Toward the Synthesis of Pentaheptite Substructure: The Cyclopenta[ <i>ef</i> ]heptalene to Phenanthrene Rearrangement

Novel carbon allotropes have long been pursued in synthetic chemistry and materials science, and graphene has been the most well-investigated carbon nanostructure in the past decade. However, an analogous carbon pentaheptite with equal numbers of pentagons and heptagons has not yet been synthesized because designing and synthesizing nonhexagon motifs along two dimensions is challenging. During the synthesis of a pentaheptite substructure (or named nanopentaheptite, <strong>NPH</strong>), which contain four pentagon-heptagon pairs and one heptalene unit, unexpected cyclopenta[<em>ef</em>]heptalene-into-phenanthrene rearrangements were observed for the first time, yielding two novel compounds (<strong> NPH-R1</strong> and <strong>NPH-R2</strong>). Experimental and theoretical results demonstrated that the <strong>NPH</strong> series exhibits narrower energy gaps than that of their hexagonal analog with similar size (e.g., hexa-peri-hexabenzocoronene, <strong>HBC</strong>). Our work reported herein, based on the azulene chemistry, provides new insights into the preparation of novel carbon allotrope pentaheptite nanostructures.