Strain and Hückel Aromaticity: Driving Forces for a Promising New Generation of Electron Acceptors in Organic Electronics

Straining at the leash: The main features of electron-accepting materials with a 9,9′-bifluorenylidene backbone are strain relief and a gain in aromaticity. These dimers (see picture) exhibit absorption near the red spectral region (ca. 600 nm) and HOMO (5.58–5.06 eV) and LUMO (3.37–3.09 eV) energy levels, which, together with high solubility and thermal stability render these materials attractive acceptors for bulk heterojunction (BHJ) solar cells.