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1998
Electroactivity enhancement by redox matching in cobalt salen-based conducting polymers.
Advanced Materials (Weinheim, Germany),vol.10,p.1100–1104,1998The synthesis and the electrochem. properties of 2 polymer-transition metal hybrids are reported. The oxidn. potential of the Co-contg. N,N-ethylenebis(salicylidenimine) polymer films was investigated by cyclic voltammetry, and the influence of the film thickness on the redox potential was analyzed. The test results for both polymers are shown and discussed in detail. It was concluded from the exptl. data, that the Co-polymer hybrids display a high cond. and sensitivity to coordinating ligands. Moreover, possible application fields for the hybrid materials are outlined. [on SciFinder(R)] -
1998
Studies of Thermotropic Properties and the Mesophase of Mixtures of n-Alkanoates and Perfluoro-n-alkanoates of Dimolybdenum (M&M).
Chemistry of Materials,vol.10,p.1758–1763,1998Mixts. of Mo2(O2C(CH2)nCH3)4, where n = 6 (=Oct) and 7 (=Non), form a columnar mesophase upon heating and the crystal-to-liq. crystal phase transition temps. differ little from those predicted by the Shroder-van Laar equation for an ideal mixt. The XRD of the solid sample obtained from cooling a 1:1 mixt. of Mo2(Oct)4 and Mo2(Non)4 conformed to a triclinic cell with lattice parameters intermediate between those of the pure compds. Similarly in the mesophase the intercolumnar sepn. (d) was an intermediate distance. The phase behavior of mixts. of Mo2(non)4 and Mo2(O2C(CH2)10CH3)4 [Mo2(Dod)4] were more complex, but in all instances a mesophase was obsd. whereas that of the pure Mo2(Dod)4 shows only a cryst. solid to isotropic phase transition. Mixts. of Mo2(Oct)4 and Mo2(O2C(CF2)6CF3)4 [Mo2(Octf)4] also form a columnar mesophase with an intercolumnar sepn. intermediate between that of the pure compds. The mesophase is an optically pos. material with the largest component of the index of refraction coincident with the columnar axis. The solid to mesophase transition temps. varied significantly from that predicted for an ideal mixt. These results are discussed from facile carboxylate scrambling to produce Mo2(O2CR)4-n(O2CR’)n, where n = 0-4 in both the mesophase and the resultant solid soln. [on SciFinder(R)] -
1998
Columnar Mesophases from Nondiscoid Metallomesogens: Rhodium and Iridium Dicarbonyl Diketonates.
Chemistry of Materials,vol.10,p.438–443,1998Nondiscoid Rh and Ir dicarbonyl $\beta$-diketonate complexes organize in columnar liq. crystal phases with hexagonal disordered structures (Colhd). The shape and dipolar attributes of these materials produce highly correlated antiparallel pairwise arrangements. This organization was studied by x-ray diffraction and miscibility studies. The liq. crystallinity is stabilized by decreasing the no. of C atoms in the side chains, and single component room-temp. liq. crystals were developed. The nature of the metal is also important, and Ir mesogens exhibited higher clearing points than the analogous Rh materials. [on SciFinder(R)] -
1998
Experimental and theoretical study of ring substituent induced effects on the structural and optical properties of poly(p-pyridylvinylene phenylenevinylene)s.
Journal of Chemical Physics,vol.109,p.2031–2037,1998Optical absorption and photoluminescence spectroscopy have been carried out on a new class of (phenylene) ring substituted p-pyridylvinylene phenylenevinylene polymers used as active materials in light emitting diodes. The effects of the ring substitutions on the optical absorption and photoluminescence energies are qual. explained through the use of semiempirical quantum chem. modeling of the ring torsion angles. Reduced aggregation through the use of “strap” substituents on the phenylene rings also is discussed. [on SciFinder(R)] -
1998
Exciplex emission from bilayers of poly(vinyl carbazole) and pyridine based conjugated copolymers.
Journal of Chemical Physics,vol.108,p.7842–7848,1998We present photoluminescence and electroluminescence studies of bilayers and blends formed from poly(vinyl carbazole) (PVK) and poly(pyridyl vinylene phenylene vinylene) (PPyVPV) copolymer derivs. Bilayers of PVK and the PPyVPV copolymers have a photoluminescence emission which cannot be assigned to either the photoluminescence of PVK or the PPyVPV layer. The blends of the two polymers show a similar new photoluminescence emission for a large range of concns. Absorption and photoluminescence excitation spectra confirm that the addnl. feature is an excited state species which results from an exciplex at the polymer/polymer interface. Bilayer light-emitting devices utilizing the PPyVPV copolymers show an electroluminescence spectrum consistent with emission from the exciplex. The efficiency of the bilayer devices as compared to single layer devices increases by over three orders of magnitude due to the exciplex formation and the elimination of exciton formation near the luminescence quenching electrodes. [on SciFinder(R)] -
1998
Palladium Couplings on Metallocalix[4]arenes: An Efficient Synthesis of New Functionalized Cavities.
Journal of Organic Chemistry,vol.63,p.8561–8564,1998An efficient one-pot procedure has been developed for the prepn. of rigid non-fluxional dialkoxy tungsten calix[4]arenes I [HOCHRCHROH = ethyleneglycol, methylcatechol, (S)-(-)-1,1′-bi-2-naphtho -
1998
Synthesis of Ethynyl-Substituted Quinquephenyls and Conversion to Extended Fused-Ring Structures.
Journal of Organic Chemistry,vol.63,p.1676–1686,1998An organometallic coupling electrophile-induced cyclization strategy for the synthesis of p-terphenyl compds. has been extended to the synthesis of p-quinquephenyl systems. In this work the authors report the synthesis of various polycyclic arom. systems contg. nine annelated rings including the synthesis of functionalized polycyclic arom. systems. An interesting side reaction which leads to an indenyl spiro ring system is also described. This side reaction can be suppressed by changing the electrophile (from H+ to I+) or by modification of the cyclization precursor. The UV-vis and fluorescence spectra of several of these polycyclic aroms. and the p-quinquephenyl precursors are also reported. [on SciFinder(R)] -
1998
Fluorescent Porous Polymer Films as TNT Chemosensors: Electronic and Structural Effects.
Journal of the American Chemical Society,vol.120,p.11864–11873,1998The synthesis, spectroscopy, and fluorescence quenching behavior of pentiptycene-derived phenyleneethynylene polymers, 1-3, are reported. The incorporation of rigid three-dimensional pentiptycene moieties into conjugated polymer backbones offers several design advantages for solid-state (thin film) fluorescent sensory materials. First, they prevent $π$-stacking of the polymer backbones and thereby maintain high fluorescence quantum yields and spectroscopic stability in thin films. Second, reduced interpolymer interactions dramatically enhance the soly. of polymers 1-3 relative to other poly(phenyleneethynylenes). Third, the cavities generated between adjacent polymers are sufficiently large to allow diffusion of small org. mols. into the films. These advantages are apparent from comparisons of the spectroscopic and fluorescence quenching behavior of 1-3 to a related planar electron-rich polymer 4. The fluorescence attenuation (quenching) of polymer films upon exposure to analytes depends on several factors, including the exergonicity of electron transfer from excited polymer to analytes, the binding strength (polymer-analyte interactions), the vapor pressure of the analyte, and the rates of diffusion of the analytes in the polymer films. Films of 1-3 are particularly selective toward nitro-arom. compds. The dependence of fluorescence quenching on film thickness provides an addnl. criterion for the differentiation of nitro-arom. compds. from other species, such as quinones. In short, thinner films show a larger response to nitro-arom. compds., but show a lower response to quinones. Such differences are explained in terms of polymer-analyte interactions, which appear to be electrostatic in nature. The rapid fluorescence response (quenching) of the spin-cast films of 1-3 to nitro-contg. compds. qualifies these materials as promising TNT chemosensory materials. [on SciFinder(R)] -
1998
Na+ Specific Emission Changes in an Ionophoric Conjugated Polymer.
Journal of the American Chemical Society,vol.120,p.5187–5192,1998The emission and absorption characteristics of a conjugated poly(phenylene bithiophene) and a monomeric model compd. were investigated as a function of [Li+], [Na+], [K+], and [Ca2+]. The calix[4]arene bithiophene receptor that is present in both compds. provides selectively for Na+ and the absorption and emission characteristics are not affected by Li+, K+, or Ca2+. Both systems display absorption spectra which are relatively insensitive to Na+; however, the Stokes shift of the emission is reduced by added Na+•. For the model system, increasing [Na+] provides a shift of the emission that is consistent with an equil. mixt. of bound and unbound receptor. The polymer displays a larger shift in the emission in response to Na+ and due to multiple binding sites lacks an isoemissive point. The chain length of the polymer also has an effect on this behavior. This behavior may be due to energy migration to regions of the polymer which do not have bound Na+ and can relax to lower energy conformations. This description is also borne out by the redn. in the lifetimes of the excited states with increasing [Na+] for both the polymer and the model system. This mechanism may provide a route to systems which can function as digital indicators at crit. concns. of analytes. [on SciFinder(R)] -
1998
Porous Shape Persistent Fluorescent Polymer Films: An Approach to TNT Sensory Materials.
Journal of the American Chemical Society,vol.120,p.5321–5322,1998Spin-cast films of a pentiptycene-derived phenyleneethynylene polymer, 2, display a fast fluorescence response (seconds) to vapors of 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 1,4-benzoquinone (BQ). The fluorescence attenuation of 2 is dependent on the time of exposure to these quenchers and on the thickness of thin films. Thinner films show a better response to TNT and DNT but the opposite is true for BQ. Such differences are attributed to different polymer-analyte interactions. For comparison, corresponding studies on an alkoxy-substituted phenylenethynylene polymer, 3, were also carried out. The results indicate that 2 is superior to 3 as a fluorescent chemosensor in terms of sensitivity, selectivity, solvent soly. and solvent stability. [on SciFinder(R)] -
1997
Color variable bipolar/ac light-emitting devices based on conjugated polymers
Applied Physics Letters,vol.70,p.3215,1997The fabrication is reported of color variable bipolar/a.c. LEDs based on conjugated polymers. The devices consist of blends of pyridine-phenylene and thiophene-phenylene based copolymers sandwiched between the emeraldine base form and the sulfonated form of polyaniline. ITO and Al are used as electrodes. The devices operate under either polarity of driving voltage with different colors of light being emitted, red under forward bias, and green under reverse bias. The relative fast time response allows the rapid switching of colors and a.c. operation. [on SciFinder(R)] -
1997
Exciplex emission in bilayer polymer light-emitting devices.
Applied Physics Letters,vol.70,p.1644–1646,1997Photoluminescent and electroluminescent studies of bilayer heterojunctions formed from poly(pyridyl-vinylene-phenylene-vinylene) (PPyVPV) derivs. and poly(vinyl carbazole) (PVK) show an emission peak which cannot be ascribed to either the PPyVPV or PVK layer. This peak results from an exciplex at the bilayer interface as demonstrated through studies of absorption and photoluminescence excitation spectra. The photoluminescence efficiency of the exciplex is greater than 20%. The electroluminescence spectrum from the bilayer devices is entirely due to exciplex emission, with internal efficiencies initially achieved exceeding 0.1%. [on SciFinder(R)] -
1997
Liquid-Crystalline Heterocyclic Phthalocyanine Analogs Based on Thiophene.
Chemistry of Materials,vol.9,p.535–538,1997The syntheses and mesomorphism of novel octaalkoxymethyl-substituted tetra-2,3-thiophenoporphyrazines, heterocyclic phthalocyanine analogs in which the benzene rings are replaced by thiophene rings, are reported. NMR anal. indicates that these materials exist as isomeric mixts. due to the asymmetry induced by the thiophene ring. Among the attractive features of liq.-cryst. 2,3-thiophenoporphyrazines over phthalocyanines are their lower mesomorphic and isotropic temps. These features resulted in liq. crystallinity at room temp. Similar to liq.-cryst. phthalocyanine derivs., metalation with Cu raises the transition temps. [on SciFinder(R)] -
1997
Novel light-emitting devices based on pyridine-containing conjugated polymers.
IEEE Transactions on Electron Devices,vol.44,p.1263–1268,1997The authors present novel light-emitting devices based on several pyridine-contg. conjugated polymers and copolymers in various device configurations. The high electron affinity of pyridine-based polymers improves stability and electron transport properties of the polymers and enables the use of relatively stable metals such as Al as electron injecting contacts. Bilayer devices utilizing poly(9-vinyl carbazole) (PVK) as a hole-transporting/electron-blocking polymer show dramatically improved efficiency and brightness as compared to single layer devices. This is attributed to charge confinement and exciplex emission at the PVK/emitting polymer interface. The incorporation of conducting polyaniline network electrode into PVK reduces the device turn-on voltage significantly while maintaining the high efficiency. Two novel device configurations that enable the use of high work function metals as electrodes are pointed out. [on SciFinder(R)] -
1997
Near IR photoluminescence in mixed films of conjugated polymers and fullerenes.
Journal of Chemical Physics,vol.107,p.2308–2312,1997The photoinduced electron transfer between conjugated polymers and a series of functionalized fullerenes was studied. A new photoluminescence signal was obsd. in the near IR (∼1.4 eV). This weak IR photoluminescence does not result from direct excitation of the fullerene, but from radiative electron-hole recombination between the fullerene excited state and the polymer ground state. The intensity of this recombination luminescence depends on the electrochem. nature of the functional group; it is obsd. only for fullerenes with first redn. potential higher than that of C60. [on SciFinder(R)] -
1997
Conducting Polymetallorotaxanes: Metal Ion Mediated Enhancements in Conductivity and Charge Localization.
Journal of the American Chemical Society,vol.119,p.12568–12577,1997In this paper polymers are described of two metallorotaxane systems, Rot(1,M) and Rot(2,M) (M = Zn2+ or Cu1+), which are formed by complexing a macrocyclic phenanthroline, a 5,5′-bis([2,2′-bithiophen]-5-yl)-2,2′-bipyridine (ligand 1) or 5,5′-bis(3,4:3′,4′-bis(ethylenedioxy)[2,2′-bithiophen]-5-yl)-2,2′-bipyridine (ligand 2), and Zn2+ or Cu1+ ions. The corresponding polymetallorotaxanes, PolyRot(1,M) and PolyRot(2,M), are produced by oxidative polymn. of Rot(1,M) and Rot(2,M). Investigations of the electrochem., conducting, and optical properties of the metallorotaxanes and polymetallorotaxanes as well as related nonrotaxane polymers Poly(1) and Poly(2) are reported. The combined electrochem. and cond. studies of PolyRot(1,M) and PolyRot(2,M) indicated that the polymetallorotaxane’s redox and conducting properties were dramatically affected by the Lewis acidic and redox properties of the coordinated metal ions. The Lewis acidity produces charge localization and a redox conduction process in both polymetallorotaxane systems. The matching of the polymer and Cu1+/2+ couple redox potentials in PolyRot(2,Cu) resulted in a Cu1+/2+ contribution to cond. The metal-free PolyRot(1) and PolyRot(2) were produced by extg. the metal ions, and these polymers reversibly bound Zn2+ or Cu2+ ions in soln. The Cu2+ dopes the films of PolyRot(2) and Poly(2), which have lower oxidn. potential than those of PolyRot(1), to produce 106-107-fold cond. increases. In the case of PolyRot(1) and Poly(1), the rotaxane structure was demonstrated to be key for reversible complexation of metal ions. [on SciFinder(R)] -
1997
Directed Electrophilic Cyclizations: Efficient Methodology for the Synthesis of Fused Polycyclic Aromatics.
Journal of the American Chemical Society,vol.119,p.4578–4593,1997A versatile method for the synthesis of complex, fused, polycyclic, arom. systems in high chem. yield is described. Construction is achieved using a general two-step synthetic sequence. Pd-catalyzed Suzuki and Negishi type cross-coupling chemistries allow for the prepn. of non-fused skeletal ring systems in yields consistently >80%. The crit. ring-forming step, which generally proceeds in very high to quant. yield, utilizes (4-alkoxyphenyl)ethynyl groups and is induced by strong electrophiles such as trifluoroacetic acid and iodonium tetrafluoroborate. The reaction in essence produces phenanthrene moieties which are integrated into extended polycyclic arom. structures. Fused polycyclic benzenoids as well as benzenoid/thiophene systems may be prepd. by this methodol. The scope of the described cross-coupling/cyclization chem. including mechanistic insights and problematic side reactions are described. [on SciFinder(R)] -
1997
Pyridine-based conjugated polymers. Photophysical properties and light-emitting devices.
Macromolecular Symposia,vol.116,p.27–38,1997The authors study the photophys. properties of the pyridine-based polymers poly(p-pyridyl vinylene) (PPyV) and poly(p-pyridine) (PPy). The primary photoexcitations in the pyridine-based polymers are singlet excitons. The authors observe direct intersystem crossing (ISC) on picosecond time scales with the vol. d. of triplet excitons varying with the sample morphol. (film or powder). These effects are demonstrated clearly by examg. the millisecond photoinduced absorption characteristics of powder and film forms of PPyV. The pyridine-based polymers were shown to be promising candidates for polymer light-emitting devices, both conventional diode device and sym. configured ac light-emitting (SCALE) device. Here the authors examine the role of insulating layers and their interfaces with the emitting layer and electrodes in the SCALE device operation, with emphasis on the central role of the polymer-polymer interfaces. [on SciFinder(R)] -
1997
New approaches to sensory materials: molecular recognition in conjugated polymers new transduction methodology.
NATO ASI Series, Series C: Mathematical and Physical Sciences,vol.492,p.133–141,1997A review, with 12 refs., is given on the use of conjugated polymers for the development of new sensory methodologies. [on SciFinder(R)] -
1997
Discrete Intensity Jumps and Intramolecular Electronic Energy Transfer in the Spectroscopy of Single Conjugated Polymer Molecules
Science,vol.277,p.1074–1077,1997Single-mol. fluorescence spectroscopy of a multichromophoric conjugated polymer (mol. wt. ∼ 20,000) revealed surprising single-step photobleaching kinetics and acute jumps in fluorescence intensity. These jumps were shown not to result from spectral diffusion and were attributed to fluctuations in the quantum yield of emission for the mols. The data indicate efficient intramol. electronic energy transfer along the polymer chain to a localized fluorescence-quenching polymer defect. The defects are created by reversible photochem. of the polymer. These findings have implications for the use of conjugated polymers in light-emitting diode displays and sensors. [on SciFinder(R)]
