Synthesis of zwitterionic open-shell bilayer spironanographenes

Abstract

Molecular nanographenes (NGs) are nanoscale graphene fragments obtained by organic synthetic protocols. Here we report the bottom-up synthesis of two spiro-NGs formed by two substituted hexa-peri-hexabenzocoronenes (HBCs), spiro-NG and F-spiro-NG. The X-ray crystal structure of the deca-tert-butyl-functionalized spiro-NG shows a bilayer disposition of the HBCs in face-to-face contact. By contrast, F-spiro-NG, which features tert-butyl substituents on one HBC unit, and fluorine on the other HBC unit, is an electron donor–acceptor bilayer NG. The structural assembly of the donor and acceptor graphenic layers enables an electron-transfer process that leads to the formation of a zwitterionic open shell, paramagnetic species constituted by a radical cation and a radical anion located in the donor and the acceptor HBCs, respectively. Magnetic and spectroelectrochemical experiments, together with theoretical calculations, support the persistent/dominant charge-separated nature of F-spiro-NG. Furthermore, photoconductivity measurements show a significant increase of the charge carrier mobility in the case of F-spiro-NG (Σμ = 6 cm2 V−1 s−1) compared with spiro-NG.