A1 - Köpnick, Thomas A1 - Stiller, Burkhard A1 - Mercer, Frank W. T1 - Study of thin films made from aromatic polyamides with silicon and phenylquinoxaline rings in the main chain N2 - Aromatic polyamides containing silicon and phenylquinoxaline rings in the main chain have been prepared by polycondensation reaction of a silicon-containing diacid chloride, namely bis(p-chlorocarbonylphenyl) -diphenylsilane, with various aromatic diamines having preformed phenylquinoxaline units. These polymers were easily soluble in polar aprotic solvents, such as N-methylpyrrolidinone (NMP) and dimethylformamide (DMF), and in tetrahydrofurane. They showed high thermal stability with decomposition temperature being above 450°C and glass transition temperature in the range of 253-304°C. Polymer solutions in NMP were processed into thin films having the thickness of tens of nanometer to 10 mm, by spin-coating onto glass plates or silicon wafers. The films had strong adhesion to substrates and exhibited very smooth surfaces, free of pinholes, in atomic force microscopy (AFM) studies. The free-standing films had dielectric constant in the range of 3.48-3.69. Thermal treatment of the films up to 350°C rendered them completely insoluble in organic solvents, while maintaining their smoothness and strong adhesion to the silicon substrate, and with no Tg in DSC experiments. Their FTIR spectra did not show any changes compared to the untreated films, meaning that polymers maintain their structural integrity at high temperature. Ó 1999 Elsevier Science S.A. All rights reserved. Y1 - 1999 ER - TY - JOUR A1 - Okatova, Olga V. A1 - Andreeva, Katharina A. A1 - Schulz, Burkhard A1 - Lavrenko, Peter N. T1 - Degradation of poly(m-phenylene oxadiazole) in concentrated sulfuric acid Y1 - 1999 ER - TY - JOUR A1 - Lavrenko, Peter N. A1 - Okatova, Olga V. A1 - Schulz, Burkhard A1 - Andreeva, Katharina A. A1 - Strelina, Irina A. T1 - Hydrodynamic and dynamo-optical properties of poly(1,3-phenylene-1,3,4-oxadiazole) molecules in sulphuric acid N2 - Translational diffusion of the macromolecules, intrinsic viscosity and flow birefringence induced in dilute solutions of poly(1,3-phenylene-1,3,4-oxadiazole) (PMOD) in conc. sulphuric acid has been investigated. Molecular-weight dependences of hydrodynamic and dynamo-optical properties are established over the M range from 8.1 103 to 87 103. Experimental data agree well with the theories developed for translational friction and intrinsic viscosity of the wormlike chains with the following molecular parameters: mass per chain unit ML = 22.7 Dalton/Å, the Kuhn segment length A = 59 ± 4 Å, the chain diameter d = 4 ± 1.5 Å. Hindrance to intramolecular rotation is characterized by the parameter s = 1.7. The shear optical coefficient was found to be approximately 1.7 times lower the value of that obtained in the same solvent for the para-phenylene isomer of this polymer, being in good agreement with higher equilibrium flexibility of the PMOD molecule chains in solutions as determined herein from the hydrodynamic data. Y1 - 1999 ER - TY - JOUR A1 - Reiche, Jürgen A1 - Freydank, Anke-Christine A1 - Helms, Andreas A1 - Geue, Thomas A1 - Schulz, Burkhard A1 - Brehmer, Ludwig A1 - Stiller, Burkhard A1 - Knochenhauer, Gerald T1 - Vacuum deposition films of oxadiazole compounds : formation and structure investigation N2 - The search for alternative routes of organic thin film formation is stimulated by the outstanding properties of these films in such fields as nonlinear optics, photonic data processing and molecular electronics. The formation of highly ordered multilayer structures by thermal vacuum deposition (VD) of organic compounds is an essential step toward the application of supramolecular organic architectures in technical systems. The VD of an amphiphilic substituted 2,5- diphenylene-1,3,4-oxadiazole 1 onto silicon substrates at defined temperature was used for the formation of ultrathin films. The structural data obtained for the VD-films of oxadiazole 1 by means of X-ray reflectivity, X-ray grazing incidence diffraction and atomic force microscopy (AFM) investigations indicate the formation of well ordered oxadiazole multilayers. The structure of the VD-multilayers is compared with that of Langmuir-Blodgett (LB) films and thermally treated LB-multilayers prepared from the same compound. Y1 - 1999 ER - TY - JOUR A1 - Lavrenko, Peter N. A1 - Andreeva, Katharina A. A1 - Strelina, Irina A. A1 - Garmonova, Tatjana I. A1 - Schulz, Burkhard T1 - Optical anisotropy and flexibility of poly(m-phenylene oxadiazole) in sulfuric acid Y1 - 1999 ER - TY - JOUR A1 - Orgzall, Ingo A1 - Lorenz, Bernd A1 - Mikat, Jürgen E. R. A1 - Reck, Günter A1 - Knochenhauer, Gerald A1 - Schulz, Burkhard T1 - Phase transition in 1,3,4-oxadiazole crystals under high pressure N2 - Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investigated at pressures up to 5 GPa using Raman and optical spectroscopy as well as energy dispersive X-ray techniques. At ambient pressure DNO shows an orthorhombic unit cell (a = 0.5448 nm, b = 1.2758 nm, c = 1.9720 nm, density 1.513 g cm-3) with an appropriate space group Pbcn. From Raman spectroscopic investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by absorption spectroscopy and X-ray measurements. Molecular modeling simulations have considerably contributed to the interpretation of the X-ray diffractograms. In general, the nearly flat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characterized by a stack-like arrangement of the DNO molecules. Only the mutual position of these molecular stacks changes due to the transformations so that this process may be described as a topotactical reaction. Phases II and III show a monoclinic symmetry with space group P21/c with cell parameters a = 1.990 nm, b = 0.500 nm, c = 1.240 nm, ß = 91.7°, density 1.681 g cm-3 (phase II, determined at 1. 1 GPa) and a = 1.890 nm, b = 0.510 nm, C = 1.242 nm, ß = 89.0°, density 1.733 g cm-3 (phase 111, determined at 2.0 GPa), respectively. The high-pressure phase IV stable at least up to 5 GPa shows again an orthorhombic structure with space group Pccn with corresponding cell parameters at 2.9 GPa: a = 0.465 nm, b = 1.920 nm, c = 1.230 nm and density 1.857 g cm-3 . For the first phase a blue pressure shift of the onset of absorption by about 0.032 eV GPa has been observed that may be explained by pressure influences on the electronic conjugation of the molecule. In the intermediate and high-pressure phases II-IV the onset of absorption shifts to increased wavelengths due to larger intermolecular interactions and enhanced excitation delocalization with decreasing intermolecular spacing. Y1 - 1999 ER -