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 application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.(1) Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.(2-5) Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of alpha-6T (alpha-6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Huckel theory (EHT) band structure calculations show that alpha-6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Gamma. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as kappa-(ET)(2) Cu(NCS)(2).