Polycyclic aromatic hydrocarbons (PAHs) are a family of organic compounds comprising two or more fused aromatic rings which feature manifold applications in modern technology. Among these species, those presenting an open-shell magnetic ground state are of particular interest for organic electronic, spintronic, and non-linear optics and energy storage devices. Within PAHs, special attention has been devoted in recent years to the synthesis and study of the acene and fused acene (periacene) families, steered by their decreasing HOMO-LUMO gap with length and predicted open-shell character above some size. However, an experimental fingerprint of such magnetic ground state has remained elusive. Here, we report on the in-depth electronic characterization of isolated peripentacene molecules on a Au(111) surface. Scanning tunnelling spectroscopy, complemented by computational investigations, reveals an antiferromagnetic singlet ground state, characterized by singlet-triplet inelastic excitations with an experimental effective exchange coupling (Jeff) of 40.5 meV. Our results deepen the fundamental understanding of organic compounds with magnetic ground states, featuring perspectives in carbon-based spintronic devices.

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• časopisecké články MeSH

The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open-shell character has so far been missing. Here, we present the on-surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many-body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet-triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.

• Klíčová slova
• Diradicals, Higher Acenes, Inelastic Excitation, Many-Body Calculations, Open-Shell Compounds,
• Publikační typ
• časopisecké články MeSH

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. peri-heptacene (n=7, 7-PA), obtained on an Au(111) substrate under ultra-high vacuum conditions. Our high-resolution 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 (Jeff ) of 49 meV.

• Klíčová slova
• Carbon Magnetism, Open-Shell Character, Periacenes, Scanning Tunneling Microscopy, Surface Chemistry,
• Publikační typ
• časopisecké články MeSH

The synthesis of nanographenes (NGs) with open-shell ground states have recently attained increasing attention in view of their interesting physicochemical properties and great prospects in manifold applications as suitable materials within the rising field of carbon-based magnetism. A potential route to induce magnetism in NGs is the introduction of structural defects, for instance non-benzenoid rings, in their honeycomb lattice. Here, we report the on-surface synthesis of three open-shell non-benzenoid NGs (A1, A2 and A3) on the Au(111) surface. A1 and A2 contain two five- and one seven-membered rings within their benzenoid backbone, while A3 incorporates one five-membered ring. Their structures and electronic properties have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy and scanning tunneling spectroscopy complemented with theoretical calculations. Our results provide access to open-shell NGs with a combination of non-benzenoid topologies previously precluded by conventional synthetic procedures.

• Klíčová slova
• STM, nanographenes, nanomagnetism, nc-AFM, on-surface synthesis, open-shell character, polycyclic aromatic hydrocarbons,
• Publikační typ
• časopisecké články MeSH

Electron and nuclear magnetic resonance spectroscopies are indispensable and powerful methods for investigating the molecular and electronic structures of open-shell systems. We demonstrate that the NMR and EPR parameters are extremely sensitive quantitative probes for the electronic spin density around heavy-metal atoms and the metal-ligand bonding. Using relativistic density-functional theory, we have analyzed the relation between the spin density and the EPR and NMR parameters in paramagnetic iridium(II/IV) complexes with a PNP pincer ligand. As the magnetic-response parameters for compounds containing 5d transition metal(s) are heavily affected by spin-orbit coupling, relativistic effects must be included in the calculations. We have used a recent implementation of the fully relativistic Dirac-Kohn-Sham (DKS) method employing the hybrid PBE0 functional and an implicit solvent model to calculate EPR parameters and hyperfine NMR shifts. The modulation of the metal-ligand bond by the trans substituent (-Cl or ≡N) and the electronic spin structure around the central metal atom and ligands are shown to be reflected in the "long-range" through-bond Fermi-contact (FC) contributions to the ligand 13C and 1H hyperfine couplings. Interestingly, the hyperfine coupling constant of the ligand atom L ( AL) bonded directly to the iridium center changes its sign because of the dominating role of the paramagnetic spin-orbit (PSO) term. Furthermore, the electronic g-shift and the PSO contribution to the ligand AL are shown to invert their signs when nitrogen is substituted for chlorine, reflecting the different formal metal oxidation states and the change in metal-ligand bond character. A full understanding of the substituent effects is provided by using chemical bond concepts in combination with a molecular-orbital (MO) theory analysis of the second-order perturbation theory expression for the EPR parameters. Our findings are easily transferable to other systems containing d-block elements and beyond. Relativistic DFT calculations of magnetic-resonance parameters are expected to frequently assist in future experimental observations and the characterization of hitherto unknown unstable or exotic species.

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• časopisecké články MeSH

Triangulene and its homologues are promising building blocks for high-spin low-dimensional networks with long-range magnetic order. Despite the recent progress in the synthesis and characterization of coupled triangulenes, key parameters such as the number of organic linking units or their dihedral angles remain scarce, making further studies crucial for an essential understanding of their implications. Here, we investigate the synthesis and reactivity of two triangulene dimers linked by two (Dimer 1) or one (Dimer 2) para-biphenyl units, respectively, on a metal surface in an ultra-high vacuum environment. First-principles calculations and model Hamiltonians reveal how spin excitation and radical character depend on the rotation of the para-biphenyl units. Comprehensive scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations confirm the successful formation of Dimer 1 on Au(111). Non-contact atomic force microscopy (nc-AFM) measurements resolve the twisted conformation of the linking para-biphenyl units for Dimer 1. On the contrary, the inherent flexibility of Dimer 2 induces the planarization of the para-biphenyl, resulting in the spontaneous formation of two additional five-membered rings per dimer connected by a single C-C bond (Dimers 2'). Furthermore, scanning tunneling spectroscopy (STS) measurements confirm the antiferromagnetic (S=0) coupling of the observed dimers, underscoring the critical influence of dihedral angles and structural flexibility of the linking units in π-electron magnetic nanostructures.

• Klíčová slova
• open-shell character, scanning tunneling microscopy, surface chemistry, triangulenes, π-electron magnetism,
• Publikační typ
• časopisecké články MeSH

Phenalenyl is known for its highly delocalized radical structure, making it a fundamental building block in the construction of polyradical compounds. This study explores how different connection topologies between phenalenyl units via acetylenic bridges modulate the polyradical character, as well as the electronic and magnetic properties of the resulting systems. The connection type depends on the atom occupation pattern of the phenalenyl singly occupied orbital (SOMO). Three types of connections are defined that induce different π conjugation strength. Linear di- and tetra-phenalenyl chains and cyclic tri- and tetra-phenalenyl aggregates have been investigated. High-level multireference averaged coupled cluster (MR-AQCC) calculations were performed to describe the electronic structures of these compounds. The polyradical character of the oligomers is assessed using descriptors such as singlet-triplet splitting, effectively unpaired electrons (NU). Additionally, the harmonic oscillator model of aromaticity (HOMA), multicenter index (MCI), fluctuation index (FLU), nucleus-independent chemical shifts (NICS (1)), and the anisotropy of the current-induced density (ACID) analysis are employed to characterize the influence of the phenalenyl linkages on aromaticity. Results indicate that bridges enabling stronger interaction between the SOMOs of phenalenyl units lead to a reduction in polyradical character. Aromaticity analysis corroborates these findings, revealing decreased aromaticity in rings where electron interaction occurs through the bridge. On the contrary, choosing bridging types of weak interaction leads to strong open shell character providing candidates for molecular magnetism. A comparison with the predictions of Ovchinnikov's rule is carried out both to rationalize the outcomes of the quantum chemical calculations and to highlight limitations of the rule, particularly in the treatment of quasi-degenerate states.

• Publikační typ
• časopisecké články MeSH

Relativistic effects significantly affect various spectroscopic properties of compounds containing heavy elements. Particularly in Nuclear Magnetic Resonance (NMR) spectroscopy, the heavy atoms strongly influence the NMR shielding constants of neighboring light atoms. In this account we analyze paramagnetic contributions to NMR shielding constants and their modulation by relativistic spin-orbit effects in a series of transition-metal complexes of Pt(II), Au(I), Au(III), and Hg(II). We show how the paramagnetic NMR shielding and spin-orbit effects relate to the character of the metal-ligand (M-L) bond. A correlation between the (back)-donation character of the M-L bond in d10 Au(I) complexes and the propagation of the spin-orbit (SO) effects from M to L through the M-L bond influencing the ligand NMR shielding via the Fermi-contact mechanism is found and rationalized by using third-order perturbation theory. The SO effects on the ligand NMR shielding are demonstrated to be driven by both the electronic structure of M and the nature of the trans ligand, sharing the σ-bonding metal orbital with the NMR spectator atom L. The deshielding paramagnetic contribution is linked to the σ-type M-L bonding orbitals, which are notably affected by the trans ligand. The SO deshielding role of σ-type orbitals is enhanced in d10 Hg(II) complexes with the Hg 6p atomic orbital involved in the M-L bonding. In contrast, in d8 Pt(II) complexes, occupied π-type orbitals play a dominant role in the SO-altered magnetic couplings due to the accessibility of vacant antibonding σ-type MOs in formally open 5d-shell (d8). This results in a significant SO shielding at the light atom. The energy- and composition-modulation of σ- vs π-type orbitals by spin-orbit coupling is rationalized and supported by visualizing the SO-induced changes in the electron density around the metal and light atoms (spin-orbit electron deformation density, SO-EDD).

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• časopisecké články MeSH

Open-shell nanographenes (NGs), also known as molecular π-magnets, have recently garnered attention for their potential in spintronics and quantum computing. Tailoring of such NGs at the atomic level allows the control of their magnetic interactions. We report here the on-surface synthesis of a dibenzo-fused rhomboidal NG with predominant zigzag edges featuring an open-shell (antiferromagnetic) character and a high value of magnetic exchange coupling (MEC) on Au(111) surfaces. Scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) confirm its chemical structure. Scanning tunneling spectroscopy (STS) measurements, complemented by state-of-the-art theoretical calculations, reveal the open-shell character of the NG, observed as singlet-triplet inelastic excitations. Furthermore, molecular chains consisting of these NGs were fabricated with tunable periodicities through the functionalization of the precursor, showing the absence of MEC between adjacent units, which provides deeper insights into the behavior of open-shell systems and preservation of individual magnetic entities within π-conjugated structures.

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• časopisecké články MeSH

Open-shell nanographenes exhibit unconventional π-magnetism arising from topological frustration or strong electron-electron interaction. However, conventional design approaches are typically limited to a single magnetic origin, which can restrict the number of correlated spins or the type of magnetic ordering in open-shell nanographenes. Here we present a design strategy that combines topological frustration and electron-electron interactions to fabricate a large fully fused 'butterfly'-shaped tetraradical nanographene on Au(111). We employ bond-resolved scanning tunnelling microscopy and spin-excitation spectroscopy to resolve the molecular backbone and reveal the strongly correlated open-shell character, respectively. This nanographene contains four unpaired electrons with both ferromagnetic and anti-ferromagnetic interactions, harbouring a many-body singlet ground state and strong multi-spin entanglement, which is well described by many-body calculations. Furthermore, we study the magnetic properties and spin states in the nanographene using a nickelocene magnetic probe. The ability to imprint and characterize many-body strongly correlated spins in polyradical nanographenes paves the way for future advancements in quantum information technologies.

• Publikační typ
• časopisecké články MeSH