Ferrocenyl-substituted diketopyrrolopyrroles (DPPs) with the symmetrical π-bridged donor–acceptor (D–π–A) frameworks D′–π–D–A–D–π–D′ and D′–A′–D–A–D–A′–D′ as well as unsymmetrical D–A–D–π–D′ and D–A–D–A′–D′ frameworks were designed and synthesized by a Pd-catalyzed Sonogashira cross-coupling reaction followed by a [2+2] cycloaddition–retroelectrocyclization reaction. The tetracyanobutadiene (TCBD) DPPs 7 and 8 have redshifted absorption bands and lower HOMO–LUMO gap values relative to those of ferrocenyl-DPPs 5 and 6. The electronic absorption spectra show a systematic redshift in the intramolecular charge transfer (ICT) band from the ferrocene to the diketopyrrolopyrrole core. The photophysical, computational, and electrochemical properties were also investigated and show substantial donor–acceptor interactions between the ferrocene and DPP moieties. The symmetrical ferrocenyl-DPPs have more thermal stability than the unsymmetrical derivatives.

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.