Design and Synthesis of Low HOMO–LUMO Gap N-Phenylcarbazole-Substituted Diketopyrrolopyrroles

Abstract

The use of the electron-withdrawing cyano group to lower energy levels and extend light absorption of compounds for optoelectronic applications is well known. The use of cyano group incorporation has been reported to improve the electron mobility in organic photovoltaics to increase performance. Recently, we have reported that materials based on multiple acceptors including 1,1,4,4-tetracyanobutadiene (TCBD) and diketopyrrolopyrrole (DPP) possesses excellent photophysical characteristics such as strong light absorption, co-planar structure and good photochemical stability, which can be utilized as non-fullerene electron acceptors for bulk heterojunction organic solar cells. To examine the effect of TCBD on the optical properties of DPPs, we designed and synthesized ethyne-bridged N-phenylcarbazole-substituted DPPs and their TCBD derivatives by Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition–retroelectrocyclization reactions, respectively. The photophysical, thermal, electrochemical and computational properties of these compounds were investigated, which show substantial donor–acceptor interactions between the N-phenylcarbazole and DPP moieties. The N-phenylcarbazole-substituted DPPs exhibit excellent thermal stability. The TCBD derivatives showed redshifted, broad charge-transfer bands in the visible region with low HOMO–LUMO gaps compared to the corresponding ethyne-bridged N-phenylcarbazole-substituted compounds. The electrochemical study revealed additional reduction waves in the TCBD-bridged compounds, which correspond to the TCBD moiety itself.