The Antiferromagnetic Spin Coupling in Non-Kekulé Acenes-Impressive Polyradical Character Revealed by High-Level Multireference Methods.

Complete active space (CASSCF) and multireference (MR-CISD(Q) and MR-AQCC) calculations were performed for non-Kekulé analogues of acenes, dimethylenepolycyclobutadienes, with lengths of up to eight cyclobutadiene (CBD) units. Multireference calculations predict that the most stable energy state of the system is either triplet (if there is an odd number of CBD units) or singlet (if there is an even number of CBD units) due to antiferromagnetic spin coupling, which thus violates Hund's rule in larger molecules. We also show an impressive polyradical character in the system that increases with the size of the molecule, as witnessed by more than eleven unpaired electrons in the singlet state of the molecule with eight CBD units. Together with the small energy gap between singlet and higher multiplicity energy states even above the triplet state, this demonstrates the exceptional polyradical properties of these π-conjugated oligomeric chains.