A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.

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The heteroatom‐stabilized annularly fused hexapyrrolohexaazacoronene shown (N blue, F green) was synthesized and isolated as a representation of a new class of nitrogen‐containing molecules with extended π systems. The reversible redox behavior, a consequence of the interior N atoms, hints at potential applications in organic electronics.

The solid-state structures of a series of 2,5-dialkyl-substituted poly(p-phenyleneethynylene)s (PPEs 4, R = hexyl, dodecyl, isopentyl, ethylbutyl, ethylhexyl, H) and of 2,5-didodecyl(1,4-phenylenebutadiynylene) were investigated by X-ray powder and electron diffraction. In addition, X-ray single-crystal structures of the monomeric (1,4-dipropynyl-2,5-dialkyl)benzenes were obtained and compared to the structures of the corresponding polymers. In the case of the ethylhexyl-substituted PPE, the packing of the monomer resembles that of the polymer in the solid state. All of the examined PPEs show lamellar morphologies, in which the extended geometry of the side chains determines the value of the lamellar spacing obtained by powder diffraction. A second d value, which could be obtained for all samples represents the π−π-stacking distance of the main chains. This distance is around 3.8 Å, a typical value for stacking of aromatic systems. It implies strong electronic interaction of the chromophoric backbones, leading to the observed sharp aggregate band in CHCl3/MeOH solutions and in thin films. From the scattering data, staggered packing of the benzene rings and the phenyleneethynylene main chains in PPEs 4 was inferred. This packing avoids repulsive electrostatic interactions between adjacent conjugated phenyleneethynylene chains and at the same time leads to optimal packing of the side chains.

By Peng Gao, Dirk Beckmann, Hoi Nok Tsao, Xinliang Feng, Volker Enkelmann, Martin Baumgarten, Wojciech Pisula, and Klaus Müllen* Organic field-effect transistors (OFETs) have been emerging as low-cost alternatives to conventional silicon-based transistors in the past decades.[1] Ring-fused coplanar acenes, such as pentacene, have been shown to yield high-performance field-effect transistors (FET), qualifying them as active materials in p-type organicelectronic devices.[2] However, pentacene suffers from rapid degradation in ambient conditions, due to photoinduced decomposition and formation of dimeric Diels-Alder adducts on the electron-rich central ring.[3] Therefore, great efforts have been devoted to improving the stability by designing new five-ring-fused heteroacenes with high charge-carrier mobilities.[4] It is noted that the incorporation of fused thiophene units in heteroacenes widens the HOMO-LUMO gap and stabilizes the HOMO level due to the formation of kinked substructures.[5] However, most of these materials encountered tedious synthesis, demanding purification processes and, more importantly, difficult introduction of suitable alkyl chains for solubility. Accordingly, devices based on heteroacenes have been mainly fabricated by the physical vapor deposition (PVD) technique, [4a-g,i-n] which restrict their practical applications in large-area printable OFETs.In our search for superior semiconductors for OFETs, we focused our attention on heteroacenes with more fused thiophene units. Recently, we reported the synthesis of solution-processable benzo [1,2-b;4,5-b 0 ]bis[b]benzodithiophene derivatives, which exhibited very high performances in comparison to devices prepared by PVD.[4h] In this communication, we present the facile synthesis of a new five-ring-fused pentacene analog, with four symmetrically fused thiophene-ring units (dithieno [2,3-d;2 0 ,31) that can be substituted by different alkyl substituents in a simple way. In an example, a solution-processed OFET based on DTBDT with two linear hexyl chains (1b) is presented, revealing a high charge-carrier mobility of up to 1.7 cm 2 V À1 s À1 and an on/off ratio of 10 7 . This is one of the most promising small molecules for solution-processible organic semiconductors.As demonstrated in Scheme 1, a series of DTBDT with two alkyl chains (linear or branched) in the molecular long-axis direction were synthesized via two-step reactions. Stille coupling between 1,4-dibromo-2,5-bis(methylsulfinyl)benzene (3) and corresponding (5-alkylthiophen-2-yl)trimethylstannane afforded precursors 2a-d in good yields. Subsequent intramolecular ring-closure was performed in an excess of pure triflic acid, where the as-formed clear solution was poured into water to give a yellowish powder as precipitate, followed by filtering, drying, and reflux in pyridine. The final DTBDTderivatives 1a-d were achieved in an overall yield of ca. 75% after purification by flash column chromatography. Their chemical structures were fully characterized by 1 H NMR, mass spectroscopy (MS...

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X-ray structure determinations have been carried out and analyzed for two conformational polymorphs of α-truxillic acid (2). Polymorph I (cell parameters known since 1959) is isolated upon recrystallization of 2 from acetic acid−petroleum ether solution, while polymorph II (fully characterized in 1993) is the product of a one-phase solid-state reaction of tail-irradiated α-cinnamic acid. For polymorph II, two redeterminations of mixed crystals containing solid solutions of α-cinnamic acid and 2 (68.1 and 89.5%, respectively) were performed. Analysis of the structural parameters of the 1993 and present redeterminations of the polymorph II structures, as well as comparison to data from the Cambridge Structural Database, confirm that claims of errors in the 1993 structure determination of polymorph II are unfounded. X-ray quality crystals of the β-polymorph of trans-cinnamic acid (cell parameters known since 1964) have been obtained from Et2O/petroleum ether solutions; the crystal structure is quite similar to that of 4-chlorocinnamic acid.