Recent progress in on-surface chemistry has enabled the synthesis of novel polyradical molecules with interesting electronic structure, which are hardly available in solution chemistry. Moreover, the possibility to characterize their electronic structure with scanning tunneling spectroscopy (STS) with the unprecedented spatial resolution opens new possibilities to understand their nontrivial electronic structure. However, experimental STS maps of molecules on surfaces are interpreted using one-electron STM theory within the framework of one-electron molecular orbitals nowadays. Although this standard practice often gives relatively good agreement with experimental data for closed-shell molecules, it fails to address multireference polyradical molecules. In this manuscript, we provide multireference STM theory including out-of-equilibrium processes of removing/adding an electron within the formalism of many-electron wave functions for the neutral and charged states. This can be accomplished by the concept of so-called Dyson orbitals. We will discuss the examples where the concept of Dyson orbitals is mandatory to reproduce experimental STS maps of polyradical molecules. Finally, we critically review the possibility of the experimental verification of the so-called SOMO/HOMO inversion effect using STS maps in polyradical molecules. Namely, we will demonstrate that experimental STS measurements cannot provide any information in case of strongly correlated molecules about the ordering of one-electron molecular orbitals and, therefore neither about the SOMO/HOMO inversion effect.

• Publication type
• Journal Article 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.

• Keywords
• open-shell character, scanning tunneling microscopy, surface chemistry, triangulenes, π-electron magnetism,
• Publication type
• Journal Article MeSH