6,6'-Biindeno[1,2-b]fluorene: an open-shell indenofluorene dimer

Nakano et al. reported that the antiaromatic indenofluorene (IF) isomers are diradicaloid molecules having varying degrees of open-shell character, with indeno[1,2-b]fluorene displaying a weaker diradical character index (y₀ = 0.072). Unlike 6,12-trimethylsilylethynyl disubstituted [1,2-b]IF, the 6,12-aryl disubstituted [1,2-b]IF derivatives did not show any experimental evidence of diradical properties. This raised the question of whether an [1,2-b]IF dimer would prefer a closed-shell or an open-shell ground state. To address it, herein we report the synthesis of a 6,6'-biindeno[1,2-b]fluorene derivative, which is an [1,2-b]IF dimer, constructed by linking two [1,2-b]IF units with a C-C single bond at carbons 6 and 6' bearing the largest orbital coefficients for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). The C6-C6' linkage effectively narrowed the HOMO-LUMO gap while the strong desire to avoid s-indacene antiaromaticity restored two Clar sextets in two proaromatic para-quinodimethane subunits, resulting in an open-shell bifluorenylidene-type diradicaloid (y0 = 0.268) ground state with minor tetraradical character index (y₁ = 0.007). The open-shell nature was confirmed by single crystal X-ray and electron paramagnetic resonance analyses, and supported by theoretical calculations.