Crown-ether (CR) ring molecules are known as host molecules for capturing guest species inside the ring. So far CR molecular films have only been grown by drop-casting a CR solution on an inert substrate in air which offers little control over the molecular structure. Here we report the successful growth of a well-ordered CR molecular array on an atomically flat and clean noble metal Cu(111) substrate at 300 K in ultra-high vacuum (UHV), using 4,4′,5,5′-tetrabromodibenzo[18] crown-6 ether (Br-CR). The adsorption, self-assembly, and electronic structures of Br-CR were studied by means of UHV low-temperature scanning tunneling microscopy and spectroscopy, low electron energy diffraction, and angle-resolved ultraviolet photoemission spectroscopy. We found that (1) the Br-CR ring, which is bent both in the crystal and gas phase, flattens upon adsorption on Cu(111). Density functional theory reveals that the two benzene groups of the molecule lie flat on the surface such as to maximize the substrate-molecule interaction. (2) The moderate molecule−substrate interaction allows thermal diffusion of the Br-CR molecules, resulting in the formation of self-assembled monolayer islands with 7 × 4 superstructure. (3) While the deposition of 0.05 ML Br-CR forms multidomain islands with disordered defects, a drastic improvement occurred at 0.25 ML, where only atomically flat single-domain islands were grown. This Br-CR flat ring cavity array could become a template for designing novel two-dimensional arrays of desired guest atoms, ions, or functionalized molecules.
On-surface metalation of metal-free π-conjugated planar molecules, such as metal-free tetraphenylporphyrin (2HTPP), using 3d transition metals prepared in ultrahigh vacuum (UHV), attracted significant attention as a newly developed bottom-up method to create a building block for 1-nm-size nano devices, one-or two-dimensional nano networks, as a precursor of further on-surface synthesis or a magnetic atom array aligned on a well-ordered molecular template with an extreme low impurity concentration. Experimental and theoretical studies for this type of on-surface metalation via dehydrogenation (e.g., 2HTPP + Fe → H 2 + FeTPP) have been successfully demonstrated by depositing 3d metal atoms on metal-free π-conjugated planar molecular array at 300 K with subsequent annealing of 350−700 K in UHV. Significantly, photoemission spectroscopy study for the on-surface metalation process at cryogenic temperatures suggested a precursor adcomplex state different from the normal metalation state. In this study, we demonstrate that on-surface metalation at cryogenic temperature reduces thermal diffusion of both metal atoms and molecules, which allows gentle adsorption of the 3d metal atoms inside the molecule, providing precursor adcomplex states while preserving the original well-ordering molecular array structure. Using scanning tunneling microscopy (STM) we investigated Fe deposition on a well-ordered metal-free tetraphenylporphyrin (2HTPP) monolayer array islands with an Au(111) substrate temperature maintained at 78.5 K. Through direct STM topographic and spectroscopy imaging supported by density functional theory calculations, we identify three types of TPP appearance: (1) original 2HTPP, (2) 2HTPP with an Fe atom on top (precursor adcomplex "α" state), and (3) 2HTPP with an Fe atom inside (precursor adcomplex "γ" state, where a clear LUMO peak change was observed).
In this combined experimental and theoretical work, we report on the evolution of the skyrmion radius and its destruction in the system Co/Ru(0001) when an out-of-plane magnetic field is applied. At low fields, skyrmions are metastable and display an elliptical instability in which along the short axis, the spin texture approaches that of the spin-spiral phase and the long axis expands in order to go back to the spin-spiral ground state. At high fields, we observe round skyrmions of finite size up to the collapse field B c , where they are destroyed and the topological charge is annihilated. We estimate B c via numerical methods based on magnetization dynamics simulations parametrized by density functional theory calculations and compare it to experimental scanning tunneling microscopy observations obtained at % 30 mK.
Peptide bond formation at the peptidyl transferase center on the ribosome is a crucial phenomenon in life systems. In this study, we conceptually propose possible roles of the RNA tetraplex as a scaffold for two aminoacyl minihelices that enable peptide bond formation. The basic rationale of this model is that "parallel" complementary templates composed of only 10-mer nucleotides can position two amino acids in close proximity, which is conceptually and essentially similar to the situation observed in ribosomes. Using supportive experimental data, we discuss the origin and evolution of peptide bond formation in early biological systems.
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