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K Müllen

Publications and source records attributed to K Müllen.

32 records · Page 2Linked to original sources
PubMed2001

Polyfluorenes with polyphenylene dendron side chains: toward non-aggregating, light-emitting polymers.

A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.

Journal Article↗
PubMed2001

New aspects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: a universal solvent-free sample preparation.

A method of solvent-free sample preparation is shown to be of universal applicability for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Results obtained were compared with those of traditional solvent-based sample preparation for MALDI-MS in order to demonstrate their similarities with respect to accuracy, sensitivity and resolution for polymers such as polystyrene and poly(methyl methacrylate) in a mass range from 2 to 100 kDa. The results revealed that there is fundamentally no difference in the quality of the obtained mass spectra, and we conclude that the mechanism of desorption and ionization remains unchanged. However, the solvent-free sample preparation turned out to have some advantages over the traditional method in certain cases: quick and easy applicability is shown for polyetherimide avoiding time-consuming optimization procedures. In particular, industrial pigments that are insoluble in common solvents were characterized without interfering signals from fragments. The method even showed improvements with respect to reproducibility and mass discrimination effects in comparison to traditional sample preparation. Additionally, this contribution provides new insight regarding the analyte/matrix preorganization for the desorption step which now appears to be independent of crystallinity.

Animals↗
PubMed2001

1- , 2-, and 3-dimensional polyphenylenes--from molecular wires to functionalised nanoparticles.

Polyphenylenes, ranging from one-dimensional wire-like conjugated polymers to two-dimensional disc-shaped polyaromatic hydrocarbons and three-dimensional sphere-like dendrimers, have been prepared using methods that allow synthetic control of their molecular and supramolecular order in order to optimise their physical, especially optical and electrical, properties. 1D-conjugated polymers can be used as emitting materials in LEDs, with their colours tuned so as to give emission across the whole visible spectrum. Their supramolecular order can be manipulated by attachment of bulky sidechains to suppress aggregation, and by formation of rod-coil block copolymers. 2D polycyclic aromatic hydrocarbons form stable columnar mesophases with high charge carrier mobilities. Their size, shape, and substitution patterns can be altered so as to maximise their intra- and intercolumnar order. 3D polyphenylene dendrimers can be prepared in ways that enable control of their shape, and their surfaces can be selectively functionalised in ways that permit them to act as functionalised nanoparticles.

Journal Article↗
PubMed2000

Synthesis and self-assembly of functionalized hexa-peri-hexabenzocoronenes.

Monolayers of hexa-alkyl substituted derivatives of hexa-peri-hexabenzocoronene (HBC) 1b have previously been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is expected that different functional groups (electron donating or withdrawing) connected to the aromatic core will influence the packing pattern and possibly the current-voltage characteristics as well. In order to provide suitable model systems, a new synthetic approach to synthesize functionalized HBC derivatives has been developed. This was accomplished by [4 + 2]-cycloaddition of suitably bromo-substituted diphenylacetylenes and 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones followed by an oxidative cyclodehydrogenation with iron(III) chloride/nitromethane. Using this strategy three different substitution patterns were synthesized: 2-bromo-5,8,11.14,17-pentadodecylhexa-pecri-hexabenzocoronene (2a), 2,5-dibromo-8,11,14,17-pentadodecylhexa-peri-hexabenzocoronene (2b), and 2,11-dibromo5,8,14,17-pentadodecylhexa-peri-hexa-benzocoronene (2c). These bromo-substituted HBC derivatives were subjected to palladium catalyzed coupling reactions to give donor (alkoxy, amino) as well as acceptor (ester, cyano) substituted derivatives. The self-assembly of these new HBC derivatives was studied in the bulk as well as at an interface. DSC, optical microscopy, and X-ray diffraction revealed the existence of columnar mesophases. The bulk structure in the mesophase is largely insensitive to changes of the substitution pattern; however, in situ scanning tunneling microscopy at the solid-fluid interface between an organic solution of the HBC derivative and highly oriented pyrolytic graphite reveals different packing patterns of the first adsorbed monolayer.

Journal Article↗
PubMed2000

New thermotropic dyes based on amino-substituted perylendicarboximides.

We report on an facile multigram synthesis of novel thermotropic dyes. When these dyes are heated they undergo an irreversible change from a photostable primary color to a photostable secondary color. Our concept is based on thermally initiated fragmentation to afford a chromophore with a donor and an acceptor substituent. We used thermally unstable alkoxycarbonyl substituents as masking groups for the strong electron-donating primary amino group attached to a perylenedicarboximide chromophore. On heating the alkoxycarbonylated dyes, the parent primary amino-functionalized dyes are regenerated in almost quantitative yield by elimination of an alkene and carbon dioxide. This process is accompanied by a large bathochromic shift in the absorption spectrum. Variations in the alkoxycarbonyl substitution lead to different reaction temperatures for the color change in this class of materials. The synthetic route used for the amino-functionalized perylene imide dyes involves the palladium-catalyzed amination of bromo-substituted precursors.

Journal Article↗
PubMed2000

Scanning tunneling microscopy: a unique tool in the study of chirality, dynamics, and reactivity in physisorbed organic monolayers.

Scanning tunneling microscopy (STM) is applied to study organic monolayers, physisorbed at the liquid-graphite interface. Due to the very local nature of the probing, the structure of these adlayers has been imaged with very high detail. The high resolution allowed us to investigate the effect of molecular chirality on the monolayer formation and provided a unique way to study chemical reactions at the liquid-graphite interface. Making use of a fast scanning mode, dynamic processes in these adlayers have been visualized.

Absorption↗
PubMed1999

Fluorescence Detection from Single Dendrimers with Multiple Chromophores.

The differences in the fluorescence behavior of a polyphenylene dendrimer with eight peryleneimides chromophores (1) and a single hexaphenylperyleneimide chromophore have been investigated at a single-molecule level through the combination of ultrasensitive fluorescence detection and microscopy.

Journal Article↗
PubMed1999

Template-Mediated Synthesis of Polycyclic Aromatic Hydrocarbons: Cyclodehydrogenation and Planarization of a Hexaphenylbenzene Derivative at a Copper Surface.

The deposition of polycyclic aromatic hydrocarbons on substrate surfaces is a key step for using such disklike molecules in nanoelectronic devices. An alternative to the (frequently problematic) preparation by evaporation or deposition from solution is to use the substrate surface as a template for the planarization that accompanies the cyclodehydrogenation (see picture).

Journal Article↗
PubMed1999

Highly Ordered Columnar Structures from Hexa-peri-hexabenzocoronenes-Synthesis, X-ray Diffraction, and Solid-State Heteronuclear Multiple-Quantum NMR Investigations.

Improved long-range ordering in the columnar mesophase of hexa(para-n-dodecylphenyl)hexabenzocoronene 1 has been achieved by inserting phenyl rings between the extended aromatic core of hexabenzocoronene and the alkyl side chains, which are needed to form liquid crystalline phases. The long-range hexagonal order of the columns is demonstrated by X-ray scattering, while the improved packing of the aromatic cores within the columns and the molecular mobility is probed by a newly developed heteronuclear multiple-quantum MAS NMR technique.

Journal Article↗