Synthesis of electron-deficient polymers based on sulfur-bridged dithienylboranes as a building block

Abstract

π-Conjugated materials incorporating tricoordinate boron are known to possess relatively deep LUMO energy levels owing to the interaction between the empty p orbital of boron and the π* orbital. However, polymers with simple triarylborane structures do not have deep LUMO energy levels for applications in electron-accepting optoelectronic materials, such as n-type semiconductors. In this study, we synthesized new p–π* conjugated polymers by copolymerizing thiaborin units containing boron and sulfur atoms and diketopyrrolopyrrole (DPP), a well-known acceptor unit. The resulting polymers exhibited extended conjugation through the boron p orbital and strong absorption in the visible to near-infrared region. Although these polymers did not show n-type semiconductor behavior, they possessed deep LUMO energy levels lower than −3.6 eV. Furthermore, titration experiments using an amine base revealed that the polymer containing a sulfone unit has strong Lewis acidity, as evidenced by changes in the UV-vis absorption spectrum upon coordination of the base.