We report the structural and electrical characterization of two new p-channel organic semiconductors, 5,5'-bis(2-tetracenyl)-2,2'-bithiophene (1) and 5,5'-bis(2-anthracenyl)-2,2'-bithiophene (2). Both compounds exhibited a high degree of thermal stability with decomposition temperatures of 530 degrees C and 425 degrees C for 1 and 2, respectively. The thin-film structures of 1 and 2 were examined using wide-angle X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXD), and atomic force microscopy (AFM). Films of 1 and 2 pack in similar triclinic unit cells with the long axes of the molecules nearly perpendicular to the substrate. Thin-film transistors (TFTs) based on 1 and 2 exhibit contact-corrected linear regime hole mobility as high as 0.5 cm2/Vs and 0.1 cm2/Vs, respectively. The specific contact resistance at high gate voltages for gold top contacts was 2 x 10(4) Ohms cm and 3 x 10(4) Ohms cm for 35 nm thick films of 1 and 2, respectively. Long-term air stability tests revealed less degradation of the electrical properties of 1 and 2 in comparison to pentacene. Variable temperature measurements revealed activation energies as low as 22 and 27 meV for 1 and 2, respectively. The temperature and gate voltage dependence of the mobility are discussed in terms of a double exponential distribution of trap states and a model accounting for the layered structure of the organic films. The enhanced air and thermal stability over pentacene, combined with good electrical performance characteristics, make 2 a promising candidate for future organic TFT applications.
The coordination chemistry of ancillary amidinate ligands with a pendant pyridine functionality is described. Reaction of p-toluoyl-or p-t Bu-benzoyl chloride with 2-aminoethylpyridine generates the amides 2-Py-(CH 2 ) 2 NHCO(p-RPh) (R ) Me 1a; t Bu 1b); these amides are then converted to the amidines 2-Py-) by reaction with PCl 5 followed by R′NH 2 . The amidines 2a,b were characterized by 1 H NMR and IR spectroscopy and elemental analyses, and 2b was characterized by X-ray crystallography. Reaction of 2a or 2b with homoleptic metal-alkyls or -amides yields the mono-or bis(amidinate) complexes (L Me ) 2 Mg (3a), (L tBu )AlMe 2 (4), (L tBu )Zr(CH 2 Ph) 3 (5), and (L Me ) 2 La[N(SiMe 3 ) 2 ] (6). All metal complexes were characterized by 1 H NMR and IR spectroscopy, elemental analyses, and X-ray crystallography. The X-ray crystal structures of compounds 3-6 show them to be monomeric, with the pendant pyridine coordinated intramolecularly in all cases. The tridentate amidinate coordinates meridionally to the metal center except in the case of the lanthanum derivative 6, where an approximate facial geometry is observed.
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