The field of synthetic metals is, and remains, highly influential for the development of organic semiconductor materials. Yet, with the passing of time and the rapid development of conjugated ...materials in recent years, the link between synthetic metals and organic semiconductors is at risk of being forgotten. This review reflects on one of the key concepts developed in synthetic metals – heteroatom interactions. The application of this strategy in recent organic semiconductor materials, small molecules and polymers, is highlighted, with analysis of X‐ray crystal structures and comparisons with model systems used to determine the influence of these non‐covalent short contacts. The case is made that the wide range of effective heteroatom interactions and the high performance that has been achieved in devices from organic solar cells to transistors is testament to the seeds sown by the synthetic metals research community.
Intermolecular heteroatom interactions are well‐known phenomena in organic charge transfer salts and are responsible for bulk metallic conductivity and even superconductivity. In this review, it is seen that the same interactions can be applied in organic semiconductors to increase bulk dimensionality through intermolecular contacts, and control molecular conformation through intramolecular interactions.
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Strategies for the design and construction of non-linear, 2D and 3D conjugated macromolecules are presented in this critical review. The materials, termed here as star-shaped structures, feature a ...core unit which may or may not provide conjugated links between arms that radiate like spokes from a central axle. The arms of the macromolecules consist of linear oligomers or irregular conjugated chains lacking a formal repeat unit. The cores range from simple atoms to single or fused aromatic units and can provide a high level of symmetry to the overall structure. The physical properties of the star-shaped materials can be markedly different to their simple, linear conjugated analogues. These differences are highlighted and we report on anomalies in absorption/emission characteristics, electronic energy levels, thermal properties and morphology of thin films. We provide numerous examples for the application of star-shaped conjugated macromolecules in organic semiconductor devices; a comparison of their device performance with those comprising analogous linear systems provides clear evidence that the star-shaped compounds are an important class of material in organic electronics. Moreover, these structures are monodisperse, well-defined, discrete molecules with 100% synthetic reproducibility, and possess high purity and excellent solubility in common organic solvents. They feature many of the attributes of plastic materials (good film-forming properties, thermal stability, flexibility) and are therefore extremely attractive alternatives to conjugated polymers (210 references).
Conspectus One of the most desirable and advantageous attributes of organic materials chemistry is the ability to tune the molecular structure to achieve targeted physical properties. This can be ...performed to achieve specific values for the ionization potential or electron affinity of the material, the absorption and emission characteristics, charge transport properties, phase behavior, solubility, processability, and many other properties, which in turn can help push the limits of performance in organic semiconductor devices. A striking example is the ability to make subtle structural changes to a conjugated macromolecule to vary the absorption and emission properties of a generic chemical structure. In this Account, we demonstrate that target properties for specific photonic applications can be achieved from different types of semiconductor structures, namely, monodisperse star-shaped molecules, complex linear macromolecules, and conjugated polymers. The most appropriate material for any single application inevitably demands consideration of a trade-off of various properties; in this Account, we focus on applications such as organic lasers, electrogenerated chemiluminescence, hybrid light emitting diodes, and visible light communications. In terms of synthesis, atom and step economies are also important. The star-shaped structures consist of a core unit with 3 or 4 functional connection points, to which can be attached conjugated oligomers of varying length and composition. This strategy follows a convergent synthetic pathway and allows the isolation of target macromolecules in good yield, high purity, and absolute reproducibility. It is a versatile approach, providing a wide choice of constituent molecular units and therefore varying properties, while the products share many of the desirable attributes of polymers. Constructing linear conjugated macromolecules with multifunctionality can lead to complex synthetic routes and lower atom and step economies, inferior processability, and lower thermal or chemical stability, but these materials can be designed to provide a range of different targeted physical properties. Conventional conjugated polymers, as the third type of structure, often feature so-called “champion” properties. The synthetic challenge is mainly concerned with monomer synthesis, but the final polymerization sequence can be hard to control, leading to variable molecular weights and polydispersities and some degree of inconsistency in the properties of the same material between different synthetic batches. If a champion characteristic persists between samples, then the variation of other properties between batches can be tolerable, depending on the target application. In the case of polymers, we have chosen to study PPV-type polymers with bulky side groups that provide protection of their conjugated backbone from π–π stacking interactions. These polymers exhibit high photoluminescence quantum yields (PLQYs) in films and short radiative lifetimes and are an important benchmark to monodisperse star-shaped systems in terms of different absorption/emission regions. This Account therefore outlines the advantages and special features of monodisperse star-shaped macromolecules for photonic applications but also considers the two alternative classes of materials and highlights the pros and cons of each class of conjugated structure.
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We demonstrate the nonaqueous self-assembly of a low-molecular-mass organic gelator based on an electroactive p-type tetrathiafulvalene (TTF)–dipeptide bioconjugate. We show that a TTF moiety ...appended with diphenylalanine amide derivative (TTF-FF-NH2) self-assembles into one-dimensional nanofibers that further lead to the formation of self-supporting organogels in chloroform and ethyl acetate. Upon doping of the gels with electron acceptors (TCNQ/iodine vapor), stable two-component charge transfer gels are produced in chloroform and ethyl acetate. These gels are characterized by various spectroscopy (UV–vis–NIR, FTIR, and CD), microscopy (AFM and TEM), rheology, and cyclic voltammetry techniques. Furthermore, conductivity measurements performed on TTF-FF-NH2 xerogel nanofiber networks formed between gold electrodes on a glass surface indicate that these nanofibers show a remarkable enhancement in the conductivity after doping with TCNQ.
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Chirality induction into a uniform, star-shaped fluorene oligomer with a central truxene moiety (T3) was achieved using circularly polarized light in the presence of achiral fluorene or phenanthrene. ...Induction into T3 alone was difficult, suggesting that close chain packing realized through interactions of T3 with small molecules plays a role in chirality induction.
This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of ...solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1−T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (E ox = +0.74 to 0.80 V, E red = −2.66 to 2.80 eV vs Fc/Fc+), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (E g CV = 3.20−3.40 eV). Close band gaps of 3.05−3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400−420 °C) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from T g = 63 °C for T1 to 116 °C for T4) and show very efficient blue photoluminescence (λPL = 398−422 nm) in both solution (ΦPL = 70−86%) and solid state (ΦPL = 43−60%). Spectroelectrochemical experiments reveal that compounds T1−T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (∼340−400 nm) to colored (from red to purple color; ∼500−600 nm) in the oxidized state.
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The present work describes the structure and binding of adducts of N,N′-diacetylpiperazine with halogens and interhalogens based on combination of different experimental methods and quantum chemical ...calculations. On the basis of conductometric and spectro-photometric experimental results, behavior of complexes in the acetonitrile solution was described. The iodine adduct with N,N′-diacetylpiperazine fully degrades into components. Adducts of interhalogens I–X (X = Cl or Br) with N,N′-diacetylpiperazine in acetonitrile partially dissociate to anionic X–I–X− and cationic species. In the solid state, molecules are connected via CO···I, C–H···I, and Cl···Cl attractive interactions. N,N′-diacetylpiperazine···dihalogen complex is stabilized by simultaneous CO···I and C–H···I interactions. Such binding mode allows to explain the problems of the direct halogenation of acetyl-containing compounds with molecular halogens as reagents. We believe that the observed binding pattern can be used as prototypical for future design of halogeno complexes.
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The fused, bicyclic molecule, 2,1,3‐Benzothiadiazole (BT), has become a key ingredient in the design of new organic semiconductors for light emission and energy harvesting applications. Here, the ...synthesis is reported of a series of trigonal, star‐shaped compounds comprising a truxene core and three quater‐dialkylfluorene arms into each of which a BT unit is inserted sequentially at each possible position (T4BT‐A to T4BT‐E). Analysis of the resulting electronic properties shows that as a consequence of conjugative coupling to the core and the resulting symmetry there are three distinct locations for the BT unit and the influence that these locations have on light emission and other spectroscopic characteristics is discussed. The systematic variation in photophysical properties for the different structural isomers helps to clarify the influence of BT unit addition to 9,9‐dialkylfluorene chains. It also helps to establish a design template for the construction of donor‐acceptor conjugated materials with targeted properties. For T4BT‐E with a BT unit at the terminal position of each arm, the photoluminescence quantum efficiency is significantly reduced and no amplified spontaneous emission is observed under typical pumping conditions. Theoretical calculations assist in understanding the variation in behaviors among the T4BT‐X family of compounds, especially in relation to their photoluminescence decay times and the Raman scattering intensities of their dominant BT‐unit‐centred molecular vibrations.
2,1,3‐benzothiadiazole (BT) units are systematically incorporated into star‐shaped trigonal molecules comprising a truxene core and three quaterfluorene arms. Five isomers are synthesized corresponding to the symmetric insertion of a single BT unit into each of the possible positions within the arms. Three BT locations are identified by comparison of absorption and photoluminescence (PL) spectra, supported by theoretical calculations. Additional experimental and theoretical characterizations reveal the influence of BT position on Raman, photoluminescence, and stimulated emission properties.
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