Current Organic Chemistry - Volume 14, Issue 18, 2010

Since the first discovery of conducting polymers in 1970s, functional π-systems such as oligomers, polymers, dendrimers and polycyclic aromatics have received intense scientific and industrial interest due to their tunable optical and electronic properties. Functional π-systems can find many practical applications for such as flexible electronic devices (e.g. organic field effect transistor (OFET), organic light-emitting diode (OLED)), clean energy (e.g., bulk heterojunction solar cells, dye-sensitized solar cells, batteries and capacitors), display technology (e.g. electrochromic display), sensing (chemical sensor and biosensor) and dyestuff industry. Significant progress on functional π-systems has been achieved in recent years and it makes the commercial applications of these materials possible in coming years. So this special issue will focus on the recent progress on the molecular design and material synthesis of some new functional π-systems. Among various materials for OLEDs, stable blue light-emitting materials are especially important for a full color RGB display. Professor Grimsdale provides a review on the synthesis and OLED applications of a type of phenylene-based conjugated polymers. A review from Dr. Chen is focused blue-emitting materials based on small molecules and oligomers. Besides fluorescent materials, electro-phosphorescent polymers have recently attracted intensive interest for high-efficiency OLEDs and Professor Pei at ULCA provides a review on this topic. Professor Tang gives a review on a special type of luminescent materials which show unusual aggregation-induced emission (AIE) phenomenon. This kind of materials has promising applications for OLEDs, chemical sensors and biological probes. Professor Huang give a review on another special type of semiconductors based on spirocyclic aromatic hydrocarbons which can serve as amorphous semiconductors with high glassy transition temperatures. Organic semiconductors with high charge carrier mobilities are highly important for OFET and solar cell applications. Among various organic semiconductors, thiophene-based materials usually exhibit good performance and thus Professor Liu gives a review on the synthesis and device applications of polythiophene-based semiconductors. In addition, oligoacenes are also key building blocks for the construction of efficient semiconductors for electronic devices and Dr. Chi provides a comprehensive review on the synthetic chemistry of oligoacenes and heteroacenes. Organic near infrared (NIR) dyes have a lot of applications such as for laser protection, solar cells, bioimaging and nonlinear optics. The guest editor Dr. Wu contributes a review on the synthetic chemistry of soluble and stable NIR dyes based on polycyclic aromatics. Together these contributions provide a timely update on the current chemistry of functional π-systems.

Polythiophenes (PTs) are one of the most important classes of conjugated polymers employed as semiconductors in organic field-effect transistors (OFETs). In the last few years, versatile thiophene-based polymers have received great interest for applications in large-area, flexible, and low-cost electronics attributing to their excellent optoelectronic properties. Significant progress has been made in PT-based semiconductors for high performance transistors. We review here the achievements gained in conjugated PT-based semiconductors and demonstrate their structural modifications and design principles. We highlight the most important semiconducting polymers created from PT derivatives obtained through rational monomer design, controlled π-conjugation and appropriately positioned pendant side-chain distribution. Finally, we discuss remaining challenges and outline our views on future developments.

Organic light emitting diodes (OLEDs), as an emerging technology for display and solid state lighting application, have many advantages including self-emission, lightweight, flexibility, low driving voltage, low power consumption, and low production cost. With the advancement of light emitting materials development and device architecture optimization, mobile phones and televisions based on OLED technology are already in the market. However, to obtain efficient, stable and pure blue emission than producing lower-energy colors is still one of the important subjects of these challenges. Full color and pure white light can be achieved only having stable blue emitting materials. To address this issue, significant effort has been devoted to develop novel blue light emitting materials in the past decade aiming at further improving device efficiency, color quality of emission light, and device lifetime. This review focuses on recent efforts of synthesis and device performance of small molecules, oligomers and polymers for blue emission of organic electroluminescent devices.

Acenes, linearly fused aromatic hydrocarbons, have been the subjects of extensive research because of interesting electronic properties associated with their linear delocalized π-system. The potential applications of these acene-based materials in electronic devices such as field-effect transistor (FET), light-emitting diode (LED) and solar cell have been recently exploited. This review will focus on the synthetic approaches toward acenes and heteroacenes, and in particular, some new synthetic strategies towards higher order acenes and heteroacenes will be highlighted.

It has been a textbook knowledge that chromophore aggregation generally quenches light emission. While in 2001, we discovered a novel phenomenon of aggregation-induced emission (AIE): a series of propeller-shaped molecules were found nonluminescent in the solution state but emissive in the aggregate and solid states. Attracted by the intriguing phenomenon and its fascinating perspectives, we launched a research program on developing new AIE luminogens and exploring their high-tech applications. In this review, we present our recent works on developing a novel AIE system based on tetraarylethenes. We show that restriction of intramolecular rotation is the main cause for the AIE effect. Utilizing such treasured AIE characteristics, the tetraarylethenes can find an array of applications, including fluorescence sensors, biological probes, and active materials in the fabrication of organic light-emitting diodes.

Electrophosphorescent polymers are an important class of light-emitting materials that combine high electroluminescence efficiency of organometallic phosphohrescent complexes and solution processability of polymers. This review surveys the progress in the design, synthesis, characterization, photoluminescence and electroluminescence of electrophosphorescent polymers reported in the recent literatures. It focuses on polymers with phosphorescent guest covalent bonded to the polymers as a repeating unit in the main chain, a side chain, or polymer chain termini. The polymer backbones either have an extended π-electron conjugation or are non-conjugated. Important electrophosphorescent polymers and their performance in light emitting diodes are described.

Currently, how to obtain near-infrared (NIR) dyes is intriguing and attractive but full of challenges. Although some cyanine dyes have been commercialized, near infrared dyes with intensive NIR absorption and/or emission, good chemical and photo-stability, and high solubility still remain as challenging targets. Certain polycyclic aromatic compounds such as rylenes, porphyrins, and phthalocyanines represent a key class of near-infrared dyes and also give rise to outstanding physical and chemical properties after appropriate chemical modification. This review highlights and updates the principal synthetic strategies used to obtain polycyclic aromatics-based soluble and stable near-infrared dyes. We will also summarize the physical properties of these newly developed NIR dyes, including absorption, emission, quantum yield, solubility and stability.

Spirocyclic aromatic hydrocarbons (SAHs) or spirocyclic arenes are functional building blocks in constructing amorphous organic/ polymeric semiconductors with high glass transition temperatures for organic electronic devices. In this review, SAHs were categorized into three classes - mono-spirocyclic aromatic hydrocarbons (MSAHs), heteroatom-containing spirocyclic aromatic hydrocarbons (HSAHs), and polyspirocyclic aromatic hydrocarbons (PSAHs) based on MSAHs and HSAHs. Various synthetic strategies of SAHs, viz., o-halobiaryl method, diarylfluorene method, dibiarylketone method, one-pot domino synthesis, tandem dilithiation method, alkyne-fused aromatization, allene method, and other related routes were summarized.

Phenylene-based polymers are a widely studied class of blue-emitting materials for use in polymer light-emitting diodes intended for display applications. This review looks at the various types of such polymers that have been synthesised and the problems that have arisen in obtaining stable and efficient blue emission from them. The emission colour from phenylene-based polymers can be controlled by linking some or all of the phenylene units with short carbon or nitrogen bridges, and it has been found that by varying the amount and type of bridges pure blue emission can be obtained. Both aggregation of chains, and the formation of emissive defects cause instability of the emission colour and it is shown that this can be surmounted by appropriate choice of substituents and synthetic routes. In particular the formation of defects is promoted by hydrogens on the bridgeheads, and polymers with aryl substituents at the bridgeheads have been found to much more stable than those with alkyl substituents. High device efficiency can be obtained by incorporation of appropriate charge transporting substituents, which also may help with reducing problems with colour stability. It is shown that by correct design of the polymer structure and appropriate synthetic strategy it is possible to obtain efficient, stable, pure blue emission from phenylene- based polymers.