The crystal structure of the high-temperature polymorph of α–hexathienyl (α–6T/HT)

Abstract

a-hexathienyl ( a -6 T ) 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 a -6 T (a-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 Hiickel theory (EHT) band structure calculations show that a -6 T / H T is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at T. 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 /c-(ET) 2 Cu(NCS) 2 .Structural studies on single crystals of oligomers of thiophene have so far been carried out only for terthienyl 6 and dimethylquaterthienyl. 7 Previous structural studies of the hexamer are all based on the Rietveld method using powder diffraction data. For systems this complicated, the Rietveld refinement has to be strongly constrained, which may cause the refinement to lock in a local minimum and bias the result. It is therefore highly desirable to use single crystals for any structural study. Crystal growth, on the other hand, has proven very difficult, with little success in the past. From the proposed and refined structures of a-hexathienyl, a variety of different monoclinic unit cells have been reported: a = 45.38 A, b = 7.86 A, c = 6.03 A, and f3 = 99.1 °5; a = 5.98 A, b = 7.80 A, c = 50.28 A, and / 3 = 111.3°; or a = 5.67 A, b = 7.80 A, c = 47.2 A, and /3 = 1OO.6 0 . 4 The main difference is in the variation of the longest axis, indicating a different packing of the molecules and even possible conformational departures from planarity. 2 In addition, a mesophase was reported 8 and analyzed. 5 This high-temperature polymorph, obtained by prolonged heating at 307 °C, shows a very different powder pattern than that of sublimed a -6 T . These puzzling facts prompted us to further investigate the various phases of a -6 T .The starting material a -6 T was synthesized differently from most reported synthesis, in a manner pro-ducing the highest-purity and highest-melting material to date. 1 Crystals of a high-temperature polymorph, a-6T/HT, were prepared by controlled melt growth in a special microscope heating stage, permitting melt...

Interchain Interactions in Organic π-Conjugated Materials: Impact on Electronic Structure, Optical Response, and Charge Transport

“…Such a molecular arrangement would, at first sight, be expected to lead to only a weak overlap between the electronic wavefunctions of the individual molecules. A similar chain packing is observed in the monoclinic single crystals of the widely studied 6T in both the low-(LT) [85] and high-(HT) [86] temperature phases (note that the terminology used to label these two phases refer to the fact that LT is crystallized by vapor deposition at a temperature source lower than that used for HT; both phases are actually stable over the whole temperature range typically probed by mobility measurements). In contrast, the crystal structure of BDT [87] displays a strong orbital overlap for molecules stacked along the b-axis (Fig.…”

Thermal Characterization of Sexithiophenes Regioselectively Functionalized with Electron Donor Methylsulfanyl Groups

“…Compound 4 displays two subsequent crystallization and melting processes, which are likely to pertain to two different crystalline phases. Polymorphism has already been observed for T6 itself, [20,21] while conformational polymorphism has been found in single crystals of a shorter homologue of sexithiophene 4, namely the tetra (methylsulfanyl) quaterthiophene having two head-tohead and one tail-to-tail junction. [17] It is significant that both conformational polymorphs of this quaterthiophene have melting points near 150 8C, while its regioisomer with one inner head-to-head junction and two lateral head-to-tail junctions (i. e. with the same substitution pattern as sexithiophene 1) is a viscous liquid at room temperature.…”

From Chiral Islands to Smectic Layers: A Computational Journey Across Sexithiophene Morphologies on C60

“…a ( b ) corresponds either to the axis c ( b ) of the original LT cell, [ 70 ] or to the axis b ( a ) of the original HT cell. [ 71 ] Lengths are given in Å, angles in degrees. The uncertainty on the cell parameters calculated for VD MLs is of 0.2 Å for distances and 3 degrees for angles.…”