Effects on the aromaticity and on the biradicaloid nature of acenes by the inclusion of a cyclobutadiene linkage

Biphenylene is a well-characterized compound that includes the union of both aromatic and antiaromatic units: benzene and cyclobutadiene, respectively. In this work, we extend this structural theme to acene analogues with a cyclobutadiene linkage in a central position. At the single reference domain, perturbation theory and DFT calculations were applied to characterize the electronic structure of the extended biphenylene systems, as well as of some acenes of interest for comparison. Multireference calculations were also used to provide more accurate information about the energetic stability and insights into the radical character. Using descriptors such as the singlet–triplet splitting, the number of effectively unpaired electrons, fractional occupation number weighted density analysis and aromaticity indexes, the different compounds have been compared with respect to biradicaloid character and chemical stability through structural and energetic approaches. The results indicate that biphenylene derivatives should possess similar reactivities and stabilities as their half acenes fragments.