Functional (Supra)Molecular Nanostructures - ruben-group

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Habilitation Dr. Mario Ruben ULP Strasbourg The so obtained free-standing molecular architecture (in contrast to the previously obtained densely-packed monolayers) of gridlike [Co II 4L4] 8+ metal ion were used as an experimental platform for the study of the electronic properties at the single-molecule level (see chapter 7.2.). B) Surface-based Hydrogen-bonded Nanostructures (in collaboration with Prof. J. V. Barth EPF Lausanne, Switzerland) Although relatively weak, Hydrogen-bonding interactions play a prominent role in organizing matter into hierarchical ordered complex structures. Their reversibility and relative structural flexibility together with the omnipresence of water as a preferred solvent enable them to play a key role especially in the self-organized metastable structures of living matter. In material science, many examples have been used Hydrogen-bonding concepts to tune material structures as well as to generate functionalities. Recently, the weakness and reversibility of the Hydrogen bond was applied in the design of materials with self-adaptive properties and functionalities. Only few research work deals with the transfer of the mainly in bulk studies developed rules of Hydrogen bonding concepts to near-surface conditions on organic or metallic substrates. The amide bond is one of the dominating hydrogen bonding groups in living matter as it interconnects amino acids in peptides. The maximisation of stabilization energy by the formation of configurations of a polypeptide chain with maximal number of -C=O---H-N- bonds (binding energy 21-42 kJ/mol per bond) leads to the fundamental secondary structural motifs as α-helix and ß-sheet. [5] 17
Habilitation Dr. Mario Ruben ULP Strasbourg Figure 9. Molecular structure of N, N-diphenyl-oxamide (Oxamide) (left) and 1D ribbon-like solid state structure of the molecule obtained by single crystal X ray diffraction studies (right). In natural systems, the amid groups –C(=O)-NH- are distanced from each other by the amino acid backbone. Vicinal linking of two amide groups together with hydrophobic side blocking by phenyl substitution (as shown in the molecular structure of N, N- diphenyl- oxamide, Oxamide, in Figure 9) will generate an additional secondary hydrogen bonding motive (beside of the natural α-helix and β-sheet): the 1D linear ribbon. To test this concept, the Oxamide molecule was synthesized following literature protocols [6] and investigated in the solid state by single crystal X-ray diffraction studies (Figure 9). [7] Within the crystal lattice, the Oxamide molecule forms the 1D ribbon motive within by adapting a s-trans configuration around the central oxalic C-C-bond so enabling so the formation of two amide hydrogen bonds per molecule. In order to prove the persistence of this ribbon hydrogen bonding motif under epitaxical conditions, the Oxamide was sublimed onto a clean gold (111) surface. Due to a weak absorbate-substrate interaction, the molecules order to molecular ribbons only below a temperature of 17 K. Within the accuracy of the STM data, basically the same structural features as found in the solid state (1.2 nm width, molecule-to-molecule distance within the chain of 0.5 nm) could be observed (Figure 10). Three different spatial orientations of the ribbons reflect the symmetry of the gold substrate. By the same token, templating effects of the underlying surface are apparently responsible for restricting the maximum lengths of the 18
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