Sigurd Schrader

OLEDs based on new oxadiazole derivatives

The absorption, photoluminescence and electroluminescence properties of systematically modified poly(1,3,4-oxadiazole)s are reported and compared with low molar mass compounds. Poly(1,3,4-oxadiazole)s are functionalised by introducing pendent alkyl side chains and tetraphenylsilane or hexafluoroisopropylidene group (6F) into the main chain, respectively. The photo-and electroluminescence of single layer devices was found to be in the blue and green spectral range. A further strategy was to start with the optimisation of the substituents of low molar mass compounds followed by bonding the optimised structure to a flexible PMMA main chain. It was demonstrated that it is possible to preserve the optical properties of the oxadiazole unit and at the same time to improve film forming properties of the final polymer.

Ellipsometric and atomic force microscopic investigations on poly(para-phenylenevinylene) and poly(phenylquinoxaline) thin films.

Abstract We have investigated the optical properties of poly( para -phenylenevinylene) (PPV) prepared via the standard precursor route and poly(phenylquinoxalines) (PPQ). The complex refractive indices are (1,75-i.0.004) for PPQ and (1,62-i.0.002) for PPV. Atomic Force Microscopy and multiple angle of incidence ellipsometric investigations are reported.

Influence of alkoxy substituents on the exciton binding energy of conjugated polymers

Abstract Scanning tunneling microscope (STM) spectroscopy and optical absorption measurements were used to determine the exciton binding energy (Eb) of poly(p-phenylenevinylene) (PPV). We find Eb=0.48±0.14 eV, which is significantly higher than the value of Eb reported previously for an alkoxy-PPV derivative. Furthermore, an analysis of photoluminescence (PL) and STM excited cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV derivatives, reveals that the energy separation between the dominant peaks in the vibronic progression is significantly smaller in the alkoxy-substituted compounds. The reduction of Eb appears to be related to the influence of the alkoxy side-chains, inducing a reduction of both the Coulomb interaction as well as the molecular structural relaxation energy of the exciton.

Gas-phase deprotonation energies of sulfuric acid, perchloric acid, chlorosulfuric acid and fluorosulfuric acid

Gas-phase deprotonation energies (DEs) of sulfuric acid, perchloric acid, chlorosulfuric acid and fluorosulfuric acid have been calculated by applying the {MP2(fc)/6-311++G(d,p)//6-31+G(d)+ZPVE[6-31+G(d)]} abinitio MO level of calculation. The DEs obtained increase in the following order: H2SO4 > HClO4 > FSO3H > ClSO3H. Combining these results with experimental data and previous density functional theory (DFT) findings, we recommend for the DEs: 308 ± 3 kcal mol−1, 300 ± 3 kcal mol−1, 298 ± 3 kcal mol−1 and 295 ± 3 kcal mol−1, respectively. It is remarkable that chlorosulfuric acid is predicted to be a stronger acid in the gas phase than fluorosulfuric acid, in agreement with DFT and in contrast to previous supposition.

Sensitivity analysis of ellipsometry applied to uniaxial optical films

Abstract A quantitative study of the sensitivity of ellipsometry applied to uniaxial optical films is reported. A general formalism for optimization of the sensitivity for a layered sample has been developed which is based on a combined variation of wavelength and angle of incidence of the probe beam. For the air/oligophenylenevinylene(OPV 5 )/crystalline silicon system a numerical calculation is performed to determine sensitivity of ellipsometric angles and correlation of variables.

Monte Carlo simulations of charge transport in molecular solids : a modified Miller Abrahams type jump rate approach

Abstract Charge transport properties in disordered organic materials show some universal characteristics which are related to the “intrinsic” degree of disorder both energetically and spatially rather than to the dominance of impurity effects. This behaviour is usually explained in terms of charge transport via elementary hopping processes between localised “sites” in terms of Gaussian Disorder Model. Quantum mechanical models for the charge transfer in donor–acceptor complexes in the diabatic approximation suggest electronic transfer rates which are dependent on the energy difference between initial and final state as well as some reorganisation energy in both directions of the electron transfer, i.e., independent of the sign of the energy difference. This contrasts a ubiquitous factor 1 for jumps downward in energy and gives rise to what we believe is a more accurate description of the dynamic properties of charge transport. In order to clarify the influence of such a modification on the macroscopic charge transport, we set up dynamic MC calculations. The temperature and field dependence of the mobility for both jump rate expressions are compared. We study the effect of spatial and energetic Gaussian disorder and discuss implications for the interpretation of experimental data.

New chromophores and polymers for second-order nonlinear optics

Linear conjugated, fluorine-containing chromophores are differing from conventional chromophores like disperse red one by a reduced linear refractive index. Second order nonlinear optical susceptibilities of asymmetrically substituted fluorine-containing dyes reach, however, values as high as those of DR I or of similar chromophores. This can be concluded from measurements of solvatochromy of absorption and fluorescence spectra, electroabsorption measurements, and from quantum chemical calculations carried out for a series of azomethine and azo dyes. The chemical constitution of the conjugated part of investigate chromophores has an important influence both on linear and non nonlinear optical properties. So azomethine dyes show a blue shifted absorption in comparison to the absorption of azo chromophores. This goes along with lower off-resonant second order susceptibility. Introduction of fluorine into the acceptor part of the molecule by attaching a CF3- group does not lead to a significantly lower second order susceptibly in comparison to non-fluorinated chromophores. Perfluorination of the acceptor part result in case of the investigated molecules in an increased second order susceptibility as compared to the non-fluorinated chromophore. Polymers which contain these chromophores are either prepared by physical doping or by covalent bonding of the dyes to the polymeric main chain.In case of physically doped polymers the classes of thermally stable allylic polymers and of polyphenyl quinoxalines (PPQs) were used as matrix material. These polymers show miscibility with the investigated chromophores up to high concentrations and are haracterized by glass transition temperatures between 140 degrees C and 180 degrees C or 180 degrees C and 350 degrees C (PPQs) dependent on chemical structure and on dye concentration. Polymers which contain the active chromophores in side chains were prepared on basis of maleic acid anhydride or as novolak epoxy amine aduct polymer. The former can be utilized for preparation of waveguides by means of Langmuir-Blodgett technique while the later provide films of excellent quality by spin-coating and subsequent corona poling. Second order susceptibilities of the polymer films as determined by second harmonic generation reach values between 5 and 27 pm/V dependent on chromophore concentration and preparation conditions.

Structural and optical properties of self-assembled multilayers based on organic zirconium bisphosphonates

Abstract Multilayer films of zirconium terphenylbisphosphonic acid (TPBP) and zirconium quarterphenylbisphosphonic acid (QPBP) were prepared on ITO substrates via self-assembly. This deposition technique requires covalent attachment of a phosphonic acid anchoring layer appropriate for the ITO substrate. The functionalized substrate is then alternately exposed to aqueous ZrOCl2 and TPBP/QPBP solutions to deposit multilayer films. Investigation of these films by ellipsometry shows a linear increase of film thickness with the number of deposited layers. The average layer thickness indicates that the molecules of the phosphonic acid are inclined by 45–60° depending on the deposition conditions, this was proven by near-edge X-ray absorption fine structure (NEXAFS) measurements. For optical characterization fluorescence spectra of these films were recorded and analyzed. In order to test the ability of phosphonate films to act as electroluminescent materials, single layer devices have been prepared by using the ITO as anode and evaporating an aluminum cathode on top of the self-assembled multilayer film. Current–voltage and electroluminescence–voltage characteristics were recorded.

Dye-fluoropolymer nanocomposite film deposition in vacuum

Abstract Dye-filled fluoropolymer nano-composite films were deposited by means of vacuum co-deposition using either thermal evaporation of bulk material both of dye and polymer or by using laser beam evaporation of composite targets prepared as mixture of dye and polymer. We studied optical properties of two types of dyes, phthalocyanines (Pc) and 1,2-dihydroxyantraquinone (Aq) in polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE) matrix. Absorption spectra of the films composed of PTFE and metal-free phthalocyanine (H 2 Pc) revealed that the chromophores form aggregates in the polymer matrix. A gradual change of dye concentration in the PTFE matrix towards lower values is connected with a change in structure of H 2 Pc aggregates. At high dye concentration, the 615 nm absorption band is more pronounced, while at low dye concentration the absorption band at 686 nm dominates. This effect is due to different size and structural states of dye nano-particles. Laser beam evaporation of pure VOPc and of dye with polymer leads to formation of phase 2 aggregates, usually formed only at high temperature, but here even at ambient substrate temperature. Using various evaporation methods and deposition conditions, it is possible to control dye aggregation in the polymer matrix and, therefore, optical film properties.

Proton affinities of some polyfluoroalkanes in comparison to the unsubstituted alkanes. The estimation of the proton affinities of polytetrafluoroethylene and polyethylene by applying theoretical methods

Abstract Applying ab initio and semiempirical quantum chemical methods, the proton affinities ( PA ) for some simple polytetrafluoroethylene ( PTFE ) models, and also for propane as a polyethylene ( PE ) model, have been estimated. Using AM1, a systematic (more than 20–30 kcal mol −1 ) overestimation of the proton affinities was found. In contrast, a much better agreement with ab initio data was obtained by using MNDO and PM3. While the former values are too high, an underestimation by PM3 was established. Reliable results are obtained, however, when the mean values from MNDO and PM3 for PA s are used. Further investigations were carried out, applying MNDO and PM3 to larger perfluorinated alkanes and all three semiempirical methods to alkanes. Using the mean PA s from MNDO and PM3, convergence values of 116 kcal mol −1 were obtained for PTFE and 154 kcal mol −1 for PE . According to the theoretical results, protonation of alkanes leads to a C=H bond cleavage in agreement with experiment. Ab initio MPn/6-31G*//3–21G ( n = 2,3) geometries of protonated propane, where all bonds are kept after protonation, correspond to a transition state. The calculated PA of approximately 135 kcal mol −1 falls between the respective values for perfluorinated alkanes and alkanes. Neither by ab initio nor by semiempirical procedures was it possible to locate an alkane/proton complex corresponding to a minimum. Consequently, one may conclude that protonation of PTFE is accompanied by moderate PA without bond cleavage, while PE may be protonated only by destruction of the C=H bond. The energy difference of these two processes is high (38 kcal mol −1 ).