Large N-Heteroacenes: New Tricks for Very Old Dogs? Bunz, Uwe H. F.; Engelhart, Jens U.; Lindner, Benjamin D. ...
Angewandte Chemie (International ed.),
April 2, 2013, Letnik:
52, Številka:
14
Journal Article
Recenzirano
Azaacenes have been known for a very long time, either as N,N′‐dihydro compounds or in their oxidized form as 4 n+2π systems, but only recently have processable and charcterizable derivatives been ...sought. In the last three years synthetic routes to large N‐heteroacenes have been developed. In particular, the Pd‐catalyzed coupling of aromatic diamines with activated aromatic dihalogenides has enabled simple access to numerous new azaacenes. Since 2010, azapentacene and stabile oligoazahexacene have been synthesized, as well as a symmetrical tetraazapentacene, which acts as an excellent electron‐transport material for thin‐film transistors.
Electron‐transporting alternatives to pentacene? Since 2010, several research groups have succeeded in synthesizing substituted diaza‐ and tetraazapentacenes as well as structurally similar tetraaza‐ and hexaazahexacenes. The symmetrical tetraazapentacene (see structure; C gray, H white, N blue, Si tan) has been investigated as an electron‐transporting material in thin‐film transistors.
Conspectus N-Heteroacenes and N-heteroarenes are the heterocyclic congeners of the acenes and arenes, in which one or several perimeter C–H bonds have been substituted by pyridine-type nitrogen ...atoms. They are formally segments out of N-doped nanographenes. Position and number of the nitrogens vary greatly, making N-heteroacenes and N-heteroarenes define a vast class of N-nanographene segments; they display modular electronic and structural properties. The nitrogen atoms in the perimeter lead to finely tunable frontier molecular orbital positions and therefore improved electron affinity and higher oxidative stability but conversely also require and allow different synthetic approaches than those reported for the synthesis of their hydrocarbon and nanographene analogues. The chemistry of N-heteroarenes, despite being known for more than a century, has made significant progress in the last years and established these materials both as powerful n-channel semiconductors in thin film transistors and as useful emitters in organic light emitting diodes (OLEDs) and in photovoltaic devices. The electronegative nitrogen atoms impart a deep LUMO into the azaacenes and azaarenes, improve electron injection, and enable powerful electron transport but also charge separation in bulk-heterojunction type organic photovoltaic (OPV) devices. At the same time, azaacenes and azaarenes are fundamentally exciting materials that push the limits of structure and stability, constantly displaying novel topologies and structures as variations of a simple leitmotif; we expect a bright future for esthetically pleasing yet highly functional N-heterocyclic species. Firstly, we discuss novel structures and structural elements that have evolved during the last years in N-heteroacene and N-heteroarene chemistry and delineate their properties. An important aspect is the oligomerization or better multimerization of azaacene and azaarene units into novel and surprising topologies, in which multiple azaarenes or azaacenes are stitched together. Examples are tetrahedral assemblies of tetraazapentacenes but also cyclic tetramers of different types of azaacenes and linearly bent, S-shaped, formally dimeric species. An exciting aspect of the exploration of the structural manifold of azaacenes is their electronic interaction in such assemblies and their solid-state microstructure. A further aspect of this work is the increase in size of the azaacenes and concepts that allow stabilization of the larger congeners. The attachment of four benzo units to the azaacene core is a powerful concept that stabilizes tetraazaheptacenes and should also be useful to achieve persistent tetraazanonacenes. Secondly, we describe the success of N-heteroacenes and N-heteroarenes in organic electronic devices; specifically, the use of symmetrical halogenated tetraazapentacenes as superb n-channel transistor materials with air stable and persistent radical anions as charge carriers; we discuss the structural reason for their success. Use of azaacenes and azaarenes is not restricted to transistors, but they are also applied in bulk heterojunction photovoltaic devices and in brightly emitting OLEDs. Azaacenes and azaarenes are attractive segments out of hetero-nanographenes and objects of study, starting from fundamental structural and topological questions, ranging to powerful applications in organic electronics. The general interest in azaacenes is witnessed by the constantly increasing number of groups who discover and work on these materials as novel functional and flexible species.
This Review details synthetic routes toward and properties of insoluble polymeric organic semiconductors obtained through desolubilization strategies. Typical applications include fixation of ...donor–acceptor bulk-heterojunction morphologies in organic photovoltaic cells, cross-linking of charge transport materials and active emitters in light emitting diodes or similar devices, and immobilization of morphologies in field effect transistors. A second important application is the structuring of organic semiconductors, using them as photoresists. After desolubilization, removal of the nonirradiated resist leads to elevated, micron-sized features of the semiconductor. In this Review, different strategies for desolubilization are covered. By photochemical or thermal cleavage of solubility-mediating groups such as esters, sulfonium salts, amides, ethers, and acetals or by retro-Diels–Alder reactions, volatile elimination products and the insoluble semiconductor are formed. In another case, desolubilization is achieved by cross-linking via functional groups present in the polymer side chains including vinyl, halide, silicone, boronic acid, and azide functionalities, which polymerize thermally or photochemically. Alternatively, small molecular additives such as photoacids, oligothiols, or oligoazides result in network formation in combination with compatible functional groups present in the immobilizable polymers. Advantages and disadvantages of the respective methods are discussed.
The Larger Linear N‑Heteroacenes Bunz, Uwe H. F.
Accounts of chemical research,
06/2015, Letnik:
48, Številka:
6
Journal Article
Recenzirano
The close structural and chemical relationship of N-heteroacenes to pentacene suggests their broad applicability in organic electronic devices, such as thin-film transistors. The superb materials ...science properties of azaacenes result from their improved resistance toward oxidation and their potential for electron transport, both of which have been demonstrated recently. The introduction of nitrogen atoms into the aromatic perimeter of acenes stabilizes their frontier molecular orbitals and increases their electron affinity. The HOMO–LUMO gaps in azaacenes in which the nitrogen atoms are symmetrically placed are similar to those of the acenes. The judiciously placed nitrogen atoms induce an “umpolung” of the electronic behavior of these pentacene-like molecules, i.e., instead of hole mobility in thin-film transistors, azaacenes are electron-transporting materials. The fundamental synthetic approaches toward larger azaacenes are described and discussed. Several synthetic methodologies have been exploited, and some have been newly developed to assemble substituted azaacenes. The oldest methods are condensation-based. Aromatic o-diamines are coupled with o-dihydroxyarenes in the melt without solvent. This method works well for unsubstituted azaacenes only. The attachment of substituents to the starting materials renders these “fire and sword” methods less useful. The starting materials decompose under these conditions. The direct condensation of substituted o-diamines with o-quinones proceeds well in some cases. Fluorinated benzene rings next to a pyrazine unit are introduced by nucleophilic aromatic substitution employing hexafluorobenzene. However, with these well-established synthetic methodologies, a number of azaacene topologies cannot be synthesized. The Pd-catalyzed coupling of aromatic halides and aromatic diamines has therefore emerged as versatile tool for azaacene synthesis. Now substituted diaza- and tetraazaacenes, azapentacenes, azahexacenes, and azaheptacenes are accessible. Pd-catalysis-based coupling methods for both activated and nonactivated o-dihalides have been developed. The larger azaacene representatives were unknown before but are of conceptual and theoretical interest. Azaacenes, particularly the symmetrical bis(triisopropylsilylethynyl)-substituted tetraazapentacene, are primarily used in organic field-effect transistors, but smaller azaacenes shine in the field as organic light-emitting diode (OLED) emitters. Diazatetracenes and substituted benzoquinoxalines are successful, improving electron injection and increasing OLED brightness, as compared to that of pure tetracenes. On the basis of the acene framework, nitrogen atoms in the acene perimeter and aggregation-precluding molecular appendages create solid-state fluorescent species. Azaacenes are expanding the range and complementing the purview of acenes in organic electronic applications. They enlarge the profiles of acenes with respect to synthetic strategies, structures, properties, and applications.
N-Heteroacenes Bunz, Uwe H. F.
Chemistry : a European journal,
July 13, 2009, Letnik:
15, Številka:
28
Journal Article
Recenzirano
The synthesis and the property evaluation of several large N‐heteroacenes are discussed. Issues of stability and aromaticity are compared and investigated and a historical perspective of the field is ...given. Some of the larger heterocyclic materials that are evaluated in this concept article have been around for more than one hundred years, yet their chemistry and properties are not well known/understood.
Aromatic or not? An account of the coexistence of formally antiaromatic 4n π heterocycles and their 4n+2 π perimeters is presented. In a significant number of cases both the formally antiaromatic and the aromatic species are known and have been isolated and investigated. In the diazatetracene case both species are stable, isolable, and interconvertible (see scheme).
The Palladium Way to N-Heteroacenes Bunz, Uwe H. F.; Engelhart, Jens U.
Chemistry : a European journal,
March 24, 2016, Letnik:
22, Številka:
14
Journal Article
Recenzirano
Odprti dostop
Novel synthetic methodologies allow increasingly efficient access to known organic materials, as well as the preparation of otherwise inaccessible species. Pd‐catalyzed coupling of aromatic dihalides ...to ortho‐diaminoarenes furnishes embedded stable N,N′‐dihydropyrazines expediently and in often excellent yields. The embedded N,N′‐dihydropyrazines can then be oxidized by MnO2 to give substituted azatetracenes, azapentacenes, azahexacenes, and azaheptacenes, which are soluble, processable, and stable. This powerful Pd‐catalyzed methodology allows the preparation of azaacenes, including diaza‐, tetraaza‐ and hexaazaacenes. In combination with a suitable Pd precursor, Buchwald‐type biarylphosphines have been shown to give excellent results. Activated dihalides such as 2,3‐dihaloquinoxalines are coupled easily under simplified conditions, whereas 2,3‐dibromoacenes require more stringent conditions and advanced catalyst precursors. Pd catalysts effect the assembly of azaacenes with otherwise difficult to obtain substitution patterns. High yields and flexibility make this method most attractive.
Palladium catalysis is a new and powerful way to construct azaacene frameworks. It allows coupling of aromatic ortho‐diamines to aromatic ortho‐dihalides, resulting in embedded N,N′‐dihydropyrazines. This reaction works well for N,N′‐dihydropyrazines that are part of larger (≥4 ring), linearly annulated dihydroacene types. Oxidation of these N,N′‐dihydropyrazines then gives azaacenes.
Gold nanoparticles (NPs) efficiently quench adsorbed fluorophores. Upon disruption of such complexes by an analyte, fluorescence turn‐on is observed. By judicious choice of the functionalized NP and ...the fluorophore, these complexes display different responses to analytes, thus leading to versatile yet simple array‐based sensor platforms. Using this strategy, we can identify proteins in buffer and serum, distinguish between both different species and different strains of bacteria, and differentiate between healthy, cancerous, and metastatic human and murine cells.
Disruption desired: Different monolayer‐protected nanoparticle–fluorophore constructs are used in indicator‐displacement assays to spy on proteins, bacteria, cells, and ions (see picture). The modus operandi involves disruption of the preformed quencher–fluorophore complexes, leading to partial and analyte‐dependent fluorescence turn‐on. Small libraries of nanoparticle–fluorophore complexes get the sensing job for different biological analytes done.
A sensor array containing six non-covalent gold nanoparticle-fluorescent polymer conjugates has been created to detect, identify and quantify protein targets. The polymer fluorescence is quenched by ...gold nanoparticles; the presence of proteins disrupts the nanoparticle-polymer interaction, producing distinct fluorescence response patterns. These patterns are highly repeatable and are characteristic for individual proteins at nanomolar concentrations, and can be quantitatively differentiated by linear discriminant analysis (LDA). Based on a training matrix generated at protein concentrations of an identical ultraviolet absorbance at 280 nm (A280 = 0.005), LDA, combined with ultraviolet measurements, has been successfully used to identify 52 unknown protein samples (seven different proteins) with an accuracy of 94.2%. This work demonstrates the construction of novel nanomaterial-based protein detector arrays with potential applications in medical diagnostics.
Be planar! The planar oligofurans (see picture, lower; C gray, H white, O red) are, despite a lack of solubilizing alkyl groups, quite soluble. They are also highly fluorescent and surprisingly ...stable, and might give the oligothiophenes (upper; S yellow) a run for their money when seeking applications in organic electronics.
Rapid and sensitive detection of pathogens is a key requirement for both environmental and clinical settings. We report here a colorimetric enzyme–nanoparticle conjugate system for detection of ...microbial contamination. In this approach, cationic gold nanoparticles (NPs) featuring quaternary amine headgroups are electrostatically bound to an enzyme β-galactosidase (β-Gal), inhibiting enzyme activity. Analyte bacteria bind to the NP, which releases the β-Gal and restores its activity, providing an enzyme-amplified colorimetric readout of the binding event. Using this strategy, we have been able to quantify bacteria at concentrations of 1 × 102 bacteria/mL in solution and 1 × 104 bacteria/mL in a field-friendly test strip format.
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