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Ullrich Scherf

Publications and source records attributed to Ullrich Scherf.

15 recordsLinked to original sources
PubMed2006

Diphenyloligothiophene-based dichromophoric macrocycles and their ability to form donor/acceptor complexes.

A series of four dichromophoric rigid macrocycles 6a-6d, two with diphenyloligothiophene chromophores, the other two with more electron-rich diphenyl-EDOT or diphenyl-bis-EDOT chromophores, have been synthesized. The absorption spectrum of the diphenyl-bis-EDOT based macrocycle 6d displayed the most pronounced vibronic resolution with a well-resolved 0-0 transition, indicating a fully planarized geometry of the diphenyl-bis-EDOT chromophores. The (1)H NMR spectra of the macrocycles displayed weak to moderate chemical shifts of characteristic signals upon addition of pi-conjugated oligonitro-9-fluorenone acceptors. X-ray single-crystal analysis showed that columnar pi-stacked donor/acceptor complexes are formed with the stacks composed of alternating donor and acceptor molecules. The stoichiometry of the crystalline, dark-colored complexes was found to be 1:1 by elemental analysis and integration of the (1)H NMR peaks. The complex formation is accompanied by remarkably large Stern-Volmer constants of fluorescence quenching.

Journal Article↗
PubMed2006

Interaction between the conjugated polyelectrolyte poly{1,4-phenylene[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} copolymer and the lecithin mimic 1-O-(l-Arginyl)-2,3-O-dilauroyl-sn-glycerol in aqueous solution.

In this paper, the interaction between the water-soluble conjugated polyelectrolyte poly{1,4-phenylene[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} copolymer and the amino acid glyceride conjugate 1-O-(L-arginyl)-2,3-O-dilauroyl-sn-glycerol dichlorohydrate (a mimic for the phospholipid lecithin) has been studied in aqueous solution by electronic spectroscopy (absorption and fluorescence) and small-angle neutron scattering (SANS). A significant increase in the polymer fluorescence and blue shift in its emission are observed on association with the surfactant. This is suggested to be due to breakup of polymer aggregates. In addition, the spectroscopic and photophysical data suggest this is followed by the vesicle to ribbon transition characteristic of this surfactant, leading to incorporation of single chains of the polymer within mixed polymer-surfactant aggregates. Support for this comes from preliminary SANS measurements, from which evidence for polymer dissolution and formation of two-dimensional structures has been obtained.

Journal Article↗
PubMed2006

Preferential oxidative addition in Suzuki cross-coupling reactions across one fluorene unit.

The Suzuki-type cross-coupling reaction of 2,7-dihalofluorenes with 1 equiv of arylboronic acid and Pd2(dba)3/P(t-Bu3) as a catalyst system is investigated. The exclusive formation of the diarylated coupling product demonstrates that "preferential oxidative addition" is also applicable to fluorene monomers due to a controlled intramolecular motion of the regenerated Pd(0) catalyst across the "large" distance between the 2- and the 7-position of one fluorene monomer.

Journal Article↗
PubMed2006

Solubilization of polyelectrolytic hairy-rod polyfluorene in aqueous solutions of nonionic surfactant.

We report on the solubilization, phase behavior, and self-organized colloidal structure of a ternary water-polyfluorene-surfactant (amphiphile) system comprised of polyelectrolytic poly{1,4-phenylene[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) in nonionic pentaethylene glycol monododecyl ether (C12E5) at 20 degrees C. We show in particular how a high amount (milligrams per milliliter) of polyfluorene can be solubilized by aqueous C12E5 via aggregate formation. The PBS-PFP and C12E5 concentrations of 0.31 x 10(-4)-5 x 10(-4) M and 2.5 x 10(-4)-75 x 10(-4) M, respectively, were used. Under the studied conditions, the photoluminescence (PL), surface tension, static contact angle, and (pi-A) isotherm measurements imply that D2O-PBS-PFP(C12E5)x realizes three phase regimes with an increasing molar ratio of surfactant over monomer unit (x). First, for x < or = 0.5, the mixture is cloudy. In this regime polymer is only partially dissolved. Second, for 1 < or = x < or = 2, the solution is homogeneous. In this regime polymer is dissolved down to the colloidal level. Small-angle neutron scattering (SANS) patterns indicate rigid elongated (polymer-surfactant) aggregates with a diameter of 30 A and mean length of approximately 900 A. The ratio between contour length and persistence length is less than 3. Third, for x > or = 4, the solution is homogeneous and there is cooperative binding between polymer and surfactant. Surface tension, contact angle, and surface pressure remain essentially constant with increasing x. A PL spectrum characteristic of single separated polyfluorene molecules is observed. SANS curves show an interference maximum at q approximately 0.015 A(-1), indicating an ordered phase. This ordering is suggested to be due to the electrostatic repulsion between polymer molecules adsorbed on or incorporated into the C12E5 aggregates (micelles). On dilution the distance between micelles increases via 3-dimensional packing. In this regime the polymer is potentially dissolved down to the molecular level. We show further that the aggregates (x = 2) form a floating layer at the air-water interface and can be transferred onto hydrophilic substrates.

Fluorenes↗
PubMed2006

How single conjugated polymer molecules respond to electric fields.

Conjugated polymers find applications in a range of devices such as light-emitting diodes, field-effect transistors and solar cells. The elementary electronic response of these semiconductors to electric fields is understood in terms of nanoscale perturbations of charge density. We demonstrate a general breaking of spatial charge symmetry by considering the linear Stark effect in the emission of single chromophores on individual chains. Spectral shifts of several nanometres occur due to effective dipoles exceeding 10 D. Although the electric field does not ionize the exciton, some molecules exhibit field-induced intensity modulations. This quenching illustrates the equivalence of charge symmetry breaking and polaron-pair or charge-transfer-state formation, and provides a microscopic picture of permanent charging, which leads to doping and exciton dissociation in actual devices. In addition to using this tuneable emission in single-photon electro-optic modulators, hysteresis in the Stark shift suggests a route to designing nanoscale memory elements such as molecular switches.

Journal Article↗
PubMed2005

Synthesis and spectroscopy of an oligophenyl based cruciform with remarkable pi-pi assisted folding.

A facile route has been developed for the preparation of a new family of oligophenyls based on a 2,5,2',5'-tetra-aryl substituted biphenyl structural motif. The cruciform terphenyl dimer 2,5,2',5'-tetra(4-tert-butylphenyl)-1,1'-biphenyl () has been prepared in a two step protocol as a representative of this interesting class of materials. The thermal behaviour of the cruciform was analysed by DSC and shows that forms an amorphous glass when cooled from the isotropic melt. Subsequent heating reveals a glass transition temperature at 130 degrees C. X-Ray single crystal structure analysis of 2,2'-bis(4-tert-butylphenyl)-1,1'-biphenyl () and shows that both these molecules with a quater-phenyl substructure adopt a folded solid-state structure. Examining the (1)H NMR spectra of and reveals that the interactions that induce this folding in the solid-state are sufficiently strong to bias foldamer formation also in solution. Consequently, it is reasonable to assume that the folded conformation within the lattice is due to intramolecular pi-pi interaction rather than being imposed by crystal packing. The optical properties of the cruciform terphenyl dimer are discussed relative to the linear analogue 1,4-bis(4-tert-butylterphenyl)benzene ().

Biphenyl Compounds↗
PubMed2005

Assembling and orientation of polyfluorenes in solution controlled by a focused near-infrared laser beam.

Ordered fibril- and particle-like assemblies of poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) can be formed by photon force of a focused near-infrared laser beam during the drying process of its tetrahydrofuran solution on a glass substrate. These formations have been achieved controllably by combining laser irradiation with convection in the cast solution; that is, when viscous drag of the solution in the convection is stronger than the photon force, the fibril-like assemblies can be formed. Molecular orientation in the assemblies differs from that in self-assembled fibril-like structures, and maybe it can be controlled by the polarization direction of the focused laser beam. We have demonstrated that the length and width of the assemblies can be controlled by the irradiation time, the laser power, the concentration of the solution, and the convection rate in the solution. On the other hand, when the viscous drag of the solution in the convection is weak compared to the photon force, particle-like assemblies in which molecular orientation is controlled by polarization direction are formed.

Journal Article↗
PubMed2005

The efficiency of light-emitting electrochemical cells.

We report on the efficiency behavior of light-emitting electrochemical cells (LECs) fabricated from a methyl-substituted ladder-type poly(p-phenylene) (mLPPP) that was blended with a crown ether based solid state electrolyte. Unlike organic light-emitting diodes (oLEDs) utilizing mLPPP as an active layer, the LECs suffer from a loss of efficiency at elevated current densities. From scan rate dependent studies we deduce that this efficiency drop is not only due to device decomposition upon high voltage operation and we also reveal the intrinsic mode of LEC operation. The decreasing width of the intrinsic region between the p- and n-type doped zones upon ongoing pin-junction formation causes distinct (either field or electrode induced) luminance quenching effects.

Journal Article↗
PubMed2005

Spin-conserving carrier recombination in conjugated polymers.

The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4-300 K enables a direct quantification of spin conversion in coulombically bound electron-hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined.

Electrons↗
PubMed2004

A universal picture of chromophores in pi-conjugated polymers derived from single-molecule spectroscopy.

Single-molecule spectroscopy can provide insight into the fundamental photophysics of large macromolecules containing tens of thousands of carbon atoms by circumventing disorder broadening. We apply this technique to comparatively ordered ladder-type poly(para-phenylene) and highly disordered poly(phenylenevinylene) (PPV), both of which are materials of substantial technological interest. Identical spectroscopic features are observed on the single-chromophore level, independent of the chemical structure or the chain morphology. Both materials exhibit narrow fluorescence lines down to 0.5 nm wide, which we attribute to the single-chromophore zero-phonon line, accompanied by a discrete vibronic progression providing a signature of the chemical structure. The chromophore units display spectral diffusion, giving rise to dynamic disorder on the scale of the linewidth. Although the energetic range of spectral diffusion is small, it can influence intramolecular excitation energy transfer and thus the overall molecular emission. The spectral diffusion dynamics of single chromophores are identical in both material systems and follow a universal Gaussian distribution. In the case of emission from multiple chromophores situated on the molecule, which we observe for PPV, spectral diffusion follows Lorentzian-like statistics. The fundamental difference between the two materials is the possibility of coherent interchromophoric coupling in PPV, resulting in strong spectral broadening caused by aggregation or superradiance. Such behavior is absent in the ladder-type polymers, where the linewidth of the emissive species is identical for all molecules. Our results demonstrate that structure-property correlations in conjugated polymers derive mainly from chain morphology rather than chromophoric properties and should be considered extrinsic in nature.

Journal Article↗
PubMed2003

Novel approaches to polymer blends based on polymer nanoparticles.

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.

Electric Power Supplies↗