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

Ferrocenyl-substituted diketopyrrolopyrroles (DPPs) with the symmetrical p-bridged donor-acceptor (D-p-A) frameworks D'-pi-D-A-D-pi-D' and D'-A'-D-A-D-A'-D' as well as unsymmetrical D-A-D-pi-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 red-shift 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.