Triangulenes are triangular-shaped polybenzenoid hydrocarbons, which can be considered graphene flakes. Their edges are formed by n zig-zag units to give an [n]-triangulene. The compounds have an open-shell character and are difficult to prepare in solution, but they can be fabricated using on-surface synthesis. The total spin of triangulenes increases with their size, but direct proof of this increasing total spin can be difficult to obtain due to the very weak magnetic anisotropy of carbon systems.

Diego Peña, Universidade de Santiago de Compostela, Spain, José Ignacio Pascual, CIC nanoGUNE-BRTA, Donostia-San Sebastián, Spain, and Ikerbasque, Basque Foundation for Science, Bilbao, Spain, and colleagues have used on-surface synthesis to prepare a large nitrogen-doped triangulene, aza-[5]-triangulene (A5T, cationic form pictured above on the right) on an Au(111) surface and investigated its spin state.

The team first prepared a precursor that is substituted with three dimethylphenyl groups (pictured above on the left) in solution. This precursor was then deposited onto an Au(111) surface and planarized at high temperatures via cyclodehydrogenation and dehydrogenation reactions. The team heated the covered gold substrate to 330 ºC to obtain A5T. The product was visualized using low-temperature scanning tunneling microscopy (STM). The team obtained a bond-resolved image that showed the honeycomb lattice of the A5T and confirmed the successful synthesis.

To investigate the spin state of the product, the team used tunneling spectroscopy at 1.2 K and measured a low-bias differential conductance signal over various parts of the molecule. Due to the Kondo effect, in which magnetic impurities affect the conductance of a metal, these measurements allowed them to draw some conclusions about the spin state. The researchers combined these results with quantum-chemical calculations and found that the aza-triangulene has a quintet ground state (S = 2) instead of the quartet state (S = 3/2) that would be expected for the neutral species. They concluded that the aza-triangulene exist in a cationic state (A5T⁺) on the gold surface, formed by electron transfer to the metal substrate.

• On‐Surface Synthesis and Characterization of a High‐Spin Aza‐[5]‐Triangulene,
  Manuel Vilas-Varela, Francisco Romero-Lara, Alessio Vegliante, Jan Patrick Calupitan, Adrian Martinez, Lorenz Meyer, Unai Uriarte-Amiano, Niklas Friedrich, Dongfei Wang, Natalia Koval, Maria Sandoval-Salinas, David Casanova, Martina Corso, Emilio Artacho, Diego Peña, Jose Ignacio Pascual,
  Angew. Chem. Int. Ed. 2023.
  https://doi.org/10.1002/anie.202307884