THE CRYSTAL-STRUCTURE OF THE HIGH-TEMPERATURE POLYMORPH OF ALPHA-HEXATHIENYL (ALPHA-U-6T HT)/

alpha-hexathienyl (alpha-6T) is a highly promising material for applic ation in thin film transistor devices. Recently, record high mobilitie s, together with record high current on/off ratios, have been reported .(1) Thus far, structural information on this exciting material is ske tchy. The crystal structures of several such hexamers have been invest igated, but only with powder samples, since the crystal growth has pro ven exceedingly difficult.(2-5) Powder Rietveld refinements on these m aterials are severely hampered by the large number of overlapping refl ections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of alph a-6T (alpha-6T/HT), as determined from a single-crystal structure anal ysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Ext ended Huckel theory (EHT) band structure calculations show that alpha- 6T/HT is an indirect gap semiconductor, with the conduction band minim um at Y and the valence band maximum at Gamma. The conduction and vale nce bands both show a remarkable degree of dispersion along X and Y fo r a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic supercond uctors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as ka ppa-(ET)(2) Cu(NCS)(2).