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

Abstract

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.

Stable Organic (Bi)Radicals by Delocalization of Spin Density into the Electron‐Poor Chromophore Core of Isoindigo

“…Since the discovery of the first biradicaloid polycyclic aromatic hydrocarbon (PAH) by Tschitschibabin in 1907, the interest in organic open shell π‐conjugated compounds has increased tremendously due to their unique optical, electronic and magnetic properties . Usually, open‐shell PAHs exhibit redox amphoterism, comparatively small HOMO–LUMO energy gaps, enhanced second hyperpolarizability and large two‐photon absorption cross‐sections which makes them interesting for the development of new ambipolar field‐effect transistors, second generation solar cells based on singlet fission, non‐linear optics, energy storage or electronic devices as well as for organic spintronics . Generally, the degree of biradical character y can range from zero (completely closed‐shell) to one (completely open‐shell) with overall spin angular momenta of S= 0 for singlet and S= 1 for triplet states which can be predicted by applying Ovchinnikov's parity model .…”

Synthesis of the Unknown Indeno[1,2‐a]fluorene Regioisomer: Crystallographic Characterization of Its Dianion

“…Recently the synthesis and study of diradical compounds has garnered renewed attention as these species could be useful as promising materials in molecular spintronics, nonlinear optics, and singlet fission . The contribution of the open‐shell form to the ground‐state structure is described by the diradical character index, y , with y= 1 defining a pure open‐shell (OS) system and y= 0 describing a pure closed‐shell (CS) system ,. With the potential to re‐aromatize the quinoidal motif (e.g., 1 , 1′′ ) to a diradical structure (e.g., 1′ ), IFs make an interesting case study.…”

“…Unfortunately, attempts to form single crystals for definitive structural proof were fruitless. However, prior studies have shown that IF derivatives can be reduced easily to monoanion and dianion states to secure crystallographic data ,. Importantly, such reductions do not alter the carbon skeleton, and therefore X‐ray crystallography of the reduced product would reveal the molecule's carbon connectivity and provide the structural confirmation that 6 is the missing IF regioisomer.…”

5,10-Dimesityldiindeno[1,2-a:2′,1′-i]phenanthrene: a stable biradicaloid derived from Chichibabin's hydrocarbon

“…Simple biradicaloid structures, such as 1 itself, 4 tend to be unstable under ambient conditions, and extensive effort has been given to developing chemically robust open-shell systems. The diindeno[1,2-b:1 0 ,2 0 -i]anthracene (DIAn) derivative 3, reported recently by Haley et al 5,6 exem-plies key strategies used to stabilize p-biradicaloids, namely extended ring fusion, steric blocking, and the use of conjugated substituents, such as ethynyl groups. 3 represents the family of stabilized diindenoarene biradicaloids, [7][8][9] which comprises indenouorenes [10][11][12][13][14] and their larger congeners, e.g.…”