Synthetic Approaches to Conjugated Polycyclic Hydrocarbons Featuring the Antiaromatic Indeno[2,1-c]fluorene Scaffold

Fully conjugated indeno[2,1-c]fluorene, a fragment of fullerene-C60, is a structural isomer of the 20π indenofluorene family, which is viewed as the antiaromatic analogue of the aromatic acene(s). Compared to the linear indeno[1,2-b]fluorene isomer featuring a para-quinoidal s-indacene core, the helical [2,1-c]IF features a para-quinoidal as-indacene core. It received relatively less attention than [1,2-b]IF despite the high electron affinity, small HOMO-LUMO energy gap, diradical character, and red-shifted UV-vis absorption, which are potentially important properties for organic optoelectronics applications. The stable [2,1-c]IF scaffold was first reported in 2013; however, an unstable [2,1-c]IF-embedded polycyclic hydrocarbon was originally disclosed in 1998. Lately, [2,1-c]IF derivatives have received significant attention in physical organic and organic materials chemistry as novel synthetic approaches emerged. Herein, we outline these synthetic approaches to [2,1-c]IF and its π-extended derivatives featuring the C/U-shaped [2,1-c]IF unit, including heterocyclic counterparts, exhibiting tunable antiaromaticity, helicity, open-shell traits and semiconducting properties.