Interplay of Aromaticity and Coulomb Repulsion in the Electronic Structure of a Dimeric Pentafulvalene Upon Multi-Electron Uptake

Multielectron reduction of finite one-dimensional (1D) π-conjugated hydrocarbons remains poorly understood because such frameworks are usually unstable due to charge accumulation in the absence of electron-withdrawing substituents. Here, we investigate the reduction of dibenzopentafulvalene dimers ([2]DBFs), a rare class of quasi-1D conjugated hydrocarbons that exhibit unusual stability toward multi-electron reduction and contain four indenylidene subunits that can each become aromatic upon reduction, raising the question of where the excess electrons reside. Chemical reduction of [2]DBFs with alkali metal-based reductants afforded dianionic and tetraanionic species, which were characterized by electronic absorption spectroscopy, NMR spectroscopy, single-crystal X-ray diffraction, and complementary quantum chemical calculations. The dianions accommodate excess negative charge at the outer indenylidene subunits, where Coulomb repulsion is minimized by charge separation while gaining 10π Hückel aromaticity. In contrast, upon four-electron reduction, all four indenylidene subunits become aromatic, and the excess charge becomes more broadly distributed and can extend to the end-capping phenyl groups even at the expense of the aromatic character of the end-capping benzene rings. The solid-state structures further reveal that the observed charge distributions are strongly influenced by the spatial arrangement of counter cations. These results identify local aromatization, Coulomb repulsion, and countercation position as competing factors that determine where electrons reside in multiply charged finite 1D π-conjugated hydrocarbons.