The road to bis-periazulene (cyclohepta[def]fluorene): Realizing one of the long-standing dreams in nonalternant hydrocarbons

The chemistry of nonalternant hydrocarbons has recently experienced a considerable resurgence. As the relationship between naphthalene and azulene shows, an important design concept for a nonalternant framework is to replace the two hexagons of an alternant system with a pentagon and heptagon pair. The pursuit of the nonalternant isomers of pyrene, which has seven possible nonalternant isomers, has been the most significant related project. Through enormous dedication by many pioneering works, six of the seven possible nonalternant isomers of pyrene have been isolated and characterized as stable aromatic molecules, differentiating themselves from pyrene in terms of optoelectronic properties. However, the only unsynthesized isomer, bis-periazulene (cyclohepta[def]fluorene), had remained an uncharacterized hydrocarbon until 2022, despite many synthetic and theoretical investigations since it was first reported in 1955. In this review, we summarize the chemistry of the pyrene's nonalternant isomers, including our recent achievements in synthesizing and characterizing bis-periazulene derivatives. The historical perspectives, synthetic approaches, fundamental properties, and comparisons to related systems are documented.