348 research outputs found
An Improved Synthesis of Poly(p-phenylenebutadiynylene)s
ABSTRACT: A new methodology is described for the synthesis of poly(p-phenylenebutadiynylene)s based on the Pd/Cu-catalyzed, benzoquinone-mediated homocoupling of terminal acetylenes. Homopolymers synthesized from the 2,5-dialkoxy-1,4-diethynylbenzene monomers 1,4-bis(decyloxy)-2,5-diethynylbenzene and 1,4-bis(hexadecyloxy)-2,5-diethynylbenzene were largely insoluble, with the soluble portion from the polymerization of 1,4-bis(hexadecyloxy)-2,5-diethynylbenzene exhibiting a number-average molecular weight of 14,000. Completely soluble polymers were obtained from these precursors by the random copolymerization of these monomers. The materials exhibited number-average molecular weights ranging from 67,000 to over 150,000. The ultraviolet-visible and emission spectra of these polymers were examined and found to be very similar to those of structurally analogous poly(p-phenyleneethynylene)s and smaller poly(p-phenylenebutadiynylene)s reported by Kijima et al. (J Mat Chem, 1998, 8, 2165
Spray-Layer-by-Layer Carbon Nanotube/Electrospun Fiber Electrodes for Flexible Chemiresistive Sensor Applications
Development of a versatile method for incorporating conductive materials into textiles could enable advances in wearable electronics and smart textiles. One area of critical importance is the detection of chemicals in the environment for security and industrial process monitoring. Here, the fabrication of a flexible, sensor material based on functionalized multi-walled carbon nanotube (MWNT) films on a porous electrospun fiber mat for real-time detection of a nerve agent simulant is reported. The material is constructed by layer-by-layer (LbL) assembly of MWNTs with opposite charges, creating multilayer films of MWNTs without binder. The vacuum-assisted spray-LbL process enables conformal coatings of nanostructured MWNT films on individual electrospun fibers throughout the bulk of the mat with controlled loading and electrical conductivity. A thiourea-based receptor is covalently attached to the primary amine groups on the MWNT films to enhance the sensing response to dimethyl methylphosphonate (DMMP), a simulant for sarin nerve agent. Chemiresistive sensors based on the engineered textiles display reversible responses and detection limits for DMMP as low as 10 ppb in the aqueous phase and 5 ppm in the vapor phase. This fabrication technique provides a versatile and easily scalable strategy for incorporating conformal MWNT films into three-dimensional substrates for numerous applications.United States. Army Research Office. Institute for Soldier Nanotechnologies (Contract No. DAAD-19–02–0002
Red Phosphorescence from Benzo[2,1,3]thiadiazoles at Room Temperature
We describe the red phosphorescence exhibited by a class of structurally simple benzo[2,1,3]thiadiazoles at room temperature. The photophysical properties of these molecules in deoxygenated cyclohexane, including their absorption spectra, steady-state photoluminescence and excitation spectra, and phosphorescence lifetimes, are presented. Time-dependent density functional theory calculations were carried out to better understand the electronic excited states of these benzo[2,1,3]thiadiazoles and why they are capable of phosphorescence.National Science Foundation (U.S.) (1122374)United States. Dept. of Energy. Office of Basic Energy Sciences (DE-FG02-07ER46474
One-Pot Regiodirected Annulations for the Rapid Synthesis of Ï-Extended Oligomers
We demonstrate the broad applicability of the annulation protocol combining, in one pot, a direct arylation and cross aldol condensation for the straightforward synthesis at gram-scale of Ï-extended thiophene-based scaffolds. The regiospecific direct arylation drives the subsequent cross-aldol condensation proceed under the same basic conditions, and the overall protocol has broad applicability in the synthesis of extended aromatics wherein the thiophene ring is annulated with furans, pyridines, indoles, benzothiophenes, and benzofurans. These scaffolds can be further elaborated into Ï-extended, highly fluorescent oligomers with a central deficient benzothiadiazole unit with up to nine aromatic rings through coupling reactions
Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals
We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.open2
Fluorofluorophores: Fluorescent Fluorous Chemical Tools Spanning the Visible Spectrum
“Fluoro” refers to both fluorescent and fluorinated compounds. Despite the shared prefix, there are very few fluorescent molecules that are soluble in perfluorinated solvents. This paucity is surprising, given that optical microscopy is a ubiquitous technique throughout the physical sciences and the orthogonality of fluorous materials is a commonly exploited strategy in synthetic chemistry, materials science, and chemical biology. We have addressed this shortage by synthesizing a panel of “fluorofluorophores,” fluorescent molecules containing high weight percent fluorine with optical properties spanning the visible spectrum. We demonstrate the utility of these fluorofluorophores by preparing fluorescent perfluorocarbon nanoemulsions.National Science Foundation (U.S.) (ECCS-0939514
Functionalized Poly(3-hexylthiophene)s via Lithium–Bromine Exchange
Poly(3-hexylthiophene) (P3HT) is one of the most extensively investigated conjugated polymers and has been employed as the active material in many devices including field-effect transistors, organic photovoltaics and sensors. As a result, methods to further tune the properties of P3HT are desirable for specific applications. Herein, we report a facile postpolymerization modification strategy to functionalize the 4-position of commercially available P3HT in two simple steps–bromination of the 4-position of P3HT (Br–P3HT) followed by lithium−bromine exchange and quenching with an electrophile. We achieved near quantitative lithium–bromine exchange with Br–P3HT, which requires over 100 thienyl lithiates to be present on a single polymer chain. The lithiated-P3HT is readily combined with functional electrophiles, resulting in P3HT derivatives with ketones, secondary alcohols, trimethylsilyl (TMS) group, fluorine, or an azide at the 4-position. We demonstrated that the azide-modified P3HT could undergo Cu-catalyzed or Cu-free click chemistry, significantly expanding the complexity of the structures that can be appended to P3HT using this method.National Science Foundation (U.S.) (ECCS-0939514
Nucleophilic Functionalization of the Calix[6]arene Para- and Meta-Position via p‑Bromodienone Route
It is here demonstrated that the p-bromodienone route,
previously reported for calix[4]arenes, is also effective for the functionalization
of the calix[6]arene macrocycle. Thus, alcoholic O-nucleophiles can be
introduced at the calix[6]arene exo rim. In addition, the reaction of a
calix[6]arene p-bromodienone derivative with an actived aromatic substrate,
such as resorcinol, led to the first example of a meta-functionalized,
inherently chiral calix[6]arene derivativ
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