Open-Shell Benzenoid Polycyclic Hydrocarbons

Graphene has been found to be an attractive material since its discovery in 2004 and has drawn enormous interest among researchers owing to its intrinsic electronic and magnetic properties. An indefinitely large graphene sheet, if cut along the two edge directions, can generate two distinct, well-defined small graphene nanoribbons (GNRs), which may be classified into “zigzag” edge (1, transpolyacetylene) and “armchair” edge (2, cis-polyacetylene) structures (Figure 1.1). If graphene is cut in an all-zigzag fashion or in a triangular shape, the smallest unit that would come out is a phenalene unit (3) with one unpaired electron. The larger frameworks when cut in a zigzag shape will indefinitely lead to a series of non-Kekulé (i.e., open-shell) polycyclic hydrocarbons (PHs, 4–6) possessing one or more unpaired electrons, which can be termed as “open-shell graphene fragments” and are very interesting in terms of academic research. The number of unpaired electrons, or radicals, increases with the size of the framework, starting from phenalenyl monoradical to high-spin polyradical systems (3–6, Figure 1.1). If a graphene sheet is cut in an all-armchair fashion, it would lead to “all-benzenoid PHs,” as their structures can be represented as fully aromatic sextet rings (the six-membered rings highlighted in blue background) without additional double bonds (7). Even though the PHs of such category are larger in size with extended conjugation, they generally show high stability due to the stabilization through the existence of more number of Clar’s aromatic sextets (8). The studies on hexa-peri-hexabenzocoronene and other extended all-benzenoid PHs have provided information on armchair-edged GNRs at the molecular level [1]. Another intriguing class of PHs would be rectangular-shaped GNRs, which are characterized by existence of both zigzag and armchair edges (9, Figure 1.1) with a typical Kekulé (i.e., closed-shell) structure. Their structures can be formulated by cycles in a monocyclic system symbolizing benzenoid aromatic sextet rings according to Clar’s aromatic sextet rule [2]. Therefore, the more Clar’s sextets the molecule can have, the more stable the system will be. Interestingly, recent theoretical and experimental work indicate that for rectangular PHs with extended