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Dicyclopentaannelated Hexa‐ peri ‐hexabenzocoronenes with a Singlet Biradical Ground State
Author(s) -
Chen Qiang,
Baumgarten Martin,
Wagner Manfred,
Hu Yunbin,
Hou Ian ChengYi,
Narita Akimitsu,
Müllen Klaus
Publication year - 2021
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.202102932
Subject(s) - singlet state , antiaromaticity , chemistry , aromaticity , electron paramagnetic resonance , molecule , density functional theory , hexa , moiety , ground state , open shell , photochemistry , benzene , computational chemistry , crystallography , stereochemistry , organic chemistry , atomic physics , nuclear magnetic resonance , excited state , physics
Synthesis of two dicyclopentaannelated hexa‐ peri ‐hexabenzocoronene (PHBC) regioisomers was carried out, using nonplanar oligoaryl precursors with fluorenyl groups: m PHBC 8 with two pentagons in the “ meta ”‐configuration was obtained as a stable molecule, while its structural isomer with the “ para ”‐configuration, p PHBC 16 , could be generated and characterized only in situ due to its high chemical reactivity. Both PHBCs exhibit low energy gaps, as reflected by UV‐vis‐NIR absorption and electrochemical measurements. They also show open‐shell singlet ground states according to electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) calculations. The use of fully benzenoid HBC as a bridging moiety leads to significant singlet biradical characters ( y 0 ) of 0.72 and 0.96 for m PHBC 8 and p PHBC 16 , respectively, due to the strong rearomatization tendency of the HBC π‐system; these values are among the highest for planar carbon‐centered biradical molecules. The incorporation of fully unsaturated pentagons strongly perturbs the aromaticity of the parent HBC and makes the constituted benzene rings less aromatic or antiaromatic. These results illustrate the high impact of cyclopentaannelation on the electronic structures of fully benzenoid polycyclic aromatic hydrocarbons (PAHs) and open up a new avenue towards open‐shell PAHs with prominent singlet biradical characters.