Electronic structure origins of radical character in triangular fused acenes: sextet stabilization vs. antiaromaticity release

Abstract

Open-shell hydrocarbons are of great interest in molecular materials, yet their electronic structures remain challenging to describe. Here we investigate triangular acenes, formed by fusing three identical linear acenes through cyclobutadiene linkers into a threefold symmetric framework. Using density functional and multiconfigurational methods, we show that triangular acenes display a stronger radical character than their linear counterparts, which increases with molecular size. Analysis of singlet–triplet gaps, unpaired electron numbers, and NICS(1) aromaticity indices reveals that this behavior arises from two cooperative effects: Clar's sextet stabilization and the release of cyclobutadiene antiaromaticity. Fractional occupation densities further indicate a redistribution of unpaired electrons from cyclobutadiene units in smaller molecules to the acene cores in larger systems. These results establish triangular acenes as a distinct class of multiradicaloid hydrocarbons, offering new insights for the design of open-shell π-conjugated materials.