A Biradical Balancing Act: Redox Amphoterism in a Diindenoanthracene Derivative Results from Quinoidal Acceptor and Aromatic Donor Motifs.

The reduced and oxidized states of an open-shell diindeno[b,i]anthracene (DIAn) derivative have been investigated by experimental and theoretical techniques. As a result of moderate biradical character and the ability of cyclopenta-fused scaffolds to stabilize both positive and negative charges, DIAn exhibits rich redox chemistry with four observable and isolable charged states. Structural and electronic properties of the DIAn system are brought to light by UV-vis-NIR and Raman spectroelectrochemical measurements. Aromatization of the diindeno-fused anthracene core upon successive single-electron injections is revealed through single-crystal X-ray diffraction of radical anion and dianion salts. We present a rare case where the pseudoaromatic/quinoidal ground state of a neutral biradical polycyclic hydrocarbon leads to a stable cascade of five redox states. Our detailed investigation of the transformation of molecular structure along all four redox events provides a clearer understanding of the nature of charge carriers in ambipolar organic field-effect transistors.