Tuning of the HOMO–LUMO Gap of Symmetrical and Unsymmetrical Ferrocenyl-Substituted Diketopyrrolopyrroles

Abstract

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.