  Fusion of three benzene rings in a triangular fashion gives rise to the
smallest open-shell graphene fragment, the phenalenyl radical, whose
${\pi}$-extension leads to an entire family of non-Kekul\'e triangular
nanographenes with high-spin ground states. Here, we report the first synthesis
of unsubstituted phenalenyl on a Au(111) surface, which is achieved by
combining in-solution synthesis of the hydro-precursor and on-surface
activation by atomic manipulation, using the tip of a scanning tunneling
microscope (STM). Single-molecule structural and electronic characterization
confirm its open-shell S = 1/2 ground state that gives rise to Kondo screening
on the Au(111) surface. In addition, we compare the phenalenyl's electronic
properties with those of triangulene, the second homolog in the series, whose S
= 1 ground state induces an underscreened Kondo effect. Our results set a new
lower size-limit in the on-surface synthesis of magnetic nanographenes that can
serve as building blocks for the realization of new exotic quantum phases of
matter.
