Nanoribbons with Non-Alternant Topology from Fusion of Polyazulene: Carbon Allotropes Beyond Graphene

Various two-dimensional (2D) carbon allotropes with non-alternant topologies, such as pentaheptites and phagraphene, have been proposed. Predictions indicate that these metastable carbon polymorphs, which contain odd-numbered rings, possess unusual (opto)electronic properties. However, none of these materials has been achieved experimentally due to synthetic challenges. In this work, by using on-surface synthesis, nanoribbons of the non-alternant graphene allotropes, phagraphene and tetra-penta-hepta(TPH)-graphene have been obtained by dehydrogenative C-C coupling of 2,6-polyazulene chains. These chains were formed in a preceding reaction step via on-surface Ullmann coupling of 2,6-dibromoazulene. Low-temperature scanning probe microscopies with CO-functionalized tip and density functional theory calculations have been used to elucidate their structural properties.