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U H Bunz

Publications and source records attributed to U H Bunz.

8 recordsLinked to original sources
PubMed2001

Organometallic dehydro[14] annulenes containing Vollhardt's cyclobutadiene: are CpCo-complexed cyclobutadienes more aromatic then benzene?

Pd-catalyzed coupling of 1,2-diethynyl-3,4-(bistrimethylsilyl)cyclobutadienecyclopentadienylcobalt to a series of 1-iodo-2-(trimethylsilylethynyl)benzenes and 1-chloro-4-trimethylsilylbut-1-ene-3-yne is followed by desilylation with potassium carbonate. Cu(OAc)(2)-promoted oxidative ring closure leads to dehydro[14]annulenes and dehydro[14]benzoannulenes fused to a cyclobutadiene(cyclopentadienylcobalt) complex. Five of these fused dehydroannulenes were structurally characterized. (1)H NMR spectroscopy of the organometallic dehydro[14]annulenes incorporating the (bistrimethylsilyl)cyclobutadiene(cyclopentadienylcobalt) unit suggested that the aromaticity of the fused cyclobutadiene complex might be stronger than that of benzene according to the ring-current criterion.

Journal Article↗
PubMed2001

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 1. Rotationally interrupted conjugation in the excited states of 1,4-bis(phenylethynyl)benzene.

A series of photophysical measurements and semiempirical calculations were carried out with 1,4-bis(phenylethynyl)benzene in search of evidence on the effects of phenyl group rotation and chromophore aggregation of oligo- and poly(phenyleneethynylene)s. It is suggested that planarization gives rise to relatively modest shifts of ca. 20-30 nm, which preserve the vibronic structure of the monomer and retain a high emission quantum yield. In contrast, it is proposed that aggregation gives rise to larger shifts and loss of vibronic structure.

Journal Article↗
PubMed2001

Oligonucleotide-directed assembly of materials: defined oligomers.

A nano-architectural system that has high variability while maintaining component specificity is described. Tetraphenylcyclobutadiene(cyclopentadienyl)cobalt complexes and phenyleneethynylene trimers were synthesized and subsequently modified with oligonucleotides utilizing standard phosphoramidite chemistry. The resulting oligonucleotide modified organics (OMOs) were characterized by UV-vis spectroscopy, fluorescence spectroscopy, and phosphate analysis. Hybridization of these OMOs resulted in a series of self-assembled oligomeric hybrids of varying length and topology. These hybrids were characterized by melting temperature, polyacrylamide gel electrophoresis, and fluorescence spectroscopy. This model system demonstrates the power of DNA to self-assemble modules of interest-independent of the module itself.

DNA↗
PubMed2001

Conjugated organometallic polymers containing Vollhardt's cyclobutadiene complex: aggregation and morphologies.

Organometallic polymers were prepared by acyclic diyne metathesis (ADIMET) or by Pd-catalyzed coupling of 1,3-diethynylcyclobutadiene(cyclopentadienyl)cobalt with a suitably substituted diiodobenzene. The polymers obtained by Heck coupling show a degree of polymerization (Pn) of 20-60. The monomers for ADIMET were made by the Pd-catalyzed coupling of [1,3-bis(trimethylsilylethynyl)-2,4-bis(trimethylsilyl)cyclobutadiene](cyclopentadienyl)cobalt to 1-bromo-2,5-dialkyl-4-propynylbenzenes in the presence of KOH in yields of 40-48%. The monomers carry hexyl, ethylhexyl, and (S)-3,7-dimethyloctyl side chains. Polymerization of the propynylated monomers furnishes organometallic polymers with a Pn of up to 230 arylene-ethynylene units. The polymers were fully characterized by polarizing microscopy, transmission electron microscopy, circular dichroism, differential scanning calorimetry, and X-ray diffraction (XRD). They show nematic, lyotropic liquid crystalline phases as well as chiroptical properties from which aggregation in poor solvents and in the solid state can be concluded. Lamellar or irregular honeycomb-shaped morphologies in these organometallic polymers can be detected by electron microscopy.

Journal Article↗
PubMed2001

Poly(p-phenyleneethynylene)s by alkyne metathesis.

Alkyne metathesis was effected by mixtures of Mo(CO)(6) and phenols at elevated temperatures based on an improved version of the method reported by Mortreux and Mori. When propynylated arenes are subjected to these reaction conditions, the formation of diarylalkynes is often quantitative and likewise allows the synthesis of alkyne-bridged macrocycles and polymers of high molecular weight. Alkyne metathesis made high-molecular weight poly[p-(dialkylphenylene)ethynylene]s (PPEs) accessible that show promise as active layers in organic semiconductor devices.

Journal Article↗
PubMed2001

Spectroelectrochemical study of the oxidative doping of polydialkylphenyleneethynylene using iterative target transformation factor analysis.

Iterative target transformation factor analysis (ITTFA) was used to determine the spectra of the individual species generated during the oxidative p-doping of films of poly(para-phenyleneethynylene) (PPE). UV-visible spectra of PPE films on transparent electrodes were obtained in-situ during an anodic sweep. ITTFA identified 4 species present during the oxidation, which we assign as neutral polymer, polaron species, bipolaron species, and a species formed by further bipolaron reaction. The region of electrochemical stability for each of these species was identified and their potential-dependent profiles were obtained. This work is the first deconvolution of conjugated polymer spectroelectrochemistry.

Electrochemistry↗