Singlet Biradical Versus Triplet Biradical/Zwitterion Characteristics in Isomers of C6−C5−C6−C7−C6-Fused Pentacyclic Aromatic Hydrocarbons Revealed through Reactivity Patterns

Among various polycyclic aromatic hydrocarbons, C₆−C₅−C₆−C₇−C₆ fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6 , where the radicaloid carbons are linked through a para-phenylene, forms a head-to-head dimer like a singlet biradical. In contrast, isomer 7 , where the para-linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C₆−C₅−C₆−C₆−C₆ ring contraction product 13 . Cation 14 , the protonated form of 7 , was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules.