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2007
Directed growth of poly(isobenzofuran) films by chemical vapor deposition on patterned self-assembled monolayers as templates.
Langmuir,vol.23,p.2483–91,2007This paper describes a method to direct the formation of microstructures of poly(isobenzofuran) (PIBF) by chemical vapor deposition (CVD) on chemically patterned, reactive, self-assembled monolayers (SAMs) prepared on gold substrates. We examined the growth dependence of PIBF by deposition onto several different SAMs each presenting different surface functional groups, including a carboxylic acid, a phenol, an alcohol, an amine, and a methyl group. Interferometry, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), and optical microscopy were used to characterize the PIBF films grown on the various SAMs. Based on the kinetic and the spectroscopic analyses, we suggest that the growth of PIBF is surface-dependent and may follow a cationic polymerization mechanism. Using the cationic polymerization mechanism of PIBF growth, we also prepared patterned SAMs of 11-mercapto-1-undecanol (MUO) or 11-mercaptoundecanoic acid (MUA) by microcontact printing (microCP) on gold substrates as templates, to direct the growth of the PIBF. The directed growth and the formation of microstructures of PIBF with lateral dimensions of 6 microm were investigated using atomic force microscopy (AFM). The average thickness of the microstructures of PIBF films grown on the MUO and the MUA patterns were 400 +/- 40 nm and 490 +/- 40 nm, respectively. SAMs patterned with carboxylic acid salts (Cu2+, Fe2+, or Ag+) derived from MUA led to increases in the average thickness of the microstructures of PIBF by 10%, 12%, or 27%, respectively, relative to that of control templates. The growth dependence of PIBF on the various carboxylic acid salts was also investigated using experimental observations of the growth kinetics and XPS analyses of the relative amount of metal ions present on the template surfaces. -
2007
Quantification of amplified quenching for conjugated polymer microsphere systems.
Langmuir,vol.23,p.112–5,2007A series of nonaggregating carboxylate-functionalized poly(phenylene ethynylene)s (PPEs) have been synthesized for immobilization via electrostatic adsorption onto Eu3+-polystyrene microspheres with a mean diameter of 0.2 microm. This system is shown to constitute a ratiometric system that measures fluorescence quenching with high fidelity. The fluorescence quenching properties of the polymer-coated particles in response to methyl viologen and a naphthyl-functionalized viologen have been investigated in aqueous solutions to study the influence of electrostatic and hydrophobic interactions with pentiptycene-incorporated as well as macrocycle-containing polymers. -
2007
Enhanced Luminescence from Emissive Defects in Aggregated Conjugated Polymers.
Macromolecules,vol.40,p.8833–8841,2007Degradation experiments and model studies suggested that the longer lived green fluorescence from an aggregated poly(p-phenylene ethynylene) (PPE) was due to the presence of highly emissive, low-energy, anthryl defect sites rather than the emissive conjugated polymer excimers proposed in a previous report. After elucidating the origin of the green fluorescence, additional anthryl units were purposely incorporated into the polymer to enhance the blue-to-green fluorescence color change that accompanied polymer aggregation. The improved color contrast from this anthryl-doped conjugated polymer led to the development of crude solution-state and solid-state sensors, which, upon exposure to water, exhibited a visually noticeable blue-to-green fluorescence color change. -
2007
Main-Chain Calix[4]arene Elastomers by Ring-Opening Metathesis Polymerization
Macromolecules,vol.40,p.7437–7440,2007Tert-butyl- and adamantyl-substituted alkene-bridged calix[4]arene monomers were synthesized by ring-closing metathesis (RCM). All the three possible conformers (cone, partial cone, and 1,3-alternate) were used as comonomers with cyclooctene and norbornene in ring-opening metathesis polymn. (ROMP). The resultant polymers were high-mol.-wt., transparent and stretchable materials with high calixarene incorporation (up to 25 mol% or 70 wt%) and low glass transition temps. [on SciFinder(R)] -
2007
Ordering of Poly( p -phenylene ethynylene)s in Liquid Crystals
Macromolecules,vol.40,p.19–25,2007Poly(phenylene ethynylene)s were designed and synthesized that are completely sol. in nematic liq. cryst. solvents at mol. wts. (GPC, relative to PS) of more than 100,000. These materials experience a chain extended rod conformation under these conditions with the polymer chains highly aligned with the liq. crystal director and an enhanced conjugation length detd. by optical spectroscopy. Structure-property relationships are investigated that reveal the role of steric congestion about the polymer mainchain in detg. the order parameter of the polymers. It was also found that the order polymer parameters increased with increasing mol. wts. [on SciFinder(R)] -
2007
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2007An efficient, modular synthesis of triptycene derivatives is presented, in which the triptycene ring system is constructed from readily available anthraquinone and alkyne starting materials. A rhodium-catalyzed alkyne cyclotrimerization reaction serves as the key step in this new method for the preparation of these useful unnatural products.
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2007
Synthesis and characterization of fluorescent acenequinones as dyes for guest-host liquid crystal displays.
Organic letters,vol.9,p.997–1000,2007Syntheses and spectroscopic properties of alkoxy-substituted para-acenequinones are reported. These compounds showed excellent alignment in nematic liquid crystals as evidenced by polarized UV-vis absorption and fluorescence measurements. [structure: see text] -
2007
Energy migration in conjugated polymers: the role of molecular structure.
Philosophical transactions. Series A, Mathematical, physical, and engineering sciences,vol.365,p.1589–606,2007Conjugated polymers undergo facile exciton diffusion. Different molecular structures were examined to study the role of the excited state lifetimes and molecular conformations on energy transfer. There is a clear indication that extended fluorescence lifetimes give enhanced exciton diffusion as determined by fluorescence depolarization measurements. These results are consistent with a strong electronic coupling or Dexter-type energy transfer as the dominating mechanism. The control of polymer conformations in liquid crystal solvents was also examined and it was determined that more planar conformations gave enhanced energy transfer to emissive low band-gap endgroups. -
2007
Templated polypyrrole electro-polymerization: Self-assembled bundles of bilayer membranes of amphiphiles and their actuation behavior
Synthetic Metals,vol.157,p.733–738,2007The electrochem. properties of conducting polymers are highly dependent on the microstructure. The authors report a method to produce specific microstructures of polypyrrole through electropolymn. in the presence of the amphiphile N-{11-(2-hydroxyethyldimethylammonium)undecanoyl}-N,N’-dioctyl-L-glutamate, bromide, which forms supramol. hydrogels with pyrrole in aq. soln. These hydrogels were used as templates during polypyrrole electropolymn. to give microstructures composed of the bundles of bilayer membranes. The highly porous nature of these films resulted in electrochem. properties superior to polypyrrole deposited under the same condition without use of an amphiphilic template. Anal. of the scan rate dependence on cyclic voltammogram reveals that the porous templated films facilitate fast diffusion of dopant ions. The actuation properties were also studied in aq. solns. contg. Na p-toluenesulfonate electrolyte. The strains displayed by the template polypyrrole films were twice those synthesized without the use of a template. [on SciFinder(R)] -
2007
Iptycene-derived pyridazines and phthalazines.
The Journal of organic chemistry,vol.72,p.10166–80,2007The synthesis of new heterocyclic oligo(phenylene) analogues based on soluble, nonaggregating 1,2-diazines is reported. Improved palladium-catalyzed reductive coupling methods were developed to allow for the preparation of large quantities of iptycene-derived bipyridazines and biphthalazines, and the controlled synthesis of well-defined oligomers up to sexipyridazine. Crystallographic, spectroscopic, and computational evidence indicate that in these analogues, hindrance at the ortho position is relaxed relative to poly(phenylenes). The resulting building blocks are promising for incorporation in conjugated electronics materials, and as new iptycene-derived ligands for transition metals. -
2006
Trace Hydrazine Detection with Fluorescent Conjugated Polymers: A Turn-On Sensory Mechanism
Advanced Materials,vol.18,p.1047–1050,2006Exposure of hydrazine vapor to conjugated-polymer films reduces the small no. of oxidized trap sites that quench excitons. This scheme is effective for trace hydrazine detection with a detection limit of <1 ppm. The introduction of an oxidizing dopant quenches the polymer emission and permits detection of hydrazine with larger on/off ratios. [on SciFinder(R)] -
2006
Structure and Morphology of Poly(isobenzofuran) Films Grown by Hot-Filament Chemical Vapor Deposition
Chemistry of Materials,vol.18,p.6339–6344,2006We describe hot-filament chem. vapor deposition of polyisobenzofuran (PIBF) films and characterize their chem. structure and surface morphol. The precursor monomer, 1,2,3,4-tetrahydro-1,4-epoxy-naphthalene, is pyrolyzed by flowing it over an array of hot filaments held at three different temps. (680, 738, and 800 °C). The produced intermediate, iso benzofuran , is deposited onto a silicon substrate as thin films of PIBF. Fourier transform IR spectroscopy and XPS revealed that the films prepd. at 800 °C possess a chem. structure and a compn. different from those prepd. at lower filament temps. (680 and 738 °C). Increasing filament temp. also leads to the formation of defect domains in the polymer films. By at. force microscopy, we observe a decrease in surface roughness and in the av. size of polymer grains in PIBF domains as the filament temp. is increased. Defect domains exhibit a rougher surface as well as larger size polymer grains than those in PIBF domains. Spectroscopic and microscopic results suggest that growth of the polymer in the defect domains proceeds by a mechanism different from that in the PIBF domains. [on SciFinder(R)] -
2006
Chemoresistive Gas-Phase Nitric Oxide Sensing with Cobalt-Containing Conducting Metallopolymers
Chemistry of Materials,vol.18,p.5649–5651,2006Nitric oxide (NO) was implicated in a wide range of biol. processes including cellular signaling, coronary artery dilation, immune system response, and neurotransmission. With this in mind, the development of direct, sensitive, and selective sensing schemes for NO has attracted great interest in recent years. The development of a selective, ppm level, gas-phase NO detection system based on chemoresistive changes in a cobalt-contg. metallopolymer film device is discussed. These films are easily prepd. and elec. interfaced, and showed detection limits of NOx <1 ppm. [on SciFinder(R)] -
2006
Facile in Situ Silver Nanoparticle Formation in Insulating Porous Polymer Matrices
Chemistry of Materials,vol.18,p.21–25,2006The in situ formation of well-dispersed silver nanoparticles in an insulating polymer matrix is reported. Poly(aryl ether)s (PAEs) incorporating an oxadiazole moiety were synthesized, and they demonstrated selective interaction with silver ions. Polymer-ion thin films were easily spin cast and exposed to hydrazine vapors to induce metal nucleation. Introducing a triptycene monomer into the polymer backbone resulted in a highly porous matrix and prevented nanoparticle aggregation. This provides a generalized approach to the facile prepn. of well-dispersed metal nanoparticles embedded thin films. [on SciFinder(R)] -
2006
Dark-field oxidative addition-based chemosensing: new bis-cyclometalated PtII complexes and phosphorescent detection of cyanogen halides.
Journal of the American Chemical Society,vol.128,p.16641–8,2006Heavy metal complexes that are phosphorescent at room temperature are becoming increasingly important in materials chemistry, principally due to their use in phosphorescent organic light-emitting devices (OLEDs). Their use in optical sensory schemes, however, has not been heavily explored. Homoleptic bis-cyclometalated Pt(II) complexes are known to undergo oxidative addition with appropriate electrophiles (principally alkyl halides) by either thermal or photochemical activation. We have applied this general reaction scheme to the development of a phosphorescence-based sensing system for cyanogen halides. To carry out structure-property relationship studies, a series of previously unreported Pt(II) complexes was prepared. Most of the complexes (excluding those that incorporated substituents on the ligands that forced steric crowding in the square plane) were strongly orange-red phosphorescent (Phi = 0.2-0.3) in a room-temperature oxygen-free solution. These sterically demanding ligands also accelerated the addition of cyanogen bromide to these complexes but slowed the addition of methyl iodide, indicating that the oxidative addition mechanisms for these two electrophiles is different. The lack of solvent-polarity effect on the addition of BrCN suggests a radical mechanism. Oxidative addition of BrCN to the metal complexes in solution or dispersed in poly(methyl methacrylate) gave blue-shifted emissive Pt(IV) complexes. The blue-shifted products give a dark-field sensing scheme that is in sharp contrast to energy transfer-based sensing schemes, which have limited signal-to-noise because of the presence of lower-energy vibronic bands of the energy donor that can overlap with the emission of the acceptor. -
2006
Increased alignment of electronic polymers in liquid crystals via hydrogen bonding extension.
Journal of the American Chemical Society,vol.128,p.15058–9,2006Crucial for the development of enhanced electrooptic materials is the construction of highly anisotropic materials. Nematic liquid crystals are able to control the chain conformation and alignment of poly(phenylene ethynylene)s (PPEs), producing electronic polymers with chain-extended planar conformations for improved transport properties. Here, we show that the dichroic ratio, and hence polymer alignment, increases dramatically when interpolymer interactions are introduced by end capping the PPE with hydrogen bonding groups. This increased order can be readily turned off by the introduction of a competing monofunctionalized hydrogen bonding compound. The formation of hydrogen bonds between the polymers results in the formation of gels and elastomers which may be of interest for future applications. -
2006
Conjugated polymers in an arene sandwich.
Journal of the American Chemical Society,vol.128,p.12426–7,2006A series of poly(p-arylene butadiynylene)s containing zero, one, and two co-facial pi-pi interactions per repeat unit were synthesized and characterized. A surprisingly selective and high-yielding Diels-Alder cycloaddition of anthracene and nonsymmetric, sterically hindered anhydrides proved essential to generating the cofacial arene-containing monomers. Single-crystal X-ray structures display nearly parallel cofacial arenes that are within the van der Waals contact distances. The precursor molecules with cofacial arenes undergo reversible one- and two-electron oxidations to the radical cation and dication in CH2Cl2. The anhydrides were converted to N-alkyl imides to increase the solubility. High-molecular weight poly(p-arylene butadiynylene)s were prepared via Pd/Cu(I)/benzoquinone oxidative coupling of the diacetylene monomers. The resulting polymers are highly emissive in solution and thin films. The ionization potentials were measured using ultraviolet photoelectron spectroscopy with thin films. Last, fluorescence measurements of polymer thin films during continuous irradiation indicate that the most hindered polymer is more resistant to photobleaching. -
2006
TOTAPOL: a biradical polarizing agent for dynamic nuclear polarization experiments in aqueous media.
Journal of the American Chemical Society,vol.128,p.11385–90,2006In a previous publication, we described the use of biradicals, in that case two TEMPO molecules tethered by an ethylene glycol chain of variable length, as polarizing agents for microwave driven dynamic nuclear polarization (DNP) experiments. The use of biradicals in place of monomeric paramagnetic centers such as TEMPO yields enhancements that are a factor of approximately 4 larger (epsilon approximately 175 at 5 T and 90 K) and concurrently the concentration of the polarizing agent is a factor of 4 smaller (10 mM electron spins), reducing the residual electron nuclear dipole broadening. In this paper we describe the synthesis and characterization by EPR and DNP/NMR of an improved polarizing agent 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL). Under the same experimental conditions and using 2.5 mm magic angle rotors, this new biradical yields larger enhancements (epsilon approximately 290) at lower concentrations (6 mM electron spins) and has the additional important property that it is compatible with experiments in aqueous media, including salt solutions commonly used in the study of proteins and nucleic acids. -
2006
Probing a conjugated polymer’s transfer of organization-dependent properties from solutions to films.
Journal of the American Chemical Society,vol.128,p.9030–1,2006The dynamic transfer of a conjugated polymer’s organization-dependent properties from the solution state to the solid film state was probed by circularly polarized luminescence (CPL) and circular dichroism (CD) spectroscopy. Different supramolecular organizations within films and aggregate solutions of a chiral poly(p-phenylenevinylene) derivative led to opposite CPL and CD spectra. These dramatic property differences were controlled by regulating the polymer’s self-assembly through solvent selection and film annealing. Therefore, different processing conditions can greatly affect the functional properties of conjugated polymer films employed in various optoelectronic applications.
