Tetrafluorenofulvalene: A Sterically Frustrated Open-Shell Alkene

Abstract

A typical π bond is weakened by oxidation and reduction, corresponding respectively to the removal of electrons from bonding orbitals and addition of electrons to antibonding orbitals, and by unpairing of the bonding electrons, e.g. in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules no longer hold. In particular, the bond in TFF becomes significantly stronger in the tri- and tetraanion, which can be generated by chemical reduction. While the triplet state of TFF does contain a weaker alkene bond than the singlet, in the quintet state the bond order increases significantly, yielding a flatter structure. The behavior of TFF originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin paring.