A Triply Negatively Charged Nanographene Bilayer with Spin Frustration.

Abstract

There is resurgent interest in atomically precise nanographene radical ions composed of fused hexagons. Two monoradical anions can readily form a doubly negatively charged π-dimer, but to our knowledge, no triply negatively charged π-dimer species have been isolated till now. Herein we report the largest open-shell grapheneic bilayer and also the first example of triply negatively charged radical π-dimer. Upon three-electron reduction, bilayer nanographene fragment molecule (C96H24Ar6)2 (Ar = 2,6-dimethylphenyl) (12) was transformed to a triply negatively charged species 123•-, which has been characterized by single-crystal X-ray diffraction, EPR spectroscopy and SQUID measurements. 123•- features a 96-center-3-electron (96c/3e) pancake bond with a doublet ground state, which can be thermally excited to a quartet state. It is the largest open-shell nanographene bilayer species till now constructed with 34 π-fused rings with 96 conjugated sp2 carbon atoms. Spin frustration is observed with the frustration parameter f = |θcw|/TN > 31.8 at low temperatures in 123•-, which indicates graphene upon reduction doping may behave as quantum spin liquids.