Rational design of diketopyrrolopyrrole-based oligomers with deep HOMO level and tunable liquid crystal behavior by modulating the sequence and strength of the donor moiety

A family of narrow-band gap π-conjugated oligomers and isomers based on diketopyrrolopyrrole and difluorobenzothiadiazole coupled with an oligothiophene or thiazole ring have been successfully synthesized. They exhibited intensive absorption bands (300-900 nm) and deep-lying HOMO energy levels (-5.41 to -5.60 eV) due to donor-acceptor interactions and multiple fluorine substituents. The sequence and intensity of the electron donor moiety play an important role in determining bulk molecular properties, such as the photophysical properties, the HOMO/LUMO energy levels and mesomorphic properties. Compared with the isomer DTFB2T, D2TFBT with two n-hexyl-substituted thiophenes as the terminal groups exhibited good liquid crystal behavior with Smectic phase when cooled from 180 °C, and it generated a large area of the liquid-crystalline phase at about 100 °C. However, we did not observe such behaviors in DTFB2T due to the different sequence of the moiety.