  Synthesis of one-dimensional molecular arrays with tailored stereoisomers is
challenging yet has a great potential for application in molecular opto-,
electronic- and magnetic-devices, where the local array structure plays a
decisive role in the functional properties. Here, we demonstrate construction
and characterization of dehydroazulene isomer and diradical units in
three-dimensional organometallic compounds on Ag(111) with a combination of
low-temperature scanning tunneling microscopy and density functional theory
calculations. Tip-induced voltage pulses firstly result in the formation of a
diradical species via successive homolytic fission of two C-Br bonds in the
naphthyl groups, which are subsequently transformed into chiral dehydroazulene
moieties. The delicate balance of the reaction rates among the diradical and
two stereoisomers, arising from an in-line configuration of tip and molecular
unit, allows directional azulene-to-azulene and azulene-to-diradical local
probe structural isomerization in a controlled manner. Furthermore, our
theoretical calculations suggest that the diradical moiety hosts an open-shell
singlet with antiferromagnetic coupling between the unpaired electrons, which
can undergo an inelastic spin transition of 91 meV to the ferromagnetically
coupled triplet state.
