Multi-redox indenofluorene chromophores incorporating dithiafulvene donor and ene/enediyne acceptor units

Abstract

Large donor–acceptor scaffolds derived from polycyclic aromatic hydrocarbons (PAHs) with tunable HOMO and LUMO energies are important for several applications, such as organic photovoltaics. Here, we present a large selection of PAHs based on central indenofluorene (IF) or fluorene cores and containing various dithiafulvene (DTF) donor units that gain aromaticity upon oxidation and a variety of acceptor units, such as vinylic diesters, enediynes, and cross-conjugated radiaannulenes (RAs) that gain aromaticity upon reduction. In some cases, the DTF units are expanded by pyrrolo annelation. The optical and redox properties of these compounds, in some cases carbon-rich, were studied by UV–vis absorption spectroscopy and cyclic voltammetry. Synthetically, the work explores IF diones or fluorenone as central building blocks by subjecting the carbonyl groups to a variety of reactions; that are, phosphite- or Lawesson’s reagent-mediated olefination reactions (to introduce DTF motifs), Ramirez/Corey–Fuchs dibromo-olefinations followed by Sonogashira couplings (to introduce enediynes motifs), and Knoevenagel condensations (to introduce the vinylic diester motif). By a subsequent Glaser–Hay coupling reaction, a RA acceptor unit was introduced to provide a DTF-IF-RA donor–acceptor scaffold with a low-energy charge-transfer absorption and multi-redox behavior.