Advanced Materials

Cover image for Advanced Materials

November, 2000

Volume 12, Issue 22

Pages 1655–1704

    1. Semiconducting (Conjugated) Polymers as Materials for Solid-State Lasers (pages 1655–1668)

      M. D. McGehee and A. J. Heeger

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1655::AID-ADMA1655>3.0.CO;2-2

      Light-emissive polymers are outstanding laser materials because they are intrinsically “four-level” systems, they have high luminescence efficiencies, they emit in the visible region, and they can be processed into optical quality films. The important materials issues are reviewed and the prospects for making polymer diode lasers are discussed. The Figure shows two luminescent polymers used as gain materials in lasers (see also inside front cover).

    2. Photo-Induced Reversible Emission Switching in Pyridine/Europium(III) Chloride/Organic Fluorophore System (pages 1669–1671)

      E. Vaganova, S. Yitzchaik, L. Shapiro, M. Sigalov and V. Khodorkovsky

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1669::AID-ADMA1669>3.0.CO;2-D

      A blue–red reversible optical switch has been created by combining EuIII, pyridine, and a fluorescent organic chromophore. The initial solution exhibits the expected blue emission of the chromophore until UV irradiation leads to red emission (see Figure), a process that is ascribed to protonation of the pyridine and formation of a charge-transfer complex with the chromophore in the excited state, allowing the red EuIII photoluminescence to be emitted.

    3. A Concept for the Fabrication of Penetrating Carbon/Silica Hybrid Materials (pages 1671–1675)

      H. Müller, P. Rehak, C. Jäger, J. Hartmann, N. Meyer and S. Spange

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1671::AID-ADMA1671>3.0.CO;2-M

      Nanostructured carbon/silica hybrid materials and poly(furfuryl alcohol)/silica xerogel are prepared and characterized here. In the key synthetic step, cation-induced polycondensation of furfuryl alcohol occurs simultaneously with a sol–gel process involving tetraethoxysilane and water to yield the xerogel, which is subsequently thermally transformed to the carbon/silica material (see Figure).

    4. Surface Relief Gratings in Host–Guest Supramolecular Materials (pages 1675–1678)

      T. Ubukata, T. Seki and K. Ichimura

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1675::AID-ADMA1675>3.0.CO;2-Z

      Surface relief gratings are one of the most promising applications for azobenzene-containing polymers (azo-polymers). Systems investigated to date have contained single-component azo-polymers. The introduction of a liquid-crystal molecule to the system, presented here, gives rise to a striking enhancement of surface relief grating formation, due to the strong molecular cooperativity and increased fluidity.

    5. Fluorophores Related to the Green Fluorescent Protein and Their Use in Optoelectronic Devices (pages 1678–1681)

      Y. You, Y. He, P. E. Burrows, S. R. Forrest, N. A. Petasis and M. E. Thompson

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1678::AID-ADMA1678>3.0.CO;2-H

      Molecular analogues of the green fluorescent protein (GFP) fluorophore have been designed by these authors (see Figure, X=NC4H9 or O). It is shown that by varying the donor and acceptor groups of the oxazolone (X=O) analogue the fluorescence emission can be tuned from blue to green to orange. The utility of these artificial fluorophores is explored by incorporating them into organic LEDs.

    6. Synthesis of Mesoporous Aluminosilicates with Enhanced Stability and Ion-Exchange Capacity via a Secondary Crystallization Route (pages 1681–1685)

      R. Mokaya

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1681::AID-ADMA1681>3.0.CO;2-K

      Mesoporous aluminosilicate materials have previously had the disadvantage of being structurally less stable than zeolites, which are crystalline and more ordered. A new approach to Al-containing MCM-41 is presented here: This two-step synthesis procedure involves adding Al to the synthesis gel during a second crystallization step, which ensures the incorporation of Al as an aluminosilicate layer at or near the surface of the pore walls of existing MCM-41 crystallites, resulting in Al–MCM-41 materials with good hydrothermal and mechanical stability and enhanced ion-exchange capacity.

    7. Superconductivity, Antiferromagnetism, and Phase Diagram of a Series of Organic Conductors: λ-(BETS)2FexGa1–xBryCl4–y (pages 1685–1689)

      H. Tanaka, H. Kobayashi, A. Kobayashi and P. Cassoux

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1685::AID-ADMA1685>3.0.CO;2-X

      Organic conductors based on the bis(ethylenedithio)tetraselenafulvalene(BETS) are of interest because they display not only superconducting and magnetic transitions but also, as recently reported, unprecedented superconductor to insulator transitions. Here is presented the electrical and magnetic properties for two series of λ-(BETS)2FexGa1–xBryCl4–y, one system with fixed Br content, the other with fixed Fe content. This systematic study includes the superconductor to insulator transition, and also a superconductor to metal transition, which has never been previously reported for an organic conducting system.

    8. Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion (pages 1689–1692)

      A. C. Arango, L. R. Johnson, V. N. Bliznyuk, Z. Schlesinger, S. A. Carter and H.-H. Hörhold

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1689::AID-ADMA1689>3.0.CO;2-9

      Record high efficiencies for a polymer-based solar cell are reported here for an inorganic/organic composite photovoltaic device containing a single polymer film rather than a polymer/polymer or polymer/nanoparticle blend. The high efficiencies are reported to be due to device construction (see Figure) and the electronic properties of the phenylamino-p-phenylenevinylene-based polymer used.

    9. Encapsulation of Poly(N-Isopropyl Acrylamide) in Silica: A Stimuli-Responsive Porous Hybrid Material That Incorporates Molecular Nano-Valves (pages 1692–1695)

      G. V. Rama Rao and G. P. López

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1692::AID-ADMA1692>3.0.CO;2-C

      Incorporation of a smart polymer into a silica matrix can lead to smart membranes, it is demonstrated here. Polymer–silica membranes were supported on porous centrifugal filters (see Figure), the encapsulated polymer molecules acting as “nano-valves”. Permeation experiments with aqueous solutions of poly (ethylene glycol) show that the membranes function as thermally switchable molecular filters.

    10. Sol-Gel-Derived Mesoporous Silica Films with Low Dielectric Constants (pages 1695–1698)

      S. Seraji, Y. Wu, M. Forbess, S. J. Limmer, T. Chou and G. Z. Cao

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1695::AID-ADMA1695>3.0.CO;2-V

      Mesoporous silica films with low dielectric constants and possibly closed pores have been achieved with a multiple step sol-gel processing technique. Crack-free films with approximately 50 % porosity and 0.9 μm thicknesses were obtained, a tape-test revealing good adhesion between films and substrates or metal electrodes. Dielectric constants remained virtually unchanged after aging at room temperature at 56 % humidity over 6 days.

    11. Photopolymerizable Cholesteric Liquid Crystals—New Materials for Holographic Applications (pages 1698–1700)

      U. Theissen, S. J. Zilker, T. Pfeuffer and P. Strohriegl

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1698::AID-ADMA1698>3.0.CO;2-D

      A novel polymerizable cholesteric liquid crystal (CLC) is presented as a candidate for holographic data storage. In contrast to classical photopolymers, which require a spatially varying intensity, the CLC is able to store polarization holograms (see Figure) with a constant intensity but spatially varying polarization (RCP and LCP denote right- and left-circularly polarized light, respectively) (see also cover).

    12. Electroluminescence from Well-Defined Tetrahedral Oligophenylenevinylene Tetramers (pages 1701–1704)

      M. R. Robinson, S. Wang, G. C. Bazan and Y. Cao

      Version of Record online: 1 DEC 2000 | DOI: 10.1002/1521-4095(200011)12:22<1701::AID-ADMA1701>3.0.CO;2-U

      Organic light-emitting diodes (LEDs) with small turn-on voltages are fabricated here using molecules based on the tetrakis(4-styryldistyrylbenzene)methane framework (see Figure). The synthesis of these molecules is reported, their absorption and luminescence properties are compared, and the effect of device architecture on performance is investigated.