Aromaticity of neutral and doubly charged polyacenes.

The aromatic character of neutral and doubly charged polyacenes was explored in terms of the harmonic oscillator model of aromaticity (HOMA) and bond resonance energy (BRE). Doubly charged species of polyacenes are different in global and local aromaticity from the neutral species. Neutral species are fairly uniform in local aromaticity, whereas doubly charged species are more aromatic in the edge rings than in the central rings. Higher polyacenes have been predicted to exist as singlet-state diradicals. It was found that these polyacene diradicals are quite similar in global and local aromaticity to the closed-shell molecular dications. The possible instability of higher polyacenes in closed-shell electronic configuration is associated with the high-lying highest occupied molecular orbital (HOMO) that contributes little to global aromaticity.