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2002
Triptycene-containing bis(phenylethynyl)benzene nematic liquid crystals.
Journal of Materials Chemistry,vol.12,p.3407–3412,2002A new class of nematic liq. crystals with triptycenes built into a p-dialkoxybis(phenylethynyl)benzene mesomorphic core were prepd. Triptycenes are appended on the center or terminal ring of the mesogen, leading to sym. liq. crystals and reduced symmetry liq. crystals, resp. Both types displayed monotropic behavior, with the asym. compds. having unusual phase behavior, lacking distinct crystn. transitions, and forming a glassy mesophase. A chiral analog is nonmesomorphic, but induced chiral nematic phases when doped into achiral triptycene-contg. analogs. Rotation is phys. hindered normal to the director and, hence, this new liq. crystal architecture may allow for the synthesis of a single component liq. crystal that displays a biaxial nematic phase. [on SciFinder(R)] -
2002
Exploiting the versatility of organometallic cross-coupling reactions for entry into extended aromatic systems.
Journal of Organometallic Chemistry,vol.653,p.215–222,2002Several pendant arenes, e.g. I (R = Me, MeO), were prepd. using a variety of Pd-catalyzed cross-coupling techniques. Under strongly acidic conditions, these arenes underwent double annulations to afford the corresponding polycyclic aroms., e.g. II, in good to excellent yields. The scope of this chem. also includes heteroarom. and heteroat. substituted bis(arylethynyl) arenes thus providing an electronically diverse array of extended arom. systems. [on SciFinder(R)] -
2002
The Interplay of Bent-Shape, Lateral Dipole and Chirality in Thiophene Based Di-, Tri-, and Tetracatenar Liquid Crystals
Journal of the American Chemical Society,vol.124,p.12742–12751,2002A range of mesogenic mols. varying in both bend angle and strength of lateral dipole were synthesized, and their phase behavior was characterized by polarizing microscopy, thermal anal., x-ray diffraction, and electrooptical measurements. The authors find the general destabilization of the liq. crystallinity caused by strong lateral dipolar groups and the bent mol. shape are off-set in mesomorphic tetracatenars, which display stable nematic, smectic, columnar, and cubic mesophases. The broad mesomorphism of the tetracatenars contg. lateral dipoles and their incompatibility with chiral induction are explained by considering that loosely correlated dimers exist within the mesophases. Chiral mesophases of derivs. with strong lateral dipoles were achieved by attaching fewer or different side chains to each end of the mesogen. [on SciFinder(R)] -
2002
Conjugated Polymer Liquid Crystal Solutions: Control of Conformation and Alignment.
Journal of the American Chemical Society,vol.124,p.9670–9671,2002Fluorescent poly(phenylene vinylene)s and poly(phenylene ethynylene)s contg. rigid triptycene groups were prepd. using Suzuki and Sonogashira protocols. The triptycene groups impart extraordinary soly. to conjugated polymers even in the absence of flexible side chains; addnl. t-Bu groups were introduced, which were found to further enhance soly. Stable solns. of poly(phenylene vinylene)s and poly(phenylene ethynylene)s in nematic liq. crystal 1-(trans-4-hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT -
2002
Structural Control in Thin Layers of Poly(p-phenyleneethynylene)s: Photophysical Studies of Langmuir and Langmuir-Blodgett Films.
Journal of the American Chemical Society,vol.124,p.7710–7718,2002We present the relationship between the spatial arrangement and the photophys. properties of fluorescent polymers in thin films with controlled structures. Eight surfactant poly(p-phenyleneethynylene)s were designed and studied. These detailed studies of the behavior of the polymers at the air-water interface, and of the photophys. properties of their transferred LB films, revealed key structure-property relationships. Some of the polymers displayed $π$-aggregates that are characteristic of an edge-on structure at the air-water interface. Monolayer LB films of these polymers showed greatly reduced quantum yields relative to soln. values. Other polymers exhibited a highly emissive face-on structure at the air-water interface, and did not form $π$-aggregates. The combination of pressure-area isotherms and the surface pressure dependent in situ UV-vis spectra of the polymers at the air-water interface revealed different behavioral details. In addn., the UV-vis spectra, fluorescence spectra, and quantum yields of the LB films provide design principles for making highly emissive films. [on SciFinder(R)] -
2002
Functionalizable Polycyclic Aromatics through Oxidative Cyclization of Pendant Thiophenes.
Journal of the American Chemical Society,vol.124,p.7762–7769,2002We present a general strategy for obtaining large sulfur-contg. polycyclic aroms. from thienyl precursors through iron(III) chloride mediated oxidative cyclizations. By placing thienyl moieties in close proximity to adjacent arenes, we have directed the oxidized intermediates into controlled cyclization pathways, effectively suppressing polymer formation. Utilizing these cyclized compds. and their thienyl precursors, we have studied cyclization/polymn. pathways of polymers such as poly(2). The unsubstituted positions $alpha$ to the sulfur atoms within these arom. cores allowed for efficient halogenation and further functionalization. As a demonstration, we prepd. a series of arylene-ethynylene polymers with varying degrees of chromophore aromatization and used them to probe the effects of synthetically imposed rigidity on polymer photophys. behavior. The symmetries and effective conjugation pathways within the monomers play a key role in detg. photophys. properties. We obsd. that rigid, aromatized chromophores generally led to increased excited-state lifetimes by decreasing radiative rates of fluorescence decay. [on SciFinder(R)] -
2002
Using Ïnternal Free Volume” to Increase Chromophore Alignment.
Journal of the American Chemical Society,vol.124,p.3826–3827,2002Triptycenes have general applicability for increasing the alignment of fluorescent and dichroic dyes in LC hosts. Dyes contg. varying nos. of triptycenes were synthesized to study the effect of free-vol. alignment of triptycenes on the alignment of dyes. These dyes were designed such that multiple triptycenes could be incorporated and the triptycene-free vol. is coincident to the aspect ratio of the dye, allowing a cooperative effect to increase their overall av. alignment. With increasing triptycene incorporation, a stepwise increase in the alignment parameters of each dye was seen. It was also found that the attachment of one triptycene group has a negligible effect on the optical switching response times of the dyes. This can be a powerful tool for designing dyes with higher alignments for a variety of applications including guest-host reflective LCDs and holog. data storage. [on SciFinder(R)] -
2002
Enhanced conductivity and dielectric absorption in discotic liquid crystalline columnar phases of a vanadyl complex.
Liquid Crystals,vol.29,p.133–139,2002A liq. cryst. vanadyl complex was studied by DSC, polarizing optical microscopy, the reversal current technique, x-ray diffraction and frequency domain dielec. spectroscopy. The compd. exhibits three columnar phases: rectangular ordered (Colro), rectangular disordered (Colrd), and hexagonal disordered (Colhd), all of which show a dielec. relaxation process at low frequencies. In the Colro low temp. phase this process seems to be connected with a slow relaxation of polarized polymeric chains inside the columns (mHz frequency range). However, in the Colhd high temp. disordered phase this relaxation is faster (Hz range). It is interesting that the liq. cryst. phases studied show enhanced cond. which changes by four orders of magnitude from 10-9 S m-1 in the orientationally disordered crystal (an ODIC phase) to 10-5 S m-1 in the Colhd high temp. phase. Such a value of the cond. is typical for semiconducting materials. [on SciFinder(R)] -
2002A novel approach to the fabrication of poly(p-phenylene ethynylene) (PPE) brushes on oxidized silicon surfaces using ring-opening metathesis polymn. was demonstrated. Chem. bound 71-110 AA thick PPE brushes were constrained from forming aggregates and gave higher emission quantum yields than spin-cast films. [on SciFinder(R)]
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2002
Self-amplifying semiconducting polymers for chemical sensors.
MRS Bulletin,vol.27,p.446–450,2002The ability of excited states (excitons) to migrate rapidly and efficiently through conjugated polymers makes these materials ideal for use in sensors based on fluorescence quenching or amplification of fluorescence signals. The structural features of the conducting polymers allow for design of highly sensitive fluorescent sensors for specific analytes such as the explosive trinitrotoluene (TNT) and to create assemblies that control energy transfer along a predetd. path. The principles involved have broad utility in the design of sensory materials and of electronic devices and display components based on electronic polymers. [on SciFinder(R)] -
2002
Polymer light-emitting devices: Light from insulated organic wires.
Nature materials,vol.1,p.151–2,2002In an contribution to an article in the same issue, the advantages of the rotaxane approach (precise mol. insulation technique) in the design of polymer light-emitting devices are highlighted. Semiconducting polymers are threaded through cyclodextrins and are capable of both emitting light and transporting charge. [on SciFinder(R)] -
2001
Poly(naphthodithiophene)s: robust, conductive electrochromics via tandem cyclization-polymerizations.
Advanced Materials (Weinheim, Germany),vol.13,p.1775–1780,2001To achieve highly stable electrochromic materials, 4,5-bis(2-/3-thiophenyl)-1,2-dialkoxybenzene precursors were prepd., cyclized (aromatized), and the resulting naphthodithiophene monomers were subjected to oxidative polymn. (chem. as well as electrochem.). The precursors, monomers, and polymers were characterized by spectroscopic and electrochem. methods. [on SciFinder(R)] -
2001
Minimization of free volume. Alignment of triptycenes in liquid crystals and stretched polymers.
Advanced Materials (Weinheim, Germany),vol.13,p.601–604,2001Two triptycene derivs. were incorporated into a polyvinyl chloride (PVC) film or into the nematic phase of 4-(trans-4-pentylcyclohexyl)benzonitrile. The supramol. alignment taking place was visualized by polarized UV-vis and IR spectroscopy. The director axis of the nematic liq. crystal or the stretching direction of the polymer film are parallel to the the long axis of the guest mol. The degree of alignment for the 2 triptycene derivs. was detd. via the aspect ratio in comparison to anthracene. The alignment provided by the uniaxial PVC film was lower than that for the LC. A threading mechanism was proposed for the supramol. arrangement based on the minimization of the free vol. [on SciFinder(R)] -
2001A review, with 28 refs., on the development of polymers as 1D photonic crystals with focus on self-assembled block copolymers. The use of plasticizer and homopolymer blends of diblock copolymers to increase periodicity and the role of self-assembly in producing 2D and 3D photonic crystals are discussed. The use of inorg. nanoparticles to increase the dielec. contrast and of a biasing field during self-assembly to control the long-range domain order and orientation are outlined. In-situ tunable materials fabricated via a mechanochromic materials system are also described. The inherent optical anisotropy of extruded polymer films and side-chain liq.-cryst. polymers provides flexibility for novel optical designs. [on SciFinder(R)]
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2001
Allosteric fluoride anion recognition by a doubly strapped porphyrin.
Angewandte Chemie, International Edition,vol.40,p.3372–3376,2001A highly selective allosteric fluoride recognition system is described consisting of a doubly strapped porphyrin (I) that contains 2 small hydrogen-bonding cavities not being able to bind larger anions. Compd. I was prepd. by condensation of strap moieties with a $alpha$$beta$$alpha$$beta$-tetrakis(2-aminophenyl)porphyrin under high diln. conditions. By comparison with related strapped porphyrins contg. 4 linkers the cofacial distance between the phenyls in the straps and the porphyrin was estd. to be 3-4 AA creating a small pocket suitable for F- binding but excluding larger anions. The proximate location of the strap moieties to the porphyrin in I was confirmed by NMR spectroscopy in CDCl3. The Soret band of I in dichloromethane was split into 2 bands of equal intensity indicating that the strap presented a significant perturbation caused by $π$-$π$ interactions of the bis(dithienyl)-benzene moieties with the porphyrin. Upon addn. of F- to a soln. of I in DMSO the split Soret band shifted to a single band with a simultaneous red shift of the Q band whereas no absorbance changes were obsd. upon exposure of larger anions including Cl-, Br-, J-, CN-, and H2PO3-. The plots of absorbance changes vs. TBAF showed a sigmoidal curve indicating that the F- binding to I is cooperative. This binding was analyzed with the Hill equation and further characterized using Scatchard plots. Given the obsd. 1:2 binding stoichiometry energy-minimized geometries were computed suggesting that the cavities in I are contracted. The calcd. structure with 2 bound F- caused an expansion of the cavity that sepd. the planes of the porphyrin and the straps by 4.9 AA. From studies of the influence of a second coexisting halide ion on the UV/Vis spectrum of the I(F2-) complex it was estd. that the affinity of I to F- was ∼104 times higher than for other halides. A conducting polymer based upon I displays both electrochem. and cond. responses to F- and no response to Cl-. [on SciFinder(R)] -
2001
Using novel fluorescent polymers as sensory materials for above-ground sensing of chemical signature compounds emanating from buried landmines
IEEE Transactions on Geoscience and Remote Sensing,vol.39,p.1119–1128,2001 -
2001
Dinuclear pincer-palladium(II) complexes and their use as homogeneous or heterogeneous catalyst for the aldol reaction of methyl isocyanoacetate.
Journal of Molecular Catalysis A: Chemical,vol.166,p.265–273,2001Two new bimetallic Pd complexes I (R = t-Bu, Ph) have been synthesized and characterized and their catalytic activity checked for the aldol reaction of aldehydes with Me isocyanoacetate. Each palladium atom is coordinated to an SCS-type ligand and the two pincer units are linked by a chiral spacer. The catalytic aldol reaction of Me isocyanoacetate with aldehydes proceeds quickly but no significant diastereoselectivity and enantioselectivity is found. The comparison with a homologous mononuclear Pd complex shows no differences with the bimetallic compds., concluding that there is no cooperativity between the metal centers. Two silica-supported catalysts prepd. with a bimetallic compd. show catalytic activity with very minor enantioselectivity. [on SciFinder(R)] -
2001
Oxidative Cyclization of Bis(biaryl)acetylenes: Synthesis and Photophysics of Dibenzo[ g,p ]chrysene-Based Fluorescent Polymers
Journal of the American Chemical Society,vol.123,p.12087–12088,2001 -
2001
Directing Energy Transfer within Conjugated Polymer Thin Films
Journal of the American Chemical Society,vol.123,p.11488–11489,2001 -
2001
Excited-State Lifetime Modulation in Triphenylene-Based Conjugated Polymers
Journal of the American Chemical Society,vol.123,p.11298–11299,2001
