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Linear- and Angular-Shaped Naphthodithiophenes: Selective Synthesis, Properties, and Application to Organic Field-Effect Transistors

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posted on 06.04.2011, 00:00 by Shoji Shinamura, Itaru Osaka, Eigo Miyazaki, Akiko Nakao, Masakazu Yamagishi, Jun Takeya, Kazuo Takimiya
A straightforward synthetic approach that exploits linear- and angular-shaped naphthodithiophenes (NDTs) being potential as new core structures for organic semiconductors is described. The newly established synthetic procedure involves two important steps; one is the chemoselective Sonogashira coupling reaction on the trifluoromethanesulfonyloxy site over the bromine site enabling selective formation of o-bromoethynylbenzene substructures on the naphthalene core, and the other is a facile ring closing reaction of fused-thiophene rings from the o-bromoethynylbenzene substructures. As a result, three isomeric NDTs, naphtho[2,3-b:6,7-b′]dithiophene, naphtho[2,3-b:7,6-b′]dithiophenes, and naphtho[2,1-b:6,5-b′]dithiophene, are selectively synthesized. Electrochemical and optical measurements of the parent NDTs indicated that the shape of the molecules plays an important role in determining the electronic structure of the compounds; the linear-shaped NDTs formally isoelectronic with naphthacene have lower oxidation potentials and more red-shifted absorption bands than those of the angular-shaped NDTs isoelectronic with chrysene. On the contrary, the performance of the thin-film-based field-effect transistors (FETs) using the dioctyl or diphenyl derivatives were much influenced by the symmetry of the molecules; centrosymmetric derivatives tend to give higher mobility (up to 1.5 cm2 V−1 s−1) than axisymmetric ones (∼0.06 cm2 V−1 s−1), implying that the intermolecular orbital overlap in the solid state is influenced by the symmetry of the molecules. These results indicate that the present NDT cores, in particular the linear-shaped, centrosymmetric naphtho[2,3-b:6,7-b′]dithiophene, are promising building blocks for the development of organic semiconducting materials.