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

Nakano reported that the antiaromatic indenofluorene (IF) isomers are diradicaloid molecules having varying degrees of open-shell character, with indeno[1,2-]fluorene displaying a weaker diradical character index ( = 0.072). Unlike 6,12-trimethylsilylethynyl disubstituted [1,2-]IF, the 6,12-aryl disubstituted [1,2-]IF derivatives did not show any experimental evidence of diradical properties. This raised the question of whether a [1,2-]IF dimer would prefer a closed-shell or an open-shell ground state. To address this, herein we report the synthesis of a 6,6'-biindeno[1,2-]fluorene derivative, which is a [1,2-]IF dimer, constructed by linking two [1,2-]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 -indacene antiaromaticity restored two Clar sextets in two proaromatic -quinodimethane subunits, resulting in an open-shell bifluorenylidene-type diradicaloid ( = 0.268) ground state with minor tetraradical character index ( = 0.007). The open-shell nature was confirmed by single crystal X-ray and electron paramagnetic resonance analyses, and supported by theoretical calculations.