Thienothiophene-centered ladder-type -systems that feature distinct quinoidal -extension

Quinoidal rc-conjugated structures, a kind of fundamental subunits for organic rc-systems, may produce some intriguing optical, electronic and magnetic properties of polycyclic hydrocarbons (PHs). Herein, we report two thienothiophene-centered ladder-type polycyclic molecules ( 1 and 2 ), which possess one quinoidal thienothiophene moiety and two para -quinodimethane ( p -QDM) subunits, respectively. As theoretically and experimentally studied, while 1 is a fully closed-shell molecule, 2 owns an open-shell structure along with partial contribution of tetraradical state that is induced by the resonance of ( p -QDM). Moreover, although 2 has a larger rc-conjugated skeleton and open-shell electronic state, it exhibits larger bandgap and blue-shifted absorption. On the other hand, the reversible oxidation activity of 1 enables the preparation of its dication 12 + , and the studies on its single-crystal and aromatic structures demonstrate that its two positive charges are delocalized onto the oxygen atoms, thus achieving fully rc-extended structure and near-infrared absorption. This study not only gains insight into quinoidal rc-subunits, but also provides an important basis for the development of antiaromatic and open-shell rc-electron materials. (c) 2024 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.