Computational Study of the Effect of π-Congestion on the Singlet Biradical Character and Stacked Antiaromaticity in Acene Dimers

We demonstrated the computational study of the π-congestion effects in benzene, naphthalane, anthracene, teteracene, and pentacene dimers close with two positions at Ca···Ca’ and Cb···Cb’ to elucidate the singlet biradical characters as well as the stacked antiaromaticity. By extending the π-system from benzene to pentacene, the singlet biradical character increase whereas the stacked antiaromaticity decrease at near the transition state of [4+4]pericyclic reaction in ground state. Especially, the pentacene dimer exhibits large biradical character y0 = 61% with a weak antiaromaticity of NICSxx = +11 ppm at the distance of 2.80 Å between π-planes. These calculated results provided a unique idea to extract the similar electronic and magnetic natures as seen in transition state where the bond-order of the sp2 and sp3 carbon changes, like the graphite-to-diamond transformation.