An Open‐Shell Coronoid with Hybrid Chichibabin–Schlenk Conjugation

• Published in: Angewandte Chemie International Edition 2021-10, Vol.60 (41), p.22496-22504
• Main Authors: Prajapati, Bibek, Dang, Duy‐Khoi, Chmielewski, Piotr J., Majewski, Marcin A., Lis, Tadeusz, Gómez‐García, Carlos J., Zimmerman, Paul M., Stępień, Marcin
• Format: Article
• Language: English
• Subjects: aromaticity; Computer applications; Conjugation; coronoids; Energy gap; Infrared absorption; molecular magnetism; Near infrared radiation; NMR; Nuclear magnetic resonance; radicaloids; spectroscopy; Switches
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202109273

A hexaradicaloid molecule with alternating Kekulé and non‐Kekulé connectivities between adjacent spin centers was obtained by fusing two conjugation motifs in Chichibabin and Schlenk hydrocarbons into a coronoid structure. 1H NMR, ESR, and SQUID experiments and computational analyses show that the system has a singlet ground state with a significant hexaradicaloid character (γ0=0.826, γ1=γ2=0.773). It has multiple thermally accessible high‐spin states (up to the septet), with uniform energy gaps of ca 1.0 kcal mol−1 between consecutive multiplicities. In line with its open‐shell character, the coronoid has a small electronic band gap (ca. 0.8 eV) and undergoes two consecutive one‐electron oxidations at low potentials, yielding cationic forms with extended near‐infrared absorption. The hexaradicaloid, which combines open‐shell and macrocyclic contributions to its π conjugation, is an example of a design strategy for multistate spin switches and redox‐amphoteric NIR dyes. An expanded, 18‐ring coronoid molecule, inspired by the structures of Chichibabin and Schlenk hydrocarbons, features a cross‐over of Kekulé and non‐Kekulé conjugation, resulting in a distinct hexaradicaloid character, easy accessibility of high‐spin states, and redox amphoterism.