On‐Surface Synthesis and Characterization of a High‐Spin Aza‐[5]‐Triangulene

Abstract

Triangulenes are a class of open‐shell triangular graphene flakes with total spin increasing with their size. In the last years, on‐surface‐synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in‐solution and on‐surface synthesis of a large nitrogen‐doped triangulene (aza‐[5]‐triangulene) on a Au(111) surface, and the detection of its high‐spin ground state. Bond‐resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero‐bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high‐spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S=2), instead of the quartet state (S=3/2) expected for the neutral species. This confirms that electron transfer to the metal substrate raises the spin of the ground state. We further provide a qualitative description of the change of (anti)aromaticity introduced by N‐substitution, and its role in the charge stabilization on a surface, resulting in an S=2 aza‐triangulene on Au(111).

Predicting Organometallic Intermediates in the Surface-Assisted Ullmann Coupling of Chrysene Isomers

“…For example, hexagonal rings created by ch26 have been observed previously in the case of 4,4 ′′ -dibromo-m-terphenyl on a Cu(111) surface [9] as well as with other 120 • -bent halogenated polyphenyl tectons used for the surface-assisted synthesis of honeycombene and kekulene macrocycles [34]. This directional-bonding approach has also been applied to the fabrication of more complex hexagonal cyclic structures, including cycloarene [35] and triangulene-based nanostars [36]. In the case of trimolecular aggregates of ch36, the 60 • bonding pattern has been found to be effective in the Ullmann coupling of 3,12-dibromo-7,8-diaza [5]helicene on an Ag(111) surface, resulting in the production of nitrogen-doped nanographene with [18] annulene pores (triangular trimers) [37].…”

Reverse intersystem crossing mechanisms in doped triangulenes

“…Geometry optimization of the 16 studied molecules has been performed with the M06-2X exchange–correlation functional 38 and the 6-31G(d) basis set, as it has shown good performance in the study of planar conjugated compounds. 39–43 Transition energies to the low-lying excited singlet and triplet states have been computed at the time-dependent DFT (TDDFT) level with the Tamm–Dancoff approximation (TDA) 44 in combination with three exchange–correlation functionals: M06-2X, 38 B3LYP, 45–47 and CAM-B3LYP. 48 Excitation energies have been also computed at the spin–flip TDDFT (SF-TDDFT) 49 level (within the TDA) from the Kohn–Sham triplet state configuration with the PBE50 functional, 50 as previously recommended.…”

Atomic-Limit Mott Insulator in [4]Triangulene Frameworks

“…As one fascinating class of these nanographenes, [ n ]­triangulene is an open-shelled triangular graphene molecule with a zigzag edge of n -unit length. , Since all edge atoms of [ n ]­triangulene belong to the same graphene sublattice, based on the Lieb theorem, its three edges prefer a ferromagnetic coupling , and hold a spin moment linearly increased with the edge length . Nowadays, atomically precise [ n ]­triangulenes with different sizes ( n = 2, 3, 4, 5, 7) have been synthesized through an on-surface growth under an ultrahigh vacuum. − The distinctive magnetic excitations of [ n ]­triangulene in different configurations (monomer, dimer, trimer, ring, and chain) are systematically investigated through scanning probe microscopy measurements. − However, it is still a hard task to synthesize the two-dimensional (2D) [ n ]­triangulene, because of the highly reactive nature of radicals. Recently, Fischer et al reported progress in this field and successfully synthesized 2D [4]­triangulene frameworks ([4]-TGFs), which were also proposed to be excitonic insulators, exhibiting a bottom-up approach to explore the 2D correlated phases in pure carbon materials.…”