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On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes.

The design of 2D metal–organic frameworks (2D MOFs) takes advantage of the combination of the diverse electronic properties of simple organic ligands with different transition metal (TM) centers. The strong directional nature of the coordinative bonds is the basis for the structural stability and the periodic arrangement of the TM cores in these architectures. Here, direct and clear evidence that 2D MOFs exhibit intriguing energy‐dispersive electronic bands with a hybrid character and distinct magnetic properties in the metal cores, resulting from the interactions between the TM electronic levels and the organic ligand π‐molecular orbitals, is reported. Importantly, a method to effectively tune both the electronic structure of 2D MOFs and the magnetic properties of the metal cores by exploiting the electronic structure of distinct TMs is presented. Consequently, the ionization potential characteristic of selected TMs, particularly the relative energy position and symmetry of the 3d states, can be used to strategically engineer bands within specific metal–organic frameworks. These findings not only provide a rationale for band structure engineering in 2D MOFs but also offer promising opportunities for advanced material design. [ABSTRACT FROM AUTHOR]

Porphyrins are promising multifunctional units particularly interesting for the realization of molecular nanodevices. Their structural variety allows to create precursors suitable for the on‐surface polymerization of porphyrin blocks. The corresponding increased stability and improved transport properties of the formed polymerized molecular nanostructures make them practically worthwhile. For the case of 2D porphyrin materials, the effect of polymerization on the magnetic properties of transition metal ions has not been reported yet. Therefore, details on the properties of an extended covalent nickel tetraphenylporphyrin network formed via Ullmann coupling on the Cu(111) surface are reported. By using photoelectron and absorption spectroscopies together with density functional theory calculations, it is systematically evolving how the functional properties of the Ni centers are changed within a polymerized molecular structure in comparison to single‐molecule nickel tetraphenylporphyrin derivatives that build the 2D molecular network. A model that explains the differences in the electronic and magnetic properties observed for the Ni centers in both structures based on the additional rigidity characteristic of the molecular layer after polymerization is drawn. [ABSTRACT FROM AUTHOR]

Topological insulators (TIs) are semiconductors with protected electronic surface states that allow dissipation‐free transport. TIs are envisioned as ideal materials for spintronics and quantum computing. In Bi14Rh3I9, the first weak 3D TI, topology presumably arises from stacking of the intermetallic [(Bi4Rh)3I]2+ layers, which are predicted to be 2D TIs and to possess protected edge‐states, separated by topologically trivial [Bi2I8]2− octahedra chains. In the new layered salt Bi12Rh3Cu2I5, the same intermetallic layers are separated by planar, i.e., only one atom thick, [Cu2I4]2− anions. Density functional theory (DFT)‐based calculations show that the compound is a weak 3D TI, characterized by Z2=(0;0001), and that the topological gap is generated by strong spin–orbit coupling (Eg,calc. ∼ 10 meV). According to a bonding analysis, the copper cations prevent strong coupling between the TI layers. The calculated surface spectral function for a finite‐slab geometry shows distinct characteristics for the two terminations of the main crystal faces ⟨001⟩, viz., [(Bi4Rh)3I]2+ and [Cu2I4]2−. Photoelectron spectroscopy data confirm the calculated band structure. In situ four‐point probe measurements indicate a highly anisotropic bulk semiconductor (Eg,exp. = 28 meV) with path‐independent metallic conductivity restricted to the surface as well as temperature‐independent conductivity below 60 K. [ABSTRACT FROM AUTHOR]

[Display omitted] • The strong molecular pinning stabilizes the macrocycle of cobalt-octaethylporphyrin and cobalt-tetraphenylporphyrin molecules in a planar conformation. • The charge transfer from the metal substrate induces a reduction of the chelated cobalt ion. • The functional cobalt ion cores in porphyrin molecules are redox-active when interacting with nitrogen dioxide, which is potentially interesting for sensorics applications. Porphyrins may easily meet the diverse requirements for their employment in a variety of molecular-based nanodevices, extending from gas sensing to photochemistry, due to their remarkable chemical and structural flexibility. In this context, self-assembled layers of cobalt β -octaethylporphyrins and meso -tetraphenylporphyrins on inert metal substrates exhibit distinct periphery-driven conformations, with the macrocycle adsorbed in either a planar or saddle-shaped configuration (PC and SSC, respectively). Here, by means of x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine spectroscopy (NEXAFS) supported by density functional theory (DFT) calculations, we demonstrate that the adsorbate behaviour on the chemically more reactive copper surface is diametrically different, as the strong molecular pinning stabilizes the macrocycle of the differently substituted molecules in a planar conformation. At the same time, the peripheral substitution in the cobalt porphyrins shows a minor effect in the charge donation to the unoccupied macrocycle molecular orbital, but it has a stronger impact on the stabilization of the Co 3d z 2 -based orbital. Moreover, the reduction of the Co(II) species, occupying the molecular core, is also ascribed to the molecular pinning. All these features are crucial for the metal-porphyrin array exploitation in nanotechnology applications, as their reactivity may be retained while the optoelectronic properties are fully tunable. Indeed, when the organic layer is exposed to hazardous nitrogen dioxide (NO 2), the low-valence cobalt metallic center selectively interacts with even small amounts of gas. This coordination induces the macrocycle's ring distortion in tetraphenylporphyrin, while the octaethylporphyrin maintains a flat conformational geometry. [ABSTRACT FROM AUTHOR]