Electrochemical redox properties of bridged and non-bridged annulenediones

The present article is fully confined to cyclic voltammetric measurements of a series of bridged and non-bridged annulenediones (quinones of large conjugated rings of aromatic character). The evaluation of their electrochemical redox properties shed light on remarkable and interesting conclusions such as aromaticity, electrostatic repulsion, disproportionation constants, and stability of their reduced charged intermediates. The results reveal that in the case of the non-bridged quinones 1–4, as the number of fused rings increases the reduction becomes gradually more difficult because the aromatic stabilization of the ‘quinone’ on conversion to aromatic ‘hydroquinone’ system decreases. However, since all studied annulenediones 5–11 possess a ‘C₂’ bridge that keeps the macrocycles flat, causing better aromaticity, they are relatively easily reduced at both E₁ and E₂ potentials and therefore, they could be considered as quinones of aromatic systems. On the other hand, when the bridge is longer (as in 12 and 13) the macrocycles tend to bend, causing a decrease in the degree of aromaticity and as a consequence, their reduction becomes more difficult.