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2004
Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals
Science and Technology of Advanced Materials,vol.5,p.437–441,2004The lasing characteristics were studied of a dye-doped nematic layer sandwiched by 2 polymeric cholesteric liq. 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. [on SciFinder(R)] -
2004
Defining Space Around Conjugated Polymers: New Vistas in Self-Amplifying Sensory Materials
Synlett,p.149–154,2004A review. Synthetic strategies to control interchain electronic communications within conjugated polymers (CPs) are described. Novel chem. architectures built on iptycenes, metallorotaxanes, and canopied pyrroles restrict the dimensionality of electronic structures responsible for exciton and charge transport. Structure-property relationships emerging from studies of selected systems are discussed, focusing on their implications for the sensitivity of these materials as sensors. [on SciFinder(R)] -
2004
New $beta$-linked pyrrole monomers: approaches to highly stable and conductive electrochromic polymers.
Tetrahedron,vol.60,p.7141–7146,2004An efficient synthetic route to $beta$-linked dipyrrole monomers has been developed. Electrochem. polymn. of these monomers leads to the incorporation of polycyclic arom. residues into a polymer backbone. The resulting conjugated polymer films are electroactive, robust electrochromic materials that are highly delocalized in their oxidized forms. [on SciFinder(R)] -
2004
Phase and orientational ordering of low molecular weight rod molecules in a quenched liquid crystalline polymer matrix with mobile side chains.
The Journal of chemical physics,vol.120,p.11316–26,2004We study the phase diagram and orientational ordering of guest liquid crystalline (LC) rods immersed in a quenched host made of a liquid crystalline polymer (LCP) matrix with mobile side chains. The LCP matrix lies below the glass transition of the polymer backbone. The side chains are mobile and can align to the guest rod molecules in a plane normal to the local LCP chain contour. A field theoretic formulation for this system is proposed and the effects of the LCP matrix on LC ordering are determined numerically. We obtain simple analytical equations for the nematic/isotropic phase diagram boundaries. Our calculation show a nematic-nematic (N/N) first order transition from a guest stabilized to a guest-host stabilized region and the possibility of a reentrant transition from a guest stabilized nematic region to a host only stabilized regime separated by an isotropic phase. A detailed study of thermodynamic variables and interactions on orientational ordering and phases is carried out and the relevance of our predictions to experiments and computer simulations is presented. -
2003
A fluorescent self-amplifying wavelength-responsive sensory polymer for fluoride ions.
Angewandte Chemie, International Edition,vol.42,p.4803–6,2003The authors have introduced a new system for the detection of fluoride ion and successfully amplified the response using exciton migration in a semiconducting org. polymer. In contrast to most semiconductive-polymer sensor schemes that rely on changes in emission intensity, this sensory system uses a new fluorescence signal. The approach of directly interconnecting the indicator electronically with the polymer’s band structure is a promising alternative to FRET schemes and the authors intend to apply this approach to other analytes of interest. [on SciFinder(R)] -
2003A new nonionic water-sol. fluorescent conjugated poly(phenylene ethynylene) is reported with hydroxyl and amide side chains surrounding an arom. polymer backbone. [on SciFinder(R)]
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2003
How doping a cholesteric liquid crystal with polymeric dye improves an order parameter and makes possible low threshold lasing.
Journal of Applied Physics,vol.94,p.279–283,2003Lasing conditions in a dye-doped cholesteric liq. crystal (ChLC) were studied in view of optical modes for the light propagating in ChLCs using a polymeric dye with the transition dipole moment parallel to the local director of the ChLC host. Lasing always occurs at the lower-energy edge of the photonic gap. This is because that the optical eigen mode at the lower-energy gap is linearly polarized parallel to the director, while it is perpendicular at the higher-energy gap. Because of this well-defined lasing condition, low-threshold lasing was successfully achieved. [on SciFinder(R)] -
2003
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2003
Cofacially constrained organic semiconductors.
Journal of Polymer Science, Part A: Polymer Chemistry,vol.41,p.3693–3702,2003The synthesis, spectroscopy, and electrochem. is presented of bis[oligo(thiophene)] monomers where the sterics of covalently attached subunits enforced oblique spatial orientations. The synthetic scheme applied to a variety of chromophores; std. bromination and cross-coupling chem. afforded bis(terthienyl)benzene systems in high yield. Model systems were prepd. consisting of mono(terthiophene)s to probe the effects that van der Waals contacts impose during electropolymn. and methyl-blocked analogs to examine the electrochem. properties of derivs. that do not undergo polymn. The extent of delocalization between chromophores as deduced from electrochem. studies is discussed and viable electrochromic polymers are demonstrated. [on SciFinder(R)] -
2003
Molecular design of free volume as a route to low-kappa dielectric materials.
Journal of the American Chemical Society,vol.125,p.14113–9,2003Polymers incorporating the triptycene subunit were prepared for the molecular-level design of low dielectric constant (low-kappa) materials that can be used to manufacture faster integrated circuits. Triptycenes having restricted rotation by multiple point attachment to the polymer backbone are shown to introduce free volume into the films, thereby lowering their dielectric constants. The triptycene containing polymers exhibit a number of desirable properties including low-water absorption and high thermal stability. Systematic studies wherein comparisons are made between two separate classes of triptycene polymers and their non-triptycene containing analogues demonstrate that proper insertion of triptycenes into a polymer backbone can give rise to a reduction in the material’s dielectric constant while also improving its mechanical properties. These characteristics are desired by the semiconductor industry for the next generation of microprocessors and memory to provide insulation of the increasingly shrinking features. -
2003
Polarized photoluminescence from poly(p-phenylene-ethynylene) via a block copolymer nanotemplate.
Journal of the American Chemical Society,vol.125,p.9942–3,2003A new approach based on a conjugated polymer/block copolymer guest/host system for the generation of polarized photoluminescence is reported. Synthetic modification of a poly(p-phenylene-ethynylene) (PPE) conjugated polymer is used for domain-specific incorporation into a cylindrical morphology block copolymer host matrix. Subsequent ordering of the host nanostructure via roll cast processing templates uniaxial alignment of the guest PPE. The ordered films are optically anisotropic displaying both polarized absorption with a dichroic ratio of 3.0 at 440 nm and polarized emission with a polarization ratio of 7.3 at 472 nm. -
2003
Defining space around conducting polymers: reversible protonic doping of a canopied polypyrrole.
Journal of the American Chemical Society,vol.125,p.6870–1,2003A canopy-shaped pyrrole derivative 2 was prepared, in which a sterically demanding pendant group is juxtaposed to the pyrrole fragment to minimize interstrand pi-pi stacking interactions in the resulting polymer. Anodic polymerization of 2 afforded highly conductive poly(2), the electronic structure of which was probed by various spectroelectrochemical techniques. A limited charge delocalization within poly(2) translates into a well-defined conductivity profile, properties important for resistivity-based sensing. Notably, the bulk conductivity was precisely modulated by a rapid and reversible deprotonation and reprotonation of the polymer backbone. -
2003
Fluorescent detection of chemical warfare agents: functional group specific ratiometric chemosensors.
Journal of the American Chemical Society,vol.125,p.3420–1,2003Indicators providing highly sensitive and functional group specific fluorescent response to diisopropyl fluorophosphate (DFP, a nerve gas (G-agent) simulant) are reported. Nonemissive indicator 2 reacts with DFP to give a cyclized compound 2+A- that shows a high emission due to its highly planar and rigid structure. Very weak emission was observed by the addition of HCl. Another indicator based on pyridyl naphthalene exhibits a large shift in its emission spectrum after reaction with DFP, which provides for quantitative ratiometric detection. -
2003
A proton-doped calix[4]arene-based conducting polymer.
Journal of the American Chemical Society,vol.125,p.1142–3,2003Segmented conducting polymers based upon a calix[4]arene scaffold are reported. The cone conformation creates a zigzag orientation of the polymer segments. Their acid-dependent conductivities are similar to the strong pH conductivity dependence of polyaniline which is said to be acid dopable. On the other hand, they have a segmented structure that imposes greater localization of the carriers. The conductivity of such a system can be considered to result from rapid self-exchange between discrete units. Hence, electron exchange between radical cations and p-diquinone salts produces the high conductivity of these polymers. -
2003
Self-amplifying sensory materials: Energy migration in polymer semiconductors.
Macromolecular Symposia,vol.201,p.127–134,2003A review. Signal amplification for ultra-sensitive detection was achieved by energy migration in conjugated semiconducting polymeric assemblies. Crit. to optimizing this effect is the synthesis of non-aggregate polymers, the multi-dimensional directional transport of excited states (excitons), and extending the intrinsic excited state lifetime of conjugated polymers. We developed new water-sol. non-ionic conjugated polymers for use in biosensory applications, which can be used to provide highly sensitive/specific ultra-trace detection that is immune to specificity problems that plage ionic conjugated polymers. [on SciFinder(R)] -
2003
Measuring Local Optical Properties: Near-Field Polarimetry of Photonic Block Copolymer Morphology
Physical Review Letters,vol.90,p.016107,2003Ultrahigh mol. wt. polystyrene-b-polyisoprene block copolymers (BCs), noted for their photonic behavior, were imaged using transmission near-field scanning optical microscopy (NSOM) and NSOM polarimetry. The authors’ improved scheme for polarization modulation (PM) polarimetry, which accounts for optical anisotropies of the NSOM aperture probe, enables mapping of the local di-attenuation and birefringence (with sep. aligned di-attenuating and fast axes) in these specimens with sub-diffraction limited resoln. PM-NSOM micrographs illuminate the mesoscopic optical nature of these BC specimens by resolving individual microphase domains and defect structures. [on SciFinder(R)] -
2002
Conducting polymers of tungsten(VI)-oxo calixarene: Intercalation of neutral organic guests.
Advanced Materials (Weinheim, Germany),vol.14,p.368–371,2002A series of host-guest complexes of tungsten(VI)-oxo calixarenes substituted with two bithiophene groups at the upper rim were prepd. The bithienyl tungsten(VI)-oxo calixarenes were prepd. by Stille coupling of the complex with 2-tributylstannyl bithiophene and hydrolysis of the glycol ligand yielding the monomer precursor as the DMSO adduct. Upon removal of DMSO, the complex readily reacted with substituted formamides to produce the host-guest adduct monomers. These adducts undergo electrochem. polymn. that results in conducting polymers that contain guest mols. inside the calixarene cavity. The cond. of the polymers is dependent on the nature of the guest, being significantly lower for the disubstituted formamide complexes. [on SciFinder(R)] -
2002
Three-dimensional electronic delocalization in chiral conjugated polymers.
Angewandte Chemie, International Edition,vol.41,p.4225–30,2002 -
2002A sensor for nitric oxide is reported that uses a novel redox matching mechanism to induce a resistance change upon binding this important ligand to cobalt. [on SciFinder(R)]. This work was made possible by a Postdoctoral Fellowship from the Japan Society for the Promotion of Science to T. Shioya and the generous financial support of the Office of Naval Research.
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2002
Effects of Desymmetrization on Thiophene-Based Bent-Rod Mesogens.
Chemistry of Materials,vol.14,p.4543–4549,2002The synthesis and characterization of various thiophene-based bent-rod liq. crystals are reported, and the effects of varying lateral dipole and core desymmetrization upon mesophase behavior are described. Incorporation of desymmetrized core 7 into the mol. framework has very different consequences depending upon whether n-alkoxy or tetracatenar-type end groups were used. Tetracatenar-type mesogens 8-11 are significantly less mesogenic than the previously reported sym. series 3. When sym. straight-chain compds. 13-17 and unsym. straight-chain compds. 18-21 were studied, however, the desymmetrized core gave rise to mesophases with much broader temp. ranges. Variable temp. x-ray diffraction of these compds. suggests the formation of antiparallel dimers of mols. within the liq. crystal phase, and this may explain the relatively stable mesophases formed by these compds. and their incompatibility with chiral induction. The effects of altering the lateral substituents were also explored, and 3,4-difluorothiophene-based compds. 24-27 exhibit broad nematic mesophases. [on SciFinder(R)]
