Extended Tetrathiafulvalene Multi-Redox Systems Incorporating 4,5-Dihydroazuleno[2,1,8-ija]azulene Cores

The introduction of 4,5-dihydroazuleno[2,1,8-ija]azulene as a central core between two 1,4-dithiafulvene (DTF) units provides a novel class of extended tetrathiafulvalene (TTF) electron donors. Herein we present the synthesis of such compounds with the azulenoazulene further expanded by annulation to benzene, naphthalene, or thiophene rings. Moreover, unsymmetrical donor-acceptor chromophores with one DTF and one carbonyl at the central core are presented. The effect of the odd-membered rings of the polycyclic aromatic hydrocarbon cores on the geometrical features and the extent of bond conjugation in these systems were investigated by UV-Vis absorption spectroscopy, (spectro)electrochemical studies, X-ray and electron diffraction, and computational studies. Altogether these studies reveal that, upon oxidation, the non-planar core of the bis(benzo)-fused dihydro-azulenoazulene-extended TTF generates a large 22π-aromatic system (together with two 6π-aromatic 1,3-dithiolium appendages) characterized by significant bond length equalizations. The dication formed by oxidation is EPR silent, indicating a singlet ground state. Computations reveal that the closed-shell singlet species and its open-shell singlet counterpart are nearly degenerate and of lower energy than a triplet species. According to cyclic voltammetry, the compounds can be oxidized further and present multi-redox systems.