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Biomedical subjects

Klaus Müllen

Publications and source records attributed to Klaus Müllen.

At least 19 recordsLinked to original sources
PubMed2003

Ordered architectures of a soluble hexa-peri-hexabenzocoronene-pyrene dyad: thermotropic bulk properties and nanoscale phase segregation at surfaces.

An alkylated hexa-peri-hexabenzocoronene with a covalently tethered pyrene unit serves as a model to study self-assembling discotic pi-system dyads both in the bulk and at a surface. Wide-angle X-ray scattering, polarized light microscopy, and differential scanning calorimetry revealed bulk self-assembly into columnar structures. Relative to a control without a tethered pyrene, the new dyad exhibits a more ordered columnar phase at room temperature but with dramatically lowered isotropization temperature, facilitating homeotropic alignment. These two features are important for processing such materials into molecular electronic devices, e.g., photovoltaic diodes. Scanning tunneling microscopy at a solution-solid interface revealed uniform nanoscale segregation of the large from the small pi-systems, leading to a well-defined two-dimensional crystalline monolayer, the likes of which may be employed in the future to study intramolecular electron transfer processes at surfaces, on the molecular scale.

Journal Article↗
PubMed2003

A polyphenylene dendrimer-detergent complex as a highly fluorescent probe for bioassays.

The synthesis of a polyphenylene dendrimer carrying three perylenemonoimide dyes as well as one biotin group is presented. Due to the hydrophobic polyphenylene scaffold, this dendrimer is insoluble in water thus preventing investigations in aqueous media. However, the use of an appropriate detergent results in the formation of well-defined supramolecular dendrimer-detergent complexes being soluble in aqueous media. The dendrimer-detergent complexes have a constant hydrodynamic radius of 7.1 nm measured by light scattering and fluorescence correlation spectroscopy and exhibit a high stability in the presence of blood serum proteins. The specific binding of the dendrimer-detergent complexes carrying a single biotin group to the protein streptavidin is demonstrated using a magnetic bead assay.

Benzene Derivatives↗
PubMed2003

Stereodynamics and characterization of the hexa(4-n-dodecylbiphenylyl)benzene hexaanion that includes a twisted benzene core.

Hexa(4-n-dodecylbiphenylyl)benzene (HDBB) was reduced by a series of alkali metals in THF under high vacuum. Three reduction states were identified by NMR spectroscopy, namely the dianion, tetraanion and hexaanion. The NMR spectra of HDBB(6-) revealed a remarkable distortion of symmetry, which is interpreted by adoption of a twisted conformation of the central benzene ring and a slow rotation of the inner phenylene rings of the biphenyl units. Due to the surprising thermal stability of the hexaanion, a dynamic NMR investigation revealed the pseudorotation of the twisted conformation and the phenylene rotation mentioned above.

Journal Article↗
PubMed2003

Uniaxial alignment of the columnar super-structure of a hexa (alkyl) hexa-peri-hexabenzocoronene on untreated glass by simple solution processing.

Uniaxially aligned, thin films of a discotic columnar thermotropic liquid crystalline material can be prepared by a simple solution zone-casting method, without the need for modified surfaces or traditional alignment techniques. Atomic force microscopy together with X-ray diffraction reveals single-crystalline-like order over several square centimeters, far exceeding the requirements for application of such films in organic molecular electronic devices such as field-effect transistors.

Journal Article↗
PubMed2003

Attaching perylene dyes to polyfluorene: three simple, efficient methods for facile color tuning of light-emitting polymers.

The emission color of fluorene-based polymers can be facilely tuned across the whole visible spectrum by copolymerization with perylene dyes. Methods are demonstrated for incorporation of the dyes in the polymer mainchain, at the chain termini, or as side chains. Efficient energy transfer causes the emission to come solely from the dye units. Efficient LEDs have been made from the copolymers with dyes in the mainchain.

Journal Article↗
PubMed2003

Alpha-helical-within-discotic columnar structures of a complex between poly(ethylene oxide)-block-poly(l-lysine) and a hexa-peri-hexabenzocoronene.

Poly(ethylene oxide)-block-poly(l-lysine) (PEO-PLL) was complexed with an amphiphilic hexa-peri-hexabenzocoronene (HBC). This produced a thermotropic liquid crystalline material (PEO-PLL-HBC), which was investigated by FTIR spectroscopy and differential scanning calorimetry as well as by wide- and small-angle X-ray scattering. It was found that the poly(l-lysine) blocks form an alpha-helical secondary structure. Each helix is surrounded symmetrically by six discotic columns of HBC, which gives an alpha-helical-within-discotic column structural entity. The dense packing of these entities produces hexagonal sublattices (formed by the columns) in the frame of a two-dimensional hexagonal lattice (formed by the helices). An order-order transition from a columnar structure Col1 to Col2 was found at 54 degrees C. The unit cell constants are 5.75 nm (Col1) and 6.60 nm (Col2). The larger unit cell size of Col2 was explained by a higher intracolumnar order of the latter in which the packing distance of the disklike HBC cores is well-defined (0.353 nm). PEO-PLL-HBC combines essential features of liquid crystals with a basic structural element of proteins into a single material.

Biomimetic Materials↗
PubMed2003

Energy transfer within perylene-terrylene dendrimers evidenced by polychromatic transient absorption measurements.

The time dependent spectral properties of a first and a second generation dendrimer with peryleneimide chromophores at the rim and a terrylenediimide chromophore in the core were investigated by time resolved polychromatic transient absorption measurements. The obtained results of the dendritic structures were compared with those of three model compounds. In the perylene-terrylene dendrimers a very fast energy transfer process was observed. Besides energy transfer singlet-singlet annihilation could be observed. It could be concluded that competition between the rates of energy transfer and of singlet-singlet annihilation is dependent on the dendrimer generation.

Journal Article↗
PubMed2003

Changes in post-source decay fragmentation behavior of poly(methyl methacrylate) polymers with increasing molecular weight studied by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

In order to investigate the systematic changes in fragmentation behavior of poly(methyl methacrylate) (PMMA) with increasing molecular weight, alkali-metal cationized PMMA 20-mer, 60-mer and 100-mer were selected for post-source decay (PSD) fragmentation study by matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry. PMMA polymers were cationized with lithium, potassium and cesium cations to explore the influence of the cation size on the fragmentation behavior of the polymers. All PMMA polymers could be fragmented by MALDI-PSD and fragmentation of the MALDI ionized synthetic polymer of molecular weight 10 kDa is reported here for the first time. It was shown that an increasing molecular weight of the PMMA chain required an increase in the size of the cation to improve the intensity and the number of the fragments in the PSD spectrum. Some instrumental parameters had to be optimized prior to a successful PSD analysis of the largest PMMA polymers.

Journal Article↗
PubMed2002

Excitation energy transfer in dendritic host-guest donor-acceptor systems.

We report on a study of a physically formed host-guest system, which was designed to be investigated by fluorescence energy transfer. All donor and acceptor molecules used were cyanine dyes. Investigation was performed at the ensemble level as well as at the single-molecule level. The ensemble measurements revealed a distribution of binding sites as well for the donor as for the acceptor. Accordingly, we found a distribution of the energy transfer efficiency. At the single-molecule level, these distributions are still present. We could discriminate entities that show very efficient energy transfer, some that do not show any energy transfer and systems whose energy transfer efficiency is only about 50%. The latter allowed the time-resolved detection of energy transfer of single entities through the acceptor decay. Finally, we discuss the observation that the energy transfer efficiency fluctuates as a function of time.

Journal Article↗
PubMed2002

Antibunching in the emission of a single tetrachromophoric dendritic system.

The photophysics of a dendrimer containing four chromophores are investigated at the single-molecule level. First, the multichromophoric character of single dendrimers' absorption is probed by modulating the linear polarization of the excitation beam. Subsequently, using circular polarization, the same dendrimers are excited, and their fluorescence transients are recorded. Using pulsed excitation in combination with the classical Hanbury-Brown and Twiss coincidence setup the presented data demonstrate that efficient singlet-singlet annihilation ensures that always only one photon is emitted even when several excitations are generated in an individual multichromophoric molecule.

Journal Article↗
PubMed2002

Pyrolysis in the mesophase: a chemist's approach toward preparing carbon nano- and microparticles.

A mild pyrolytic method is proposed for the generation of different carbon micro- and nanoparticles that are either unprecedented or have never been reported under the present experimental conditions. A hexa-alkyl-substituted hexa-peri-hexabenzocoronene serves as a graphite-like starting compound that melts into a discotic liquid crystalline phase prior to heat-induced cross-linking and dehydrogenation. An essential feature of the process is that the liquid crystalline order persists even above 400 degrees C, i.e., during alkyl chain cleavage. The present approach bears a resemblance to carbomesophase formation during graphitization starting from pitch. The pyrolysis products are characterized by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, optical microscopy, selected area electron diffraction (SAED), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM).

Journal Article↗
PubMed2002

Single-crystal structures of polyphenylene dendrimers.

A series of first-generation polyphenylene dendrimers based on three different cores were prepared by Diels-Alder cycloaddition and their single-crystal structures were determined. Consisting exclusively of interlocked, twisted phenyl rings, these polyphenylene nanostructures have exciting structural and dynamic properties. Single crystals of dendrimers, suitable for X-ray structure analysis, were grown from different solvent mixtures by slow evaporation at room temperature. It should be pointed out that one of the described polyphenylene dendrimers represents up to now the biggest oligophenylene nanostructure from which crystallographic data is available.

Journal Article↗
PubMed2002

Photoinduced electron transfer in a rigid first generation triphenylamine core dendrimer substituted with a peryleneimide acceptor.

The electron-transfer process of a first generation dendrimer with a triphenylamine core substituted with one peryleneimide chromophore at the rim (N1P1) was investigated by steady-state and time-resolved spectroscopic techniques in two different solvents of medium and low polarity. Single photon counting experiments showed a fast charge separation and a thermally activated back reaction, which is uncommon for a polyaryl bridge or long-distance through-space electron transfer. The four exponential fluorescence decay can be traced to the presence of two subsets of molecules, which are constitutional isomers of N1P1. Although formally N1P1 resembles a donor-bridge-acceptor compound, detailed analysis of the data shows that the electron transfer occurs by a through-space mechanism. This amine core dendrimer has peculiar and unique characteristics resulting in the observation of efficient back transfer and delayed peryleneimide fluorescence in diethyl ether at 293 K and very long-lived charge recombination luminescence at 77 K.

Journal Article↗
PubMed2002

Epitaxial composite layers of electron donors and acceptors from very large polycyclic aromatic hydrocarbons.

Large polycyclic aromatic hydrocarbons (PAHs) can be considered as nanographenes, whose electron donating or accepting properties are controlled by their size and shape as well as functionalities in their periphery. Epitaxial thin films of them are targets for optoelectronic applications; however, large PAHs are increasingly difficult to process. Here we show that epitaxial layers of very large unsubstituted PAHs (C(42)H(18) and C(132)H(34)), as well as a mixed layer of C(42)H(18) with an electron acceptor, can be obtained by self-assembly from solution. The C(132)H(34) is by far the largest nanographene that up to now has been processed into ordered thin films; due to its size it cannot be sublimed in a vacuum. Scanning tunneling microscopy (STM) studies reveal that the interaction with the substrate induces a strong perturbation of the electronic structure of the pure donor in the first epitaxial monolayer. In a second epitaxial layer with a donor acceptor stoichiometry of 2:1 the molecules are unperturbed.

Journal Article↗
PubMed2002

Intramolecular Förster energy transfer in a dendritic system at the single molecule level.

The photophysics of a dendrimer containing four donor chromophores and one acceptor chromophore are studied at the single-molecule level. Upon excitation of the donors exclusive acceptor emission is observed due to efficient Förster energy transfer. For 70% of the molecules donor emission is observed after bleaching of the acceptor, leading to a reduction of the Förster energy transfer efficiency. Furthermore, we demonstrate that in this molecular system the donor chromophores do not bleach by a triplet-sensitized photooxidation.

Journal Article↗