  Phenalenyl is a radical nanographene with triangular shape that hosts an
unpaired electron with spin S = 1/2. The open-shell nature of phenalenyl is
expected to be retained in covalently bonded networks. Here, we study a first
step in that direction and report the synthesis of the phenalenyl dimer by
combining in-solution synthesis and on-surface activation and its
characterization both on Au(111) and on a monolayer of NaCl on top of Au(111)
by means of inelastic electron tunneling spectroscopy (IETS). IETS shows
inelastic steps that, together with a thorough theoretical analysis, are
identified as the singlet-triplet excitation arising from interphenalenyl
exchange. Two prominent features of our data permit to shed light on the nature
of spin interactions in this system. First, the excitation energies with and
without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating a
significant renormalization of the spin excitation energies due to exchange
with the Au(111) electrons. Second, a position-dependent bias-asymmetry of the
height of the inelastic steps is accounted for by an interphenalenyl
hybridization of the singly occupied phenalenyl orbitals that is only possible
via third neighbor hopping. This hybridization is also essential to activate
kinetic interphenalenyl exchange. Our results set the stage for future work on
the bottom-up synthesis of spin S = 1/2 spin lattices with large exchange
interaction.
