  Nitrogen heteroatom doping into a triangulene molecule allows tuning its
magnetic state. However, the synthesis of the nitrogen-doped triangulene
(aza-triangulene) has been challenging. Herein, we report the successful
synthesis of aza-triangulene on the Au(111) and Ag(111) surfaces, along with
their characterizations by scanning tunneling microscopy and spectroscopy in
combination with density functional theory (DFT) calculations. Aza-triangulenes
were obtained by reducing ketone-substituted precursors. Exposure to atomic
hydrogen followed by thermal annealing and, when necessary, manipulations with
the scanning probe, afforded the target product. We demonstrate that on Au(111)
aza-triangulene donates an electron to the substrate and exhibits an open-shell
triplet ground state. This is derived from the different Kondo resonances of
the final aza-triangulene product and a series of intermediates on Au(111).
Experimentally mapped molecular orbitals match perfectly with DFT calculated
counterparts for a positively charged aza-triangulene. In contrast,
aza-triangulene on Ag(111) receives an extra electron from the substrate and
displays a closed-shell character. Our study reveals the electronic properties
of aza-triangulene on different metal surfaces and offers an efficient approach
for the fabrication of new hydrocarbon structures, including reactive
open-shell molecules.

Materials Science