Donor-acceptor structures have recently gained great popularity for the design of low band gap polymeric organic
semiconductors. Presented here is a first systematic study of organic semiconductors based on columnar liquid crystals
that consist of discotic and board-shaped donor-acceptor structures. The discotic benzotristhiophenetricarboxamide and
hexaphenyldiquinoxalinophenazine derivatives form hexagonal columnar mesophases over wide temperature ranges
while the board-shaped tetraphenylquinoxalinophenanthrophenazine derivative displays rectangular and hexagonal
columnar mesophases. All compounds are designed to preferentially conduct electrons and not holes because the central
acceptor parts, which are the sole contributors to the unoccupied frontier orbitals, show strong intermolecular electronic
interactions within columnar stacks whereas the donor parts at the periphery of the cores, the sole contributors to the
occupied frontier orbitals, are too far apart to provide sufficient electronic interactions. The absence of hole conduction
is confirmed by charge carrier mobility measurements based on time-resolved microwave conductivity and time-of-flight
methods that reveal intrinsic electron mobility values of about 10-2 cm2 V-1 s-1. The board-shaped compound is also
highly fluorescent not only in solution but also in its mesophases. Interstingly, the fluorescence quantum yield of its
mesophases reversibly increases with decreasing temperature, which is reasoned with changes in molecular mobility and
intracolumnar packing within columnar stacks.
The field-effect mobility in two isomers of thieno[f,f']bisbenzothiophene was studied as a function of structure. Both regioisomers exhibit substantial mobilities up to 0.12 cm2/Vs, a value that is at most one order of magnitude lower than the best known organic transistors based on silicon dioxide gate insulators. The devices based on these materials exhibit another phenomenon, namely a shift in the threshold voltage during operation. This shift differs from usually observed threshold voltage changes in amount and irreversibility. In this paper, we present possible explanations for the observed behavior.