S‐shaped para‐Quinodimethane‐Embedded Double [6]Helicene and Its Charged Species Showing Open‐Shell Diradical Character. Issue 67 (9th November 2020)

• Record Type: Journal Article
• Authors: Jiang, Qing; Han, Yi; Zou, Ya; Phan, Hoa; Yuan, Liu; Herng, Tun Seng; Ding, Jun; Chi, Chunyan
• Is Part Of: Chemistry. Volume 26:Issue 67(2020)
• Page Start: 15613
• Page End: 15622
• Publication Date: 2020-11-09
• Subjects: diradicaloid -- helicenes -- isomerization -- polycycles -- quinodimethanes; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202002952
• Languages: English
• ISSNs: 0947-6539

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. Abstract : Double‐helical diradicaloid : A new S‐shaped para ‐quinodimethane‐embedded double [6]helicene shows unique open‐shell diradical character. X‐ray crystallographic analysis shows that it has a double helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions. Furthermore, an anti / syn isomerization process was observed on the NMR timescale with a moderate interconversion energy barrier. The dication and dianion of 1 also show open‐shell diradical character with small singlet–triplet energy gaps.