Near-infrared absorbing tetracyanobutadiene-bridged diketopyrrolopyrroles

Abstract

In order to investigate the effects of end capping donors and secondary acceptors on the photonic and electrochemical properties of diketopyrrolopyrrole (DPP), the tetracyanobutadiene (TCBD) bridged D–A¹–A²–π–D type DPPs V–VII have been designed and synthesized by a [2+2] cycloaddition–retroelectrocyclization reaction. Optical, thermal, electrochemical and computational studies are performed and compared with their para N-phenyl carbazole-based analogue DPP-VIII. The TCBD bridged DPPs V–VIII exhibit electronic absorption in the visible to near infrared (NIR) region with low HOMO–LUMO gap values compared to their acetylene bridged precursors DPPs I–IV. The DPPs V–VIII exhibit excellent thermal stability. Electrochemical studies reveal additional reduction waves in DPPs V–VIII corresponding to two successive reductions of the TCBD moiety. The introduction of electron-withdrawing TCBD to acetylene-bridged DPPs extends the absorption and lowers the HOMO–LUMO gap which is supported by computational calculations.