Monolayers of a tetralactam macrocycle, which are commonly used as building blocks in the synthesis of rotaxanes or catenanes, are deposited on a Au(111) surface by using vapor deposition. Due to self‐organization, 2D highly ordered supramolecular networks form. From scanning tunneling microscopy (STM) and concomitant density‐function theory calculations, two structurally different phases are found. In both phases, pairs of hydrogen bonds between the amide groups of next‐neighbor macrocycles are responsible for the structural arrangement of the macrocycles. The structure of both phases differs from that of bulk lattice planes, which reveals that the Au(111) surface acts as a template for the growth of the specific 2D structures. These networks of tetralactam macrocycles possibly open a route to study mechanical interlocking processes or guest/host interactions of the molecules in further detail by using STM.
Despite their rigid scaffold, tetralactam macrocycles (TLMs) display a remarkable degree of conformational flexibility, as revealed by analysis of the corresponding X-ray crystal structures. This flexibility is not limited to the rotatability of the TLM amide groups but also applies to the m-xylene rings, and it thus has a great impact on the overall shape of the macrocycle cavity. The conformational properties of the TLMs give rise to a broad variety of intermolecular hydrogen-bonding patterns, including infinite ladders, an interesting catemer motif, and short C-HO=C hydrogen bonds. These results are in accord with previous theoretical calculations, support a structural model proposed earlier for an interpretation of scanning tunneling microscopy images, and substantially contribute to the understanding of the adaptability of macrocyclic scaffolds, which is crucial for guest binding or templated syntheses with TLMs.
The formation of intermolecular complexes of two large moleculessa macrocycle and a semiaxle, which have been used in templated syntheses of amide rotaxanesswas studied by scanning tunneling microscopy (STM) and density functional theory (DFT). These experiments mimic the so-called "threading process", which is based on intermolecular recognition and which is essential for the rotaxane synthesis in solution. First, ordered monolayers of a tetralactam macrocycle (TLM), i.e. the rotaxane wheel, are prepared on a Au(111) surface. Then, semiaxles (SA) are deposited on top of these ordered TLM layers at ca. 140 K. In solution, the SA molecule threads into the TLM cavity by formation of three hydrogen bonds between the amide groups of both molecules. On the Au(111) surface, the scenario is similar, although different in detail due to geometric restrictions given by the underlying Au(111) surface and conformational energy barriers due to the confinement of the TLM geometry in the ordered monolayer structure. Three distinct and defined adsorption sites of the SA molecules with respect to the TLM molecules exist. Notably, the population of these sites is assisted by interaction with the STM tip. Two sites are compatible with a structural model, in which the tail of the SA molecule binds into the TLM cavity, in one case with three H bonds, one to the terminal NH 2 group of the SA and two to the central amide group. This SAsTLM adsorption complex formed at low temperatures is metastable and dissociates at higher temperatures. These results demonstrate the possibility to study intermolecular complex formation by STM.
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