The COLUMBUS program system provides the tools for performing high-level multireference (MR) computations, including the multireference configuration interaction (MRCI) method and its multireference averaged quadratic coupled cluster (MR-AQCC) extension, allowing computations on a wide range of fascinating atomic and molecular systems, including the treatment of open-shells and complicated excited state phenomena. The inclusion of spin-orbit coupling (SOC) directly within the MRCI step enables the description of systems containing heavy elements, such as lanthanides and actinides, whose properties are strongly influenced by SOC. Analytic energy gradients and nonadiabatic couplings at the correlated MRCI level provide the foundation for a variety of dynamics studies, giving insight into ultrafast photochemistry. New and ongoing method developments in COLUMBUS include the computation of spin densities, improved descriptions of ionic states, enhancements to the AQCC method, and the porting of COLUMBUS to graphical processing units (GPUs). New external interfaces enable an enhanced description of electronic resonances and molecules in strong laser fields. This work highlights these new developments while providing a detailed account of the diverse applications of COLUMBUS in recent years.

• Publikační typ
• časopisecké články MeSH

Triangulenes are prototypical examples of open-shell nanographenes. Their magnetic properties, arising from the presence of unpaired π electrons, can be extensively tuned by modifying their size and shape or by introducing heteroatoms. Different triangulene derivatives have been designed and synthesized in recent years thanks to the development of on-surface synthesis strategies. Triangulene-based nanostructures with polyradical character, hosting several interacting spin units, can be challenging to fabricate but are particularly interesting for potential applications in carbon-based spintronics. Here, we combine pristine and N-doped triangulenes into a more complex nanographene, TTAT, predicted to possess three unpaired π electrons delocalized along the zigzag periphery. We generate the molecule on a Au(111) surface and detect direct fingerprints of multiradical coupling and high-spin state using scanning tunneling microscopy and spectroscopy. With the support of theoretical calculations, we show that its three radical units are localized at distinct parts of the molecule and couple via symmetric ferromagnetic interactions, which result in a S = 3/2 ground state, thus demonstrating the realization of a molecular ferromagnetic Heisenberg spin trimer.

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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

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.

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• 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

Surface-assisted synthesis has become a powerful approach for generation of molecular nanostructures, which could not be obtained via traditional solution chemistry. Nowadays there is an intensive search for reactions that could proceed on flat surfaces in order to boost the versatility and applicability of synthesized nano-objects. Here we propose application of atomic hydrogen combined with on-surface synthesis in order to tune the reaction pathways. We demonstrate that atomic hydrogen could be widely applied: (1) as a cleaning tool, which allows removal of halogen residues from the surface after Ullmann couplings/polymerization, (2) by reaction with surface organometallics to provide stable hydrogenated species, and (3) as a reagent for debromination or desulfurization of adsorbed species.

• Klíčová slova
• atomic hydrogen, graphene nanoribbon, hydrogenation, molecular polymers, on-surface synthesis, organometallic state,
• Publikační typ
• časopisecké články MeSH