The field of chemical sensing is becoming ever more dependent upon novel materials. Polymers, crystals, glasses, particles, and nanostructures have made a profound impact and have endowed modern sensory systems with superior performance. Electronic polymers have emerged as one of the most important classes of transduction materials; they readily transform a chemical signal into an easily measured electrical or optical event. Although our group reviewed this field in 2000,1 the high levels of activity and the impact of these methods now justify a subsequent review as part of this special issue. In this review we restrict our discussions to purely fluorescence-based methods using conjugated polymers (CPs). We further confine our detailed coverage to articles published since our previous review and will only detail earlier research in this Introduction to illustrate fundamental concepts and terminology that underpin the recent literature.

Chemical sensors based on amplifying fluorescent conjugated polymers.

Lanthanide-based luminescent hybrid materials.

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

This article provides a review about electroluminescence from organic materials and deals in detail with organic light-emitting diodes (OLEDs), light-emitting electro-chemical cells (LECs) and electrogenerated chemilumi-nescence (ECL) reflecting different electrooptical appli-cations of conjugated materials. It is written from an organic chemist's point of view and pays particular attention to the development of organic materials involved in corresponding devices. In recent years a substantial amount of both academic and industrial research has been directed to organic electroluminescence in an effort to improve the processability and tunability of organic materials and the longevity of OLEDs and LECs. On the eve of the commercialization of organic electrolumi-nescence this review provides an overview of lifetimes and efficiencies attained and reflects materials and device concepts developed over the last decade. In this context electrogenerated chemiluminescence is discussed with respect to its importance as a versatile tool to simulate the fundamental electrochemical processes in OLEDs.

The electroluminescence of organic materials

A series of soluble alternating polyfluorene copolymers with different main chain structures and those of the same main chain structure polyfluorene-co-alt-phenylene with different functional groups attached at the 2- and/or 5-positions of the phenylene ring were synthesized by a palladium-catalyzed Suzuki coupling reaction. All 10 polymers had the band gaps ranging from 2.81 to 3.35 eV, corresponding to blue-light emission. Through controllable modification for both the main chain structures and the side chains, not only the optical and electronic properties of the blue emissive polymers had been tuned, but also the structure−property relationships, especially the HOMO and LUMO energy level engineering, had been studied. Relatively high PL efficiency in both solution and film states, good thermal stability, and relatively high glass transition temperatures were demonstrated on these polymers. In general, polymers with the main chain structure of polyfluorene-co-alt-phenylene were found to have higher Φfl...

Blue-Light-Emitting Fluorene-Based Polymers with Tunable Electronic Properties

Three conjugated polymers comprised of 9,9-dioctylfluorene and 2,2‘-bipyridine, which are alternatively linked by the C−C single bond (P1), vinylene bond (P2), or ethynylene bond (P3), have been synthesized via the Suzuki reaction, the Wittig−Horner reaction, and the Heck reaction, respectively The optical, electrochemical, and other physical properties of the polymers are dependent on the linkers The polymer linked by the C−C single bond exhibits a much larger Stokes shift compared with the other two polymers, indicative of higher extended and rigid backbone conformations in the polymers linked by the vinylene and ethynylene bonds All the three polymers are sensitive to the existence of a variety of transition metal ions due to the chelation between the 2,2‘-bipyridyl moieties and the metal ions For the metal ions which have moderate and weak coordination ability with the 2,2‘-bipyridyl moieties, an obvious difference in response sensitivity is observed among the three polymers:  P1 has the highest s

Design and Synthesis of Bipyridyl-Containing Conjugated Polymers: Effects of Polymer Rigidity on Metal Ion Sensing

A novel series of soluble alternating conjugated copolymers comprised of 9,9-dihexylfluorene and substituted bithiophene or thiophene moieties were synthesized by palladium-catalyzed Suzuki coupling reaction. In the backbone structure of the polymers, the thiophene rings in the bithiophene moieties are β-substituted with decyl chains, and the bithiophene moieties are in two different coupling configurations of head-to-head and tail-to-tail regioregularities. For the thiophene moieties, they are either mono- or disubstituted with decyl chains. By means of the controllable structure modifications, the electronic properties of the polymers were tuned. In addition, the structure−property relationship of the backbone structure was studied. All of the polymers demonstrate efficient blue-to-green light emission, good thermal stability, and relatively high glass transition temperatures (Tg ∼ 78 °C). For the green-light-emitting polymers comprised of bithiophene moieties, their optical and electrochemical properti...

Synthesis, Characterization, and Structure−Property Relationship of Novel Fluorene−Thiophene-Based Conjugated Copolymers

A novel class of monodisperse conjugated polymer nanoparticles have been readily prepared by the facile reprecipitation of poly{1,3-bis[2-(3-alkylthienyl)]azulene} or poly{1,3-bis[2-(3-alkoxythienyl)]azulene}. The multicomponent poly(bithiophene-alt-azulene) macromolecules were efficiently self-assembled into a wide range of size-tunable nanoparticles from a few tens to five hundred nanometers via the hydrophobic and π-stacking effects in the mixed chloroform/methanol solutions. Electronically altered polymer structures with different alkyl or alkoxy substitutes exhibited variable self-assembling behaviors to precisely tune the size and the optical/electronic properties of nanoparticles. A strong size dependence of continuous bathochromic absorption and significant enhanced emission were observed with the increase of particle size. A linear relationship between the absorption or fluorescence intensities and the particle size was demonstrated as well, and this is very useful to probe the intermolecular int...

Aggregation-Driven Growth of Size-Tunable Organic Nanoparticles Using Electronically Altered Conjugated Polymers

Two poly(p-phenylenevinylene) (PPV) based polymers functionalized with an electron-deficient oxadiazole segment as the side chain by mimicking the chemical structure of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) have been successfully synthesized through the Gilch route and Wittig method. The obtained polymer II, which is a copolymer, is completely soluble in conventional organic solvents. However, polymer I, which is a homopolymer, is not soluble in any common organic solvents tried. The structure and purity of II have been characterized by FT-IR, 1H NMR, 13C NMR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV−vis and photoluminescence (PL) spectroscopy, and electrochemical analysis. The TGA results indicate that II has very high thermal stability, while DSC investigation demonstrates that the glass transition temperature (Tg) of II is higher than 200 °C, which might be a merit for the long-life operation of light-emi...

Yee-Hing Lai

Papers

Blue-Light-Emitting Fluorene-Based Polymers with Tunable Electronic Properties

Design and Synthesis of Bipyridyl-Containing Conjugated Polymers: Effects of Polymer Rigidity on Metal Ion Sensing

Synthesis, Characterization, and Structure−Property Relationship of Novel Fluorene−Thiophene-Based Conjugated Copolymers

Aggregation-Driven Growth of Size-Tunable Organic Nanoparticles Using Electronically Altered Conjugated Polymers

Photoluminescent poly(p-phenylenevinylene)s with an aromatic oxadiazole moiety as the side chain : Synthesis, electrochemistry, and spectroscopy study