Breaking Global Diatropic Current to Tame Diradicaloid Character: Thiele's Hydrocarbon Under Macrocyclic Constraints

• Published in: Angewandte Chemie International Edition 2024-04, Vol.63 (17), p.e202400780-n/a
• Main Authors: Banachowicz, Piotr, Das, Mainak, Kruczała, Krzysztof, Siczek, Miłosz, Sojka, Zbigniew, Kijewska, Monika, Pawlicki, Miłosz
• Format: Article
• Language: English
• Subjects: Benzene; Cyclophane; Dihedral angle; diradicaloid; macrocycle; Magnetic properties; NMR; Nuclear magnetic resonance; Optical properties; phenylenes; pi-conjugation; Quinones; Theoretical analysis; Thiele's hydrocarbon
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202400780

A diradical/biradical character of organic derivatives is one of the key aspects of contemporary research focusing on the fundamental studies followed by potential applicability relying on the unique optical, electronic, or magnetic properties assigned to unpaired electrons. A precise involvement of two p‐phenylenes into a cyclophane‐like conjugated, diatropic system creates a flexible molecule with the two different characters of both subunits (benzene and quinone) imprinting into the structure a Kekulé delocalized system. A dynamic of both carbocyclic subunits, and their mutual interaction generates a singlet open‐shell state (J=−1.25 kcal/mol) as documented spectroscopically (NMR and EPR). The extended theoretical analysis has proved a correlation between dihedral angle and the diradicaloid character that shifts from a closed‐shell singlet to an open‐shell state, eventually showing the y0=0.86 for 78 degrees and ΔEST=−0.34 kcal/mol. Two p‐phenylene units with dissimilar resonance character (benzene and quinone) are embedded in a macrocyclic system. Their mutual orientation, in terms of the dihedral angle between the carbocyclic units, can stabilize the diradicaloid character of the system. Detailed spectroscopic and theoretical analysis confirmed a presence of open‐shell singlet state with a low ΔEST of −0.34 kcal/mol.