S‐shaped <i>para</i>‐Quinodimethane‐Embedded Double [6]Helicene and Its Charged Species Showing Open‐Shell Diradical Character
Qing Jiang, Yi Han, Ya Zou, Hoa Phan, Yuan Liu, Tun Seng Herng, Jun Ding, Chunyan Chi
Abstract
Abstract Helicenes and extended helical π‐conjugated compounds have been widely studied, but most of the systems contain only aromatic benzene or heterocyclic rings, showing local aromatic character. Herein, new S‐shaped double [6]helicene 1 , which has two embedded para ‐quinodimethane ( p ‐QDM) units, is reported. Due to the existence of a proaromatic quinoidal substructure, it has open‐shell diradical character. Its model compound, C‐shaped single [6]helicene 2 containing one p ‐QDM unit, was also synthesized and compared. Their ground‐state structures and electronic properties were systematically studied by a combination of various experimental methods assisted by theoretical calculations. Compound 1 has a double‐helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions (i.e., anti form). However, an anti / syn isomerization process with a moderate interconversion energy barrier was observed on the NMR timescale. Compound 1 shows amphoteric redox behavior. It also exhibits open‐shell diradical character ( y 0 =12.1 %) and a small singlet–triplet gap. On the other hand, compound 2 has a typical closed‐shell nature. The dication and dianion of 1 also show open‐shell diradical character. The dianion of 2 and the tetraanion of 1 exhibit similar electronic structures to their respective isoelectronic structures, that is, [6]helicene and a double [6]helicene. This work provides some insights into the design and synthesis of stable helical π systems with open‐shell diradical character and magnetic activity.