Bis-periazulene: remaining non-alternant isomer of pyrene

Abstract

Abstract Bis-periazulene (cyclohepta[def]fluorene) is one of the seven possible non-alternant isomers of pyrene. Different from the other isomers, bis-periazulene had remained an uncharacterized hydrocarbon until 2022, since it was first reported in 1955. This paper documents our recent achievements in synthesizing and characterizing bis-periazulene derivatives. Its triaryl derivatives 5c‒e exhibited the superimposed electronic structures of peripheral, polarized, and open-shell π-conjugated systems. Furthermore, in contrast to previous theoretical predictions, bis-periazulene derivatives showed the singlet ground state.