Xiaojie Zhang

Synthesis, Self-Assembly, and Charge Transporting Property of Contorted Tetrabenzocoronenes

A facile route has been developed for the preparation of a new family of contorted 1.2,3.4,7.8,9.10-tetrabenzocoronenes (TBCs). A two-step cyclization reaction, i.e., oxidative photocyclization followed by FeCl(3)-mediated intramolecular cyclodehydrogenation, was carried out on the olefin precursors to obtain the final TBC compounds. These new TBC molecules have contorted conformation due to steric overcrowding as disclosed by single-crystal crystallographic analysis. Nevertheless, they showed extended π-conjugation compared with coronene and exhibited strong aggregation in solution. The thermal behavior and self-assembly of TBC-C8 in solid were studied by a combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). Compound TBC-C8 showed very good thermal and photostability and exhibited long-range ordered π-stacking in the bulk state. Moreover, uniform nanofibers with tens of micrometer length are formed in the drop-casted thin films. TBC-C8 also possesses a desirable HOMO energy level (-5.10 eV), which allows efficient charge injection from electrodes such as gold electrode. The charge carrier mobilities were determined by using the space-charge limited-current (SCLC) technique and high average hole mobility of 0.61 cm(2) V(-1) s(-1) was obtained for TBC-C8.

Adsorption behavior of humic acid onto polypyrrole-coated nylon 6,6 granules

One of the challenges in water supply is the removal of humic substances as the unit treatment processes in a conventional water treatment plant are usually not effective for this purpose. In this study, nylon 6,6 granules were surface-modified by chemical polymerization of pyrrole and were used as a granular adsorbent for removing humic acid from water. Zeta potential study showed that the coated granules have positive surface charges over a wide range of pH values. X-Ray photoelectron spectroscopy (XPS) indicated that the positive surface charges of the coated granules are attributed to the protonated nitrogen atoms (i.e. N+) in the polypyrrole (PPy) coating. Batch adsorption experiments were conducted under various conditions to examine the removal efficiency and mechanisms. PPy-coated nylon 6,6 granules were found to have much better removal of humic acid, as compared to the uncoated nylon 6,6 granules, suggesting the importance of electrostatic interactions between the positively charged granules and the negatively charged humic acid in the process. Evidence showed that although most adsorption may take place through non-specific electrostatic attraction, some adsorption occurred at specific sites and resulted in charge neutralization. It was found that low pH and high ionic concentration favored the adsorption performance and the adsorption kinetics involved both diffusion-controlled and attachment-controlled mechanisms.

A cruciform 6,6'-dipentacenyl: synthesis, solid-state packing and applications in thin-film transistors.

Organic field-effect transistors (OFETs) have attracted great interest in recent years as a potential low-cost alternative to amorphous silicon-based transistors for many applications in electronics. Charge-carrier mobility in an OFET device is strongly related to the crystalline structure of the organic semiconductor, that is, the molecular packing mode is one of the critical factors governing the intermolecular electronic interaction and charge transport. It is known that charge-carrier transport depends on being able to hop between the molecules in the region, therefore, the molecular arrangement in a good semiconductor needs to provide sufficient overlap of the p orbitals of conjugated organic molecules to enable efficient charge migration between neighbouring molecules. Typical small-molecule-based organic semiconductors, such as oligoacenes and a-oligothiophenes, generally adopt a herringbone arrangement in the crystal structure, as shown in Figure 1a. In this packing motif, unfavourable p orbital repulsion and favourable CH–p electrostatic interactions between adjacent molecules drive the molecules to adopt an edge-to-face arrangement. 3] However, this molecular packing mode also results in a reduction in the intermolecular electronic interaction. Theoretical calculations suggested that face-to-face p stacking would increase the p orbital overlap and thus enhance the charge carrier mobility by maximising electronic coupling between adjacent molecules. Therefore, in the last several years different groups have reported several strategies of molecular design to tune the packing mode from herringbone to a cofacial p-stacking motif, as illustrated in Figure 1b. In this motif, the molecules usually show a tilted (or offset) face-to-face arrangement in the crystalline form due to the molecular orbital repulsion. Anthony et al. reported that the introduction of bulky alkynyl groups into the meso positions of pentacene could disrupt aromatic edge-to-face interactions and tune the packing structure from a herringbone mode to a face-to-face p-stacking motif. Bao et al. found that tetracene with halogen atoms at the 5 and 11 positions adopted a face-to-face pstacking motif with enhanced charge transport. Holmes et al. also reported that fused a-oligothiophenes with relatively high C/H ratios can adopt a face-to-face arrangement in crystals due to the diminished CH–p interactions. The chalcogen–chalcogen interaction was introduced by Kobayashi et al. to obtain face-to-face p-stacking of oligoacenes. Moreover, typical disc-like molecules, such as triphenylene and hexabenzocoronene derivatives, generally adopt a face[a] Dr. X. Zhang, Dr. J. Luo, Dr. C. Chi, Prof. J. Wu Department of Chemistry National University of Singapore 3 Science Drive 3 (Singapore) Fax: (+65) 6779-1691 E-mail : chmwuj@nus.edu.sg [b] Dr. X. Jiang, Prof. H. Chen Zhejiang University Department of Polymer Science and Engineering Hangzhou, 310027 (PR China) E-mail : hzchen@zju.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902675. Figure 1. Illustration of different packing structures of organic semiconductors: a) a herringbone motif ; b) tilted face-to-face motif ; c) columnar p stacking; d) a possible two-dimensional p stacking from cruciform molecules.

meso-Substituted bisanthenes as soluble and stable near-infrared dyes.

Three meso-substituted bisanthenes, 4-6, were prepared in a short synthetic route from the bisanthenequinone. They exhibit largely improved stability and solubility in comparison to the parent bisanthene. All of these compounds also show near-infrared (NIR) absorption and emission with high to moderate fluorescence quantum yields. Amphoteric redox behavior was observed for 4-6 by cyclic voltammetry, and these compounds can be reversibly oxidized and reduced into respective cationic and anionic species by both electrochemical and chemical processes. In addition, compound 5 adopts a herringbone pi-stacking motif in the single crystal.

Electron-Deficient Triphenylene and Trinaphthylene Carboximides

A series of triphenylene and trinaphthylene carboximides were synthesized mainly through Diels-Alder reactions between the in situ generated radialenes and N-alkylmaleimides. High electron affinity was obtained due to the attachment of electron-withdrawing carboximide groups. These disklike molecules can assemble into one-dimensional columnar structures in solid state and their mesophase properties were characterized by DSC, POM, and X-ray diffraction. Long-range ordered columnar phase was observed, qualifying them as potential n-type semiconductors in electronic devices.

Fused Bispentacenequinone and Its Unexpected Michael Addition

Fused bispentacenequinone 2 was synthesized by photocyclization of bispentacenequinone 1. Unusual regioselective Michael addition was observed for 2 when excess aryl Grignard reagent was used. Subsequent acidification and oxidation in air gave diaryl-substituted bispentacenequinone 3. Tetra-aryl-substituted fused bispentacenequinone 4 was obtained from 3 after the second Michael addition followed by oxidation in air.

π-Conjugated oligothiophene–anthracene co-oligomers: synthesis, physical properties, and self-assembly

A new class of oligothiophene–anthracene co-oligomers, 9,10-bis(2-dodecyloligothienylvinyl)anthracene TnA (n = 1, 2 and 3), were synthesized and characterized. The photophysical and electrochemical properties of these oligomers demonstrated an increased conjugation length with increasing number of thiophene units. These oligomers also exhibited relatively larger ionization potential values compared with pentacene and sexithiophene and thus are expected to show improved stability. The strong aggregation tendency and excimer formation for T1A and obvious aggregation effect for T2A in solution were observed from the concentration and temperature-dependent fluorescence spectra. In contrast, no obvious aggregation behavior could be observed for T3A in solution. Thermal behavior and self-assembly of these co-oligomers in solid state were studied by a combination of TGA, DSC, powder X-ray diffraction and polarizing optical microscope. Liquid crystalline phases were observed in T2A and T3A at elevated temperature while only a crystalline phase was found for T1A. Single-crystal structures of T1A and T2A revealed a lamellar packing mode with ordered intermolecular anthracene-to-anthracene and oligothiophene-to-oligothiophene π–π stacking and these qualify them as potential semiconductors in electronic devices.

H-Shaped Oligothiophenes with Low Band Gaps and Amphoteric Redox Properties

H-shaped bridged oligothiophenes HT-1 and HT-2 were synthesized by two different approaches. Different from normal oligothiophenes, HT-1 and HT-2 showed low band gaps and amphoteric redox behaviors due to intramolecular charge transfer, which is further supported by time-dependent DFT calculations.