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The antiferromagnetic spin coupling in non- Kekulé Acenes–impressive poly-radical character revealed by high level multi-reference methods

Complete active space (CASSCF) and multi- reference (MR-CISD(Q) and MR-AQCC) calculations were performed for non-Kekulé analogs of acenes, dimethylenepolycyclobutadienes, of increasing length up to eight cyclobutadiene (CBD) units. Multi-reference 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), respectively, due to antiferromagnetic spin coupling thus violating Hund’s rule in larger molecules. We also show an impressive poly-radical character in the system which is increasing 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 energy states of higher multiplicities even beyond triplet, this demonstrates exceptional polyradical properties of these π-conjugated oligomeric chains.

spin coupling ; multireference methods ; polyradical character ; cyclobutadiene chains

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