Organic triplets as free radical pairs: Everything old is new again

More than a half-century ago, Michael Dewar devised a theoretical construct whereby the aromatic stabilization of conjugated hydrocarbon rings could be analyzed by imagining them to be formed by combining free radical chains. From the analysis, he also predicted that the rings would reverse their aromaticity in their first excited states. By taking the construct more literally, we were able to calculate the ground state-to-triplet energies of twisted conjugated polyenes and to elaborate upon the rules of aromaticity and anti-aromaticity for hydrocarbon rings. In the present publication, we point out that the spin densities of the triplet states of polybenzenoid hydrocarbons closely mimic those expected for free radicals, and that to some extent, the relative energies of their isomers can be understood in terms of interacting free radical networks.

Automated summary

This paper explores the rules of aromaticity and anti-aromaticity for conjugated hydrocarbon rings. By calculating energy and analyzing spin density, it is demonstrated that the triplet states of polybenzenoid hydrocarbons exhibit characteristics of free radicals, and that the relative energies of their isomers can be explained by interacting free radical networks.