Antiaromatic character of cycloheptatriene-bis-annelated indenofluorene framework mainly originated from heptafulvene segment

Fully π-conjugated polycyclic hydrocarbons with antiaromatic character have attracted research attention because of their unique properties such as narrow energy gaps, and thus should find application as optical and electronic materials. Although antiaromatic 16π-electron frameworks can be constructed by the incorporation of multiple seven-membered rings in a fused fashion to install methylenecycloheptatriene (heptafulvene) segments, the development of corresponding benzo[1,2:4,5]di[7]annulene (BDA)-containing π-conjugated systems remains challenging due to the difficulty of their molecular design and synthesis. In this study, we develop an unprecedented chemical structure of cycloheptatriene-bis-annelated indenofluorene, which possesses both BDA and indenofluorene frameworks in a fused fashion. Physical measurements and X-ray analyses, along with theoretical calculations, indicated that antiaromaticity appeared in the BDA framework. By using the conjugated polycyclic hydrocarbon possessing both seven-membered and five-membered rings, this study provides fundamental insight into the strong antiaromatic nature of heptafulvene-based BDA framework.