Dicyclopentaannelated Hexa-peri-hexabenzocoronenes with a Singlet Biradical Ground State

Qiang Chen; Martin Baumgarten; Manfred Wagner; Yunbin Hu; Ian Cheng-Yi Hou; Akimitsu Narita; Klaus Müllen

doi:10.1002/anie.202102932

Dicyclopentaannelated Hexa-peri-hexabenzocoronenes with a Singlet Biradical Ground State

Angew Chem Int Ed Engl. 2021 May 10;60(20):11300-11304. doi: 10.1002/anie.202102932. Epub 2021 Apr 7.

Authors

Qiang Chen^{1

2}, Martin Baumgarten¹, Manfred Wagner¹, Yunbin Hu^{1

3}, Ian Cheng-Yi Hou¹, Akimitsu Narita^{1

4}, Klaus Müllen^{1

5}

Affiliations

¹ Synthetic Chemistry, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
² Current address: Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford, OX1 3TA, UK.
³ Current address: College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan, 410083, P. R. China.
⁴ Organic and Carbon Nanomaterials Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa, 904-0495, Japan.
⁵ Institute of Physical Chemistry, Johannes Gutenberg-University, Duesbergweg 10-14, 55128, Mainz, Germany.

Abstract

Synthesis of two dicyclopentaannelated hexa-peri-hexabenzocoronene (PHBC) regioisomers was carried out, using nonplanar oligoaryl precursors with fluorenyl groups: mPHBC 8 with two pentagons in the "meta"-configuration was obtained as a stable molecule, while its structural isomer with the "para"-configuration, pPHBC 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 (y₀ ) of 0.72 and 0.96 for mPHBC 8 and pPHBC 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.

Keywords: dicyclopentaannelation; hexa-peri-hexabenzocoronene; low energy gap; not-fully benzenoid PAH; open-shell biradical.

Abstract

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