Synthesis and Characterization of a π-Extended Clar's Goblet.

Concealed non-Kekulé polybenzenoid hydrocarbons have no sublattice imbalance yet cannot be assigned a classical Kekulé structure, leading to an open-shell ground state with potential applications in organic spintronics. They constitute an exceedingly small fraction of the total number of polybenzenoid hydrocarbons that can be constructed for a given number of benzenoid rings, and their synthesis remains challenging. The archetype of such a system is the Clar's goblet (C₃₈H₁₈), a diradical proposed by Erich Clar in 1972 and recently synthesized on a Au(111) surface. Here, we report the synthesis of a π-extended Clar's goblet (C₇₆H₂₆), a tetraradical concealed non-Kekulé polybenzenoid hydrocarbon, by a combined in-solution and on-surface synthetic approach. By low-temperature scanning tunneling microscopy and atomic force microscopy, we characterized individual molecules adsorbed on a Cu(111) surface. We provide insights into the electronic properties of this elusive molecule, including the many-body nature of its ground and excited states, by mean-field and multiconfigurational quantum chemistry calculations.