Stacked-ring aromaticity from the viewpoint of the effective number of π-electrons

We theoretically examined the effective number of π-electrons in the closely π-stacked 4nπ electron dimers exhibiting stacked-ring aromaticity. Multi-configurational calculations for cyclobutadiene π-dimer models revealed that a double-triplet [1(T1T1)] character bearing 2 x (4n–2)π conjugated electrons + 4 formally unpaired electrons appears in the ground state at small stacking distances. Energy stabilization of the closely π-stacked dimers can be attributed to mixing intermolecular charge-transfer (CT) configurations. However, its energy gain would be insufficient for self-aggregation of the π-dimers with a stacking distance of ~3 Å.