From Open-Shell Singlet Diradicaloid to Closed-Shell Global Antiaromatic Macrocycles.

A dithieno[a,h]-s-indacene- (DTI-) based diradicaloid DTI-2Br was synthesized and its open-shell singlet diradical character was validated by magnetic measurements. On the other hand, its macrocyclic trimer DTI-MC3 and tetramer DTI-MC4 turned out to be closed-shell compounds with global antiaromaticity, which was supported by X-ray crystallographic analysis and NMR spectroscopy, assisted by ACID and 2D-ICSS calculations. Such change can be explained by a subtle balance between two types of antiferromagnetic spin-spin coupling along the π-conjugated macrocycles. The dications of DTI-MC3 and DTI-MC4 turned out to be open-shell singlet diradical dications, with a singlet-triplet energy gap of -2.90 and -2.60 kcal mol , respectively. At the same time, they are both global aromatic. Our studies show that intramolecular spin-spin interactions play important roles on electronic properties of π-conjugated macrocycles.