Synthesis and Characterization of peri‐Heptacene on a Metallic Surface

Abstract

The synthesis of long n-peri-acenes (n-PAs) is challenging as a result of their inherent open-shell radical character, which arises from the presence of parallel zigzag edges beyond a certain n value. They are considered as π-electron model systems to study magnetism in graphene nanostructures; being potential candidates in the fabrication of optoelectronic and spintronic devices. Here, we report the on-surface formation of the largest pristine member of the n-PA family, i.e. periheptacene (n = 7, 7-PA), obtained on an Au(111) substrate under ultra-high vacuum conditions. Our highresolution scanning tunneling microscopy investigations, complemented by theoretical simulations, provide insight into the chemical structure of this previously elusive compound. In addition, scanning tunneling spectroscopy reveals the antiferromagnetic open-shell singlet ground state of 7-PA, exhibiting singlet-triplet spin-flip inelastic excitations with an effective exchange coupling (J eff ) of 49 meV.

Steering Large Magnetic Exchange Coupling in Nanographenes near the Closed-Shell to Open-Shell Transition

“…12 The energetic positions of the steps are found to be at ±116, ±183, and ±190 meV for A [3,5], B [4,5] S2 in the Supporting Information), we ascribe the dI/dV steps to singlet−triplet spin excitations in the NGs. 12,17,20,30,31 We also obtained dI/dV maps at the spin excitation thresholds (Figure 3e), which are in line with the calculated spin polarization plots of the respective NGs (see the Supporting Information). Additionally, we performed dI/dV spectroscopy on A [3,5], B [4,5], and C [4,3] in larger bias windows to detect orbital resonances.…”

“…17 A distinct approach to express magnetism in NGs is to take advantage of the role of electronic correlations above a critical system size, which makes feasible that Coulomb repulsion overcomes the hybridization energy of the frontier states, giving rise to a net spin polarization. 4,20,30,31 Herein, the edge shape of the NG is very relevant, being zigzag peripheries successful for achieving spin polarizations. A recent study exemplifying this concept reports on the on-surface synthesis of rhombus-shaped NGs with zigzag peripheries, containing four and five benzenoid rings along each edge (termed [4]and [5]-rhombenes, respectively).…”

Emergence of π-Magnetism in Fused Aza-Triangulenes: Symmetry and Charge Transfer Effects

“…Mechanistic origins of magnetism in materials can be viewed as breaks in symmetry that allow for unpaired electrons. Though long-time associated with d- and f-shell electrons of metals, unpaired electrons can also emerge in p-shell electrons of carbon-based materials, conferring magnetic properties to π-conjugated structures , due to (1) sublattice imbalance , in π electrons of the graphene hexagonal lattice that introduce a nonzero net spin, (2) topological frustration , of π electrons, preventing double occupation of the same π-bond, and (3) spin polarization − of low-energy states driven by Coulomb repulsion or topological phase transitions. Engineering π-magnetism of carbon nanomaterials therefore requires precise control of the most trivial changes in the chemical structure.…”

“…Engineering π-magnetism of carbon nanomaterials therefore requires precise control of the most trivial changes in the chemical structure. On-surface synthesis, simultaneously affording atomic precision, inert environment, and substrate stabilization, , has proven effective, allowing access to a myriad of π-magnetic molecular materials , such as triangulenes, ,− graphene nanoribbons, , and other graphene nanofragments. ,− ,, …”

“…On-surface synthesis, simultaneously affording atomic precision, inert environment, and substrate stabilization, 12,13 has proven effective, allowing access to a myriad of π-magnetic molecular materials 1,2 such as triangulenes, 4,14−16 graphene nanoribbons, 7,8 and other graphene nanofragments. 5,[9][10][11]17,18 Aside from their intrinsic magnetic properties, charge transfer between carbon nanostructures and a metal surface can also generate (or modify) π-magnetism. 19,20 Adding/ removing electron(s) to/from a molecule may modify the spin balance.…”