A combined study of mesomorphism, optical, and electronic properties of donor-acceptor columnar liquid crystals

Abstract

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.