Biomedical subjects

P Günter

Publications and source records attributed to P Günter.

At least 19 recordsLinked to original sources

PubMed2001

Highly functionalized dimeric tetraethynylethenes and expanded radialenes: strong evidence for macrocyclic cross-conjugation.

A selection of dimeric tetraethynylethenes (TEEs) and perethynylated expanded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structure of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)phenyl substituents, is presented. This macrocycle, the all-carbon core of which is isomeric with fullerene C60, adopts a non-planar, "chair-like" conformation. Also a TEE dimer, carrying N,N-dimethylaniline donor substituents, has been subjected to an X-ray crystallographic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, providing fundamental insight into mechanisms of pi-electron delocalization in the acyclic and macrocyclic chromophores. Donor or donor-acceptor-substituted dimeric TEE derivatives show very strong absorptions extending over the entire UV/Vis region; their longest wavelength absorption bands have high charge-transfer character. Macrocyclic cross-conjugation in the expanded radialenes becomes increasingly efficient with increasing donor-acceptor polarization. A dual, strongly solvent-polarity-dependent fluorescence was observed for a tetrakis(N,N-dimethylaniline)-substituted dimeric TEE; this interesting emission behavior is explained by the twisted intramolecular charge-transfer (TICT) state model. Donor-substituted expanded radialenes display huge resonance-enhanced third-order nonlinear optical coefficients.

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PubMed2001

Higher order ferroic switching induced by scanning force microscopy.

We present the observation of ferroelastoelectric switching in a ferroelectric material. It is achieved in barium titanate thin film by simultaneously applying electric field and compressive stress with the tip of a scanning force microscope. For low compressive stresses, the presented measurements reveal classical ferroelectric domain reversal, i.e., the spontaneous polarization is aligned parallel to the applied electric field. However, for high compressive stresses the direction of polarization after switching is antiparallel to the poling field, demonstrating ferroelastoelectric switching.

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PubMed2001

Pt-tetraethynylethene molecular scaffolding: synthesis and characterization of a novel class of organometallic molecular rods.

The series of monodisperse Pt-bridged TEE oligomers 3a-f was prepared by oxidative Glaser-Hay oligomerization of monomer 7 under end-capping conditions. These novel molecular rods extend in length from 3.3 nm (monomeric 3a) to 12.1 nm (hexameric 3 f). Their isolation was achieved by high performance gel permeation chromatography (GPC), and their purification was best monitored by analytical GPC in combination with matrix-assisted laser-desorption-ionization mass spectrometry (MALDI-TOF MS). The mass spectra of each oligomer revealed the molecular ion or its sodium complex as parent ion together with a clean, highly characteristic fragmentation pattern. Delayed addition of the end-capping reagent PhC(triple bond)CH to the oligomerization mixture afforded polymer 10 with an average of approximately 32 repeat units and a remarkably narrow molecular weight distribution (Mw/Mn=1.06), which is indicative of a living polymerization process. UV/Vis spectral data as well as measurements of the second hyperpolarizability gamma by third harmonic generation (THG) revealed a nearly complete lack of pi-electron delocalization along the oligomeric backbone. The Pt atoms act as true insulating centers, and the Pt-C(sp) bonds hardly possess any pi character. The synthesis of the molecular rods 3a-f provides another demonstration of the power of oxidative acetylenic homocouplings for the preparation of unusual nanoarchitecture.

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PubMed2000

Synthesis and physical investigation of donor-donor and acceptor-acceptor end-functionalized monodisperse poly(triacetylene) oligomers.

Two series of monodisperse, terminally donor-donor [D-D, D=4-(dimethylamino)phenyl] and acceptor-acceptor [A-A, A=4-nitrophenyl] functionalized poly(triacetylene) (PTA) oligomers ranging from monomer to hexamer were synthesized by oxidative Hay oligomerization under end-capping conditions. The corresponding D-D and A-A end-substituted polymers with an average degree of polymerization (DP) of n approximately 18 and n approximately 12, respectively, were also prepared and served as reference points for the corresponding infinitely long polymers. These terminally functionalized PTA oligomers and polymers are yellow- to orange-colored compounds, displaying excellent solubility in aprotic solvents with melting points above 200 degrees C for the hexamers. For the 4-(dimethylamino)phenyl substituted compounds, a consistent first oxidation potential around +0.42 V versus Fc/Fc+ (ferrocene/ferricinium) was observed, whereas the 4-nitrophenyl functionalized systems underwent a reversible reductive two-electron transfer around -1.40V versus Fc/Fc+. The nature of the end-groups has a dramatic influence on the electronic absorption spectra. Saturation of the linear optical properties in the D-D series occurs at significantly shorter chain-length [effective conjugation length (ECL) of n approximately 4 monomer units] than in the A-A substituted or the previously reported Me3Si- and Et3Si-end-capped PTA oligomer series (ECL: n approximately 10 monomer units). Similar observations with respect to the ECL were made by measurement of the Raman-active v(C triple bound C) stretches. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) experiments showed that shorter oligomers of terminally D-D or A-A functionalized PTAs display higher second hyperpolarizabilities gamma than the corresponding R3Si-end-capped series (R=alkyl). Moreover, they disclose a distinct peak of the nonlinearity per monomer unit at intermediate backbone lengths. In THG experiments, the second hyperpolarizabilities for long D/A-functionalized PTA oligomers attained the same saturation values as observed for the corresponding R3Si-end-capped rods. The nonlinearities measured by DFWM of the D-D and A-A substituted PTAs were found to be larger than for the silylated ones, which can be explained by the closeness of the two-photon resonance.

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PubMed2000