Open-Shell Nonbenzenoid Nanographenes Containing Two Pairs of Pentagonal and Heptagonal Rings

Abstract

Nonbenzenoid carbocyclic rings are postulated to serve as important structural elements toward tuning the chemical and electronic properties of extended polycyclic aromatic hydrocarbons (PAHs, or namely nanographenes), necessitating a rational and atomically precise synthetic approach toward their fabrication. Here, using a combined bottom-up in-solution and on-surface synthetic approach, we report the synthesis of nonbenzenoid open-shell nanographenes containing two pairs of embedded pentagonal and heptagonal rings. Extensive characterization of the resultant nanographene in solution shows a low optical gap, and an open-shell singlet ground state with a low singlet–triplet gap. Employing ultra-high-resolution scanning tunneling microscopy and spectroscopy, we conduct atomic-scale structural and electronic studies on a cyclopenta-fused derivative on a Au(111) surface. The resultant five to seven rings embedded nanographene displays an extremely narrow energy gap of 0.27 eV and exhibits a pronounced open-shell biradical character close to 1 (y 0 = 0.92). Our experimental results are supported by mean-field and multiconfigurational quantum chemical calculations. Access to large nanographenes with a combination of nonbenzenoid topologies and open-shell character should have wide implications in harnessing new functionalities toward the realization of future organic electronic and spintronic devices.

Dicyclohepta[ijkl,uvwx]rubicene with Two Pentagons and Two Heptagons as a Stable and Planar Non‐benzenoid Nanographene

“…Thus, DHR shows good stability, being similar to PAHs with two embedded azulene units reported by Mastalerz and co‐workers . This makes DHR different from nanographenes with two pentagons and two heptagons reported recently by Müllen and Feng which are not stable in air …”

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…[15] There are av ery few successful approaches known for synthesizing PA Hs with azulene units embedded from precursor monolayers on surfaces. [16] However,t ot he best of our knowledge,t here are only af ew examples of soluble azulene-based PA Hs that have been synthesized from solution on al arger scale. [17,18] One is the example recently published by Konishi and co-workers, [17] in which the PA H contains besides one azulene core,s ix more fused sixmembered rings.V ery recently,t he same group presented as mall PA Ht hat contains two fused azulene units and two additional peri-fused six-membered rings,g iving overall four rings.…”

Spiers Memorial Lecture : Carbon nanostructures by macromolecular design – from branched polyphenylenes to nanographenes and graphene nanoribbons

“…A remarkable case is compound 85 ( Figure 14A) which has been made available by Xinliang Feng and Junzhi Liu. 206 Five-membered rings are, first, formed after 1,2-phenyl migration upon oxidative treatment of 81, and then, seven-membered rings result from Friedel-Crafts ring closures. Dehydrogenation of 84 towards the realization of fully unsaturated NG 85 incorporating two azulene units is accompanied by a drastic bathochromic shift of the absorption band of 435 nm.…”