Writing in the Journal of the American Chemical Society, a team of researchers from Spain has investigated the synthesis and properties of Aza-Triangulene. Their study opens the possibility of fabricating novel hydrocarbon structures including reactive open-shell molecules for cutting-edge applications in advanced materials science research.
Study: Aza-Triangulene: On-Surface Synthesis and Electronic and Magnetic Properties. Image Credit: Matej Kastelic/Shutterstock.com
What is Triangulene?
Theoretically devised in 1953, triangulene has received significant research attention. Triangulene is the smallest triplet-ground-state polybenzenoid. It is a biradical which is made of six benzene rings, which form a flat triangular molecule. This molecule has an even number of carbon atoms but pairing all its π-electrons is impossible. This means that forming a closed-shell structure cannot be achieved.
The unpaired electrons in this molecule lead to it possessing a high reactivity, meaning that its synthesis by conventional solution-phase chemical methods is problematic. Triangulene molecules have been described as carbon fragments by researchers, and the first successful synthesis of this material was reported in 2017. An unstable molecule, possible future uses of triangulene include applications in the field of microelectronics.
Synthesizing Triangulene
Recently, on-surface synthesis methods have been developed to synthesize triangulene. These methods are carried out under ultra-high vacuum conditions. Using these techniques, scientists can synthesize reactive carbon-based structures which contain π-radicals. A rationally designed precursor material is annealed under high-temperature conditions over a catalytic surface, forming the target product.
The structures of the products prepared using on-site synthesis methods have been precisely categorized using techniques such as noncontact atomic force spectroscopy and bond-resolving STM. Additionally, the observation of Kondo resonances, singly occupied/unoccupied molecular orbitals, and spin-flip excitations have confirmed the open-shell characteristics of both triangulene and its derivative products.
Heteroatom doping can alter the electronic and magnetic properties of carbon-based materials including triangulene. One example is the substitution of specific carbon atoms with nitrogen, which adds a π-electron, which results in different ground-state spin multiplicities than undoped triangulene.
The Study: Synthesizing Aza-Triangulene and Investigating its Properties
Despite theoretical predictions, synthesizing aza-triangulene has remained elusive. Writing in the Journal of the American Chemical Society, researchers have successfully synthesized this material and investigated its properties. Theoretical predictions for the material indicate that is energetically more favorable than conventional triangulene. The authors have reported the synthesis of nitrogen doped aza-triangulene using on-site synthesis on Au (111) and Ag(111) substrates.
Previous work by the authors has focused on a synthesis method which combines atomic hydrogen reaction followed by annealing. This work demonstrated that the method was efficient for removing oxygen atoms from ketone-substituted graphene nanoribbons on Au(111.) This inspired the authors to employ a similar method for a ketone-substituted aza-triangulene precursor material. The method resulted in complete or partial deoxygenation of the precursor material.
By-products, intermediates, and the final aza-triangulene 7 products were produced by subsequent tip-induced removal of hydrogen atoms from the prepared materials. The results of their study demonstrated a donation of an electron to the Au(111) substrate, resulting in a triplet ground state. Conversely, the Ag(111) surface donated an electron to the aza-triangulene, resulting in a closed-shell ground state.
The high work function of the Au(111) substrate surface and the highest occupied molecular orbital of hydrocarbon structure’s low binding energy explain the charge transfer, along with the effect of n-doping of graphitic nitrogen substitutes. Similar charge-transfer processes were observed in other nitrogen-doped molecules on the Au(111) substrate. In contrast, these behaviors were not observed in undoped triangulene, confirming the lower ionization of aza-triangulene compared to unsubstituted and extended triangulenes.
Electronic Properties
Alongside magnetic properties, electronic properties of aza-triangulene at the Fermi-level were investigated by the authors. A lack of Kondo resonance and spin excitation for aza-triangulene prepared on the Ag(111) substrate indicated that one electron was transferred from the substrate to aza-triangulene. A closed-shell ground state was suggested. Aza-triangulene becomes closed-shell upon the addition of an extra electron.
Conductance maps of the 7+ and 7- triangulenes differed substantially, whereas the calculated DOS of the lowest unoccupied orbital was almost identical. The authors have noted that the calculations were performed for free-standing molecules, but the experimental data were obtained on two different substrates. This suggested that both systems may be affected differently by the effect of molecule-substrate hybridization.
The authors have presented a simplistic chemical view of symmetries in relation to the molecule’s charge state. The prepared aza-triangulene adopts a conformation that minimizes the unpaired electrons at the expense of its resonance structure and symmetry. The material generates localized charges. In turn, a threefold symmetric conformation is adopted by molecules. This conformation causes charge delocalization, leading to either closed-shell or open-shell structures.
Further Reading
Wang, T et al. (2022) Aza-Triangulene: On-Surface Synthesis and Electronic and Magnetic Properties [online] Journal of the American Chemical Society | acs.pubs.org. Available at: https://pubs.acs.org/doi/10.1021/jacs.1c12618
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