Poster presentation

Emergence of π-magnetism in fused aza-triangulenes: symmetry and charge transfer effects
Martina Corso^1,2, Jan Patrick Calupitan,^1,2 Tao Wang,^1,2 Alejandro Berdonces-Layunta,^1,2 Fernando Aguilar-Galindo,³ Manuel Vilas-Varela,⁴ Diego Peña,⁴ David Casanova,^2,5 Dimas G. de Oteyza^1,2,6

1 Centro de Física de Materiales, CSIC-UPV/EHU, 20018 San Sebastián, Spain
2 Donostia International Physics Center, 20018 San Sebastián, Spain
3 Departamento de Química, Universidad Autónoma de Madrid, Madrid, Spain
4 Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
5 Ikerbasque, Basque Foundation for Science, 48009 Bilbao, Spain
6 Nanomaterials and Nanotechnology Research Center (CINN), CSIC-UNIOVI-PA, 33940 El Entrego, Spain

On-surface synthesis paved the way towards the precise fabrication and characterization of conjugated carbon-based molecular materials that exhibit π-magnetism such as triangulenes. Aza-triangulene, a nitrogen-substituted derivative of triangulene, displays rich on-surface chemistry by its opposite charge transfer effects on Au(111) and Ag(111) substrates. Aza-triangulene donates an electron to Au(111) and exhibits an open-shell triplet ground state (S=1) with spin-density distribution mostly along the edge atoms, while it receives and electron from Ag(111) and displays a closed-shell character [1]. We further exploit the resulting differences in electronic structure on the said surfaces to produce fused dimer derivatives of aza-triangulene. We assign their chemical structures by bond-resolving scanning tunnelling microscopy and characterize their electronic properties by scanning tunnelling spectroscopy supported by density functional theory calculations. We show that the closed-shell structure of aza-triangulene on Ag(111) leads to closed-shell fused dimers coupled via the sterically more accessible carbon atoms. Instead, the open-shell structure of aza-triangulene on Au(111) couples via the atoms displaying highest spin density, resulting in symmetric or asymmetric dimer products. Interestingly, whereas all fused products on Au(111) display similar charge transfer properties, only the latter displayed magnetic fingerprints (in particular a Kondo resonance). We rationalize this behaviour in terms of the molecular bonding structure and π-conjugation. In contrast to the symmetric and highly conjugated dimer, asymmetric dimers display more localized orbitals, which result in a larger Coulomb repulsion and thus promote single electron occupancies with the associated spin density and π-magnetism. These results thus identify a clear relationship between molecular symmetry, charge transfer and spin states in π-conjugated carbon-based nanostructures.

[1] Tao Wang et al., JACS 144, 4522 (2022)