  Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received
great interest owing to their unusual electronic structure and potential
applications in non-linear optics and photovoltaics. Here, we report the
generation of unsubstituted indeno[1,2-a]fluorene, the final and yet unreported
parent indenofluorene isomer, on various surfaces by cleavage of two C-H bonds
in 7,12-dihydroindeno[1,2-a]fluorene through voltage pulses applied by the tip
of a combined scanning tunneling microscope and atomic force microscope. On
bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state,
while it exhibits charge bistability between neutral and anionic states on the
lower work function surfaces of bilayer NaCl on Ag(111) and Cu(111). In the
neutral state, indeno[1,2-a]fluorene exhibits either of two ground states: an
open-shell pi-diradical state, predicted to be a triplet by density functional
and multireference many-body perturbation theory calculations, or a
closed-shell state with a para-quinodimethane moiety in the as-indacene core.
Switching between open- and closed-shell states of a single molecule is
observed by changing its adsorption site on NaCl.

Mesoscale and Nanoscale Physics