Alkyl substituted dithienothieno[2,3-d;2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophenes as solution-processable hexathiaheptacenes†
Solution-processable hexathiaheptacene dithienothieno[2,3-d;2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophenes (DTTBDT a–d) with various alkyl groups were synthesized by triflic acid induced ring closure. UV/Vis absorption, photoluminescence spectroscopy and cyclic voltammetry combined with density functional theory quantum-chemical calculations were performed to determine the photophysical and the electrochemical properties of DTTBDT a–d. In addition, the new hexathiaheptacenes show liquid crystalline properties dependent on their alkyl chain length. As observed from differential scanning calorimetry and polarized optical microscopy compared with DTTBDT a and b, DTTBDT c and d exhibit thermotropic liquid crystalline textures due to the rigid conjugated skeleton and the flexible long alkyl chains. For all four derivatives, the structural analysis indicates the formation of lamellar structures in which, surprisingly, the molecules are poorly packed. This molecular disorder is the main reason for the low performance of the p-type field-effect transistors.