Anthradithiophene-based liquid crystal molecules: High carrier mobilities enhanced by rubbed polyimides for the application in organic field-effect transistors

Abstract

We synthesize three liquid crystal molecules with angular anthradithiophene (a-ADT) as the core for the application in organic film-effect transistors (OFETs). Thiophene-octyl group and thiophene-dodecyl group are introduced on both sides of the a-ADT core to yield compounds Bis-C8T-a-ADT and Bis-C12T-a-ADT. Moreover, thiophene-dodecyl group is attached to the a-ADT core on one side to yield compound C12T-a-ADT. The synthesized compounds are measured by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) to determine their mesomorphic properties. Compound Bis-C8T-a-ADT shows nematic and smectic A phases while both compounds Bis-C12T-a-ADT and C12T-a-ADT reveal smectic A phases. The optical and electrochemical properties of the compounds are characterized by UV-vis spectroscopy and cyclic voltammetry (CV). The HOMO levels for Bis-C8T-a-ADT and Bis-C12T-a-ADT are estimated to be -5.27 eV while the HOMO level of C12T-a-ADT is estimated to be -5.34 eV. The low-lying HOMO levels of the molecules indicate their great oxidation stability in OFET devices. The carrier mobilities are observed to increase 10-1000 times using rubbed polyimide (PI), resulting from the better arrangements of the liquid crystal molecules, and excellent carrier mobilities of 1.06 and 1.02 cm(2)/Vs are obtained from compounds Bis-C12T-a-ADT and C12T-a-ADT, respectively.