Growth kinetics of racemic heptahelicene-2-carboxylic acid nanowires on calcite (104)

Einax M, Richter T, Nimmrich M, et al. Growth kinetics of racemic heptahelicene-2-carboxylic acid nanowires on calcite (104). Journal of Chemical Physics. 2016;145(13):134702.Molecular self-assembly of racemic heptahelicene-2-carboxylic acid on a dielectric substrate at room temperature can be used to generate wire-like organic nanostructures consisting of single and double molecular rows. By means of non-contact atomic force microscopy, we investigate the growth of the wire-like pattern after deposition by experimental and theoretical means. From analyzing the time dependence of the mean row length, two distinct regimes were found. At the early post-deposition stage, the mean length grows in time. Subsequently, a crossover to a second regime is observed, where the mean row length remains nearly constant. We explain these findings by a mean-field rate equation approach providing a comprehensive picture of the growth kinetics. As a result, we demonstrate that the crossover between the two distinct regimes is accomplished by vanishing of the homochiral single rows. At later stages only heterochiral double row structures remain. Published by AIP Publishing

Molecular Self-Assembly of Enantiopure Heptahelicene-2-Carboxylic Acid on Calcite (1014)

Hauke CM, Rahe P, Nimmrich M, et al. Molecular Self-Assembly of Enantiopure Heptahelicene-2-Carboxylic Acid on Calcite (1014). Journal of Physical Chemistry C. 2012;116(7):4637-4641.Chirality can have a decisive influence on the molecular structure formation upon self-assembly on surfaces. In this paper, we study the structures formed by enantiopure (M)-heptahelicene-2-carboxylic acid ((M)-[7]HCA) on the calcite (10 (1) over bar4) cleavage plane under ultrahigh vacuum conditions. Previous noncontact atomic force microscopy studies have revealed that the racemic mixture of (M)-[7]HCA and (P)-[7]FICA (1:1) self-assembles into well-defined molecular double rows that are oriented along the calcite [01 (1) over bar0] direction. Here, we investigate the enantiopure (M)[7]HCA compound, resulting in distinctly different molecular structures upon deposition onto calcite (10 (1) over bar4). In sharp contrast to the racemate, the (M)-[7]HCA enantiomer forms molecular islands with a (2 x 3) superstructure. Comparison of the results presented here for the enantiopure compound with the results previously obtained from the racemate indicates that heterochiral recognition is responsible for the formation of the unidirectional double row structures formed by the racemate

Toward Molecular Nanowires Self-Assembled on an Insulating Substrate: Heptahelicene-2-carboxylic acid on Calcite (1014)

Rahe P, Nimmrich M, Greuling A, et al. Toward Molecular Nanowires Self-Assembled on an Insulating Substrate: Heptahelicene-2-carboxylic acid on Calcite (1014). Journal of Physical Chemistry C. 2010;114(3):1547-1552.Molecular self-assembly is employed for creating unidirectional molecular nanostructures on a truly insulating substrate, namely the (10 (1) over bar4) cleavage plane of calcite. The molecule used is racemic heptahelicene-2-carboxylic acid, which forms structures, well-aligned along the [010] crystallographic direction and stable at room temperature. Precise control of both molecule-substrate and molecule-molecule interaction is required, leading to the formation of such wire-like structures of well-defined width and lengths exceeding 100 nm. This subtle balance is governed by the heptahelicene-2-carboxylic acid used in this study, allowing for both hydrogen bond formation as well pi-pi stacking

Modular pyridine-type P,N-ligands derived from monoterpenes: Application in asymmetric Heck addition

Novel (diphenylphosphinophenyl)pyridine ligands (+)-8, (+)-15, (-)-21, and (-)-26 were synthesized from (-)-β-pinene, (+)-3-carene, (+)-2-carene, and (-)-α-pinene, respectively, via Kröhnke annulation as the key step, and shown to effect ≤88% ee in Heck addition (27→28). Ligands (+)-15 and (-)-21 are quasi-enantiomeric; ligands 8 and 26 can be prepared in both enantiomeric forms from (+)- and (-)-enantiomers of α- and β-pinene, respectively. © 2001 Elsevier Science Ltd

ENANTIOSELECTIVE [2+2+2] CYCLOISOMERISATION OF ALKYNES IN THE SYNTHESIS OF HELICENES: THE SEARCH FOR EFFECTIVE CHIRAL LIGANDS

The enantioselective [2+2+2] cycloisomerisation of the aromatic triynes under nickel(0) catalysis to afford nonracemic [6]- and [7]helicene derivatives has been systematically studied. A collection of mono- and bidentate phosphines, phosphites, phosphinites and phosphinous amides possessing stereogenic units such as chiral centre, axis or plane (or their combinations) has been tested and axially chiral binaphthyl-derived monodentate MOP-type phosphine ligands were the optimal class of ligands. Nickel complexes of these ligands afforded nonracemic tetrahydro[6]helicene in up to 64% ee in a model reaction.</jats:p

Tailored Formation of N-Doped Nanoarchitectures by Diffusion-Controlled on-Surface (Cyclo)-Dehydrogenation of Heteroaromatics

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH-substrate interaction rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest