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1995
Solid-State Ordering and Potential Dependence of Conductivity in Poly(2,5-dialkoxy-p-phenyleneethynylene).
Chemistry of Materials,vol.7,p.418–425,1995Soln.-cast films of four different poly(2,5-dialkoxy-p-phenyleneethynylene) mols. with varying backbone chain lengths and varying alkoxy substituent chain lengths and 1,4-diethynyl-2,5-dihexadecyloxybenzene-9,10-dibromoanthracene alternating copolymer (I) have been characterized electrochem. and by x-ray diffraction and DSC. The polymers have varying degrees of order and crystallinity based on long-range lamellar structure. Cyclic voltammetry in liq. SO2/electrolyte shows that the onset of oxidn. for poly(2,5-dialkoxy-p-phenyleneethynylene) occurs at ∼1.05 V vs SCE with the more cryst. polymers having slower electrochem. response than the less cryst. ones. In situ characterization of the potential dependence of cond. in the same medium shows that the max. conductivities of poly(2,5-dialkoxy-p-phenyleneethynylene) range from ∼0.2 to ∼5 $Ømega$-1 cm-1, suggesting that higher cond. is assocd. with lower long-range order in the polymer films but showing little dependence on av. polymer chain length. Poly(2,5-dialkoxy-p-phenyleneethynylene) all have max. cond. at ∼1.6 V vs SCE and finite potential windows of high cond. ∼0.55 V wide, indicating that the potential of max. cond. and the width of the window of high cond. are detd. by mol. rather than bulk properties. For I, the onset of oxidn. occurs at ∼0.8 V vs SCE, the potential of max. cond. is ∼1.5 V vs SCE, and the width of the potential window of high cond. is ∼0.85 V. [on SciFinder(R)] -
1995
Blue electroluminescent devices based on soluble poly(p-pyridine).
Journal of Applied Physics,vol.78,p.4264–4266,1995We have fabricated unilayer electroluminescent devices from sol. poly(p-pyridine) (PPy). The soly. of PPy in weak acids allows direct spin casting of the polymer films. The electroluminescence spectrum peaks at 2.5 eV (497 nm) corresponding to white light weighted towards the blue end of the spectrum. The photoluminescence spectrum peaks at 2.35 eV (530 nm). The operating voltages of the devices ranged from 4 to 12 V with current densities of 6 to 8 mA/mm2. We compare our devices with similar blue emitting devices based on poly(p-phenylene). [on SciFinder(R)] -
1995
Probing Delocalization Across Alkyne-Containing Linkages: Synthesis and Cyclic Voltammetry of Bridged Phenylenediamines.
Journal of Organic Chemistry,vol.60,p.7096–7100,1995This paper presents the synthesis of N,N,N’,N’-tetraethyl-1,4-phenylenediamines bridged by ethynyl-phenyl-ethynyl and diethynyl linkages. These compds. are of interest to det. if the N,N,N’,N’-tetraethyl-1,4-phenylenediamines exhibit delocalized mixed oxidn. states when oxidized electrochem. 1-Butoxy-2,5-bis(N,N-diethylamino)-4-ethynylbenzene was a key intermediate, and nucleophilic addn. of lithium (trimethylsilyl)acetylide to 2,5-bis(N,N-diethylamino)-1,4-benzoquinone provided an efficient synthesis of this compd. This intermediate was then assembled into the redox assemblies of interest in high yield by use of palladium-catalyzed cross-coupling and copper-catalyzed oxidative homo-coupling protocols. Cyclic voltammetry of these compds. indicate that the N,N,N’,N’-tetraethyl-1,4-phenylenediamine redox units behave independently and that the only interactions are electrostatic. As a result, we conclude that the electrochem. generated radical cations and dications are highly localized. [on SciFinder(R)] -
1995
Fluorescence Studies of Poly(p-phenyleneethynylene)s: The Effect of Anthracene Substitution.
Journal of Physical Chemistry,vol.99,p.4886–4893,1995The poly(p-phenyleneethynylene) mols. synthesized by a Pd-catalyzed cross-coupling reaction of diiodobenzene derivs. and derivs. of 1,4-diethynylbenzene, are highly luminescent materials. The polymers are sol. by virtue of the -OC16H33 groups introduced on the arom. rings and by controlling their mol. wt. The fluorescent quantum yields are between 0.35 and 0.40 depending on the material. The excited-state lifetimes of the polymers are 1-2 ns, slightly shorter than that of the model compd. 1,4-diphenylethynyl-2,5-dibutoxybenzene, which has a lifetime of 3 ns. Incorporation of anthracene, coupled at the 9,10-positions by using 9,10-dibromoanthracene, into the polymer backbone decreases the quantum yield to between 0.05 and 0.27 depending on the anthracene content. In addn., low-energy electronic transitions and lower wavelength emission bands assocd. with the anthracene group are produced. The polymers harvest optical energy and transfer it to the anthracene resulting in emission from this chromophore. In the case where only terminal anthracene units, introduced by using 9-bromo-10-phenylethynylanthracene, are present, the process is very efficient with >95% of the energy being transferred to the end groups. [on SciFinder(R)] -
1995
Fluorescent Chemosensors Based on Energy Migration in Conjugated Polymers: The Molecular Wire Approach to Increased Sensitivity
Journal of the American Chemical Society,vol.117,p.12593–12602,1995The authors demonstrate herein how conjugated polymers (mol. wires) can be used to interconnect (wire in series) receptors to produce fluorescent chemosensory systems with sensitivity enhancements over single receptor analogs. The enhancement mechanism in the polyreceptor materials is based on an energy migration scheme in which excitations, diffuse along the polymer backbone. Analyte binding produces trapping sites for the excitations which results in greatly attenuated emission intensity. Three different cyclophane-based receptor systems that bind paraquat were studied. These systems are quenched by paraquat binding, and the quenching enhancements relative to a monomeric model compd. were used to det. the efficiency of energy migration. Two polymers with related poly(phenyleneethynylene) structures were studied, and the all-para system was found to exhibit more facile energy migration than the more electronically localized analog that contained meta linkages. The para polyreceptor system was found to display a 65-fold enhancement in sensitivity to paraquat as compared to a model monoreceptor fluorescent chemosensor. However, delocalization alone is not sufficient to produce facile energy migration, and the more delocalized polythiophenes appear to be less effective at energy migration than the para poly(phenyleneethynylene) material. Paraquat-induced fluorescent quenching studies on homologous polymers that lacked the cyclophane receptors were also performed. Diffusive quenching by paraquat is enhanced by energy migration. [on SciFinder(R)] -
1995
Ionoresistivity as a highly sensitive sensory probe: investigations of polythiophenes functionalized with calix[4]arene-based ion receptors.
Journal of the American Chemical Society,vol.117,p.9842–9848,1995The authors report the synthesis, optical, and electrochem. properties of a calix[4]arene-substituted polythiophene which demonstrates ion-selective voltammetric, chromic, fluorescent, and resistive responses. The ionochromic response of this polythiophene on exposure to Na+ shows an increased effective conjugation length of the polymer backbone. Despite this, Na+ induces a large pos. shift in the potential at which the polymer is oxidized (greater than +100 mV) commensurate with a large decrease in cond. (>99%). Although the calix[4]arene-substituted polythiophene exhibits no changes in the UV-visible spectrum and only minimal changes in the voltammetric responses on exposure to Li+ or K+, there are large decreases in relative conductivities (69 and 47%, resp.). Thus, although the sensory properties of this polymer are expressed via several measurable entities, the ionoresistive response is clearly the most sensitive. This sensitivity originates from the cooperative nature of carrier transport in a conducting polymers (CP) and is thus inherent in chemoresistive CPs. [on SciFinder(R)] -
1995
Design of chemoresistive sensory materials: polythiophene-based pseudopolyrotaxanes.
Journal of the American Chemical Society,vol.117,p.9832–9841,1995The authors report conducting polymer-based sensors which transduce reversible, noncovalent, and non-redox-dependent mol. recognition events into measurable changes in cond. These chemoresistive polymers are derived from bithiophenes contg. cyclophane receptors capable of forming self-assembled pseudorotaxane complexes with paraquat. The electrostatic perturbations arising from pseudopolyrotaxane formation cause a decrease in carrier mobility and thus lower the cond. The chemoresistive response was consistent with decreased carrier mobility and exhibited an enhanced sensitivity to analyte-promoted electrostatic perturbations relative to the voltammetric response. Polymer-based devices which demonstrate a real time chemoresistive response to paraquat are also reported. [on SciFinder(R)] -
1995
Method for enhancing the sensitivity of fluorescent chemosensors: energy migration in conjugated polymers.
Journal of the American Chemical Society,vol.117,p.7017–7018,1995A new method for the enhancement of a fluorescent chemosensory response using conjugated polymers is reported. This method makes use of the delocalized electronic structure of these materials which allows the excited state to migrate to an occupied receptor site. In the conjugated polymer I each repeating unit displays a cyclophane receptor which binds paraquat (Ka ≈ 1600 M-1). The binding of paraquat to I and II results in electron transfer quenching. The relative amt. of quenching in I is enhanced relative to II due to energy migration to the occupied sites. Theor., I can exhibit a max. sensitivity which is n times greater than II where n is the d.p. This effect leads to an enhancement in the static quenching consts. as detd. by the Stern-Volmer relation from 1600 M-1 for I to 105,000 M-1 for II (Mn = 122,500). Polymers lacking receptor groups also displayed enhanced dynamic quenching consts. relative to monomeric analogs. [on SciFinder(R)] -
1995
Host-Guest Mesomorphism: Cooperative Stabilization of a Bowlic Columnar Phase.
Journal of the American Chemical Society,vol.117,p.5011–5012,1995The azo-substituted calix[4]arene I (R = N:N-C6H3-(O(CH2)10H)2-3,4) in pure form does not display a stable thermotropic mesophase. A novel method is reported for stabilization of mesomorphic behavior by making use of the calix[4]arene’s receptor properties. In particular host-guest interactions between I and 1,4-(diformamide)butane (II) stabilize a bowlic hexagonal columnar phase (Bh) with very low clearing enthalpies and high fluidity. X-ray diffraction studies indicate that the Bh phase is the only mesophase present at I:II ratios of 2:1, but at ratios of 4:1 and 1:1 this phase coexists with small amts. of other unidentified phases. This system also displays the unusual properties that the range of mesomorphism is independent of the host/guest ratio and that only a fractional amt. of guest is required to stabilize the mesophase. The latter feature is indicative of a cooperative transition to the mesomorphic state. [on SciFinder(R)] -
1995
Molecular Magnetism. By Olivier Kahn.
Journal of the American Chemical Society,vol.117,p.1458,1995 -
1995
Liquid crystalline behavior in octahedral metal complexes.
Molecular Crystals and Liquid Crystals Science and Technology, Section A: Molecular Crystals and Liquid Crystals,vol.260,p.301–306,1995Octahedral liq. crystals based upon diketonate complexes of Fe+3, Mn+3, and Cr+3 are reported. These materials display columnar phases with ≥12 side chains and an ordered smectic phase with 6 side chains. [on SciFinder(R)] -
1995
Organometallic cross-coupling methods for the synthesis of complex conjugated polymers.
Polymeric Materials Science and Engineering,vol.73,p.513–514,1995The synthesis of conjugated polymers incorporating recognition elements for ions and electron deficient org. mols., e.g., polythiophenes, polyphenylenes, etc., are described. Palladium [Pd(PPh3)4] catalyzed coupling of polythiophenes which form a self-assembled pseudopolyrotaxane and synthesis of conjugated polymers with enhanced chemosensory response are described. The use of these methods for the construction of all carbon ladder polymers, i.e., graphite ribbons is outlined. [on SciFinder(R)] -
1995
Electro – and photoluminescent properties of nitrogen analogs of PPV: Poly(pyridylvinylene).
Polymeric Materials Science and Engineering,vol.73,p.473–474,1995The electroluminescence properties of three poly(pyridylvinylene) regioisomers used for LED applications are studied. [on SciFinder(R)] -
1995
Fully conjugated ladder polymers: Construction using a versatile cross-coupling/cyclization methodology.
Polymeric Materials Science and Engineering,vol.73,p.256–257,1995Ladder polymers are prepd. by postpolymn. cyclization of acetylene group-contg. polyarylenes. [on SciFinder(R)] -
1995
Photophysics of poly(p-pyridine): Blue electroluminescent devices from a soluble conjugated polymer.
Polymeric Materials Science and Engineering,vol.72,p.573–574,1995For polypyridine the Stokes shifted fluorescence spectrum and photoinduced absorption studies suggest that triplet excitons are importance for this system. We have fabricated electroluminescent devices with polypyridine as the emitting layer. The electroluminescence and photoluminescence spectra are similar and are peaked in the blue-green region. The simplicity of synthesis, the ability to cast polypyridine directly from soln., and the ability to quaternize the nitrogen site makes polypyridine of particular interest. [on SciFinder(R)] -
1994
Molecular recognition and chemoresistive materials.
Advanced Materials (Weinheim, Germany),vol.6,p.595–597,1994A review, with 10 refs., is given on the design of conducting polymeric sensory materials. They exhibit ionochromic, electrochem., or resistive responses to specific chem. signals. The integration of mol. recognition elements into polymers of pyrrole, thiophene, and bithiophene is described. [on SciFinder(R)] -
1994
Stabilization of polar superstructures by polymeric oxo-metal units: columnar liquid crystals from conically shaped dioxomolybdenum complexes.
Angewandte Chemie,vol.106,p.2378–80 (See also Angew. Chem., Int. Ed. Engl., 1994, 33(22), 2342–5),1994A study was made of ferroelec. liq. crystals from conical MoO2 complexes with polar polymer ligands. A hexagonal columnar liq. crystal phase is obsd. The heats of transition are given. A method for internal stabilization of the polar superstructure is described. [on SciFinder(R)] -
1994
Complementary Shapes in Columnar Liquid Crystals: Structural Control in Homo- and Heteronuclear Bimetallic Assemblies.
Chemistry of Materials,vol.6,p.2252–2268,1994A comprehensive study of the liq.-cryst. properties of 51 bimetallic compds. based upon 1,3,5-triketonate and 1,3,5,7-tetraketonate ligands is reported. These materials are liq. cryst. when six or more side chains are appended to the mesogenic core, and only columnar phases were obsd. Most of the liq. crystals were homonuclear dicopper complexes. Schiff-base derivs. of some of the triketones allowed for the synthesis of heteronuclear bimetallic liq. crystals. The NiCu and NiPd Schiff-base complexes are the 1st heteronuclear liq. crystals with proximate (strongly interacting) metal centers. Other heteronuclear complexes studied were not liq. cryst. due to the tendency to retain coordinated solvent or to form strongly assocd. structures in the absence of axial ligands. The use of complementary shapes was demonstrated as a means to generate av. relative organizations (correlations) between the complexes. The presence of these correlated structures was shown through comparisons of the structures, phase behavior, and the immiscibility between materials having the same phase but different shapes. Correlated structures were shown which produce av. rotations of 90° and 180° between nearest-neighbor mols. A crystal structure of one compd. confirmed that a similar superstructure was exhibited in the solid state. The correlated structures exhibit relatively short (3.29 AA) correlations between the mesogens, thereby allowing for strong intermol. interactions. The ability to control the orientation and relative position of transition metal centers in liq. crystals has applications in the design of new liq.-cryst. materials with useful magnetic and electronic properties. [on SciFinder(R)] -
1994
Transition Metals in Highly Correlated Discotic Phases: Designing Metallomesogens with Selected Intermolecular Organizations.
Chemistry of Materials,vol.6,p.101–103,1994New correlated columnar liq. crystals which are based upon square-planar 1,3-diketonate Schiff-base complexes of Pd, Ni, and Cu were developed. These compds. exhibit a no. of desirable liq.-cryst. properties such as ease of alignment, low m.ps., and low viscosity. The complexes are correlated into a discotic antiphase structure in which the complexes are oriented antiparallel. This structure facilitates very close intermesogen contacts of 3.6 AA since the antiphase structure directs the sterically bulky side chains away from those of the nearest neighbors. Vanadyl complexes are not liq. crystals. [on SciFinder(R)] -
1994
Liquid Crystalline Calixarenes.
Journal of Inclusion Phenomena and Molecular Recognition in Chemistry,vol.19,p.389–398,1994A review with 18 refs. of bowl-shaped (bowlic) liq. crystals of calixarenes and tungsten-oxo calix[4]arenes. [on SciFinder(R)]
