Pronounced electrochemical amphotericity of a fused donor-acceptor compound: a planar merge of ttf with a tcnq-type bithienoquinoxaline

Multistage organic redox systems,[1] in particular, electrochemically highly amphoteric compounds with a small HOMO/LUMO gap (HLG, for example, EoxEred<0.5 eV), are of current interest due to their potential applications in molecular electronics and optoelectronics.[2] Consequently,research efforts have been directed toward the design and synthesis of molecular systems composed of powerful electron donor (D) and acceptor (A) units. However, any strong electronic interaction in such molecules renders them neither strong electron donors nor acceptors. In practice, therefore, D and A components are often covalently linked by saturated flexible or rigid s spacers (D-s-A). Thereby, the structural D and A moieties on which the HOMO and LUMO are localized, respectively, are kept apart from each other. This hinders their strong coupling in the electronic ground state, so as to keep the HLG small.[3] Conjugated bridges of D-p-A assemblies, although providing easier steric control, generally favor stronger electronic coupling. Besides the amphoteric ground-state properties, photoinduced intramolecular charge-transfer (CT) processes may result in interesting photophysical phenomena such as longlived,charge-separated states.